Radiometry Primer: What is a Lumen, Lux, Nit, Candela?
Here are those definitions you always wanted! (Portions from: Dr. Mark W. Lund (mlund@moxtek.com).) • 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 street jive, but illuminance in sci.optics talk. 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 wives, 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. Put another way, Candelas are a measure of luminous int
