Daylight derives mainly from the thermal radiation of the sun filtered through the atmosphere. It forms a continuous spectrum comprising wavelengths from about 300 to 4500 nm with a mean color appearance of 5000 K (for Central Europe).


This radiation ranges from UV-B radiation (starting at 300 nm) to IR-B radiation (UV = ultraviolet; IR =infrared). Daylight is a mixture of direct sunlight and the light of the sky. The blue of the sky is the result of scatter in the molecules of air in the atmosphere. The spectral composition of the sky is constantly changing, depending on the time of day, point of the compass and the weather. Most natural functions of the Earth's flora and fauna are attuned to these changes and only achieve optimum efficiency in daylight. Visible light (380 to 780 nm) is the basis for visual perception in man (1 nm = 10-9 m).

Daylight in interiors

Adequate daylight and visual contact with the outside world are key components for a sense of wellbeing and motivation in people indoors. DIN 5034, Daylight in interiors describes how windows can be used to ensure adequate brightness and contact with the outside world, and the conditions to be met to achieve acceptable light conditions from natural daylight indoors.

Requirements, lighting design

The standard also includes provisions relating to protection from the sun and other planning measures. Artificial lighting in interiors can never be viewed in isolation; it is to be seen as a system for complementing natural daylight. The function of the interior and indoor working should not be hindered, but rather supported through theprovision of artificial light that is or appears to be as natural as possible.

The design of the windows (shape, size, location, direction), and the reflective properties of window ledges, ceilings, floors, walls and furnishings determine the amount of daylight penetrating the depths of a room. To achieve the right light at the workplace, artificial lighting must be employed as a complementary system. The proportion of daylight that penetrates the interior is expressed as daylight factor D, which is the ratio of illuminance at a point in the interior Ep to outdoor illuminance Ea from an unobstructed hemisphere of the same sky.

For lighting planning with daylight, the standard refers to typical skies as idealised conditions (cloudy, clear and intermediate; DIN 5034 Part 2).