Lighting

	Overview   |   Measurement of light   |   Luminance balance   |   Illuminance   |   Workstation light level   |   Transient Adaptation Factor (TAF)   |   Disability glare   |   Other considerations

Measurement of light
Special terminology and the use of both the metric and English systems tend to complicate discussions about light measurements. It will be helpful for some concepts to be discussed so that various reports found in the cited references and bibliography may be properly interpreted. The intent of this section is to give a brief explanation of certain terms used in light measurements.

In the past the reference measurement standard was actually a spermaceti wax candle of certain specifications. Today, the standard is much more precise, but the measurement terms stem from the earlier concept.

Photometric Terms

The photometric system is based on the concept of radiated flux, where flux is defined as the total amount of radiation passing through a unit area per unit time. Flux is measured in terms of its ability to stimulate the standard photopic human eye. The resultant unit is the lumen. The following illustrates this system, where:

	=	the solid angle through which flux from the point source is irradiated
	=	the incremental surface area.
	=	the radiometric energy restricted to a narrow bandwidth, .
	=	the wavelength of light in nanometers
	=	the luminous efficacy at . This is the ratio , where , is the radiometric flux.

Photometric System

Parameter	Symbol	Definition	Units
Luminous Flux			lumen (lm)
Luminous Intensity			Candela (cd)
Illuminance			Lux (lx)
Luminance			cd/m2
Energy			lumen-sec (lm x s)

Luminance: The luminance, at a point of a surface and in a given direction, is the luminous intensity of an element of the surface, divided by the area of the orthogonal projection of this element on a plane perpendicular to the given direction. The unit of luminance is the candela per square meter (cd/m² ). (For those instruments which only read in footlamberts, 1 fL equals 3.246 cd/m².)

Luminous Intensity: The candela (cd) is the unit of luminous intensity. It is the luminous intensity, in the perpendicular direction, of a surface of 1.67 10^-6 meter² of a black body radiator of 2,045K (the temperature of solidification platinum) under standard atmosphere (101,325 newtons/meter²). A photopic response filter is implied.

Luminous Flux: The lumen (lm) is the luminous flux emitted within a solid angle of 1 steradian by a point source having a uniform intensity of 1 candela. The unit of luminous flux, lumen (lm), equals 1 candela - steradian (cd x sr).

Troland: The Troland is the unit of retinal illuminance equal to that produced by viewing a surface having a luminance of 1 candle per square meter through a pupil having an area of 1 square millimeter. It is used whenever pupil area is explicitly taken into account.

Back to top