Candela is the new unit for measuring the visible light energy per unit area coming from a glowing body. It replaces the terms candle and candlepower. We call the light energy per unit area the luminous intensity. One candela is the light emitted from 1/60th of a square centimeter of surface of incandescent platinum at 2046 degrees.
The terms luminous intensity, luminous flux and illumination are easily confused. To understand light units, one must first analyze light as radiant energy.
Each glowing body gives off radiant energy in all directions. This radiant energy is composed of visible energy and invisible energy. The light energy emitted is luminous energy.
Luminous flux represents the total amount of light from a glowing body per unit time. The unit of luminous flux, English system or metric, is the lumen, the amount of light from a one candela strength source striking a one square unit spherical surface one unit of length from the candela. These measurements define a solid angle called a steradian. It is this angle which is important here and not the units in feet, meters, square feet or square meters. Thus, in the metric system, the lumen is the light striking a surface one meter square and one meter distant from a one candela source. The illustration shows the English system. The lumen is the unit in both.
Illumination is the light received by any object some distance from the glowing body emitting that light. Its formula is illumination equals candela divided by distance squared. Its units are lux and foot-candle. Measurements of illumination are made of photoelectric cells.