Photosensitive instruments which measure absolute irradiance in four spectral bands (2000–2500 A, 2500–4000 A, 4000–5000 A, and 5000–10 000 A) are described. One instrument is used to cover each range. The basic unit is a prism-type spectrometer that weighs 2 lb and measures by 15 in. This compact packaging is achieved by the use of a unique single prism that produces a straight-through optical axis. The spectral cutoffs are sharp and the electrical output is constant for a given flux of any wavelength distribution within the spectral passband. The passband with sharp wavelength cutoffs and a flat response versus wavelength is accomplished by the use of a mask with variable aperture placed in the focal plane. Microsecond response time is obtained through the use of a . phototube detector which is used as a photomultiplier or photodiode, depending upon sensitivity requirements.
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