A designer of an IR optical system needs to account for key specifications that greatly impact the complexity of the device construction and its production cost. The most important requirements are described below.

The maximum achievable theoretical resolution in wavelength infrared region

The maximum resolution is estimated with the help of Rayleigh criterion:

r=0.61λ/NA.

r - minimum size of the object that can be recognized by an optical device

λ - radiation wavelength

NA - numerical aperture of the optical device

In accordance with this criterion, the resolving power of the optical device with thermal radiation (λ – about 10 µm) is 20 times lower than that of the visible radiation optical device (λ – about 0.55 µm).