This a robust colorimeter featuring a stabilised power supply that maintains a constant light output from a 6V 0.15A tungsten lamp with infra-red filter. There are two absorbance ranges and one transmission range that are selected by a position rotary switch. As well as enabling the principles of colorimetric analysis to be taught in schools and colleges it can also be used for the quantitative determination of substances present in small amounts and to monitor the course of a reaction which is accompanied by a change in the absorption of transmitted light. An automatic gain control provides automatic zeroing by simply pressing a switch and the instrument is insensitive to normal ambient lighting. The eight colour filters are held in a rotatable filter holder and the disc can be removed cleaning or replacement. Spare filter wheels are available (B8L33431). The single cuvette holder takes 10mm path length cuvettes and is part of the subassembly moulded in plastic there is a drain hole in the base to eliminate any accidental spillages causing damage. The instruments have 0-1V output sockets fitted as standard to allow a datalogger or PC to be attached. The Philip Harris Colorimeter features an LCD display which displays the values according to the range selected.