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Re: Measuring 'Dominant Wavelength'?

Posted by Kenneth Sloan-2 on Jan 28, 2015; 4:34am
URL: http://imagej.273.s1.nabble.com/Measuring-Dominant-Wavelength-tp5011335p5011377.html

How many assumptions are you willing to make?

The human eye is no more able to compute the “dominant wavelength” of an arbitrary spectrum than a camera is.  But, the human eye did not evolve in an environment of arbitrary spectra.

If you assume “natural light”, or “black body radiation”, then there is some hope.  If you want to do this in the face of arbitrary lighting conditions - then, no - it simply can’t be done.  Either by the human visual system or a traditional camera.  

How many samples are required (across the spectrum)?  Alas, in general the answer is: an infinite number. Any claim that you can use fewer is simply an assertion about the nature of the lighting.

In fact, even the concept of “Dominant Wavelength” involves considerable assumptions.  In fact…in those cases where “Dominant Wavelength” makes sense… then human eyes and conventional RGB cameras can do the job just fine.  The difficulty comes when you try to extend this concept into domains where it doesn’t apply.  “Dominant Wavelength” is a concept that really only makes sense in a color system that is 3D - one where “Hue” is a possible dimension.  It’s really a psychological concept, not a physical one [except to the extent that psychology evolved to match a certain flavor of physical reality].

Confused, yet?  Good!

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Kenneth Sloan
[hidden email]


> On Jan 27, 2015, at 13:35 , Marcel Tschudin <[hidden email]> wrote:
>
> Hello everyone,
>
> I am new here. I am wondering whether I could use ImageJ (or an other program) for measuring in photos the 'Dominant Wavelength' of the colors within a selected pixel area. I provide here some further explanations because I am not sure whether what I intend to do would actually even be possible with photos.
>
> I would like to estimate the sun's 'Dominant Wavelength' in photos of the setting sun. For a detector like the human eye the 'Dominant Wavelength' would result from the sun's spectrum after passing the atmosphere and after passing the eye's spectral detector efficiency. It would be calculated from the detector's spectrum as Ldom, with the radiation intensity, I, at a certain wavelength, L, in increments, dL, over the visible spectrum as a ratio of two sums (integrals):
> Ldom = Sum(I*L*dL) / Sum(I*dL)
> (Because 'Dominant Wavelength' could be misinterpreted others suggest to call this the 'Balanced Wavelength' instead.)
>
> Consumer cameras do not record the spectrum, they rather approximate the detected spectral content, i.e. the color perceived by the human eye, with the RGB information. Would it now be possible to estimate the original 'Dominant Wavelength' from the available RGB information in the photo? If yes, do you know if ImageJ (or an other program) provides such a feature or a similar one?
>
> Thanks,
> Marcel
>
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