Die Beiträge von direkter und indirekt diffuser Sonnenstrahlung im UV-Spektrum

Abstract

Ultraviolet radiation differs in some respects from other parts of the solar spectrum. Spectroradiometric measurements were used to demonstrate how and to which extent direct and indirect diffuse radiation vary at certain wavelengths in the UV range depending on the solar elevation, i.e. during the course of a day. Measurements were done on the North Sea island of Sylt at 54° 55.5 N, 8° 18.6 E. UV-spectra were measured during a clear cloudless summer day using a double monochromator system (DM 150, Bentham Comp., Reading, England). ATeflon diffuser with an approximate cosine response served as input optic of the spectroradiometer. Direct solar radiation was measured when the receiving surface of the diffuser was arranged to be normal to the incident sun rays. A custom made aperture constructed according to the WMO guidelines for measurement of direct solar radiation by pyrheliometers (opening half-angle 2.5°, slope angle 1°) ensured that only the irradiance of the direct sun rays was measured. Conversely the diffuse radiation was measured when the horizontally adjusted diffuser was shaded from the direct sun rays by a shadow disk. To make both measurements comparable, the measured irradiance of the direct radiation was converted in relation to a horizontal plane. Out of the spectra the following six wavelengths were analyzed: 305 nm and 310 nm in the UVB and 330, 340, 360, and 380 nm in the UVA, respectively. Naturally both direct and indirect radiation increase with the elevation of the sun. However, they do so in a different manner. At low elevation up to about 30° indirect radiation prevails at all wavelengths in the UV range. At 305 nm and 310 nm this holds true even up to a solar elevation of almost 60° at noon. At these wavelengths indirect radiation contributes almost 80% to the global irradiance at 30° solar elevation and still about 56% at 60°. This means that indirect radiation exceeds direct radiation during the entire day. This relationship changes in the UVA range. The longer the wavelengths and the higher the sun the more important becomes the direct radiation. At 330 nm the irradiances of direct and indirect radiation are roughly equal at a solar elevation of 60°, At 340 nm this point of even shifts to about 50°, at 360 nm to 40 °, and at 380 nm to roughly 30 °. All in all, the shorter the wavelength the more the UV radiation is irradiated as diffuse indirect radiation scattered from the blue of the sky. This is of particular interest because the short wavelengths are precisely those which cause the most skin erythema.