• Ultraviolet Photography

Lens and camera tests with sparticle and monochromator

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#61 Jim Lloyd


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Posted 30 August 2018 - 09:46

Whether the filters in the range 340-360 look a bit brownish comes down I think to how much response there is in the blue channel.

If there is some (even very small) then how this appears will depend on details of software rendering

In my case there is a small amount of blue in this region in the RAW sparticle data and in the response measured using the monochromator.

So the fundemental question is whether the blue Bayer dyes do transmit some radiation in this range.

My feeling is probably yes, although for both my methods of measuring there may be alternative explanation: With the monochromator I only had the BG40 filter as I wanted to see the response in visible (particularly 400-500) so that I could then model the effect of different filters cutting at different points atound 400. If taking the work forward I would definetly look at this more. With the sparticle then there may be some small transmission outside the band of interest and then may be some contribution from reflected light.

Having said that I still think there may be some genuine transmission through the blue Bayer. I think Jonathan's work might show this (see here) and somewhere (can't find the link just now) I saw some work by Kluass Schmidt indicating blue response at short wavelengths. Also when I looked before at your (Cadmium) sparticle images - although they do look green and yellow (in this range) - they do have a little blue, even if this isnt very obvious visually.

So if there is some genuine small response in blue at these wavelengths how it appears in the final rendered image will be sensitive to processing. It will get boosted considerably by both the white balance (as overall blue reponse is low compared to red) and by gamma correction.

I have looked at my images with Rawdigger, View NX2, FastRaw viewer, Photoninja, and Lightroom and all give slightly different results which vary with different settings. They go from quite clear green and yellow, to muddy brown versions. The most recent images above are from View NX2 using the Neutral profile as I thought this would be the minimal level of processing that would be most similar to how I modelled the white balance process.

For actual photography I would tend to produce something that looked a little brighter.

Edited by Jim Lloyd, 30 August 2018 - 09:47.