Ballast: A device in a fluorescent fixture that maintains the current through the fluorescent bulb(s) at the desired constant value, sometimes also providing the necessary starting voltage and current. 
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 “usable 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, lighting fixture 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). 
CRI: See Color Rendering Index.
C.U.: See Coefficient of Utilization.
Diffuser: A device that diffuses, spreads out or scatters light in some manner to create soft light. Diffusers in most light fixtures are made from glass or acrylic. 
Efficacy: Lumens per watt.
Fluorescent Lamp: A gas-discharge lamp (bulb) that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce producing visible light. A fluorescent lamp converts electrical power into useful light more efficiently than an incandescent lamp. Lower energy cost typically offsets the higher initial cost of the lamp. The lamp fixture is more costly because it requires a ballast to regulate the current through the lamp. 
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.
LampThe 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. 
LED: Short for light-emitting diode, an LED is an electronic semiconductor device that emits light when an electric current passes through it. They are considerably more efficient than incandescent bulbs, and last much longer (50,000 hours+). 
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.
LLF: See Light Loss Factor.
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 EfficiencyNumber 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 light fixture. 
Photometric ReportDetails the output characteristics of the light 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. 
RCR: See Room Cavity Ratio.
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 light fixture can be spaced and still achieve uniform illumination. For example, a light fixture with an S/MH of 1.3 and a mounting height (distance between workplane and the fixture) of 6 feet can be placed (1.3) x (6) or 7.8 feet apart and still achieve uniform illumination.