Luminous sources can be grouped according to the values of:

•    absorbing power of energy: it is quantified in watts (W). Watts however, even if often used by manufacturers to describe their own lamps, only indicate the electricity consumption, and are not suitable to describe the lamps’ characteristics as far as their luminous emission. Also, watts do not take into account the influence of the lamp’s optic group on the projected beam.

•    luminous flux, measured in lumens (lm). It indicates the amount of total light emitted by a luminous source, weighted on the human eye sensibility to various shades of color.
•    luminous efficiency: it is the ratio of luminous flux (in lumens) to power (usually measured in watts).  It is a compromise between the two above-mentioned figures, and indicates the capability with which a luminous source can supply visible light from a certain quantity of absorbed electricity. So it is a very handy figure for comparing the emission of similar luminous sources.
•    color temperature, measured in Kelvin degrees (K°). It is simply the light color to the human eye. A low color temperature corresponds to a yellow-orange light. Going further down the spectrum, there is the red light and afterwards the infrared (which is invisible), while going up on the spectrum the light gets white, then blue, violet and ultraviolet (which is invisible). When you say that a light is hot, this actually corresponds to a low color temperature; instead, a higher temperature produces a cold light. Such definitions have a psychological rationale, since the mind is likely to match the idea of warmth to red yellow or orange, and the idea of coldness to white blue or violet.

For a complete description of the various degrees of color temperature, please refer to the picture below, bearing in mind that:

1. a candlelight has a 1800°K temperature
2. an incandescence light bulb has a 2700°K temperature
3. a halogen light bulb has a 3000°K temperature
4. the sunlight at noon has a 6500°K temperature
5. discharge lamps have a temperature between 6000°K and 7000°K therefore they approximate very well the sun’s temperature, for realistic colors in projections.