Abstract
The photodetector used is a cadmium selenide photoconductive cell and allowance is made for the temperature coefficient of the cell by applying a correction factor. Nonlinearity of the photocell response has been shown to be of negligible importance. Heating of the skin by the light source is greatly reduced by the use of fibre optic light guides and the influence on the recording of changes in oxygenation of haemoglobin is kept to a minimum by an infrared filter. The instrument has been shown to be sensitive, atraumatic and applicable to almost any skin area. Quantitative measurements of blood flow are not possible but accurate comparative measurements of changes in flow are easily obtained when the area of skin under study is compared with an adjacent control area.