4 Attachment(s)
Color correction when photographing lasers
For those of you who do not know, I am big into nonlinear and ultrafast optics--both at work at at play.
On a few occasions I have tried to photograph supercontinuum generating processes, even going so far as to try and take some video from the system at work https://www.youtube.com/watch?v=gzST4Uts6rA
So far, all of my attempts to capture the beauty of the process have been largely unsuccessful, and I have always ended up with washed out images that do not reflect what is actually going on. I think people who try to photograph lumia run into a similar problem, here the color mixing just looks 'wrong' on camera.
I think I have made a bit of progress sorting out why this is, and it has to do with the spectral response of the bayer filters on my cameras.
I managed to capture this effect clearly when photographing the output of my supercontinuum laser (NKT SuperK) which has a singlemode fiber coupled output and continuous spectral coverage across the visible and into the IR. Because it is a supercontinuum laser, the output beam is a continuous spectrum not discrete lines like a whitelight argon laser.
I collimated this beam using a reflective collimator (to avoid introducing chromatic aberrations--the endlessly single mode photonic crystal delivery fiber has a mode shape that depends on wavelength so there is some residual variation in the color across the beam--similar to the how whitelight argon lasers look) and dispersed it on a prism over ~10m to get a well dispersed 'rainbow'. I also have a IR block filter on the output mainly for eye safety (there is a significant fundamental component at 1064nm from the pump laser present at the fiber output), but also to ensure that there is no IR light that is interfering with the color rendition on my cameras. At first glance this looks very similar to the rainbows generated by dispersing sunlight, but if you look closer it does look a bit 'off' since the spectral power distribution is not that of a blackbody like the sun is.
In any case, to cut to the chase here is what the beam looks like with a bit of fog
Attachment 54663
Instead of the expected continuum, we get instead 3 distinct bands of red/green/blue with almost nonexistant cyan and a very narrow band of yellow.
That picture was taken with a samsung NX1 (S5KVB2 sensor), for which I was not able to locate a datasheet. There was however a paper published by Samsung in 2013 (1 year before the release of the S5KVB2 sensor) which included the attached QE plot
Attachment 54664
from https://pdfs.semanticscholar.org/cb7...8834208d05.pdf
This paper was for their cellphone sensors (1.5um pixel compared to 3.6um for the S5KVB2) but the spectral shape of the filters should be similar regardless.
With a bit of squinting and hand waving one can now see where the problem is. Imagine a illuminating the sensor with a tunable monochromatic light source and estimating the measured r/g/b values based on the above responsivity plot.
Below 450nm only B pixels are illuminated so we get pure blue
Between 450-530nm there is a narrow band of cyan where both B and G pixels are illuminated so we should see a band of cyan
Between 530-575nm there is nearly pure G pixels illuminated so we get pure green
Between 575-630nm there is a mix of G and R so we should see a band of yellow
And for wavelengths greater is only R pixels illuminated so we get pure R
To further explore this, I took an image of the rainbow on a white card again at a ~10m range in a dark room to get a clean image. I then I tried creating a false color image with a highly nonlinear gain applied for each r/g/b channel, so that if there was any appreciable power detected in the channel, the drawn image is at full intensity for that channel.
Attachment 54668
The sharp edge in the red spectral response is clearly visible at the yellow/green boundary, as are the large gaps in the bayer filter spectral overlap as the regions of pure red green and blue. Also, a pink dot is visible in the middle of the red region where the blue filter response starts to pick up again, presumably this is the same behavior that causes near-IR lasers to show up as purple on camera.
Compare this chart to an eye sensitivity chart (found at https://www.quora.com/Why-are-only-s...aked-human-eye where it was posted without attribution)
Attachment 54665
Here, we see that the green/red bands largely overlap, and most importantly there is always at least 2 cones giving appreciable response. The blue and green responsivity seem to match up roughly correctly with the bayer filter, but the red is completely off.
That does not quite explain why the cyan went missing, but does at least explain why all of the orange/yellow colors are missing.
Has anyone dealt with this before? I was contemplating schemes to correct for this in post, but it seems that in particular with the red channel the information needed is simply missing. Does anyone know of cameras that have RGB filters that have expanded overlap regions? Clearly it is a conspiracy from Big Camera to us laser enthusiasts down *warn
