Video cameras may be used for radiometric measurement in machine vision applications including colour measurement and shape from shading. These techniques assume that the camera provides a linear measurement of light intensity. Even when the video gamma factor is disabled, video camera response is still often nonlinear and varies from one camera to another. We describe a method of measuring a camera?s radiometric response function using unknown but stable reflectance targets and illumination. The technique does not require sophisticated equipment or precise measurement or control of physical properties. The technique simultaneously estimates the camera's radiometric response, the relative reflectance values of reflectance targets and the relative illumination levels of multiple light sources. The technique has been practically applied for colour measurement applications with an experimentally verified accuracy of 0.3%.