Candela

 

IESNA Definition: the Standard International unit of luminous intensity. One candela is one lumen per steradian. Formerly, candle.

The intensity of a light source in a specific direction is expressed in candelas (cd). Any given light source will have many different intensities, depending upon the direction considered. Since intensity is a property of the source itself, the candlepower (luminous intensity expressed in candelas) for a specified direction remains the same, regardless of distance from the source.

It is interesting to compare lamps of the same wattage with regard to luminous intensity. In the following examples, imagine the lamps aimed straight down, with 0 representing a point directly beneath each one, and 20 representing a point 20 up from 0. Although not true for all sources, the highest candela values occur at 0 in these examples:

Lamp Candelas at 0 Candelas at 20
150-W R40 Flood 1,100 cd 820 cd
150-W R40 Spot 5,800 cd 780 cd
150-W PAR38 Flood 4,000 cd 1,100 cd
150-W PAR38 Spot 11,800 cd 500 cd
 
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Glossary Terms: IESNA, steradian