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Re: Measuring 'Dominant Wavelength'?

Posted by Jean-Louis Oneto on Jan 30, 2015; 4:35am
URL: http://imagej.273.s1.nabble.com/Measuring-Dominant-Wavelength-tp5011335p5011382.html

[second try, it seems that just the picture of the equation went through
the list (I would have rather suspected the opposite !)]
Hello,
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]
[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
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>>
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>

--
Jean-Louis Oneto
email: [hidden email]


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