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Holophane luminaires can be provided with a wide range of available lamp types, wattages and manufacturers. Ordering luminaires with lamps insures system compatibility

Lighting Basics
An understanding of some of the fundamental terms in lighting technology is basic to good design practice. The more important terms and concepts are reviewed here for this purpose.


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Luminous Flux
Luminous flux is the time rate of flow of light as measured in lumens. It is a measure of the total light emitted by a source and is most commonly used for measurement of total lamp output.

Luminance (L)
Luminance, often called "brightness," is the name given to what we see. "Brightness" is a subjective sensation varying from very dim or dark to very bright. Objectively it is referred to as luminance, defined as intensity in a given direction divided by a surface's projected area as seen by the observer. The surface may be a luminaire surface or a reflecting surface, such as a wall or roadway.

The direct luminance, or brightness, of luminaires at various angles of view is a major factor in the visual comfort evaluation of an installation using those luminaires. In general it is desirable to minimize the brightness of ceiling mounted luminaires at the high vertical angles, 60-90 degrees. When the intensity is in candelas, and the projected area is in meters, the unit of luminance is candelas per square meter.

 

Luminous intensity (I)
The candela is the unit of intensity and is analogous to pressure in a hydraulic system. It is sometimes called "candlepower" and describes the amount of light (lumens) in a unit of solid angle. This unit of solid angle is called the steradian. It will be seen from Figure 1 that as the light travels away from the source, the solid angle covers a larger and larger area; but the angle itself remains the same, as does the amount of light it contains. Intensity therefore, in a given directions is constant regardless of distance. See Figure 1


Illuminance (E)
Illuminance is the quantity of light reaching a unit area of surface and is measured in footcandles or lux. As the area covered by a given solid angle becomes larger with distance from the source, the included light flux remains the same. The illumination density of light on the surface decreases, therefore, with the square of the distance. Illuminance is defined by the intensity in cndelas directed toward point P, divided by the square of the distance (D) from the source of the surface.

This formula holds only if the receiving surface is perpendicular to the source direction. If light is incident at some other angle, See Figure 2, the formula becomes:

E = illuminance in footcandles (fc) or lux
I = intensity in candelas (cd) toward point P
D = distance in feet or meters
e = angle of incidence

 




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