Glossary of commonly used Terms

COMMONLY USED ESD TERMS

"Brightness" is a psycho-physiological phenomenon that cannot be measured

directly. The term "photometric brightness" was at one time used to refer to 

luminance. However, it is no longer used for scientific or engineering purposes.

A foot-candle is equivalent to one lumen per square foot (where a lumen is a

measure of the luminous flux, or quantity of light).

A wax candle flame has a luminous intensity (or equivalently, 

candlepower) of approximately one candela. If you hold the candle one foot 

away from a surface, the illuminance of the surface at this distance due 

to the light from the candle will be approximately one foot-candle. It will be 

1/4 fc at two feet, 1/9 fc at three feet, and so on in accordance with the inverse

square law for point light sources.

In plain English, illuminance is the quantity of light arriving at a point

on a real or imaginary surface. (The point does not have to be located on a

physical surface.)

A Lumen is a quantity of light, the same kind of unit as a watt, but measured

in such a way that takes into account the sensitivity of the eye.  If you are

looking at the specification of a light source Lumens are the measured as the

entire output, no matter which direction the light is going in.

A Lux is a lumen per square meter, which we might call illumination in

Layman's terms but, when speaking technically, it is referred to as illuminance.  

It is how much light that falls on a surface.

A Nit is a lumen per square meter per steradian, which we might call

brightness to our family members, but luminance to our peers.  If a Lux of illuminance

strikes a perfectly diffusing surface it gives a Nit of luminance.

A candela is a lumen per steradian.  It is used for "point" sources so that

you can calculate how much light is cast on a surface no matter where

the surface is.