|
Accent Lighting: |
the directional to emphasize a particular object or to
draw attention to a part of the field of view. See directional
. |
|
Adaptation: |
the process by which the retina of the eye 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
of the eye to light. |
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 electric-discharge lamp to obtain the necessary circuit conditions
(voltage, current and wave form) for starting and operating; all
fluorescent and HID light sources require a ballast for proper
operation. Dimming ballasts are special ballasts which when used
together with a dimmer will vary the light output of a lamp.
|
|
Ballast Factor |
the measured
ability of a particular ballast to produce light from the lamp(s) it
powers; ballast factor is derived by dividing the lumen output of a
particular lamp/ballast combination by the lumen output of the same
lamp(s) on a reference ballast. |
| Beam
Angle: |
the angle
between the two directions for which the intensity (candlepower) is 50%
of the maximum intensity as measured in a plane through the nominal beam
centerline (center beam candlepower). |
| Beam
Spread: |
(in any
plane) the angle between the two directions in the plane in which the
candlepower is equal to a stated percent (usually ten percent) of the
maximum candlepower in the beam. |
|
Brightness: |
see
luminance. |
Candela, |
the unit of measure indicating the luminous intensity (candlepower) of a
light source in a specific direction; any given light source will have
many different intensities, depending upon the direction considered. |
|
Candlepower Distribution: |
a curve that
represents the variation in luminous intensity (expressed in
candelas) in a plane through the light center of a lamp or
luminaire; each lamp or lamp/luminaire combination has a unique set of
candlepower distributions that indicate how light will be spread.
|
|
Candlepower Distribution Curve: |
a curve,
generally polar, representing the variation of luminous intensity of a
lamp or luminaire in a plane through the light center. |
|
Center beam Candlepower |
the intensity
of light produced at the center of a reflector lamp, expressed in
candelas. |
|
Chromaticity: |
the aspect of
color that includes consideration of its dominant wavelength and purity. |
|
Coefficient of Utilization (CU): |
the ratio of
the luminous flux (lumens) from a luminaire received on the
work-plane to the lumens emitted by the luminaire's lamps alone. |
| Color
Rendering Index |
the measure of a
light source's ability to render the color of objects "correctly," as
compared with a reference source with comparable color temperature; the
scale peaks at 100. |
|
Correlated Color Temperature |
a
specification of the color appearance of a lamp relating its color to
that of a reference source heated to a particular temperature, measured
in degrees Kelvin (K); CCT generally measures the "warmth" or "coolness"
of light source appearance. |
|
Current |
a measure of
the flow of electricity, expressed in amperes (A). |
|
Cut-Off Angle: |
(of a luminaire) the
angle from the vertical at which a reflector, louver or other shielding
device cuts off direct visibility of a light source. It is the
complementary angle of the shielding angle. |
Diffuse Lighting: |
light that is not predominantly incident from any particular direction. |
|
Diffuser: |
a device to
redirect of scatter the light from a source by the process of diffuse
transmission. |
|
Direct Glare: |
glare resulting from
high luminance or insufficiently shielded light sources in the field of
view, or from reflecting areas of high luminance. It is usually
associated with bright areas such as luminaires, ceilings and windows
that are outside the visual task or region being viewed.
|
|
Direct Lighting: |
by luminaires
distributing 90 to 100 percent of the emitted light in the general
direction of the surface to be illuminated. The term usually refers to
light emitted in a downward direction. (See accent .)
|
|
Directional Lighting: |
illumination on the
work-plane or on an object predominantly from a single direction.
|
|
Disability Glare: |
glare
resulting in reduced visual performance and visibility. It is often
accompanied by discomfort. |
|
Discomfort Glare: |
glare producing
discomfort. It does not necessarily interfere with visual performance or
visibility.
|
Efficacy: |
efficiency of a light source expressed in lumens per watt (LPW or lm/W). |
|
Energy: |
a measure of
work done by an electrical system over a given period of time, often
expressed in kilowatt-hours (kWh). |
|
Enhancing Reflections: |
reflections which
enhance appearance described in such terms as sparkling, glittering,
etc. |
| ESCO: |
Energy
Saving service Company |
Fenestration: |
any opening or arrangement of openings or windows (normally filled with
media for light control) for the admission of daylight or for the
transmission of electric from one room to another room.
|
|
Fixture: |
see
luminaire. |
|
Fluorescent Lamp: |
a low
pressure mercury electric discharge lamp, tubular in shape in which a
fluorescent coating (phosphor) transforms ultraviolet energy into
visible light. |
|
Footcandle (fc): |
a unit of
illuminance equal to 1 lumen per square foot. |
|
Frequency: |
the number of
times per second that an alternating current system reverses from
positive to negative and back to positive, expressed in cycles per
second or hertz, Hz. |
General Lighting: |
designed to provide a substantially uniform illuminance throughout an
area, exclusive of any provision for special local requirements.
|
|
Glare: |
excessive
brightness that may be caused by either direct or indirect viewing of a
light source; any brightness or brightness relationship that annoys,
distracts or reduces visibility. |
Harmonic: |
an electrical frequency that is an integer multiple of the fundamental
frequency; for example, if 60 Hz is the fundamental freqency, then 120
Hz is the second harmonic and 180 Hz is the third harmonic; some
electronic devices, such as ballasts or power supplies, can cause
harmonic distortion, directly affecting power quality. |
| Hertz
(Hz): |
a unit of
freqency equal to one cycle per second; see frequency.
|
| HID |
see High
Intensity Discharge Lamps |
| High
Intensity Discharge Lamps |
a general group of
lamps consisting of mercury, metal halide and high pressure sodium
lamps. Also refered to as HID lamps. |
Illuminance: |
light arriving at a surface, expressed in lumens per unit area; 1 lumen
per square foot equals 1 footcandle, while 1 lumen per square
meter equals 1 lux. |
|
Incandescent Filament Lamp: |
a lamp in
which light is produced by a filament heated to incandescence by an
electric current. |
|
Indirect Lighting: |
by luminaires
distributing 90 to 100 percent of the emitted light upward. |
Lamp: |
a generic term for a man-made source of light; the 3 broad categores of
electric lamps are incandescent, fluorescent, and high intensity
discharge (HID). |
| Lamp
Life: |
an average rating,
in hours, indicating when 50% of a large group of lamps have failed,
when operated at nominal lamp voltage and current; manufacturers use 3
hours per start for fluorescent lamps and 10 hours per start for HID
lamps when performing lamp life testing procedures; every lamp type has
a unique mortality curve that depicts its average rated life.
|
| Lamp
Lumen Depreciation Factor (LLD): |
the
multiplier to be used in illumination calculations to relate the initial
rated output of light sources to the anticipated minimum rated output
based on the relamping program to be used. |
| Lens: |
a glass or
plastic element used in luminaires to change the direction and control
the distribution of light rays. |
|
Light: |
radiant
energy that is capable of producing a visual sensation. |
| Light
Loss Factor |
a factor used in
calculating illuminance after a given period of time and under given
conditions. It takes into account temperature and voltage variations,
dirt accumulation on luminaire and room surfaces, lamp depreciation,
maintenance procedures and atmosphere conditions. Formerly called
maintenance factor. |
| Light
Source: |
see lamp.
|
|
Louver: |
a series of baffles
used to shield a source from view at certain angles or to absorb
unwanted light. The baffles are usually arranged in a geometric pattern.
|
| Lumen
(lm): |
a unit of
luminous flux; the overall light output of a luminous source is measured
in lumens. |
| Lumen
Depreciation: |
the decrease in
lumen output of a light source over time; every lamp type has a unique
lumen depreciation curve (sometimes called lumen maintenance
curve) depicting the pattern of decreasing light output.
|
| Lumen
Maintenance: |
see lumen
depreciation. |
|
Luminaire: |
a light
fixture; the complete unit, including lamp, reflector, ballast, socket,
wiring, diffuser, and housing. |
|
Luminaire Efficiency: |
the ratio of
luminous flux (lumens) emitted by a luminaire to that emitted by the
lamp or lamps used therein. |
|
Luminance (L) |
light reflected in a
particular direction; the photometric quantity most closely associated
with brightness perception, measured in units of luminous intensity (candelas)
per unit area (square feet or square meters). |
|
Luminance Contrast: |
the
relationship between the luminances of an object and its immediate
background. |
|
Luminance Ratio: |
the ratio
between the luminances of any two areas in the visual field. |
|
Luminous Flux: |
the time rate
of flow of light. |
| Lux
(lx): |
a unit of
illuminance equal to 1 lumen per square meter. |
Matte Surface: |
a non-glossy dull surface as opposed to a shiny (specular) surface.
Light reflected from a matte surface is diffuse. |
|
Modeling: |
the effect of
using highly directional light to create form through shadows and
highlights. |
Nanometer (nm): |
a unit of length equal to 10-9 meters; commonly used as a
unit of wavelength. |
Power: |
the rate at which energy is taken from an electrical system or
dissipated by a load, expressed in watts (W); power that is
generated by a utility is typically expressed in volt-amperes
(V-A). |
| Power
Factor: |
a measure of the
effectiveness with which an electrical device converts volt-amperes to
watts; devices with power factors >0.90 are "high power factor" devices.
|
Quality of Lighting: |
pertains to the distribution of luminance in a visual environment. The
term is used in a positive sense and implies that all luminances
contribute favorably to visual performance, visual comfort, ease of
seeing, safety and esthetics for the specific visual tasks involved.
|
|
Quantity of Light: |
the product of the
luminous flux by the time it is maintained. It is the time integral of
luminous flux. |
Reference Ballast: |
a ballast specially constructed to have certain prescribed
characteristics for use in testing electric-discharge lamps and other
ballasts. |
|
Reflectance (rho): |
the
percentage of light reflected back from a surface, the difference having
been absorbed or transmitted by the surface. |
|
Reflected Glare: |
glare
resulting form specular reflections of high luminance in polished or
glossy surfaces in the field of view. |
|
Reflection: |
the process
by which flux leaves a surface or medium from the incident side. |
|
Reflector: |
a device used
to redirect the light by the process of reflection. |
|
Refraction: |
the process by which
the direction of a ray of light changes as it passes obliquely from one
medium to another. |
|
Refractor: |
a device used
to redirect the luminous flux from a source, primarily by the process of
refraction. |
|
Resistance (R): |
a measure of
resistance to flow of current, expressed in ohms.
|
Shielding: |
a general term to include all devices used to block, diffuse or redirect
light rays, including baffles, louvers, shades, diffusers and lenses. |
|
Shielding Angle: |
The complementary
angle of the cut-off angle of a luminaire. |
|
Spectral Power Distribution (SPD): |
a curve
illustrating the distribution of power produced by the lamp, at each
wavelength across the spectrum. |
|
Specular Surface: |
a shiny,
highly polished surface which reflects light at an angle equal to that
of the incident light. |
|
Supplementary Lighting: |
used to
provide an additional quantity and quality of illumination that cannot
be readily obtained by a general system and that supplements the general
level usually for specific task requirements. |
Transmission: |
the process by which incident flux leaves a surface or medium on a side
other than the incident side, the characteristics of many materials such
as glass, plastics and textiles. |
|
Transmittance: |
the ratio of the
flux transmitted by a medium to the incident flux. |
Veiling Reflections: |
reflections which partially or totally obscure the details to be seen by
reducing the contrast. |
|
Visual Comfort Probability |
a discomfort glare
calculation that predicts the percent of observers positioned in the
least favorable part of the room who would be expected to judge a
condition to be comfortable. VCP rates the luminaire in its environment,
taking into account such factors as illuminance level, room dimensions
and reflectances, luminaire type, size and light distribution, number
and location of luminaires, and observer location and line of sight. The
higher the VCP the more comfortable the environment. |
|
Visual Field: |
the location
of objects or points in a space where the head and eyes are kept fixed. |
|
Visual Surround: |
all portions
of the visual field except the visual task. |
|
Visual Task: |
those details
and objects which must be seen for the performance of a given activity,
including the immediate background of the details or objects.
|
|
Voltage (E): |
a measure of
electrical potential, expressed in volts (V). |
Watt (W): |
a unit of electrical power equal to 1 joule per second. |
| Work
Plane: |
the plane at which
work usually is done, and on which the illuminance is specified and
measured. Unless otherwise indicated, this is assumed to be a horizontal
plane 0.76 meters (30 inches) above the floor. |
