The space irradiance isodose distribution of light delivered in tissue by an interstitial optical fibre may be determined by photographing the light diffusing out of the tissue. Provided that a plane of symmetry is photographed, it is shown that there is no significant distortion of the isodose distribution measured in this way. Two series of photographs were taken to test the method. In one set, a partially reflecting film was used to simulate the presence of the missing tissue. Isodose distributions obtained were in agreement with the predictions of diffusion theory. The isodoses were 'scaled' by measuring the space irradiance at one point with a bulb-tipped optical fibre. Optical parameters are deduced from the measurements. The method, which is simple and fast in use, should be of wide applicability.