It is shown for a Second source field that, when such a field is incident upon a reclangular or a slit aperture, the spectrum of the light at an observation point in the far zone of the aperture is different from the spectrum of the ligt in the aperture. An explicit 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 size of the aperture to the offective corelation length of the light in the apertuer. Numerical examples are presented. There are some difference comparing to the results of circular aperture.