Accent Lighting - Directional lighting to emphasize a particular object or draw
attention to a display item.
Adaptation - The process by which the human eye adjusts to a change in light
Ambient Lighting - The general lighting present in an area --excluding task
lighting and accent lighting but including general lighting and daylight streaming
Amperes - A measure of electrical current. In incandescent lamps, the
current is related to voltage and power as follows: Watts (power) = Volts x Amps
American National Standards Institute (ANSI)- A consensus-based
organization which coordinates voluntary standards for the physical, electrical
and performance characteristics of lamps, ballasts, luminaires and other lighting
and electrical equipment.
Application - Also called "lighting application," it refers to the particular use the
lamp is being put to. (e.g. high-bay industrial application or retail lighting
application.) The term can also refer in a general way to "application engineering"
which deals with specific parameters and usage of light sources. (e.g. how to do
a lighting layout, where to place fixtures and so on.)
Arc - A general term for a high intensity electrical discharge occurring between
two electrodes in a gaseous medium, usually accompanied by the generation of
heat and the emission of light.
Arc Lamp - A light source containing an arc (see above). Also called a discharge
lamp, or an arc discharge lamp.
Arc Length - In High Intensity Discharge lamps this is the distance between the
electrode tips, which represents the physical length of the electrical discharge.
Atmosphere - This field designates the type of gas or vacuum filling a volume or
chamber of the lamp. This chamber might contain a filament or it might refer to
the bulb which contains the arc tube.
Baffle - A single opaque or translucent element to shield a source from direct
view at certain angles, or to absorb unwanted light.
Ballast - A device used with an electronic discharge lamp to obtain the
necessary circuit conditions for starting and operating all fluorescent and HID
Ballast Factor (BF) - This is the percentage of a lamp's rated lumen output that
can be expected when operated on a specific, commercially available ballast. For
example, a ballast with a ballast factor of 0.93 will result in the lamp's emitting
93% of its rated lumen output. A ballast with a lower BF results in less light output
and also generally consumes less power.
Base - the end of the lamp that fits into the socket. There are many types of
bases used in lamps, screw bases being the most common for incandescent and
HID lamps, while bi-pin bases are common for linear fluorescent lamps.
Bayonet - A style of bulb base which uses keyways instead of threads to
connect the bulb to the fixture base. The bulb is locked in place by pushing it
down and turning it clockwise.
Beam Angle - The angular dimension of the cone of light from lamps (such as R
and PAR types) encompassing the central part of the beam out to the angle
where the intensity is 50% of maximum. The beam angle sometimes called
"beam spread" is often part of the ordering code for the lamps. Example: The
50PAR30/HIR/NFL25 is a 50 watt PAR30 narrow flood lamp with a beam angle of
Beam Lumens - The total lumens present within the portion of the beam
contained in the beam angle.
Beam Spread - For reflector type lamps. The total angle of the directed beam (in
degrees horizontal or vertical) to where the intensity of the beam falls to 50% or
10% of the maximum candlepower value as indicated.
Bi-Pin - Any base with two metal pins for electrical contact. This is the typical
base for a fluorescent tube of 1 to 4 feet in length. It consists of 2 prong contacts
which connect into the fixture. Medium bi-pins are used with type T-8 and T-12
tubular fluorescent lamps, and miniature bi-pins are used for tubular T-5
Black Light - A popular term referring to a light source emitting mostly near UV
(320 to 400 nm) and very little visible light.
Bollard - A short, thick post with a light at its top, used for grounds and outdoor
Bulb - A loose way of referring to a lamp. "Bulb" refers to the outer glass bulb
containing the light source.
Bulb Size - Bulb shape followed by its size (the maximum diameter of the bulb
expressed in eighths of an inch). The code also includes a reference such as T8
or T5 to represent the size of the tube.
Brightness - Brightness can refer to any of several technical terms used in
lighting and is, therefore, ambiguous.
Candela (cd) - The measure of luminous intensity of a source in a given
direction. The term has been retained from the early days of lighting when a
standard candle of a fixed size and composition was defined as producing one
candela in every direction. A plot of intensity versus direction is called a candela
distribution curve and is often provided for lamps and for luminaires with a lamp
operating in them.
Candlepower - current practice is to refer to this simply as candelas.
An obsolete term for luminous intensity
Candlepower (Mean Spherical) - Initial mean spherical candlepower at the
design voltage. Mean spherical candlepower is the generally accepted method of
rating the total light output of miniature lamps. To convert this rating to lumens,
multiply it by 12.57 (4 pi)
Case Quantity - Number of product units packed in a master case. Also known
as Standard Package Quantity.
Cathode Resistance - Resistance of the cathode in a Fluorescent lamp. It is
measured "cold" before the lamp is turned on (Rc) or "hot" after the lamp is
turned on (Rh). The ratio of the hot resistance to cold resistance is also measured (Rh/Rc).
Cost of Light - Usually refers to the cost of operating and maintaining a lighting
system on an ongoing basis. The 88-8-4 rule states that (typically) 88% is the
cost of electricity, 8% is labor and only 4% is the cost of lamps.
Crest Factor (Max Current) - The ratio of the peak lamp current to average
lamp operating current (RMS). The lower the current crest factor is, the gentler
the ballast is on the lamp.
Current - A measure of the flow of electricity expressed in amperes.
Current Type (AC/DC) - Whether the operational voltage is based on Alternating
Current or Direct Current.
Daylight Harvesting - Lighting design for building interiors that makes of
daylight as a way of reducing energy consumption.
Daylight Lamp - A lamp resembling the color of daylight, typically with a color
temperature of 5500 K to 6500K
Diffused Lighting - light that is not predominantly incident from any particular
Diffuser - A device to redirect or scatter the light from a source by the process of
Dimmable - Whether or not the lamp lumens can be varied while maintaining
Dimmer, Dimming Control - A device used to lower the light output of a source,
usually by reducing the wattage it is being operated at. Dimming controls are
increasing in popularity as energy conserving devices.
Direct Lighting - If luminaries distribute 90-100% of the emitted light in the
general direction of the surface to be illuminated.
Driver (LED) - Power source and control circuitry designed to operate an LED
package, module or lamp.
Efficacy - A measurement of how effective the light source is in converting
electrical energy to LUMENS of visible light. Expressed in LUMENS-PER-WATT
(LPW) this measure gives more weight to the yellow region of the spectrum and
less weight to the blue and red region where the eye is not as sensitive.
Efficiency - The efficiency of a light source is simply the fraction of electrical
energy converted to light, i.e. watts of visible light produced for each watt of
electrical power with no concern about the wavelength where the energy is being
radiated. For example, a 100 watt incandescent lamp converts 7% of the
electrical energy into light. The efficiency of a luminaire or fixture is the
percentage of the lamp lumens that actually comes out of the fixture.
Electrical Discharge - A condition under which a gas becomes electrically
conducting and becomes capable of transmitting current, usually accompanied
by the emission of visible and other radiation. An electric spark in air is an
example of an electrical discharge, as is a welder's arc and a lightning bolt.
Electrodeless Lamps - Light sources where the discharge occurs in a chamber
with no electrodes (no metal.) The energy for the discharge is supplied by radio
frequency excitation, e.g. microwaves.
Electromagnetic Ballast - A ballast used with discharge lamps that consists
primarily of transformer-like copper windings on a steel or iron core.
Electromagnetic Interference (EMI) - High frequency electronic ballasts and
other electronic devices can produce a small amount of radio waves which can
interfere with radio and TV. Federal mandated requirements must be met for EMI
levels before an electronic device is considered FCC compliant. (FCC is the
Federal Communications Commission.)
Electromagnetic Spectrum - A continuum of electric and magnetic radiation that
can be characterized by wavelength or frequency. Visible light encompasses a
small part of the electromagnetic spectrum in the region from about 380
nanometers (violet) to 770 nanometers (red) by wavelength.
Electronic Ballast - A short name for a fluorescent high frequency electronic
ballast. Electronic ballasts use solid state electronic components and typically
operate fluorescent lamps at frequencies in the range of 25-35 kHz. The benefits
are: increased lamp efficacy, reduced ballast losses and lighter, smaller ballasts
compared to electromagnetic ballasts. Electronic ballasts may also be used with
HID (high intensity discharge) lamps.
Energy Policy Act (EPACT) - Comprehensive energy legislation passed by the
- S. Congress in 1992. The lighting portion includes lamp labeling and minimum
energy efficacy (lumens/watt) requirements for many commonly used
incandescent and fluorescent lamp types. Federal Canadian legislation sets
similar minimum energy efficacy requirements for incandescent reflector lamps
and common linear fluorescent lamps.
ESCO - Energy Savings Service Company specializing in lighting upgrades and engineering.
Eye Sensitivity - A curve depicting the sensitivity of the human eye as a function
of wavelength (or color). The peak of human eye sensitivity is in the yellow-green
region of the spectrum. The normal curve refers to photopic vision or the
response of the cones.
Field Angle - The angular dimension of the cone of light from reflectorized lamps
(such as R and PAR types) encompassing the central part of the beam out to the
angle where the intensity is 10% of maximum.
Fixture - Also known as a luminaire. A light fixture is the complete unit including
lamp, reflector, ballast, sockets, wiring, diffuser, and housing.
Flicker - The periodic variation in light level caused by AC operation that can
lead to strobe effects.
Flood - Used to refer to the beam pattern of a reflector lamp, which disperses the
light over a wide beam angle, typically 20 degrees or more. ("Flood" as opposed
5 of 12
Floodlight - A luminaire used to light a scene or object to a level much brighter
than its surroundings. Usually floodlights can be aimed at the object or area of
Fluorescence - A physical phenomenon whereby an atom of a material absorbs
a photon of light an immediately emits a photon of longer wavelength. If there is a
significant delay the phenomenon is called phosphorescence rather than
fluorescence. It is interesting that "phosphors" used in lamps exhibit
"fluorescence," not "phosphorescence."
Fluorescent Lamp - A high efficiency lamp utilizing an electric discharge through
low pressure mercury vapor to produce ultraviolet (UV) energy. The UV excites
phosphor materials applied as a thin layer on the inside of a glass tube which
makes up the structure of the lamp. The phosphors transform the UV to visible
Footcandle (fc) - A unit of illuminance or light falling onto a surface. It stands for
the light level on a surface one foot from a standard candle. One footcandle is
equal to one lumen per square foot. See also Lux.
Footcandle Meter - A device that measures the illuminance at a location
calibrated either in footcandles or in lux. (Also know as a light meter or
Frequency - The stated operating frequency in Hz of a discharge lamp.
Fovea, Foveal Vision - A small region of the retina corresponding to what an
observer is looking straight at. This region is populated almost entirely with
cones, while the peripheral region has increasing numbers of rods. Cones have a
sensitivity peaking in the yellow and corresponding to the eye response curve.
Full Spectrum Lighting - A marketing term, typically associated with light
sources that are similar to some forms of natural daylight (5000K and above, 90+
CRI), but sometimes more broadly used for lamps that have a smooth and
continuous color spectrum.
General Lighting - designed to provide a substantially uniform illuminance
throughout an area.
Glare - Visual discomfort caused by excessive brightness is called discomfort
glare. If task performance is affected it is called disability glare. Glare can be
direct glare or indirect (reflected) glare.
Halogen Lamp - A halogen lamp is an incandescent lamp with a filament that is
surrounded by halogen gases, such as iodine or bromine. Halogen gases allow
the filaments to be operated at higher temperatures and higher efficacies. The
halogen participates in a tungsten transport cycle, returning tungsten to the
filament and prolonging lamp life.
High-Bay Lighting - Lighting designed for (typically) industrial locations with a
ceiling height of 25 feet and above.
High Intensity Discharge (HID) - A general term for mercury, metal halide
and high-pressure sodium lamps. HID lamps contain compact arc tubes which
enclose various gases and metal salts operating at relatively high pressures and
High-Pressure Sodium (HPS) Lamp - HPS lamps are high intensity discharge
light sources that product light by an electrical discharge though sodium vapor
operating at relatively high pressures and temperatures.
Ignitor - An electronic device providing a high voltage pulse to initiate an
electrical discharge. Typically, the ignitor is paired with or is a part of the ballast.
Illuminance - the light level on a surface. Illuminance is measured in footcandles
or lux. The density of light (lumens/area) incident on a surface.
Illuminance Meter - A device that measures the illuminance at a location
calibrated either in footcandles or in lux. (Also know as a light meter)
Incandescent Lamp - A light source that generates light utilizing a thin filament
wire (usually of tungsten) heated to white heat by an electric current passing
Indirect Lighting - The method of lighting a space by directing the light from
luminaires upwards towards the ceiling. The light scattered off the ceiling
produces a soft, diffuse illumination for the entire area.
Induction Lighting - Gases can be excited directly by radio-frequency or
microwaves from a coil that creates induced electromagnetic fields. This is called
induction lighting and it differs from a conventional discharge, which uses
electrodes to carry current into the arc. Induction lamps have no electrodes
inside the chamber and generally, therefore, have longer life than standard
Infrared Radiation - Electromagnetic energy radiated in the wavelength range of
about 770 to 1,000,000 nanometers. Energy in this range cannot be seen by the
human eye, but can be sensed as heat by the skin.
Instant Start - A type of ballast designed to start fluorescent lamps as soon as
the power is applied. Most T8 fluorescent lamps are being operated on electronic
instant-start ballasts. Slimline fluorescent lamps operate only on instant start
Inverse Square Law - Formula stating that if you double the distance from the
light source, the light level goes down by a factor of 4, if you triple the distance, it
goes down by a factor of 9, and so on.
Isocandela Plot - A plot with lines connecting points of equal luminous intensity
around a source.
Isolux Plot (or Isofootcandle Plot) - A line plotted to show points of equal
illuminance (lux or footcandles) on a surface illuminated by a source or sources.
Kelvin - A unit of temperature starting from absolute zero, parallel to the Celsius
(or Centigrade) scale. 0C is 273K.
Kilowatt (kW) - The measure of electrical power equal to 1000
utility charges € 0.10/kWh, then the electricity cost for the 10 hours of operation
would be 10 cents (1 x € 0.10)
Lamp - The term used to refer to the complete light source package, including
the inner parts as well a the outer bulb or tube. "Lamp", of course, is also
commonly used to refer to a type of small light fixture such as a table lamp.
Lamp Types - Filament lamps: Incandescent, Halogen, Halogen-IR. Discharge
Lamps: Fluorescent, HID (High Intensity Discharge) HID Lamps: Mercury, HPS
(High Pressure Sodium), MH (Metal Halide) and CMH (Ceramic Metal Halide)
Lens - A transparent or semi-transparent element which controls the distribution
of light by redirecting individual rays. Luminaires often have lenses in addition to
Light - Radiant energy that can be sensed or seen by the human eye. Visible
light is measured in lumens.
Light Emitting Diode (LED) - A solid that directly converts electrical impulses
into light. Some LED's today incorporate fluorescent materials to change the
color characteristics of the emitted light.
Light Loss Factor - The product of all factors that contribute to lowering the
illumination level including reflector degradation, dirt, lamp depreciation over
time, voltage fluctuations, etc.
Light Meter - A device that measures the illuminance at a location
calibrated either in footcandles or in lux. (Also know as a foot-candle meter)
Light Pollution - Light that is directed to areas where it is not needed, and
thereby interferes with some visual act. Light pollution directed or reflected into
the sky creates a "dome" of wasted light and makes it difficult to see stars above
Light Trespass (Spill Light) - Light that is not aimed properly or shielded
effectively can spill out at into areas that don't want it: it can be directed towards
drivers, pedestrians or neighbors. It is distracting and annoying and can
sometimes be disabling.
Louver - A series of baffles used to shield a source from the view at certain
angles or to absorb unwanted light. The baffles are usually arranged in a
Lumens - A measure of the luminous flux or quantity of light emitted by a source.
For example, a dinner candle provides about 12 lumens. A 60-watt Soft White
incandescent lamp provides about 840 lumens.
Lumen Maintenance - A measure of how well a lamp maintains its light output
over time. It may be expressed numerically or as a graph of light output vs. time.
Luminaire Efficiency - The ratio of total lumens emitted by a luminaire to those
emitted by the lamp or lamps used in that luminaire.
Luminaire - A complete lighting unit consisting of a lamp (or lamps), ballast (or
ballasts) as required together with the parts designed to distribute the light,
position and protect the lamps and connect them to the power supply. A luminaire
is often referred to as a fixture.
Luminance - A measure of "surface brightness" when an observer is looking in
the direction of the surface. It is measured in candelas per square meter (or per
square foot) and was formerly referred to as "photometric brightness."
Lux (lx) - A unit of illuminance or light falling onto a surface. One lux is equal to
one lumen per square meter. Ten lux approximately equals one footcandle.
Mean Lumens - The average light output of a lamp over its rated life. Based on
the shape of the lumen depreciation curve, for fluorescent and metal halide
lamps, mean lumens are measured at 40% of rated lamp life. For mercury, high pressure
sodium and incandescent lamps, mean lumen ratings refer to lumens at
50% of rated lamp life.
Medium Base - Usually refers to the screw base typically used in household
incandescent lamps. There is also the medium bi-pin base commonly used in
T12 and T8 fluorescent lamps.
Mercury Lamp - A high-intensity discharge light source operating at a relatively
high pressure (about 1 atmosphere) and temperature in which most of the light is
produced by radiation from excited mercury vapor. Phosphor coatings on some
lamp types add additional light and improve color rendering.
Metal Halide Lamp - A high intensity discharge light source in which the light is
produced by the radiation from mercury, plus halides of metals such as sodium,
scandium, indium and dysprosium. Some lamp types may also utilize phosphor
Mesopic - Typically referring to nighttime outdoor lighting conditions, the region
between PHOTOPIC and SCOTOPIC vision.
Mogul Base - A screw base used on larger lamps, e.g. many HID lamps.
Monochromatic Light - Light with only one wavelength.
Mounting Height - Distance from the bottom of the fixture to either the floor or
work plane, depending on usage.
MR-16 & MR-11 - A line of low voltage compact reflector lamps used for
accent and spot lighting. The 16 and 11 refer to 16 eighths of an inch diameter
and 11 eighths.
Nanometer - A unit of wavelength equal to one billionth of a meter.
Open Circuit Voltage (OCV) - Open Circuit Voltage measured across the socket
the lamp screws into, with the ballast powered on. It is dangerous to stick a
voltmeter into such a socket without precise knowledge of the ballast because
exceedingly high voltages could be present.
Operating Voltage - For electrical discharge lamps, this is the voltage measured
across the discharge when the lamp is operating. It is governed by the contents
of the chamber and is somewhat independent of the ballast and other external
Package (LED) - An assembly of one or more LED modules
PAR Lamp - PAR is an acronym for parabolic aluminized reflector. A PAR lamp,
which may utilize either an incandescent filament, a halogen filament tube or a
HID arc tube, is a precision pressed-glass reflector lamp. PAR lamps rely on both
the internal reflector and prisms in the lens for control of the light beam.
Phosphor - An inorganic chemical compound processed into a powder and
deposited on the inner glass surface of fluorescent tubes and some mercury and
metal-halide lamp bulbs. Phosphors are designed to absorb short wavelength
ultraviolet radiation and to transform and emit it as visible light.
Photometry - The measurement of light and related quantities.
Power - The rate at which energy is taken from an electrical system or dissipated
by a load, expressed in watts.
Power Factor (PF) - A measure of the phase difference between voltage and
current drawn by an electrical device, such as a ballast or motor. Power factors
can range from 0 to 1.0, with 1.0 being ideal. Power factor is sometimes
expressed as a percent. Incandescent lamps have power factors close to 1.0
because they are simple "resistive" loads. The power factor of a fluorescent and
HID lamp system is determined by the ballast used. "High" power factor usually
means a rating of 0.9 or greater. Power companies may penalize users for using
low power factor devices.
Quartz - A name for fused silica or melted sand from which many hightemperature
containers are fashioned in the lighting industry. Quartz looks like
glass but can withstand the high temperatures needed to contain high intensity
Radiation - A general term for the release of energy in a "wave" or "ray" form. All
light is radiant energy or radiation, as is heat, UV, microwaves, radio waves, etc.
Rapid Start Circuit - A fluorescent lamp-ballast circuit that utilizes continuous
cathode heating, while the system is energized, to start and maintain lamp light
output at efficient levels. Rapid start ballasts may be either electromagnetic,
electronic or of hybrid designs. Full-range fluorescent lamp dimming is only
possible with rapid start systems.
Rated Lamp Life - For most lamp types, rated lamp life is the length of time of a
statistically large sample between first use and the point when 50% of the lamps
have died. It is possible to define "useful life" of a lamp based on practical
considerations involving lumen depreciation and color shift.
Reflectance - The ratio of light reflected from a surface to that incident upon it.
Reflector - A device used to redirect the light by the process of reflection.
Reflector Lamp - at other times, it includes all reflectorized lamps like PAR
and MR. A light source with a built-in reflecting surface. Sometimes, the term is
used to refer specifically to blown bulbs like the R and ER lamps
Resistance (R) - A measure of resistance in the flow of current, expressed in
Room Cavity Ratio (RCR) - A shape factor (for a room, etc.) used in lighting
RCR = 5H (L+W) / L x W, or, alternately,
RCR = (2.5) Total Wall Area / Floor Area.
Where H = height, L = length and W = width of the room.
the flatter the room the lower the RCR.
A cubical room will have an RCR of 10
Self-Ballasted Lamps - A discharge lamp with an integral ballasting device
allowing the lamp to be directly connected to a socket providing line voltage.
Spacing to Mounting Height Ratio - sometimes called spacing criterion. It is
OK to have fixture spaced closer than the spacing criterion suggested by the
manufacturer but not farther, or you will get dark spots in-between fixtures.
Ratio of fixture spacing (distance apart) to mounting height above the work plane.
Spectral Power Distribution (SPD) - A graph of the radiant power emitted by a
light source as a function of wavelength. SPDs provide a visual profile or "finger
print" of the color characteristics of the source throughout the visible part of the
Specular Reflection - Reflection from a smooth, shiny surface, as opposed to
Spot - A colloquial term referring to a reflector lamp with a tight beam of light,
typically around 10 degrees or less. It comes from the fact that such a lamp
produces a narrow spot of light as opposed to a wide flood of light.
System - A term referring to the lamp and ballast combination, and sometimes to
the entire lighting delivery system including the fixture, the optics, the particular
layout and the lighting controls.
T12, T8, T5 - A designation for the diameter of a tubular bulb in eighths of an inch.
T12 is 12 eighths of an inch, or 11/2 inches. T8 is eight eighths = 1 inch, and so on.
Task Lighting - Supplemental lighting provided to assist in performing a
localized task, e.g. a table lamp for reading or an inspection lamp for fabric
Total Harmonic Distortion (THD) - A measure of the distortion caused by
ballasts and other inductive loads of the input current on alternating current (AC)
power systems caused by higher order harmonics of the fundamental frequency
(60Hz in North America). THD is expressed in percent and may refer to individual
electrical loads (such as ballast) or a total electrical circuit or system in a building.
ANSI C82.77 recommends THD not exceed 32% for individual commercial
electronic ballasts, although some electrical utilities may require lower THDs on
some systems. Excessive THDs on electrical systems can cause efficiency
losses as well as overheating and deterioration of system components.
Troffer - A long, recessed lighting unit, usually installed in an opening in the
Underwriters Laboratories (UL) - A private organization which tests and lists
electrical (and other) equipment for electrical and fire safety according to
recognized UL and other standards. A UL listing is not an indication of overall
performance. Lamps are not UL listed except for compact fluorescent lamp
assemblies - those with screw bases and built-in ballasts.
Uniform Product Code (UPC) - The 12 digit code on the salable unit that is
used for scanning at the register.
Ultraviolet (UV) Radiation - Radiant energy in the range of about 100-380
nanometers (nm). For practical applications, the UV band is broken down further
- Ozone-producing - 180-220 nm
- Bactericidal (germicidal) - 220-300
- Erythemal (skin reddening) - 280-320
- "Black" light - 320-400
Valance Lighting - Lighting from light sources on a wall typically above eye
level, shielded by horizontal panels. The light may be upward or downward
Visual Task - objects and details that must be seen to perform an activity. The
task associated with seeing
Volt - A measure of "electrical pressure" between two points. The higher the
voltage, the more current will be pushed through a resistor connected across the
points. The volt specification of an incandescent lamp is the electrical "pressure"
required to drive it at its designed point. The "voltage" of a ballast (e.g. 277 V)
refers to the line voltage it must be connected to.
Voltage - A measurement of the electromotive force in an electrical circuit or
device expressed in volts. Voltage can be thought of as being analogous to the
pressure in a waterline.
Warm White - Refers to a color temperature around 3000K, providing a
Watt - A unit of electrical power. Lamps are rated in watts to indicate the rate at
which they consume energy.
Wavelength - The distance between two neighboring crests of a traveling wave.
The wavelength of light is between 400 and 700 nanometers.
Work Plane - unless otherwise indicated, it is assumed to be a horizontal plane
30 inches above the floor (table-top height) having the same area as the floor.
Plane at which work is done and at which illumination is specified and measured.