Knowledge Bank

A photometer unit has been developed for use with a variety of monochromators to convert them to spectrophotometers. The utilization of a split beam and a single phase-sensitive detector, with the two radiation beams chopped $90\circ$ out of phase, is similar to the Savitsky $system.1$ An automatic slit control has been added to maintain a constant reference-signal level from the detector over a wide spectral region for any combination of source and detector. The two containing only mirrors in order to admit specimen holders for a variety of conditions---temperature, pressure, etc. Fast response is achieved with a wideband amplifier and a minimum of filtering, the latter being facilitated by square-wave chopping of the radiation beams. The two dc voltages from the phase detector are applied to a high-impedance ratio taker whose output can then be applied to any unmodified recorder. The photometer has been first applied to a Beckman quartz monochromator. The full range 0.2 to 2.0 microns is covered with photomultipliers, a thalofide cell, and a lead sulfide cell as detectors. The unit may also be used with the Golay cell in the far infrared without modification. The spectral transmittance curve of a didymium-glass filter was run from 0.4 to 1.2 microns with a thalofide detector in 2 minutes without appreciable distortion of the small features of the spectrum, and in 40 seconds with a maximum error of 5 percent