Unfortunately, the device to take this measurement is very expensive so few hobbyists will actually be able to take these measurements within their own tank. However, several authors have taken measurements from the various lights available and these numbers can be read from a variety of publications. This measurement is given in einsteins or quanta and provide for the first time a way to compare between bulbs how much light emitted is useable by the corals. Metal halide lamps are now available in a wide variety of wattages as well as color temperatures. Wattage determines how much light is produced, with higher wattage bulbs producing more total light.
Color temperature refers to the spectrum of the bulb.
The higher the color temperature of a bulb, in degrees Kelvin, the bluer the light that will be produced. That is, a lamp that is rated at 6500 degrees Kelvin produces a greener appearing light relative to the light produced by a lamp rated at 20,000 degrees Kelvin, which produces a blue light. I nitially it was thought that the bluer lights would produce more useable light. That is that the amount of total light that they were producing was equal to that of the lower temperature lights, but that it contained a higher percentage of light with the PAR wavelengths. This may be important, as the main reason for using these bulbs is to get enhanced or vivid coloration in the SPS corals.