Does the use of two radiometers correct for irradiance changes during measurements

The method of inter-calibrating two field radiometers proposed by Duggin (1980) is critically assessed. It is claimed that by this method it is possible to correct for the effects of variations in total solar irradiation upon hemispherical directional reflectance during a series of measurements. However, this is questioned because, in theory, the non-Lambertian nature of most natural surfaces prevents such a correction. Furthermore, it is shown that in practice, intra-canopy shadow effects are likely to confound the proposed method. Uniform irradiation conditions are seen as the only suitable environment for the field measurement of reflectance factors.