Just tested the relative sensitivities of my QHY5 cameras, one colour and the other mono. Might help quantify the difference in sensitivity of a Bayer matrix camera when compared to a mono camera.
test conditions:
- source - white paper illuminated primarily by skylight from overcast sky, viewed through a refractor and defocused.
- cameras tested alternately and results checked by one repeat - identical gain and exposure settings were used.
- an IR/UV cut filter (luminance from an LRGB set) was fitted to the cameras when tested (same filter used on both)
- images were analysed in Nebulosity, with 2x2 software binning to average out Bayer filter effects and reduce fixed pattern and read noise.
Results:
- the mono camera detected 2.3x as many photons as the Bayer camera.
- the signal to noise ratios were in proportion to the square roots of the signal, so camera noise/offset was probably not a major factor.
Discussion: the Bayer matrix does not divide the light up into three equal and precise spectral regions, but does so with some overlap. It appears that this, in concert with the green bias inherent in the 1:2:1 matrix results in higher sensitivity than would be expected from a simplistic division of the input into equal thirds. This test assumes that the underlying MT9M001 chips are the same for the two cameras and that the QHY software sets them up identically - I have no way to check this. The spectral distribution of the test source was not known, but it should have been fairly representative of real targets. I also have no way of knowing if the Bayer filters used by Aptina are similar to those from other makers, but assume that they are not too different and that these results may be more generally applicable.
had previously assumed that a Bayer camera would be about 0.33 times as sensitive as a mono one. That is incorrect - in this test the Bayer QHY5 was better than that, being about 0.43 times as sensitive as the mono counterpart.
thanks for reading. Regards Ray