Knowledge Bank

An infrared spectrophotometer has been built incorporating a photometer system which uses light from the same area on the source in both the sample and reference beams. The spectrometer is equipped with four prisms, KBr, NaCl, $CaF2$, and LiF, and switches automatically from KBr to NaCl, from NaCl to $CaF2$, and from $CaF2$ to LiF in scanning the region from 25 to 2 microns. The reference beam is attenuated to match the sample beam by means of a comb mechanically coupled to the recorder pen. Balance is achieved through the agency of 5-cps chopped radiation, an ac amplifier having a voltage gain of $108$, full-wave synchronous rectification, and a dc amplifier having a voltage gain of 140 controlling the armature current of a dc motor driving the comb. The ac amplifier bandwidth is 5 cps. The residual ac signal at balance is used through a separate channel to control the slit width. Manual control of wavelength and slit width is also provided.