This International Standard is applicable to optical imaging systems in the optical spectral region from λ = 100 nm to λ = 1 μm. Theoretical reflections and the comparison with the calculation apply only to optical systems. These need not, however, be rotationally symmetric; anamorphic systems, for example, are included. Telescopic systems are also included. The title of this International Standard refers to the relative irradiance in the image field, but this standard is also applicable to determination of the relative radiant power. NOTE — For telescopic systems it will be suitable to state only the radiant power; for most imaging systems, the conversion from radiant power to irradiance will be easy. As far as measurements are concerned, this International Standard can also be applied to electro-optical systems. The two methods described differ particularly in the influence of veiling glare.