Quantifying imaging imperfections resulting from nonideal fill factor and similar pixel-scale responsivity nonuniformities
Image sensors can exhibit variations in the responsivity to light across the area of individual pixels. If the responsivity is uniform part from a region with zero sensitivity, the situation is well described by the conventional concept of a fill factor, which gives the fraction of sensitive area in the pixel. In many practical cases, such responsivity variations are gradual, due to effects such as photon scattering or photoelectron diffusion between neighboring pixels. Such gradual variations can also arise when characterizing the spatial responsivity of a complete camera, where optical blur will smear out pixel-scale variations. This paper generalizes the concept of a fill factor to cover such gradual sensitivity variations within a pixel, for an image sensor as well as for a complete camera. The resulting effective fill factor (EFF) is derived from the pixel sampling point spread function and sampling interval. It has a simple relation to an upper bound on the error in reflectance estimated from a reflected-light image. The EFF also has a simple relation to the average error in the total signal from a point source. It is noted that the spatial variation causing these image artifacts is specifically the variation of signal loss across the pixel. The EFF can supplement specification of many camera types, including thermal cameras as well as color and multispectral cameras based on filter arrays.