The rays of the sun can, in addition to the desired tanning effect，also cause undesirable reddening of the skin, erythema - in its worse form, sunburn. For one-off sunbathing, the time required for tanning is actually longer than that required for skin reddening.
Despite this, it is also possible to achieve a nice tan, without burning - quite simply by means of regular sunbathing. The reason for this is that the body reduces the preliminary stages of skin reddening relatively quickly, while the tan constantly builds itself up through repeated exposure.
On the sunbed the exact intensity of the UV light is known. Consequently the tanning plan can be adjusted to ensure that the individual stops before burning starts and then that a good tan is built up