I have realised that the transmission of a Cokin BLUE 80B photo-filter applied to the solar (outside the atmosphere) energy flux spectrum provides a remarkably good analogue for the colour of the twilight sky within the visible part of the spectrum. The plot shows a measurement of the twilight sky with a solar elevation of -8° (green line) compared with the Cokin-filtered intrinsic solar spectral energy distribution (blue line). The filter thus represents the the effect of the long atmospheric path taken by sunlight during twilight (The Blue Hour). There are differences in detail: the analogue is brighter than twilight in the far red (beyond about 680nm) and also peaks at a shorter wavelength in the blue, but is does provide a rather realistic twilight blue. The riboflavin fluorescence spectrum (red line) is of interest for animal vision but is incidental to this this post. Note: The slight wavelength shift between the two solar spectra is understood: I resampled the solar flux spectrum onto the original JAZ wavelength vector before I applied my calibration correction. This small difference is not important for this plot.
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