Candela: Systeme Internationale (SI) unit of luminous intensity. The modern calibration reference is a platinum radiator heated to just below its melting point: each square cm of such an object has 60.0 candela luminous intensity! (In old-fashioned parlance, each sq cm of the hot platinum emits as much light as 60 standard candles.) A point source of 1 candela radiates 1 lumen into a solid angle of 1 steradian. A 1 sq-cm source of 1 candela produces an illumination level of 1 foot candle at a distance of 1 foot.
It gets a little confusing at times, because there are 3 ways of measuring the light output of sources: 1) Total luminous flux, 2) Luminous intensity or surface brightness per unit area (Summing this up over a surface gives total flux, but a point source has no area so you can't really calculate its intensity.) See below. 3) Illumination level at some distance away, sometimes measured in foot-candles.
Luminous intensity is surface brightness, sometimes measured in candelas. Note that a big surface can have a high luminance at the same time that it has a low luminous intensity and vice-versa. An example will make this clear (I hope): A 40W fluorescent tube is a lot easier to stare at than a 40W incandescent. Why? The light output of the incandescent is spread over a much smaller area, so it has a much higher luminous intensity. The 40W fluor. puts out about 2000lumens spread out over about 2500sq-cm. Meanwhile, the incand. puts out only about 750lumens, but it's spread over only about 150sq-cm (assuming a frosted bulb). So, the surface of the incandescent will have about 6 times greater luminous intensity. Imagine how bright the filament itself is, with its much smaller surface area.
So, as you look at a CRT or other display, its surface brightness can be measured in candelas. It it has 60candela brightness, it's as bright as the Pt standard. Another way to look at it is that each sq-cm would emit the same luminous flux as 60 candles! However, the intensit
