http://imagej.273.s1.nabble.com/Measuring-Dominant-Wavelength-tp5011335p5011389.html
temperature. That should indeed be possible provided I can calibrate the
> In your specific case (sunset), I think you can make the following safe
> assumptions:
> 1) most (if not all) of the phenomena comes from Rayleigh scattering
> through the atmosphere
> 2) there is no reason that strong discrete spectral lines appears in the
> resulting spectra (the diverse absorption line should be negligible here)
> 2') I take for granted that you avoid strange (and rare) weather
> phenomenae like green light, boreal aurors...
> 3) the main part of the result should be close to a shift in color
> temperature, the Sun light (outside atmosphere) being very close to a black
> body radiation.
> Then, why don't you try to just make a White Balance in order to estimate
> the color temperature of your pictures? You should calibrate your camera
> first (with some pictures of calibration targets taken under controlled
> lighting), but after that several image processing programs (I use
> Rawtherapee for that, I'm not sure about ImageJ) will give you directly a
> measure of the color temperature, that you can translate in wavelength of
> the maximum black body radiation power curve using the Wien displacement
> law:
>
> \lambda_\max = \frac{b}{T}
>
> where the constant,/b/, known as Wien's displacement constant, is equal
> to2.8977721(26)×10^−3 K m.^[32] <
http://en.wikipedia.org/wiki/> Black-body_radiation#cite_note-32>
> [quoted from Wikipedia:
http://en.wikipedia.org/wiki/Black-body_radiation> ]
>
> Best regards,
> Jean-Louis
>
>
>
> On 30/01/2015 01:17, Marcel Tschudin wrote:
>
>> 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>>>>
>>> --
>>> ImageJ mailing list:
http://imagej.nih.gov/ij/list.html>>>
>>> --
>> ImageJ mailing list:
http://imagej.nih.gov/ij/list.html>>
>>
> --
> Jean-Louis Oneto
> email:
[hidden email]
>
>
>
> --
> ImageJ mailing list:
http://imagej.nih.gov/ij/list.html>