A 45° fused quartz prism is located at the head of the fore-optics tube and reflects incoming radiation (by total internal reflection) at right angles along the horizontal optical path of the instrument. The zenith prism plays an important role in selecting the appropriate source of radiation for lamp tests, sky measurements, direct sun or moon measurements, or UV spectral measurements. The prism rotates about the optical axis of the fore-optics so that radiation originating from zenith angles between 0° and 90° can be directed along the axis. In this mode, the instrument measures the radiance at the respective zenith angle. Rotation of the zenith prism is done by a computer controlled stepping motor (called the zenith motor), which acts through a set of reducing gears to position the desired direction with a precision of 0.128°.
A lamp housing containing a mercury lamp and a quartz iodine lamp is located below the zenith prism. When the zenith prism is directed so that it points downward (zenith angle of 180°), radiation from the lamps is directed along the optical path. The mercury lamp is used to check the wavelength setting of the instrument and adjust it if necessary. The quartz iodine lamp provides a radiation source to check the relative wavelength response of the instrument.
Figure 6.2 Photograph of the Mark III version of the Brewer Spectrophotometer (Courtesy of Kipp and Zonen)
Radiation incident on a horizontal Teflon diffuser on top of the cover is viewed by rotating the zenith prism to view horizontally towards the UV port shown in Fig. 6.1. The UV port consists of another 45° prism that is mounted on the fore-optics tube and positioned directly below the horizontal diffuser. The UV port prism reflects radiation transmitted through the horizontal diffuser at right angles toward the zenith prism, which in turn, reflects radiation at right angles along the optical path. In this mode, the instrument measures global irradiance.
Radiation passing along the fore-optics axis then passes through a lens that focuses radiation from infinity onto the plane of an iris. The iris is used for only direct sun or moon measurement. When the azimuth and zenith are aligned toward the sun or moon, the focused image of the target passes through the aperture of the iris, which is closed to about five solar diameters (e.g., Cede et al., 2003). This prevents most of the scattered sky radiation around the solar or lunar disk to pass through to the spectrometer. Scattered sky radiation can become important when the sun is low, or at relatively large aerosol or cloud optical depths. For all other measurements, the iris remains open and does not block any radiation passing along the optical axis.
After passing through the plane of the iris, radiation is collimated by a second lens and then passes through two filter wheels which can both hold up to six filters. The first wheel consists of an open position and neutral density filters with increasing optical depth in the five remaining positions. Progressive filter positions decrease radiation by a factor of about 3. A second six-position filter wheel inserts filters that include ground quartz for direct sun measurements and polarization filters. The ground quartz filter is used to diffuse direct sun radiation, which reduces errors that may result from the effects of slight misalignment. After passing by the filter wheels, radiation is then focused onto the entrance slit of the spectrometer.
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