برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات.

دایره المعارف تاسیسات برق (اطلاعات عمومی برق)

Lighting Language


 



Line voltage


A voltage supplied by the electric grid. In US residential buildings, this
refers to 120-volt alternating current (AC) power.


Low voltage


Some electrical devices are designed to work with voltage lower than that
supplied by the electrical system. Such devices use a transformer or power
supply to convert 120v AC power to the voltage and current needed by the
device.


CCT


Correlated color temperature indicates the relative color appearance of a
white light source, from yellowish- white or “warm” (2700-3000 K) to
bluish-white or “cool” (5000+ K).


Luminous efficacy


Light output of a light source, divided by nominal wattage, given in lumens
per watt (lm/W). Does not include driver, thermal, or luminaire optical
losses.


Luminaire efficacy


Light output of a luminaire, divided by total wattage to the power supply,
given in lumens per watt (lm/W). Luminaire efficacy accounts for all driver,
thermal, and luminaire optical losses.


The Lighting Fixture


A luminaire is a complete lighting system consisting of a
lamp(s) and ballast(s) (when applicable) together with the parts designed to
distribute the light, to position and protect the lamps, and to connect the
lamps to the power supply.


Lamps a.k.a. Light Bulbs


A generic term for a man-made source created to produce optical radiation. By
extension, the term is also used to denote sources that radiate in regions of
the spectrum adjacent to the visible. Note: Through popular usage, a portable
luminaire consisting of a lamp with shade, reflector, enclosing globe, housing,
or other accessories is also called a “lamp.” In such cases, in order to
distinguish between the assembled unit and the light source within it, the
latter is often called a “bulb” or “tube,” if it is electrically powered.


Incandescent filament lamp


A lamp in which light is produced by a filament heated to incandescence by an
electric current. Note: Normally, the filament is of coiled or coiled coil
(doubly coiled) tungsten wire. However, it may be uncoiled wire, a flat strip,
or of material other than tungsten.


Halogen Lamps


A gas-filled tungsten filament incandescent lamp containing halogens or
halogen compounds and utilizing the halogen regenerative cycle to prevent
blackening of the lamp envelope during life.


Full Wattage Linear T12 Lamps


Under the terms of the Energy Policy Act of 1992 (EPACT) many of these lamps
can no longer be manufactured due to their relative low efficacy and/or poor
color characteristics.


Reduced Wattage Linear T12 Lamps


The 1992 EPACT still permits the use of antiquated reduced wattage T12 lamps
such as the 34-watt, 48 inch lamps, or the so-called energy-saving lamps. These
lamps save up to 15 percent energy on (older) existing 40-watt electromagnetic
T12 ballasts, at the expense of a corresponding reduction in lumen output.
Reduced wattage versions of several types of T12 lamps are still available to
directly replace their full wattage T12 counterparts except in those
applications where the lamp temperature is too cold or the ballast is
unsuitable, such as some electronic and/or “shoplight” fixtures.


Slimline Lamps


Slimline lamps are similar to the T12 lamps in their energy loading, but they
use a single pin base (instead of the double or bi-pin base) and are instant
start lamps, not requiring a lamp starter. These lamps are available in several
lengths up to 2440 mm (96 in.) in T6, T8, and T12 diameters, the latter of which
is by far the most prevalent.


High Output Lamps


High output fluorescent lamps are a high-current rapid start lamp operating
at approximately 800 milli-amperes (mA). This family of lamps is commonly
applied where the standard lamp does not provide sufficient light output per
lamp length. T12 (and newer T8) lamps are available in up to 2440 mm (96 in.)
lengths and are also particularly suitable for outdoor applications. They use a
recessed double contact base. T12 high output lamps are also affected by EPACT
legislation. Reduced wattage versions of T12/HO lamps meet current legislative
requirements. The newer T8/HO tri-phosphor lamps often retrofit T12/HO with a
ballast change to save energy.


Very High Output Lamps


The 1500 mA fluorescent lamp is also of rapid start design and has the
highest current density commonly available. It is physically, but not
electrically interchangeable with the 800 mA lamp and is used when a lower
current lamp will not meet light output requirements. These lamps are also
affected by EPACT legislation. Reduced wattage versions are available meeting
legislative requirements. Lumen maintenance is inherently poor for both the T12
& the T10 versions.


T8 Lamps


The availability of higher-efficacy phosphors and different gas fill
pressures allowed the development of T8 lamps. They have become the preferred
choice in the specification of new installations of linear fluorescent lamps and
offer over 20 percent increase in efficacy over 40-watt T12 lamps. When the
system is powered by electronic ballasts, system efficacy improves further
still. T8 lamps are available in lengths similar to T12 with compatible bases
and sockets, but require a different, unique ballast. Therefore, in retrofit
situations, the ballast must be replaced. Several wattages of 4-ft bi-pin T8
lamps are available including the North American standard 32-watts. There are
also reduced wattage 4-ft T8 lamps available in 30, 28, 25 and 23 watts, which
in many cases can directly replace the 32-watt T8 with no ballast change
required.


T5 Lamps


Further developments in lamp technology have resulted in the development of
high efficacy T5 straight tube lamps employing tri-phosphor technology. Smaller,
more compact luminaires are possible using these lamps. Standard and high output
(higher power) versions are available. T5 lamp design has promoted development
of luminaires that are more efficient than those using T8 and T12 lamps. T5
lamps, whether linear or twin tube, require properly designed fixtures to
minimize source glare and visual discomfort.


T5 High Output


T5/HO lamps provide significantly greater light output than their T5
counterparts. Although they identically resemble T5 lamps in appearance and
dimension, they are not electrically interchangeable with T5 lamps. Electronic
ballasts that operate T5/HO lamps are required. Special care needs to go into
fixture designs that employ T5/HO lamps, as otherwise significant glare
potentially even greater than that of T5 may result, especially when used at low
mounting heights or in non-industrial spaces. Reduced wattage versions of the
standard 54W lamp have recently been developed which do not require a ballast
change.


Compact Fluorescent


The compact fluorescent lamp (CFL) family compromises a wide
variety of multi-tube, single-based lamps. Initially designed to physically
replace conventional 25 to 100-watt standard incandescent lamps, CFLs conserve
up to 75% energy, provide 8-12 times longer lamp life, and approach the color of
standard incandescent lamps. Today’s CFL designs include wattages and colors
which can replace conventional fluorescent lamps in size-reduced luminaires.
Examples include the 32-watt and 42-watt triple-tube lamps, which are available
in correlated color temperatures ranging from 2700 K to 6500 K. Some CFLs are
manufactured with the lamp and ballast as an integral unit with a (medium) screw
base. Others are manufactured without a ballast and are available in 2-pin or
4-pin configurations. Only 4-pin versions are dimmable. Generally the 4-pin
versions are used with electronic ballasts (either dimmable or on/off versions).
However, 2-pin lamps can also be used on electronic ballasts designed especially
for them. Both CFL types plug into appropriate lamp holders that are used in
luminaires, and can also be inserted into adapter ballasts, which generally come
with a medium screw base for insertion into a standard incandescent socket. Many
integrally ballasted and amalgam type plug-in CFLs require a warm-up time when
initially energized.


High-Intensity Discharge (HID) Lamp


The term high-intensity discharge lamp describes a wide
variety of light sources. The HID family includes high-pressure sodium, mercury,
metal halide, and ceramic metal halide lamps. HID lamps are among the most
efficacious light sources. They are characterized by compact size, long life,
and full temperature range starting and operation. HID sources are normally
designed with inner arc tubes, hard glass outer bulbs, and single ended screw
bases or bi-pin bases with the more compact versions. The inner arc tube
contains an arc discharge operating at a significantly higher pressure than
fluorescent lamps. All HID sources must be operated with a current-limiting
ballast.


These lamps have certain common performance characteristics that include



  • A warm-up period, after starting, until stable light output and electrical
    operating values are reached.

  • A period of time, after any interruption of supply voltage, during which the
    lamps must cool before they will automatically restart.


 

There are HID lamp and ballast systems available however, that will instantly
restart after a short interruption of the supply voltage. The color
characteristics of HID lamps depend on the materials in the arc stream, the
pressure at which the lamp operates, and the presence (or absence) of a phosphor
coating. HID lamps are available in either clear or coated versions. Coated
lamps are used when diffuse light is desired and in some instances, phosphors
are used to lower the color temperature and improve color rendering as well. HID
lamps include groups of lamps known as mercury, metal halide, and high pressure
sodium.


 

High Pressure Sodium (HPS) lamp


 

A high intensity discharge (HID) lamp in which light is
produced by radiation from sodium vapor operating at a partial pressure about
1.33 x 104 Pa (100 Torr). Includes clear and diffuse-coated lamps.


 

Metal Halide Lamp


 

A high intensity discharge lamp (HID) in which the major
portion of the light is produced by radiation of metal halides and their
products of dissociation – possibly in combination with metallic vapors such as
mercury. Includes clear and phosphor-coated lamps, quartz metal halide and
ceramic metal halide.


 

Mercury lamp


 

A high intensity discharge (HID) lamp in which the major portion of the light
is xiii Obsolete USA term produced by radiation from mercury operating at a
partial pressure in excess of 105 Pa (approximately one atmosphere). Includes
clear, phosphor-coated(mercury-fluorescent), and self-ballasted lamps.


 

LED


 

Light Emitting Diodes (LEDs) are solid state electronic
devices for generating light. In recent years, the architectural lighting
applications for these devices have seen rapid growth as improvements in
luminous efficacy and chromaticity have made them into viable alternatives for
some applications. Limitations in color rendering and light color appearance for
“white” LEDs still restrict their full acceptance into point source applications
where incandescent or low-voltage incandescent have previously been used. Issues
with heat management and controls integration also require design
consideration.


 

LED light engines have a typical rated life of 35,000 to 50,000 hours of
operation at which point the light output of the LED will have decreased to 70%
of its initial value. At the end of rated life, the LEDs will likely continue to
operate for an extended period time with a continuing decline in light output.
The rated life of LEDs can be affected by the quality of the electric power and
the ambient temperature of their environment. Poor quality power can negatively
affect the electronic components of the LED system. High ambient temperatures
can make it difficult for the LEDs to shed heat to maintain appropriate
operating criteria.


 

The lumen depreciation of LED light sources must be accounted for in the
initial design. The design layout must over light the space based on the
expected lumen depreciation or the luminaires must either be dimmed or maintain
a constant light output with increasing power consumption over the life of the
system. Successful dimming of LEDs requires careful coordination between the LED
vendors and the controls vendors to match up the required operational
conditions. The increased power consumption of constant output LEDs can be
significantly higher than the initial power consumption and significantly impact
the lighting power density of the project.


 

OLED


 

As of this writing, an emerging lamp technology that is
comprised of molecules of carbon and hydrogen, providing a large area source
that is thin, driven with low-voltage DC current power supplies. Relatively low
light output and expensive, although progress is rapid.


 

Electrodeless Lamps (Induction)


 

Induction fluorescent lamps are basically low pressure gas
discharge fluorescent lamps that operate without the need of electrodes. Because
there are no electrodes to fail, the lamps have lifetimes of up to 100,000 hours
but this depends on thermal management and removal of heat from the generator or
driver.


 

As with standard fluorescent lamps, light is given off by a phosphor coating
excited by ultraviolet radiation from the discharge. The lamp and ballast/driver
are part of a tuned system. Individual components may be exchanged but at the
moment, the lamp/ballast combination should be from the same manufacturer. Lamps
are available in power ranges from 40 W to 400 W. These lamps are finding
greater use in hard to reach locations and where lamp or fixture maintenance
might be especially difficult.


 

Hanging (a.k.a. Pendant)


 

A luminaire that is hung from a ceiling by supports.


 

On The Ceiling or Wall


 

A luminaire that is mounted directly on a wall or on the ceiling.


 

In the Ceiling or the Wall


 

A luminaire that is mounted above the ceiling (or behind a
wall or other surface) with the opening of the luminaire level with the
surface.


 


 

Light Distribution


 


 

Direct Component


 

That portion of the light from a luminaire that arrives at the work-plane
without being reflected by room surfaces.


 

Downward Component


 

That portion of the luminous flux from a luminaire that is
emitted at angles below the horizontal.


 

Direct Ratio


 

The ratio of the luminous flux that reaches the floor of a room cavity
directly to the downward component from the luminaire.


 

Indirect Component


 

That portion of the luminous flux from a luminaire that arrives at the
work-plane after being reflected by room surfaces.


 

Upward Component


 

That portion of the luminous flux from a luminaire that is emitted at angles
above the horizontal.


 

Ceiling Ratio


 

The ratio of the luminous flux that reaches the ceiling directly to the
upward component from the luminaire.


 

Interreflection (also called interflection)


 

The multiple reflection of light by the various room surfaces before it
reaches the work-plane or other specified surface of a room.


 

Ballast Factor (BF)


 

It is a relative measure of the light output from a particular lamp‐ballast
system and is a characteristic of such a system‐not just of the ballast alone.
Ballasts that can operate more than one type of lamp will generally have a
different ballast factor for each combination. Ballast factor is the ratio of a
lamp’s light output on a given commercial ballast, compared to the lamp’s rated
light output as measured on a reference ballast under ANSI test conditions.
Ballasts are available with either normal (conforming to ANSI specifications),
low or high ballast factors. However, ballast factor is not a measure of energy
efficiency. Although a lower ballast factor reduces lamp lumen output, the lamp
may also consume proportionally less input power. As such, the careful selection
of a lamp‐ballast system with 705 a specific ballast factor allows designers to
better minimize energy use by “tuning” the lighting levels in the space. Ballast
factor should not be confused with a similar‐sounding metric called the ballast
efficacy factor (BEF). The BEF, defined as the relative light output of a
particular lamp‐ballast combination under ANSI test conditions (this site is the
ballast factor times 100 percent) divided by the measured input power in watts,
serves as a relative measure of system efficacy and is used to compliance with
federal regulations


 

Ballast Efficiency Factor (BEF)


 

The BEF metric is used solely to show compliance with US
ballast efficacy regulations. It should not be used as a ballast specification
criterion. The BEF is not a true measure of ballast efficiency as its value
depends on the following factors:


 


     

  • Quantity of lamps operated

  •  

  • Type of gas fill in the lamp

  •  

  • Lamp tube diameter/size

  •  

  • Lamp operating frequency


 

The BEF does not indicate absolute light level. In general, BEF values are
not particularly useful to the specifier even though some ballast manufacturers
provide BEF data in their catalogs. The best method of comparing lamp‐ballast
systems is by their system efficacy. The U.S. Department of Energy (DOE) issued
a final rule amending the existing test procedures for fluorescent lamp ballasts
and establishing a new test procedure. The amendments to update a reference to
an industry test procedure. The new test procedure changes the efficiency metric
to ballast luminous efficiency (BLE), which is measured directly using
electrical measurements instead of the photometric measurements employed in the
previous test procedure. The calculation of BLE includes a correction factor to
account for the reduced lighting efficacy of low frequency lamp operation. The
test procedure specifies use of a fluorescent lamp load during testing, allowing
ballasts to operate closer to their optimal design points and providing a better
descriptor of real ballast performance compared to resistor loads.


 

Light Loss Factor (LLF)


 

(Formerly called maintenance factor) The ratio of
illuminance (or exitance or luminance) for a given area to the value that would
occur if lamps operated at their (initial) rated lumens and if no system
variation or depreciation had occurred. Components of this factor may be either
initial or maintained.


 

Note: The light loss factor is used in lighting calculations as an
allowance for lamp(s) or luminaire(s) operating at other than rated conditions
(initial) and for the depreciation of lamps, light control elements, and room
surfaces to values below the initial or design conditions, so that a minimum
desired level of illuminance may be maintained in service. Light loss factors
address losses that result in direct changes to lamp lumens, emitted luminaire
lumens, or the interreflected light delivered to the space.


 

Daylighting


 

Daylighting refers to the art and practice of admitting beam sunlight,
diffuse sky light, and reflected light from exterior surfaces into a building to
contribute to lighting requirements and energy savings through electric lighting
controls.


 

A view of the outdoors is believed to be important for human psychological
and physiological reasons. While daylight can be used to help light a space,
extra care should be taken in industrial environments to control the quantity
and distribution of the light and its associated heat gain. It should be noted
that more illuminance is sometimes needed on interior surfaces near windows to
reduce the contrasts between those surfaces and the windows.


 

Daylight’s dynamic nature makes it a complex light source. The sun’s
continuous apparent movement, coupled with changes in atmospheric conditions,
causes the solar beam and sky dome luminance distribution to vary in intensity
and spectral content. Successful daylighting requires balancing the daylight
distribution in the space throughout the entire year, providing sufficient, but
not excessive, daylight illumination levels for space activities, while
minimizing glare. It also optimizes the building envelope for the geographic
location and climate to maximize energy savings from both lighting and HVAC
systems.


 

Direct sunlight can be extremely bright, with intensity of up to
3×1027candela and luminance of about
1.6×109 cd/m². Interior illuminance levels are
typically only a few to several percent of outdoor illuminance levels in
daytime. The much higher intensities of direct sunlight, when introduced into
interior spaces, can produce painful glare and other unwanted effects. Under
clear skies, direct sunlight requires very careful management, usually being
blocked, diffused, or very carefully transmitted into interior spaces through
thoughtful architectural designs. Sunlight generates very high exterior
illuminance levels, ranging by solar incident angle from 20,000 lux (about 2,000
fc) for high incident angles, to 100,000 lux (about 10,000 fc) at normal
incident.


 

Lighting System Selection and Design


 

In designing an electric lighting system for a daylit space, the designer
must consider how to best integrate electric light with daylighting. Integration
includes selection and layout of a complete system, including lamps, ballasts,
luminaires and controls. In cases where a task-ambient approach is selected,
daylight may supply the room ambient lighting.


 

Some useful recommendations related to the integration of an electric
lighting system with daylighting are the following:


 


     

  • Select an electric light distribution that best integrates with the daylight
    delivery system and the room geometry. In an office environment, an indirect
    system will be less noticeable when dimmed, and helps brighten room surfaces
    within the non-daylit zone. A downlight system provides more localized lighting
    that may better differentiate lighting control zones, but will result in a much
    darker ceiling outside the daylight zone.


 


     

  • Balance luminances across the space. In large daylit spaces, lighting the
    interior wall that faces a window helps to balance room surface brightness.
    Indirect lighting for the ceiling serves a similar function.


 


     

  • Provide a luminaire layout and control zones that are coordinated with the
    daylight zone. This holds whether or not an automated lighting control system is
    applied. A row of luminaires along the windows can be manually switched off
    during daylight hours when provided with separate zone control.


 


     

  • Select a lamp color temperature that integrates well with daylight, while
    serving the space needs. Daylight is generally very cool, with color
    temperatures of 5000K and above. 5000 K lamps, however, may be unacceptably cool
    for non-daylight hours in a building interior. 3500 and 4100 K lamps provide
    acceptable color temperature when combined with daylighting. 3000 K will be
    noticeably warmer than daylight, but may be selected when a warm color
    temperature is desired at night. Daylight at the beginning and end of the day is
    somewhat warm and many believe that the interior should respond in a similar
    manner.


 

Daylighting and Controls


 

For a daylighting system to save energy, daylight must replace electric
lighting during daylight hours. This is accomplished by either switching or
dimming the electric lighting. Occupant control can provide some savings when
flexible personal control is provided through multi-level switching, zoned
switching or dimming; however occupants are not focused on minimizing electric
lighting energy as daylight conditions change.


 

Personal control is likely to save energy when the occupant is forced to
select an appropriate output setting upon entering a space, rather than have all
lights turn on through a single switch. An automatic lighting control system
guarantees that lighting energy savings will occur when daylight is present. A
photosensor signal can be used to adjust the electric lighting by monitoring
either exterior daylight levels, the amount of daylight passing through an
aperture, or the combined daylight and electric light within a space.


 

For proper operation, a photosensor control system must be properly designed
and calibrated. This includes establishing a controlled lighting zone that
correlates with the daylit area, then selecting, locating, and calibrating the
photosensor to accurately sense daylight levels and dim or switch the electric
lighting system accordingly. The daylit zone should receive levels of daylight
that generate significant savings in electric lighting energy. Higher daylight
levels are required with photosensor-based switching as compared to dimming.
Occupant overrides are important for achieving user satisfaction, with some
advanced systems having the capability of adjusting control based on users’
preferences.


 

Externally Reflected Daylight


 

While the sun and sky are the primary sources of daylight, externally
reflected light from the ground and adjacent structures or objects also
contributes luminous flux to daylight apertures. For a vertical window on a flat
site, the ground encompasses the lower half of the field of view. Like skylight,
ground light is usually diffuse, with its luminance a function of the ground
reflectance, the sky conditions, and shadowing and reflections provided by
surrounding objects.


 

Light reflected from the ground provides an important daylight contribution,
since it is directed through vertical apertures to the ceiling and walls. The
fraction of the total incident daylight on a vertical facade that arrives from
the ground can range from below 10% to as high as 70-80% at a ground reflectance
of 20%. The lowest fractions occur when direct sunlight strikes the facade,
while the highest occur on a facade facing away from the sun on a clear day,
when the sky is deep blue and the ground is sunlit. Under an overcast sky, the
ground contribution is generally around 20%.


 

Ground reflectance can vary significantly. Light-colored ground surfaces such
as sand and snow will result in higher ground contributions.


 

Objects such as trees, neighboring buildings, and other portions of the same
building can limit the view of the ground or sky seen from a daylight aperture.
In these situations, daylight from portions of the sky or ground is replaced by
light reflected from the obstructing object, which may either increase or
decrease the daylight delivered to a building interior.


 

Brightness


 

Attribute of a visual sensation according to which an area
appears to emit more or less light.


 

Subjective Brightness


 

The subjective attribute of any light sensation giving rise
to the perception of luminous magnitude, including the whole scale of qualities
of being bright, light, brilliant, dim, or dark.


 

Note: The term brightness often is used when referring to the
measurable luminance. While the context usually makes it clear as to which
meaning is intended, the preferable term for the photometric quantity is
luminance, thus reserving brightness for the subjective sensation.


 

Color Appearance


 

Color appearance is dependent upon the state of chromatic adaptation, the
geometric context for the object being viewed, including the background and
surrounding surfaces, the absolute luminance levels within the field of view,
and other aspects of the optical radiation stimulus and the cognitive attributes
of the observer.


 

Color Rendering Index (CRI)


 

Measure of the degree of color shift a defined set of objects undergo when
comparing their color illuminated by the light source to the color of those same
objects when illuminated by a reference source of comparable color
temperature.


 

Color Temperature


 

The absolute temperature of a blackbody radiator having a chromaticity equal
to that of the light source. A blackbody radiator is a an object which, when
heated, emits a spectrum of radiant power. Radiant power from a practical
source, particularly from an incandescent lamp, is often described by comparison
with that from a blackbody radiator.


 

Correlated Color Temperature (CCT)


 

The absolute temperature of a blackbody radiator whose chromaticity most
nearly resembles that of the light source. CCT is the nearest visual match of
source and blackbody chromaticity.


 

Absorption


 

A general term for the process by which incident flux is
converted to another form of energy, usually and ultimately to heat. (Note: All
of the incident flux is accounted for by the processes of reflection,
transmission, and absorption.)


 

Accommodation


 

This act of focusing the cornea is called accommodation.
Accommodation is always a response to an image of the target located on or near
the fovea rather than in the periphery. It is used to bring a defocused image
into focus or to change focus from one target to another at a different
distance. Any condition, either physical or physiological, that handicaps the
fovea, such as a low light level, will adversely affect accommodative
ability.


 

Adaptation


 

The process by which the all or part of the retina becomes accustomed to more
or less light than it was exposed to during an immediately preceding period. It
results in a change in the sensitivity to light.


 

Candela (cd)


 

The SI unit (metric International System of Units) of luminous intensity. One
candela is one lumen per steradian. Note: The fundamental luminous intensity
definition in the SI is the candela.


 

Discomfort Glare Rating (DGR)


 

A numerical assessment of the capacity of a number of sources of luminance,
such as luminaires, in a given visual environment for producing discomfort. It
is the net effect of the individual values of index of sensation M, for all
luminous areas in the field of view.


 

Emittance


 

The ratio of radiance in a given direction (for directional emittance) or
radiant exitance (for hemispherical emittance) of a sample of a thermal radiator
to that of a blackbody radiator at the same temperature.


 

Equivalent veiling luminance


 

The luminance of the reflected image of a bright surface that is superimposed
on a test object to measure the veiling effect equivalent to that produced by
stray light in the eye from a disability glare source. The disability glare
source is turned off when the reflected image is turned on.


 

Exitance


 

Exitance is the luminous flux density leaving a surface at a point, expressed
in lumens per unit area (lumens per square foot or meter). It can be related to
how luminous the emitting surface is or how “bright” it appears.


 

Eye Adaptation


 

The process by which all or part of the retina becomes accustomed to more or
less light than it was exposed to during an immediately preceding period. It
results in a change in the sensitivity to light. (Note: Adaptation is also used
to refer to the final state of the process, as reaching a condition of
adaptation to this or that level of luminance.)


 

Chromatic Adaptation


 

The process by which the chromatic properties of the visual system are
modified by the observation of stimuli of various chromaticity’s and
luminance’s


 

Scotopic Vision


 

Vision mediated essentially or exclusively by the rods,
generally associated with adaptation to a luminance below about 0.034
cd/m2.


 

Rods


 

Retinal receptors that continue to respond at low levels of
luminance, even below the threshold for cones. At these levels there is no basis
for perceiving differences in hue and saturation. No rods are found in the
fovea.


 

Fovea


 

A small region at the center of the retina, subtending about two degrees
which contains cones but no rods, and forms the site of most distinct
vision.


 

Photopic Vision


 

Vision mediated essentially or exclusively by the cones, generally associated
with adaptation to a luminance of at least 3.4 cd/m2.


 

Cones


 

Retinal receptors that dominate the retinal response when the luminance level
is high and provide the basis for the perception of color.


 

Mesopic Vision


 

Vision with fully adapted eyes at luminance conditions between those of
photopic and scotopic vision, i.e. between about 3.4 cd/m2 and 0.034 cd/m2.


 

Glare


 

The sensation produced by luminance within the visual field that is
sufficiently greater than that to which the eyes are adapted. Glare may cause
annoyance, discomfort or loss of visual performance and visibility.


 

Disability Glare


 

Is caused by a veiling luminance superimposed on the retinal image within the
eye, which reduces visual performance or visibility, and is often accompanied by
discomfort. Reducing illuminance at workers’ eyes and/or raising the source of
the disability glare can alleviate the problem.


 

Direct glare


 

Is glare resulting from high luminances or insufficiently shielded light
sources in the field of view. It usually is associated with bright areas, such
as luminaires, ceilings, and windows that are outside the visual task or region
being viewed. A direct glare source may also affect performance by distracting
attention.


 

Discomfort Glare


 

Produces visual discomfort without necessarily interfering with visual
performance or visibility. It occurs when luminous objects (or reflections of
luminous objects) have significantly higher luminance than the balance of the
person’s field of view. Size, luminance and angular displacement from the line
of sight are all factors. Even a source that is directly overhead, if bright
enough, can cause discomfort glare.


 

Reflected Glare


 

Results from high luminance sources or from luminous difference reflected
from specular (shiny) surfaces. “Veiling reflections” are contrast reducing
reflections from semi-specular surfaces that may reduce task visibility.


 

Illuminance


 

The areal density of the luminous flux incident at a point on a surface.


 

Luminance


 

Luminance is the amount of light coming from a surface or point, and it is
measured in candelas per square meter (cd/m2).
Some objects are self-luminous, that is, they generate light; lamps and computer
screens, for example. Other objects simply reflect light from other sources,
office walls, for example. In the latter case, the surface luminance is a
function of the surface illuminance and the reflective properties of the
surface. For example, a dark gray and a light gray surface can be lit with the
same illuminance yet will have very different luminances. This also exemplifies
why room surface finishes should also be considered as an integral part of the
lighting design.


 

The human eye responds to luminance, not illuminance. The predominance of
illuminance in recommendations is partly due to the fact that illuminance is
easier to measure and calculate.


 

Luminance Ratio


 

The ratio between the luminances of any two areas in the visual field.


 

Lumen


 

A unit of luminous flux. Radio-metrically, it is determined from the radiant.
Photometrically, it is the luminous flux emitted within a unit solid angle (one
steradian) by a point source having a uniform luminous intensity of one
candela.


 

Luminance Contrast


 

Every visual task has some combination of light and dark areas that the human
visual system must discern in order to “see” it. This is called contrast.


 

It could be a black character printed on a white page, or the shadow line of
a nose against a cheek in facial features, or a shadowy area of dirt on a floor.
Contrast may be affected by source/task/eye geometry, disability glare, shadows
and the surface characteristics of objects being viewed.


 

Refraction


 

The process by which the direction of a ray of light changes
as it passes obliquely from one medium to another in which its speed is
different.


 

Reflectance


 

The ratio of the reflected flux to the incident flux.


 

Reflection


 

A general tem for the process by which the incident flux leaves a
(stationary) surface or medium from the incident side, without change in
frequency.


 

Shadows


 

Shadows can interfere with task visibility by placing detail in darkness or
they can enhance definition of three-dimensional details. Point sources (e.g.,
incandescent or high intensity discharge lamps) create more defined shadows than
fluorescent lamps, which produce diffuse shadows.


 

Shadows cast by the structure of the task may reveal detail, or may mask what
needs to be seen. High reflectance surroundings help fill in and modify shadows,
as do luminaires with 10 percent or more up light when the ceiling cavity
reflectance is over 50 percent.


 

The presence of shadows may be desirable, and the interplay of highlight and
shadow helps to define the form of many visual tasks, as well as the
architectural environment. Lighting vertical surfaces to at least half the
horizontal illuminance level often brings the ratio of highlight to shadow into
a tolerable range for three-dimensional tasks. Some shadow will still be
present, which helps to model the task and reveal form. Since each visual task
has an optimum range of modeling, a careful evaluation of critical visual tasks
should be made to determine the effects of various ratios of horizontal vs.
vertical illuminance on visibility.


 

Transmission


 

A general term for the process by which incident flux leaves a surface or
medium on a side other than the incident side, without a change in
frequency.


 

Note: Transmission through a medium is often a combination of
regular and diffuse transmission.


 

Transmittance


 

The ratio of the transmitted flu to the incident flux. It should be noted
that transmittance refers to the ratio of flux emerging to flux incident;
therefore, reflections at the surface as well as absorption within the material
operate to reduce the transmittance.


 

Uniformity


 

Even if a regular grid pattern of identical luminaires is used, the light
distribution of luminaires is such that perfect uniformity across work surfaces
can never be produced. The ratio of average illuminance to minimum illuminance
over the practical work surface, (i.e., excluding corners and edges) for many
applications should not be lower than 1.4. Illuminance variations across an
entire space may be larger, and should produce luminances that respect the
luminance ratio limits.


 

Veiling Brightness


 

A brightness superimposed on the retinal image that reduces its contrast. It
is this ceiling effect produced by bright sources or areas in the visual field
that results in decreased visual performance and visibility.


 

Veiling Reflection


 

Regular reflections that are superimposed upon diffuse reflections from an
object that partially or totally obscure the details to be seen by reducing the
contrast. This sometimes is called reflected glare. Another kind of veiling
reflection occurs when one looks through a plate of glass. A reflected image of
a bright element or surface may be seen superimposed on what is viewed through
the glass plate.


 

Visibility


 

The quality or state of being perceivable by the eye.


 

Planned Indoor Lighting Maintenance


 

Maintenance is the link between predictive design and actual
performance over the long term. In the design of new systems, the owner’s
maintenance practices, which include relamping and luminaire cleaning, determine
the light loss factors that are applied in lighting system design calculations.
These in turn may affect the number of lamps or luminaires required in the
space. Proper maintenance can therefore help to achieve a low energy design. A
lighting system that is regularly cleaned will result in less luminaire dirt
depreciation and require fewer luminaires, which reduces the system’s initial
cost, installed lighting power, and lighting energy consumption. Similarly,
group relamping can help maintain higher lamp lumen depreciation factors.


 

In some cases, it may be necessary to educate the owner or facility operator
on the importance of good maintenance practices, and the impact these practices
have on the installed lighting power and the associated initial and operating
costs.


 

Economics


 

The cost of the lighting system is, of course, an important
area to be considered. There are direct costs related to the actual cost of the
installation and operation of the lighting system which must be carefully
weighed and balanced against factors related to quality of the lighting for a
given application. Economic analysis gives insight into the question of when a
lighting system under consideration will “pay off.” It can help the lighting
designer make decisions regarding energy conservation. Most importantly, it
provides methods for gauging the profitability of a capital investment in a
lighting system. Many metrics and techniques for answering these questions have
been proposed over the years. These methods can be classified into two
categories: first-level analysis methods, and second-level analysis methods.


 

First-level methods (such as simple payback) are attractive due to their
simplicity and can be used for quick estimates involving short payback periods.
Second-level analysis allows the comparison of all economic events in the life
of a lighting system (including initial cost, maintenance, energy cost, and
salvage value). These factors are converted into their value today, or present
value using the principle of time equivalence. The benefits and savings are
totaled and compared with the sum of the costs and disadvantages. If the first
sum is greater, the system should be purchased. If the second is greater, it
would be unprofitable to purchase the system. Of the second-level methods, Life
cycle Cost/ Benefit Analysis (LCCBA) has emerged as the most robust method (is
only robust if our information on the lesser known factors such as employee
satisfaction and productivity is complete), and the one that is accepted by
experts in managerial economics from all industries.


 

Accordingly, LCCBA is the economic analysis method recommended by IES.


 

Factors Related to Direct Cost of Light.


 

Those factors having a direct impact on estimates of the cost of light
produced by any specific lighting system include:


 


     

  1. Cost of luminaires

  2.  

  3. Lamp cost

  4.  

  5. Auxiliary equipment costs

  6.  

  7. Labor costs (of installation)

  8.  

  9. Luminous efficacy

  10.  

  11. Cost of electricity, include use and demand charges

  12.  

  13. Efficiency of auxiliary equipment

  14.  

  15. Useful life of lamps and auxiliary equipment

  16.  

  17. Replacement cost (labor plus materials)

  18.  

  19. Operating hours per year

  20.  

  21. Starting frequency

  22.  

  23. Cleaning scheduled

  24.  

  25. Maintenance program

  26.  

  27. Amortization rates

  28.  

  29. Interest rates

  30.  

  31. Taxes

  32.  

  33. Insurance

  34.  

  35. Environmental costs


 

Maximizing Energy Efficiency through System Design


 

One of the best ways to reduce lighting power is to install lighting only
where it is needed. There are many aspects to quality lighting, but the designer
should always be aware that lighting systems do not necessarily need to rely on
permanently installed ceiling systems to generate all lighting. For instance,
task / ambient systems provide permanent ceiling or wall mounted lighting
systems providing a general “ambient” level of light and rely on work station
“task” lighting to supplement the ambient light producing the resultant
recommended levels only where they are needed. It is generally accepted that
room surfaces using light finishes can also produce more pleasant environments.
Carefully integrating daylight into the space not only improves indoor
environmental quality, but may provide the opportunity to eliminate permanently
installed lighting or at least reduce the amount of permanently installed
lighting or the amount of time electric lighting is needed by the use of
automatic controls.


 

Maximizing Energy Efficiency through Equipment
Selections


 

Lighting systems (lamps and ballasts) and luminaire distributions continue to
improve, providing additional strategies for reduced energy consumption.


 

One notable example is within the fluorescent product segment. Standard T8
lamps with commonly used “standard” electronic ballasts are being replaced by
third generation “high performance” T8 systems (Consortium for Energy Efficiency
specifications) and highly efficient T5 systems, providing system efficacies
above 90 lumens per watt. The “NEMA Premium” program tests ballasts for meeting
the same qualifying specifications CEE HPT8 and allows such products that pass
to be labeled “NEMA Premium”.


 

There are notable examples in most other categories. Ceramic metal halide
sources can last over twice as long and produce twice the center beam candle
power while using half the power of incandescent accent lighting. Compact
fluorescent lamps have all but replaced incandescent lighting for general down
lighting solutions. Solid state lighting, better known as LED, is rising to meet
our lighting challenges. Standardized testing procedures through
IES
LM-79
, LM-80 and TM-21 make product quality more
identifiable. Additionally, as with HPT8, voluntary testing and standards
programs such as CALiPER, ENERGY STAR and DesignLights™ Consortium and the
Municipal Solid State Lighting Consortium offer designers and consumers clearing
houses for finding good quality products on everything from holiday light
strings to downlights to pole mounted area lighting.


 

Diffusers


 

The introduction of high brightness, low gloss LCD screens that are less
susceptible to reflected glare has led to resurgence in the popularity of lensed
luminaires. Their higher efficiencies and higher angle distributions can produce
brighter, more open visual impressions.


 

A luminaire with a diffuser spreads the light from its lamps evenly over the
diffuser surface. Since the diffuser area is much larger than the area of the
lamps, the average luminance (compared to using bare lamps) is reduced. However,
when diffusers are used, the luminance at all viewing angles near horizontal may
be as high as looking straight up at the luminaire. In a large office, this will
result in low Visual Comfort Probability (VCP) as well as possible reflections
in specular VDT screens, reducing visibility. In small offices, diffusers may be
appropriate if high luminance areas are outside a worker’s peripheral vision,
their broad distribution does not create excessive wall luminance, and the
partitions are full height and opaque.


 

Lenses


 

A luminaire with a lens may incorporate a series of small prisms that control
the photometric distribution of the light to reduce the luminance of the
luminaire in the near-horizontal viewing angles between 45° and 90° from
vertical. Depending on the exact characteristics of a specific lens, glare from
the luminaire may be reduced in large open spaces. However, with most lenses,
the reduction is not sufficient to prevent luminaire reflections in specular VDT
screens or to provide an acceptable VCP.


 

“Reflected Direct” Luminaires


 

A class of luminaires, similar in performance to lensed or diffused
luminaires, makes use of light reflecting off the inner surface of a recessed
housing from concealed or obscured sources. Typically, these luminaires exhibit
perforated metal baskets backed with diffuse acrylic to hide the lamps from
direct view and direct light onto the upper reflector surface. The upper
reflector is commonly a smooth surface with a matte, high-reflectance finish,
however, textured aluminum reflector variations are also available. This optical
system results in photometric distributions generally similar to those of
perfectly diffuse emitters. Other variations manipulate the lamp shields with
louvers or lenses for higher luminaire efficiency, but with more glare
potential.


 

Parabolic Louvers


 

Luminaires shielded with a grid of parabolic louvers having a specular
(mirror-like) finish can control luminance precisely. The grid comprises an
array of cells, each with their walls in the form of parabolic reflectors. Cell
size ranges from 1.27 cm by 1.27 cm (0.5 in. by 0.5 in.) to almost 30.48 cm by
30.48 cm (12 in. by 12 in.). The smaller cell types are usually injection-molded
plastic vacuum-metallized with aluminum. The larger cells are usually fabricated
from aluminum sheets, anodized prior to forming. A specular finish permits cells
to control luminaire light output precisely so that almost no light is emitted
above the cutoff angle. When this is the case, the louver may look darker than
the ceiling. A semi–specular or mostly diffuse louver finish may be used to
brighten the appearance of the luminaire. However, the higher luminance of more
diffuse finishes may show up as reflections in VDT screens.


 

The precision of the luminaire cutoff will also influence the overall visual
impression of the space. The more specular louvers may limit how much light
reaches the walls. Combined with the dark appearance of the ceiling, the
relatively bright areas of the horizontal work plane can produce a cave-like
impression. Supplemental accent lighting to wash or accent wall surfaces is
often necessary to maintain proper luminance ratios between various surfaces in
the space and to provide for more pleasant visual environments.


 

When judging the performance of parabolic luminaires, cutoff angles are a key
factor. Parabolic louvers can actually be described with several cutoff angles,
however two cutoff angles are better suited for performance comparison.


 

The first is the physical cutoff angle: the angle from vertical of a line
drawn from the bottom of a parabolic cell to the top of the opposite side. This
is the cutoff angle for the light passing directly through the louver.


 

The second is the optical cutoff angle: the angle from vertical at which rays
directly from the lamp that are reflected from the surfaces of the parabolic
cells cannot be seen. Optical cutoff depends on the curve of reflector surfaces
and is not always the same as the physical cutoff. For precise cutoff, the two
cutoff angles should be identical


 

Room Reflectance


 

The ratio of the flux actually reflected by a sample surface to that which
would be reflected into the same reflected-beam geometry by an ideal perfectly
diffused, completely reflecting standard surface in exactly the same way as the
sample.


 

On/Off Switching


 

On/off switching is commonly performed at the entrance to a space, as
required by code, and at other locations of convenience to the users. Switching
is generally used in spaces where lighting system operation is intended to be
either on or off, or where switching individual groups of lights can provide
sufficient adjustment of work plane illuminance and general or localized
luminance distributions.


 

Multilevel Switching


 

Multilevel switching is required by some energy codes, and involves switching
of different lamps within luminaires or different luminaires within a layout to
provide two or more illuminance levels within a space. In day lit spaces, this
provides energy savings when a user determines that full light output is not
necessary and activates only a portion of the available hardware.


 



 


 

Dimming


 

Continuous dimming is achieved by reducing the lamp current. Most
commercially available dimming ballasts are electronic, though magnetic dimming
ballasts may still be encountered in existing construction. The dimming ballast
must be able to communicate with the connected control devices, which forms the
basis for a controls protocol. Control protocols can be either analog or
digital. For more detailed information, refer to IES TM-23-11 Lighting Control
Protocols.


 

Stepped dimming can be achieved in one of two ways: 1. by switching off one
or more lamps in a multi-lamp lamp luminaires; 2. with stepped-dim ballasts.


 

Provided there are enough steps, stepped dimming can approach continuous
dimming in final effect.


 

Fluorescent Ballast


 

The ballast controls the starting of the lamp, the electrical conditions
during operation (e.g. power factor, harmonics), and is a key component of
system efficacy. The current limiting component of a ballast can be a resistor,
capacitor, inductor (a.k.a. ‘choke’), or an electronic circuit.


 

High frequency electronic ballasts should be employed for new specifications
because they have several important advantages over the magnetic types: improved
lamp and system efficacy of approximately 10%, no flicker or stroboscopic
effects, integrated starting circuitry, increased lamp life, excellent ability
to regulate lamp lumen output, integrated power factor (PF) correction, quiet
operation, comparatively light weight, many options for input voltage, and some
can be used with direct current (DC).


 

Low power factor ballasts are more common with compact fluorescent systems
than for 4-ft and 8-ft fluorescent systems. Some utilities require high power
factor equipment or have established penalty clauses in their rate schedule for
installations with low power factor.


 

Low Voltage Transformers


 

Low-voltage incandescent lamps operate at 12V or 24V and typically require
transformers to adjust the line voltage down from 120V, 277V, or 347V. The
transformers can use either magnetic coils or solid state electronics to adjust
the voltage. Depending on the application, the style of luminaire and the type
of transformer used, transformers are sized for either a single lamp or for a
group of multiple lamps. The secondary voltage provided by the transformer must
be verified for proper operation, some transformers are intentionally designed
to provide more or less than the expected design voltage to account for line
voltage variations or voltage drop through long lengths of wiring. Power
consumption of the transformer is also something to be noted and accounted for
both with the lights operating and with the lamps off but with primary
magnetizing current still flowing to the transformer.


 

Scheduling/Control zones


 

Control zones (sometimes referred to as control channels) are groups of
luminaires that are switched or dimmed together, and should be arranged to
provide flexibility, appropriate light distributions for functional and
aesthetic effects, and energy savings. Load schedules, which consider source
and/or ballast type and lighting power within each zone are used to size the
lighting circuits and lighting control equipment. For zones that are to be
dimmed, control zones consist of luminaires and/or lamps of the same type for
consistent, reliable dimming.


 

The layout of control zones is generally based on groupings of lighting
equipment according to one or more of the following:


 

• By the architectural feature, area, or task being illuminated


 

• By access to daylight to allow for energy savings in day lit zones


 

• By operating schedule for both manual and time schedule control


 

• To provide multilevel control in spaces through switching or dimming


 

• By equipment type


 

Occupancy Sensors


 

Electric lighting that remains on in spaces which are unoccupied is usually a
waste of energy. Occupancy sensors incorporate motion sensors that detect
occupancy and can switch lighting on when someone enters a space and off when
the space is vacated.


 

Occupancy sensors that automatically turn lights on the half load (requiring
manual operation to go to full load) actually saves energy over those that are
manual on only.


 

Vacancy Sensors


 

A sensor that requires the occupant to switch lighting on, which has the
potential to save additional energy, particularly in daylit spaces. The sensors
are configured to only turn lighting off when a sensor detects no activity
within a space.


 

Daylight Harvesting


 

Daylight harvesting is a broadly used term that describes the application of
energy saving daylighting strategies to maintain target interior task level
illuminances.


 

Load Shedding


 

A facility operator- or automatic control strategy in which illuminances are
lowered based on time of day when demand charges are highest or in response to
energy price spikes or utility emergency event.


 

Daylighting Control Commissioning


 

The commissioning process for a photosensor-based dimming control system
typically involves calibrating the control algorithm under a representative
daylight condition. Depending on the control algorithm applied, it may also
require a nighttime signal. For dimming control, daytime calibration involves
placing an illuminance meter at the critical task location, and then the dimmed
zone output is adjusted to produce the desired illuminance at this point. Other
task locations should then be checked to determine if they require a higher
setting from the controlled lighting zone.


 

The daylight condition used for calibration should meet the following
conditions:


 

The daylight condition should provide a dimmed zone setting slightly above
the minimum dimming level. It may be necessary to adjust blinds or shades on the
windows to achieve an appropriate daylight level.


 

The daylight condition selected should provide a relatively high S/E ratio.
If horizontal blinds are applied, they should be angled to provide a high
photosensor signal (which may be based on the ceiling illuminance at the
photosensor) in relation to the critical point illuminances. Calibration under
overcast sky conditions should be avoided with side lighting applications since
these usually provide low S/E ratios that result in under lit environments under
other sky conditions. (S/E being the ratio of the electric lighting and
daylighting levels. If the controls aren’t calibrated with a proper S/E ratio
when the electric lighting goes off the controls will read too dark of an level
and turn them on, when it will read there is now too much light in the space so
the lights will go off.)


 

Recycling


 

Modern lighting systems may contain small amounts of mercury, lead, cadmium,
other heavy metals, PCBs, corrosives, and other toxic substances. These
substances may be damaging to human health and the environment if they are
released from the products in which they are contained. In the past, ballasts
containing polychlorinated biphenyls (PCBs) and lamps containing mercury were
disposed of in the normal building solid waste stream. These toxic substances
were buried in landfills – free to react with many substances – or incinerated
to create energy. They could leach slowly into the groundwater or be emitted
into the air through the landfill’s ventilation system, or be released directly
into the atmosphere with a combustion facility’s emissions.


 

It is important to check state and local regulations to ensure proper
compliance. In addition, the use of registered and certified recycling companies
can limit liability, insure compliance with all applicable rules and
regulations, and simplify record keeping – see www.lamprecycle.org.


 



 


 

Disposal


 

Disposal of luminaires varies based on the specific
components within the luminaire. Federal and state laws govern disposal of these
products, (such as ballasts, fluorescent lamps, HID lamps, and batteries)
following federal regulations such as: the Resource Conservation and Recovery
Act (RCRA); the Toxic Substance Control Act (TSCA); the Comprehensive
Environmental Response, and Compensation and Liability Act (CERCLA, or
Superfund); and the Universal Waste Rule (UWR). While many federal regulations
pre-empt state laws, in the case of the Universal Waste Rule, the federal
guidelines are a baseline for states to adopt or exceed. Some states have
enacted standards that are more stringent than these federal rules and
regulations. And for some products, many states have passed their own laws that
are broad enough to encompass future technologies like LED. All mercury
containing lighting products are regulated by the U.S. EPA/UWR, including lamps
that contain mercury, cadmium, or lead. The US EPA hosts a municipal solid waste
state data website
http://www.epa.gov/msw/states.htm
that has information on state regulations and contacts.


 

Energy Management ChecklistEnergy Management Checklist


 

The checklist is designed to be initialed and dated by the designer and
client to signify subject review and agreement regarding an energy-conscious
lighting program.


 


 

Thermal Energy Effects


 

Every watt of lighting power introduces 3.6 kJoules (3.41 BTUs) per hour of
heat into the space. If the building is in cooling mode, air conditioning energy
(usually provided by electricity) will be required to remove this heat. If the
building is in heating mode, the heat from lighting offsets the heating provided
by the building’s main heating system (usually not electric). Most office
buildings in North America experience both heating and cooling regimes over the
course of a year, so the overall effect of lighting on thermal loads is building
and climate dependent.


 

Concepts


 

Codes and related documents might seek to control total installed lighting
power density, lighting energy use, lighting system component efficiencies, or
some combination of these. Some documents might also mandate certain control
types for certain space types, such as occupancy sensors, multilevel switching,
or daylight harvesting.


 

Emergency Lighting


 

Emergency lighting helps ensure the safety of a building’s occupants when the
normal lighting system fails. The emergency lighting system should permit
orderly, accident free building egress during an emergency situation. For those
people remaining inside, the emergency lighting should provide a feeling of
safety and comfort until the general lighting is restored.


 

The emergency electric lighting systems in present use are:


 

Separate luminaires and wiring powered by a standby or emergency generator
with an independent driving source (e.g., a diesel engine, etc.). This system
provides backup power to the normal lighting system and may provide the
necessary illumination if the normal lighting fails.


 

Common luminaires supplied by the regular utility and by a secondary source
that is activated automatically when the primary source fails. The secondary
source may be centrally located batteries or inverter equipment that are
automatically recharged after use, or it may be an on-premises auxiliary
generator that provides the necessary power within ten seconds after the primary
source fails.


 

Unit equipment supplied by the same regular utility branch circuits that
serve other lighting in the spaces being protected. Each unit is activated
automatically and independently when the utility or building electrical systems
fails because each has its own battery. These batteries are recharged after
power is restored.


 

Each state or jurisdiction is responsible for creating and adopting its own
building code. The National Fire Protection Association (NFPA) develops model
codes that jurisdictions typically adopt, with or without amendments. NFPA 70,
the National Electrical Code (NEC), establishes minimum standards for the
installation of lighting-related equipment and electrical safety as well as
power for emergency egress lighting. NFPA 101, the Life Safety Code, defines
allowable paths of egress and minimum lighting during egress conditions.


 

Exit Signs


 

Exit pathways and exit access doors must be clearly
identified with approved exit signs easily visible from any
direction of egress travel. Light sources may be inside or outside

of signs, but they must operate for no less than 90 minutes during
power interruptions. Even at locations where the exit is not
clearly visible, the route to the exit must be identified in
the same manner as the exit itself. Proper directional arrows must be

provided on all route-to-exit signs. Exit sign placement shall be such
that no point in the egress pathway is more than 100 feet from
the nearest visible exit sign. Exit signs should be located and
lighted so they are visible during the presence of smoke. Each state or
jurisdiction is responsible for creating an adopting its own building code. NFPA
101, the Life Safety Code, defines placement and visibility of exit signs.


 

Inverse-square law


 

A law stating that the illuminance E at a point on a surface varies directly
with the intensity I of a point source, and inversely as the square of the
distance d between the source and the point. If the surface at the point is
normal to the direction of the incident light, the law is expressed by E =
I/d.


 

Note: For sources of finite size having uniform luminance this gives
results that are accurate within one percent when d is at least five times the
maximum dimension of the source as viewed from the point on the surface. Even
though practical interior luminaires do not have uniform luminance, this
distance d is frequently used as the minimum for photometry of such luminaires,
when the magnitude of the measurement error is not critical.


 

Cosine law


 

A law stating that the illuminance on any surface varies as the cosine of the
angle of incidence. The angle of incidence is the angle between the normal to
the surface and the direction of the incident light. The inverse-square law and
the cosine law can be combined as E = (I cos θ)/d.


 

Cosine-cubed law


 

An extension of the cosine law in which the distance d between the source and
surface is replaced by h/cos θ , where h is the perpendicular distance of the
source from the plane in which the point is located. It is expressed by E =(I
cos3 θ)/h.


 

Work-Plane


 

The plane on which a visual task is usually done, and on which the
illuminance is specified and measured. Unless otherwise indicated, this is
assumed to be a horizontal plane 0.76 meter (30 inches) above the floor.


 

Initial Luminous Exitance


 

This term can be used in two different ways. In flux transfer it is the
density of luminous flux leaving a surface within an enclosure before inter
reflections occur


 

Note: For light sources this is the luminous exitance. For
non-self-luminous surfaces it is the reflected luminous exitance of the flux
received directly from sources within the enclosure or from daylight. In
lighting calculations it is the total exitance at time zero before depreciation
(light loss) occurs.


 

Coefficient of Utilization (CU)


 

The ratio of luminous flux (lumens) calculated as received on the work plane
to the total luminous flux (lumens) emitted by the lamps alone.


 

Zonal Cavity Coefficients


 

Coefficients used in interior lighting design to determine illuminance and
exitance (or luminance) values from room and luminaire parameters.


 

Photometry


 

The measurement of quantities associated with light, often used to refer to
the intensity (candlepower) distribution curve of a lamp or luminaire.

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