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Ballast Factor
A ratio describing a ballast’s actual lumen output vs.
rated lumen output, normally expressed as a percentage.
Coefficient of Utilization (C.U.)
Best expressed as the percentage of light that reaches the workplane or
“useable light”. Technically, it is the number of lumens that reach the
workplane divided by the total lumens generated by the lamp(s). This
metric accounts for room geometry, luminaire efficiency and ceiling,
wall, and floor reflectance.
Color Rendering Index (CRI)
The scale upon which a lamp’s ability to render true color is
measured. On a scale of
0 - 100, the higher the number, the better the color rendering. Select a
lamp with CRI of 85+ where color matching/selection is occurring, such
as vanity lighting and closets; 80+ for pleasant appearance of people
and food, such as in the kitchen and living room; 70+ for office areas;
50+ for work areas, garage and storage areas.
Color Temperature
Commonly referred to as Kelvin temperature, color
temperature is the visual warmth or coolness of a lamp. The lower the
Kelvin temperature, the visually warmer the color; the higher the Kelvin
temperature, the visually cooler the color. A lamp with a Kelvin
temperature of 2700K is considered a warm source (Warm White). Lamps
with a Kelvin temperature of 4100K and higher are considered cool
sources (Cool White).
Efficacy
Lumens per watt.
Illuminance
Luminous flux onto a surface. Illuminance is measured in lumens per
square foot or footcandles. Illuminance measured in lumens per square
meter is referred to as lux. One footcandle = approximately 10 lux.
Lamp
The light source, or 'lamp' determines the light level, color, fixture
spacing and energy use.
Lamp Lumen Depreciation
The decrease in lumen (light) output of a lamp (bulb) over time.
Light Loss Factor (LLF)
Accounts for depreciation due to dirt, lamp lumen loss, ballast
depreciation, etc. Normally in a residence, .8 is used, meaning that the
maintained or average lumen output of the system will be 80% of the
initial output due to dirt and age. However, LLF can be more or less
than .8, depending on the operating conditions of the system.
Lumen
Unit of light flow or “Luminous Flux.” All lamps have a
rated lumen output.
Luminaire
A complete lighting unit consisting of housing, wiring,
ballasts, sockets, lens, louvers, lamps, etc.
Luminaire Efficiency
Number of lumens leaving the aperture or opening of the
fixture divided by the lumens produced by the lamps. Expressed as a
percentage.
Luminance
Often called brightness. It is the amount of light to
leave a surface in a certain direction. This is measured in Footlamberts.
Luminous Intensity
Luminous flux in a certain direction. Measured in Candelas. Sometimes
called candlepower.
Mounting Height
The
distance between the workplane and the bottom of the luminaire
(Fixture).
Photometric Report
Details the output characteristics of the luminaire (fixture). This
includes C.U. (coefficient of utilization), spacing due to mounting
height ratio, and zonal lumen summary or lumen distribution.
Reflectance
The process of light “bouncing” off a surface. Formally,
it is the amount of flux leaving a surface divided by the total flux
onto the surface. For example, if 100 lumens strike a surface and 85
lumens leave the surface, the surface is 85% reflective.
Recessed Lighting
A light fixture that is
installed into a hollow opening in a ceiling. When installed it appears
to have light shining from a hole in the ceiling, concentrating the
light in a downward direction as a broad floodlight or narrow spotlight.
There are two
parts to recessed lights, the trim and housing. The
trim is the visible
portion of the lamp, the insert that is seen when looking up into the
fixture, also include the thin lining around the edge of the fixture.
The housing is the fixture itself that is installed inside the ceiling
and contains the light socket.
Room Cavity Ratio (RCR)
A number that summarizes the room’s cavity proportions. This takes into
consideration not only the room’s area, but also its shape as defined by
its dimensions of length, width, and height. For this, mere square
footage is not important. Two rooms may have the same square footage but
different wall perimeter. For example, a 3000 sq. ft. room could be 10’
x 300’ or 60’ by 50’. Consider the perimeter 620’ vs. 220’. Therefore
the 10’ x 300’ room has “more wall” to absorb light.
Formula:
5(Mounting Height) x (Length + Width) = RCR
(Length) x (Width)
Spacing to Mounting Height Ratio
(S/MH)
Enables one to calculate the maximum distance a luminaire (fixture) can
be spaced and still achieve uniform illumination. For example, a
luminaire with an S/MH of 1.3 and a mounting height (distance between
workplane and luminaire) of 6 feet can be placed (1.3) x (6) or 7.8 feet
apart and still achieve uniform illumination. |