Design of an input optic for solar UV-measurements
J.G. Schreder, M. Blumthaler, M. Huber Institute of Medical Physics, University of Innsbruck, Austria
Abstract This article outlines a new input optic for measuring spectral UV irradiance. The system provides a considerably improved measurement accuracy in comparison to traditional optics. The input optic has a weatherproof housing and is designed for use with a quartz fibre. The cosine error is less than ± 3% for incidence angles between 0° and 75° and therefore the integral cosine error for isotropic radiation is less than 2.5%.
Introduction For measuring global irradiance the incoming radiation is weighted by the cosine of the incidence angle q . This is the angle between the direction of the incidence radiation and the surface normal. Generally all standard UV input optics deviate significantly from this ideal behaviour. The deviation of the input optics response from the ideal cosine is described by the
cosine error f (1) where S(q, j) is the radiometer
signal and q,j are
respectively the incidence angle and the azimuth angle. The integral cosine error á f (2) International intercomparison campaigns (2,3) have shown that discrepancies of up to 10% in the measurement data may arise as a result of different cosine responses. Nevertheless, the measurement data are comparable if the experimental results are corrected by applying a cosine correction procedure described in (4). Depending on aerosol amount and wavelength the cosine correction factor can reach 7% ± 2% (5,6). Since these routines are restricted to cloudless sky conditions and known aerosol concentrations their use is very restricted. These routines also require the assumption of an isotropic distribution of the UV radiation in the sky. However, measurements of the spatial distribution of the sky radiance show this not to be true (7). Due to these limitations, the use of improved input optics with a very small cosine error is advised.
Material and Methods The new input optic was designed to provide a nearly perfect cosine response at all incidence angles as well as to be used in any weather conditions. The input optic consists of a weather proof housing, a shaped teflon diffuser, a quartz glass dome, an easyly to replace silica cartridge and a spirit level. The diffuser is optimised to work under the quartz dome.
The cosine response characteristic of a flat diffuser can be modified by changing it into a nonplanar form (8). To allow a specific and reproducible shaping of the diffuser it is manufactured by the use of a computer driven milling machine. A diagrammatic illustration of the input optic is shown in Figure 1 where the flange H is simultaneously the shadow ring of the diffuser. A 1000 W Xe UV lamp was used to determine the cosine response in the laboratory. The lamp is placed behind a wall with a hole to prevent stray light from reaching the input optic. The input optic is connected by a light guide with the spectroradiometer DM150 from Bentham. The input optic is mounted on a stepping motor driven rotary table, which is aligned with a laser system. This arrangement allows the variation of the incidence angle of the radiation on the input optic. The precision of a stepping motor is desirable because at large incidence angles, a small misalignment will lead to considerable errors in the cosine response. A 1° error in the incidence angle will produce an error of 5% in the cosine response (9). With this setup, errors due to misalignments are below 2% at 85°.
Results Figure 2 presents the cosine error of the newly designed global input optic in comparison to the previous input optic (flat teflon diffuser) of the Institute of Medical Physics in Innsbruck and the input optic (flat diffuser) of the Brewer MKIII #119 spectral radiometer.
It is obvious that the two flat diffusers show an increasing cosine error for increasing incidence angle. The better cosine response of the new input optic is distinctly seen. For q >50° the dome shaped part of the diffuser increases the incoming radiation. The shadow ring reduces the incoming radiation for incidence angles q >80°. The interaction of the dome and the shadow ring leads to a cosine error less than ± 3% for incidence angles between 0° and 75°. Several input optics have been constructed following the methodology described here, and their integral cosine errors are all less than 2.5%.
Conclusion Conventional input optics with flat diffusers produce systematic errors
in the measurement of global irradiance of up to 10% (2,3). To reduce these errors, a new
improved input optic was designed.
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