Simple Logarithmic Ratio-Recording Spectrophotometer Attachment

A double-beam system is described which uses a single photomultiplier tube operated in a logarithmic difference circuit to give direct readings of log transmittance. The circuit compensates for changes in the incident energy as well as variations in the spectral sensitivity of the photocathode, over a very wide range.The attachment can be used to convert any good monochromator into a recording spectrophotometer, and permits measurements of spectral densities as high as 3 with an accuracy better than 0.02 density unit.