An attempt is made to obtain the design of a photometer unit which may be used with an infrared spectrograph so that percent transmission or some related quantity may be recorded directly. Several basic methods of accomplishing this are evaluated. It is concluded that the following should characterize the design: the optical system should be symmetrical; there should be only one path through the spectrograph; only one detector and amplifier should be employed; and these should actuate, through an appropriate servomechanism, a device to maintain equality of intensity in the two radiation beams, so that a null system results. Several possible basic designs for some of the components of the photometer are discussed.
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