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The calculation is related to a screen illuminated by a small light source located at the front focal plane of an optical system. The light source dimensions are small compared to the focal length of the optical system. The aperture diameter is small compared to the light source dimensions (pinhole type aperture) and the distance from the optical system to the screen is larger than the focal length. The aperture is supposed to coincide with the primary principal plane of the system. The page calculates the irradiance on the screen as a function of the radiance of the light source, the focal length and aperture diameter of the optical system and the distance from the optical system to the screen.