http://imagej.273.s1.nabble.com/Measuring-Dominant-Wavelength-tp5011335p5011382.html
phenomenae like green light, boreal aurors...
black body radiation.
> 1) Sorry, I made a big mistake by designating the balance point in the
> response spectrum of a detector "dominant wavelength". I was not aware that
> this expression is already used in colour science. I noticed also that
> others tend to misinterpret it as the statistical modal value. I find
> "balance wavelength" is less confusing and relates even better to the
> initial purpose, but there might even be better ones.
>
> 2) I hoped to obtain from the RGB values of the sun image, which
> approximate the perceived colour of it, an estimate for the "balance
> wavelength" of the camera's response spectrum. The sun's spectrum outside
> the atmosphere is known. With a fairly high probability the result will be
> between e.g. 500nm and 700nm. The transmission through the atmosphere can
> be estimated using aerosol measurements. But here we start now to estimate
> the "balance wavelength" by simulation, and I actually wondered whether I
> could obtain from the photos an estimate independent of the simulation.
>
> Marcel
>
> On Wed, Jan 28, 2015 at 6:34 AM, Kenneth Sloan <
[hidden email]>
> wrote:
>
>> 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!
>>
>> --
>> 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
>>>
>>> --
>>> ImageJ mailing list:
http://imagej.nih.gov/ij/list.html>> --
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