a quantity permitting comparison of illuminances in the image planes of various optical systems. If the losses of light energy due to absorption and reflection in an optical system are neglected, we speak of the geometric light power, which is the square of the relative aperture of the system; that is, it is equal to (D/f)2, where D is the diameter of the entrance pupil of the system (seeSTOP) and f is the system’s focal length. When the geometric light power is multiplied by the coefficient τ, which characterizes the losses, the physical, or effective, light power is obtained. This ratio can be increased by reducing the light losses, through the use of antireflection coatings.
The illuminance E in the image plane of an axially symmetric optical system is the ratio of the luminous flux passing through the system to the area of the image and is given by the formula E = πBτsin2 u′, where B is the luminance of the object and u′ is the angular aperture of the image space. For objects that are sufficiently remote (practically at infinity), the image plane coincides with the focal plane. In this case sin u′ = D/2f, and the illuminance and, consequently, the light power can be obtained from the relation
L. N. KAPORSKII