What is Illumination?
A body that gives off light is called luminous, like all those glow-in-the-dark toys, and a body that gives off light when it is heated is called incandescent, like the filament in a light bulb. A body is illuminated when you can see it because it reflects light towards your eyes. Which of the following is which case?
- A book you are reading
- A star
- The gas in a flourescent light
- The moon
- The filament wire in a flashlight bulb
- The pixels on your computer screen
- Molten lava from a volcano
A star, the pixels of your computer screen, and the gas in a fluorescent light are luminous: they give off light but not due to heating (2,3,6). The filament of a bulb and molten lava are incandescent because they give off light due to heating (5,7). The moon and your book do not give off any light, but they are illuminated by reflected light from the sun or from your lamp (1,4).
The amount of light can be described by giving the radiant flux, the amount of energy radiated in a unit of time by an electromagnetic wave source. The part of this radiation that contributes to our being able to see (the electromagnetic radiation in only the visible range of wavelengths) is called the luminous flux, F, and is measured in lumen (lm). The total light output of a 40-watt incandescent light bulb is about 500 lm, while that of a 40-watt fluorescent tube is about 2300 lm. (Fluorescent tubes generate light much more efficiently and give off less energy as heat than incandescent bulbs do.) Illuminance, E, is the luminous flux per square meter and is measured in lm/m2 or lux. If you do not have constant illumination in all directions, you can specify the luminous intensity of a source in a particular direction, I, using the unit candela (cd). One cd is equal to 1 lumen/steradian. The steradian is the unit of solid angle, Ω, given by A/R2 where A is the area of the sphere and R is the radius of the sphere. Please notice that the luminous intensity does not depend on the distance away from the light source, but the illuminance does.