rements em- Footlamberts cd m ² ploys a photoconductive or photovoltaic silicon photodiode con- tained in a light tight housing. In front of the detector is a filter Atomic fission bomb (0.1m 6x1011 2x1012 specially designed such that the combination of detector and filter after firing 90 ft diameter ball) spectrally matches the response of the eye. In front of the filter is an Lightning flash 2x1010 6.8x1010 opal glass disc which in combination with the geometry of the Carbon arc (positive crater) 4.7x106 1.6x107 housing produces the required cosine response. The photometer head is connected to a calibrated readout unit. Tungsten filament lamp (gas 2.6x105 8.9x105 In a telephotometer used for luminance measurements the filled 16 1m measuring head incorporates a telescopic optical system focused Sun (as observed from the 4.7x1 10³ 1.6x104 onto the detector. A beam splitter enables the operator to look earth's surface at meridian) through the telescope and point the instrument at the surface whose luminance is required. In some instruments the dial of the readout Clear blue sky meter appears in the eyepiece field SO that the observer can read off Fluorescent lamp (T-12 the luminance of any part of the scene without removing the instru- cool white 430 mA medium ment from his eye. loading) 7.5.2 Moon (as observed from Radiometers and spectral analysis earth's surface) Radiometry is concerned with the emission and detection of radiant energy. The radiant energy is taken wavelength by wavelength through the entire radiation spectrum from the extreme ultraviolet (7.4) to the extreme infra-red. To detect this radiation an instrument will normally employ a detector which is responsive to radiant power Thus in direction 0, Equation 7.5 is obtained. which is expressed in watts. A radiometer might employ a photo- diode, photomultiplier or photoconductive cell as its detector. Here (7.5) the sensitivity of each of these detectors varies with wavelength. It luminance = cos 0 is therefore necessary to take account of this vari