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.

Summary A cheap, simple badge for personnel UVR exposure monitoring has been developed; it is based on measuring the optical absorbance of a polysulphone film. This has been successfully used to demonstrate quantitative differences in UVR exposure of groups of people in different environments.

1 Ultraviolet radiation (UVR)‐induced wealing was studied in four patients with solar urticaria, whose measured action spectra were within the range 300 to 700 nm. 2 Elevated histamine levels were found in blood draining wealed skin in all four patients. 3 Histological and electron microscopial studies of the irradiated skin showed evidence of mast cell degranulation. 4 These findings demonstrate an association between histamine release from mast cells and wealing in solar urticaria, and should encourage evaluation of drugs which suppress histamine release in this disorder.

Skin temperatures 24 h after irradiation at both 254 and 300 nm have been examined using the technique of thermography. Increases in skin temperature have been recorded at both wavelengths but the magnitude of these differences after 254 nm irradiation is influenced by the ambient temperature. The apparent disagreement between two groups of observers working in the field of UV erythema may well have been explained.

Blood-flow changes in human skin after ultraviolet irradiation at 250 and 300 nm have been measured by three separate methods. Those methods which measure overall blood-flow changes in the skin showed increased flow after irradiation at both wavelengths. A method which measured flow only in the superficial vessels showed a slight increase in flow after low doses of both wavelengths, but in contrast, after higher doses, this method showed a marked reduction in flow through the upper dermal vessels. This reduction in flow is probably due to stasis in these superficial vessels, perhaps secondary to vascular damage. Contrary to previous reports, blood-flow changes after irradiation at 250 and 300 nm are similar and may be mediated by identical mechanisms.