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Reduction of veiling glare in phase-correlation measurements in the lens elements of absorbing glass.

In most optical instruments the image is degraded by veiling glare, of which there are many sources. In some systems the major source is the unwanted light due to internal reflections between lens elements and this note is concerned solely with the glare from this source. It can be shown that if one or more of the lens components is made from neutral absorbing glass then the glare from internal reflections is reduced. This reduction operates additionally to any improvement already achieved by the use of reflectionreducing films and optical cements, and its magnitude does not depend critically upon the wavelength of the radiation or its angle of incidence. The light transmission will also be reduced but in many cases this will be acceptable, as the ratio between the reduction of glare and the loss of transmission is favorable. An approximate calculation follows which considers a single lens element, made from absorbing glass. The curvatures are such that, to a very rough approximation, the fractional light transmission may be considered constant over the area of lens being used. In general the reflection coefficients for an absorbing medium depend upon the complex refractive index, n — ik, and Snell's law does not apply. However, if we restrict ourselves to glasses such that thicknesses of about 3 mm produce transmissions of about 1-10%, the corresponding value of k in the above expression is very small—of the order of 1 × 10, and the usual reflection and refraction laws for transparent glasses may be used. Referring to Fig. 1, let the incident beam be of unit intensity, r = reflectivity, and x = fraction of light transmitted for one transit through the lens. It can be seen from Fig. 1 that the intensity of the image-forming light, I, is given by