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Glossary of Terms

International unit (SI) of luminous intensity; term evolved from considering a standard candle, similar to a plumber's candle, as the basis of evaluating the intensity of other light describe the relative intensity of a source.
Candlepower (CP)
Luminous intensity expressed in candelas.
Candlepower Distribution Curve
A graphic presentation of the distribution of light intensity of a lamp or luminaire.
Coefficient of Utilization (CU)
A percent of initial lamp lumens that reaches the work plane as determined by surface reflectances, room shape (room cavity ratio or RCR) and fixture efficiency.
Color Rendering Index
The method that indicates how colors will look under a given source. A color rendering index (CRI) number is assigned to a light source based on its ability to make pigments look as they would under certain test sources when compared to other sources having the same color temperature.
Color Temperature
Apparent color temperature (or correlated color temperature) of a light source-indicating its degree of warmth or coolness, with the higher number being "cool."
Cosine Law
The illuminance of any surface varies as the cosine of the angle of incidence. The angle of incidence is the angle between the normal angle to the surface and the direction of the incident light.
Digital Light Processing
Lumens per watt
Electromagnetic Spectrum
Orderly arrangement of radiant energy by wavelength or frequency. In the visible spectrum, the eye is sensitive to radiant energy between 380 nanometers (violet) and 780 nanometers (red).
Foot-candle Equations
Unit of illumination equal to one lumen uniformly incident upon an area of one square foot. Also, equal to the illuminance at a point one foot distant from a one candela source.
Foot-lambert Equation
Unit of luminance or brightness equal to one lumen uniformly reflected or emitted by an area of one square foot.
Visual discomfort caused by excessive brightness; can be direct or indirect (reflected).
Illuminance (E)
The quantity of light (measured in foot-candles, Lux, etc) at a point on a surface.
Infrared Radiation
For practical purposes, any radiant energy within the range of 770-106 nanometers. This energy is sensed as heat.
Inverse Square Law
Formula stating that illumination at a point on a surface varies directly with the intensity of a point source, and inversely as the square of the distance between the source and the point; illustrates how the same quantity of light flux is distributed over a greater area as the distance from the source to the surface is increased.
Liquid Crystal Display
Light Emitting Diode, a solid state lighting technology.
Light Loss Factor
The product of all considered factors that contribute to a lighting system's depreciated light output over a period of time, including dirt and lamp lumen depreciation.
The international unit of luminous flux or quantity of light. If a uniform point source of one candela is at the center of a sphere of one foot radius, having an opening in the surface of one square foot area, the quantity of light that passes through is called a lumen. The sphere has a total surface area of 12.57 square feet. Since, by definition, a lumen flows to each square foot of surface area, a uniform point of one candela produced 12.57 lumens.
Lumens per Watt
A ratio expressing the luminous efficacy of a light source; lumens per watt.
A complete lighting unit consisting of a lamp (or lamps) together with the parts designed to distribute the light, position and protect the lamps, and connect them to the power supply. This is sometimes referred to as a "fixture".
Luminaire Efficiency
The ratio of lumens emitted by a luminaire to those emitted by the lamp (or lamps) used.
Luminance (L)
Practically speaking, the brightness of an object, that which the eye perceives; luminance of a surface is equal to illuminance x reflectance
SI (international system) unit of illumination. One lumen uniformly distributed over an area of one square meter.
Mounting Height
Distance from the bottom of the fixture to either the floor or work plane, depending on usage.
A unit of wavelength equal to one billionth (10-9) of a meter.
The ratio of light reflected from a surface to that light incident upon it.
Room Cavity Ratio (RCR)
Takes into account length and width of a room and height from the fixtures to the work plane.
RCR = 5H (L/W) / L x W
Rear Projection Television
Spacing to Mounting Height Ratio
Ratio of fixture spacing (distance apart) to mounting height above the work plane. Sometimes called spacing criterion.
Ultraviolet Radiation
For practical purposes, any radiant energy within the range of 100-380 nanometers. Some wavelengths (180-220) produce ozone, some (220-300) are bactericidal, some (280-320) are erythermal (redden human skin) and others (320-400) cause secondary luminance (black light).
Veiling Reflections
Effective reduction in contrast between task light and its background caused by the reflection of light rays. Sometimes called "reflected glare."
Visual Comfort Probability (VCP)
VCP is a ratio of a lighting system expressed as a percent of people who, when viewing from a specific location and in a specified direction, find the system acceptable.
Visual Task
Objects and details that must be seen to perform an activity.