sensitivity function

A sensitivity function (S) is a function of wavelength characterizing the sensitivity of optics by wavelength. The domain is the full range of wavelengths of electromagnetic radiation (EMR) with the range being [0,1], "0" meaning fully attenuated (no EMR passes through ) and "1" meaning no attenuation (all passes through). Such a function is used to characterize a passband/filter: for example a B filter has a sensitivity function with a peak near 445 nm, with little attenuation in that vicinity, but in each direction from that region, falls away with some slope, eventually to 0. The sensor (e.g., CCD) also has such a sensitivity function as do other parts of the optical path, and the telescope configuration as a whole has one that results, which I believe is the convolution of that of each individual optical element along the light path. (Note: the above explanation is in terms of wavelengths, optics, and light, but the term can be used for any portion of the electromagnetic spectrum, and can be specified by frequency rather than wavelength.)

This use of the term "sensitivity function" is analogous to the use of the term for the human eye, for which a so-named function of wavelength is used to describe its sensitivity across the spectrum. For the eye, the term luminosity function is also used. For the eye, the sensitivity depends upon whether the eye is using scotopic vision (i.e., adjusted to low illumination, i.e., using "rod" photoreceptors).