The spectrum of radiation describes its composition with regard to wavelength. Licht, for example, as the portion of electromagnetic radiation that is visible to the human eye, is radiation with wavelengths in the range of approx. 380 to 780 nm (1 nm = 10-9m). The corresponding range of colours varies from violet to indigo, blue, green, yellow, orange, and red. These colours form a continuous spectrum, in which the various spectral sectors merge into each other.

A graphic presentation takes the form of a smooth curve. Thermal radiators, like incandescent lamps and the sun have such a continuous spectrum. Discontinuous radiation, on the other hand, has breaks in the spectrum. In the individual case it is comprised only of spectrum lines (discontinuous spectrum). Laser light is monochromatic light radiating at just one single wavelength.

The Lamps in common use today emit portions of light from almost all sectors of the spectrum, but the actual composition differs considerably. In some cases there is a preponderance of blue (daylight white) and in other cases red (warm white). On the one hand theactual composition is determined by the colour appearance of the light source (Color appearance), and on the other hand an object can only reflect those colours that are present in the light (Color rendition).

In addition to light, thermal radiators also emit significant portions of IR radiation in the form of heat. (It would be more accurate to say that they produce a little bit of visible light in addition to heat radiation). In the case of discharge lamps without fluorescent materials, on the other hand, the spectrum extends into the UV sector. For both cases of invisible light, it can be said that excessive exposure can cause damage to the human eye and also to objects, although it should be borne in mind that UV and IR wavelengths are natural components of daylight and contribute to man's natural response to light, because light is perceived not only by the human eye but also by the skin.