  Thin-film optical filters can nowadays be integrated onto pixels of
commercial image sensors used for spectral imaging. A drawback of having more
filters on an image sensor is a loss in spatial resolution which could be
regained by using smaller pixels. However, this work shows that miniaturization
causes a reduction in transmittance and requires a trade-off between pixel size
and filter bandwidth. This is caused by diffraction and the reduced number of
reflections in the filter. Analytical wave and ray optics models are developed
and used for fast transmittance prediction and this is validated using a
commercial spectral sensor and numerical simulations. The identified trade-off
is an intrinsic limitation of pixel-integrated thin-film filter technology and
the developed models will be useful tools for exploring new hardware and
computational solutions.
