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UltravioletPhotography

Paper: Multispectral imaging


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I was checking on recent publications by Adrian Dyer's group and I came across this:

 

Garcia et al 2015

http://journals.plos...al.pone.0125817

 

Make your self a cup of coffee, or something stronger before you start it as it will make your head hurt. It compares, in the visible region only, a normal DSLR with a multispectral camera with 128 channels. I have no idea how much that costs or whether it would work for UV but it might solve my never ending question of what our false UV colours mean and if they really do relate to wavelength or are just clever artifacts of the RGB dyes and the white balancing process.

 

Cheers,

 

Dave

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if they really do relate to wavelength or are just clever artifacts of the RGB dyes and the white balancing process.

 

Both, actually. The different dyes of the reseau have different absorption spectra in the UV as well as the visible; thus, they will respond differently to differing wavelengths. Thus, wavelength information will be to some extent be encoded as colors in the image (though perhaps not in a straightforward manner.)

 

Long ago, before RGB color photography, there was something called the Lippmann Process. A b&w plate was coated with a thick emulsion and backed with a mirror coating; the emulsion then recorded a kind of hologram of interference fringes. Illumination of the developed plate with white light reproduced the original spectrum of the reflected light recorded by the image via diffraction, resulting in a color image produced by a means completely independent of the color perception of the human eye. The technique is still practiced by a few enthusiasts (though not outside the visible--that would result in invisible final images!) Perhaps the 128-channel camera is a new way to do a very old thing.

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hyperspectral cameras have been recently developed to produce image spectrophotometers to recover reflectance spectra at individual pixel locations

 

Oh how much would I love to have such a camera ?!!!!! Very much.

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In the course of this paper the authors do note many of the factors preventing recovery of accurate spectral data from a standard digital camera RGB image: Univariance principle, metamerism, lack of uniform illumination in the field, inability of camera to discriminate between very similar colours.

And more.

Thus a "hyperspectral camera" is needed with many, many channels.

 

Or how about a SpectroPhotoMetric camera? That would be so cool.

I'm sure it's coming eventually. :D

 

It might be useful to have a handful of very narrowband filters to make a stab at this kind of thing.

But even then, you could not overcome Univariance (many signal combinations can produce an identical response).

 

The authors say:

Our results show that the degree of accuracy with which the signal spectra can be reconstructed from camera responses is dependent on the number of sensors available in the system. In particular, the RGB camera equipped with just three colour channels was unable to accurately discern between samples with a chromatic difference of about 2 JND. This result suggests that RGB cameras are better suited for large-scale examinations in colour patterns such as between-species comparisons. Nevertheless, the discrimination ellipses calculated from Newhall’s results for discrimination of small colour differences by 3 observers, also intersect for these colour samples. This suggests that differentiating between very similar colours is indeed a difficult task for trichromatic systems.

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So, we will never know how flowers are perceived by insects, especially since we have little or no way of actually knowing how the signals from the blue, green and UV receptors are processed and combined.

 

In a sort of related field, astronomers now use robots to accurately place tiny fibre optic probes in the focal plane of telescopes and measure the spectrum of each star at the other end of the fibre. I want one of those B)

 

Dave

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We can only dream about these specialized cameras and telescopes, Dave !! :D

 

The bee & insect papers are fairly detailed about the way eye receptors work (in re the pigments and the stimulation) but the details about the brain processing seem to be in progress. Of course, I have no way to really keep up with that on an ongoing basis. It would be very much too time consuming to be reading papers all the time. Whew !!

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