A Sensitivity Function (S) is a function of Wavelength characterizing the sensitivity of a telescope by wavelength (or a practical equivalent such as Frequency). The domain is the full range of wavelengths of Electromagnetic Radiation with the range being [0,1], 0 meaning fully attenuated and 1 meaning no attenuation. Such a function is used to characterize a Passband/Filter, e.g., for a B filter, in which the sensitivity function typically peaks at the central wavelength of the B filter, and falls of to 0 in both directions. A sensitivity function can also be formed to include attenuation due to other parts of the telescope such as the sensor.
The telescopic term "sensitivity function" is used by analogy with 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).
Spectral Power Distribution (SPD)