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electromagnetic spectrum
Light and warmth are our most direct observations of the sun. Sunrays however consist of a combination of rays with different characteristics and can be described in terms of frequency or

invisible 'light'
It are mainly the high-energetic sunrays with wavelengths shorter than that of visible light that can be harmful for the human body. Fortunately the atmosphere that surrounds the earth prevents the

greater part of this radiation from reaching the surface of our planet. The radiation that does reach us, mainly consists of visible light, an almost equal amount of infrared (IR-) radiation and a much smaller amount of ultraviolet (UV-) radiation. IR-radiation is what we notice as warmth while UV-radiation on the other hand is something we can neither feel nor see but that tans our skin. Most of the therapies based on artificial sunlight therefore use sun

substitutes that produce light, infrared- or ultraviolet radiation or a combination of them, all with the intention to simulate the positive influence of natural sunlight on our health.

visible 'radiation'
A radiation source never emits energy on one single wavelength only. When the main wavelength of emitted infrared- or ultraviolet radiation is not too far from that of visible light, a small amount of visible light will be emitted as well. This is something we notice as a more or less dark red respectively deep purple light, leading to the widely spread but in fact incorrect term of infrared- or ultraviolet light for the radiation itself or infrared- or ultraviolet lamp for the radiator.

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wavelength. The concepts frequency and wavelength can best be explained with help of sound. Sound is a vibration of air with which the vibration is a wave-shaped periodic change in air pressure. The number of vibrations per second is called the frequency, measured in Hertz (Hz). We are capable to notice air vibrations from roughly 20 Hz to 20 kHz (1 KiloHertz =

1000 Hz). A single wave has a minimum and a maximum value, a peak and a valley. The difference between them is called the amplitude. The greater the amplitude, the louder the sound. The wavelength of a vibration is the distance that the wave has travelled after one complete sequence, for instance the distance between two peaks. This distance depends on the speed with which the wave propagates itself through a medium. The propagation speed of sound through air is about 330 meters per second. A vibration of one Hertz in air consequently has a

wavelength of 330 meters and a vibration of 1000 Hz has a wavelength of 33 centimeters (330 m/s : 1000 Hz). In water the propagation speed of sound is about 1480 m/s. Through water sound propagates faster than it does through air and the same soundwave, with a frequency of 1000 Hz, in water therefore has a wavelength of 148 cm. The

frequency of light is in the order of 600.000 GigaHertz. The propagation speed of light through vacuum is about 300.000 kilometers per second, giving it a wavelength in the order of 500 nanometers. In older literature wavelength still is expressed in Ångström (Å) where one Ångström equals one-tenth of a nanometer. 500 nm is therefore equivalent to 5000 Å. In 1873 James Clerk proved that light is only a special appearance in a more comprehensive spectrum. With regard to

some typical characteristics this spectrum can be sub-divided in electric waves, radio waves, micro waves, infrared radiation, light, ultraviolet radiation, X-rays, gamma radiation and cosmic radiation. In this spectrum, referred to as the electromagnetic spectrum, the wavelength of electric waves is the longest while that of cosmic rays is the shortest.