Spread Function

a function of an optical device that describes the distribution of illumination of the image of a small (point) radiation source created by the device. The spread function permits evaluation of the resolving power of the device and defines the relationship between illumination distribution in the image formation of an object and intensity distribution of the same object; having determined the illumination distribution in the image and knowing the spread function, it is possible to determine the intensity distribution of the object.

By definition, an ideal optical device represents a point radiation source as a point; in this way, its spread function everywhere except for that point is equal to zero. In real devices, as a consequence of the diffraction of light and the presence of aberrations and defects in the construction of the device, the image of a point source occupies an area of finite dimensions and the spread function differs from zero according to the limits of this area.

Considerable difficulty is encountered in the calculation of the spread function in the presence of aberrations and other factors; in view of this, the spread function is usually determined experimentally.

In application to spectral devices, the concept of the spread function has a somewhat different meaning. If the spread function of a spectral device is known, it is possible to determine the true energy distribution in the spectrum of the radiation source under investigation by the results of recording of the spectrum.