Article: On Diffraction-Limited Performance For Infrared Cameras

Diffraction limits the resolution possible with an objective lens. In many optical lens calculations, commonly ray-tracing approximations are made where electromagnetic radiation is approximated as traveling in straight lines from object points to image points.

In reality, diffraction spreads each point in the image to a spot having size depends on the f-number of the lens and the wavelength of the radiation.

At the diffraction limit, typically about 20% of the focused energy must be captured by the detector pixel limiting the size of an infrared pixel to twice the wavelength being detected. The remaining 80% of the radiance is split between at least 8 surrounding pixels.