Abstract

It is shown, for a particular class of partially coherent fields, that when such a field is incident upon a circular aperture, the spectrum of the light at an observation point in the far zone of the aperture is different from the spectrum of the light in the aperture. An expression for the spectrum at such observation points is obtained, and it is shown that the difference between the two spectra depends on the ratio of the radius of the aperture to the effective correlation length of the light in the aperture. Numerical examples are presented. My results support a recent suggestion by Kandpal el al.¹ that the discrepancies found in the comparison of spectroradiometric scales maintained by standards laboratories may be due to such changes in the spectrum.

© 1990 Optical Society of America