Abstract
A general review is made of double-beam systems in absorption spectroscopy. Apparatus is described in which the two beams of radiation fall alternately on to a fast thermopile, the latter being in the primary circuit of a step-up transformer designed to work at low frequencies. The output goes to a low-noise wide-band amplifier capable of a maximum overall voltage gain exceeding 109. The amplified signals from the two beams are separated and rectified by a simple mechanical switch on the radiation interrupter shaft, thus making the amplifier selective for frequency and phase, and enabling response time to be varied between 1 and 100 sec. Finally, the ratio of the outputs is measured on a potentiometer recorder. The reference beam output is used to regulate a slit monitor, so maintaining a constant reference energy level on the thermopile through the whole spectral range. The performance of the equipment is illustrated.