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Author Topic: Accurate Astro Color - Adaptation  (Read 2682 times)

Jack Hogan

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Accurate Astro Color - Adaptation
« on: March 19, 2018, 04:20:47 AM »

Hello Color Gurus, I have a question.

Say you are an astrophotographer at a dark site looking at a patch of sky through a telescope inside a hut in complete darkness.  There is no light pollution so light from the telescope's narrow field of view strikes your eye and you perceive the wondrous colors of galaxies and nebulae within it.  With some effort and the proper tools you are able to estimate the spectral power distribution of a particular nebula. You would like to calculate what coordinates the relative tone corresponds to in an output color space it fits in - let's say Adobe RGB - to check against the same captured and rendered by a digital camera.   This is what I have been doing to obtain Adobe RGB values calculated from spectrum- but I have a question in the back of my head about Observer Adaptation that's been nagging at me for a while:

1) dot product of absolute spectral irradiance of nebula by the CIE CMF of choice;
2) Adaptation to D65
3) projection to Adobe RGB
4) application of gamma.

What is the starting white point for calculating the adaptation in 2)?  Is it illuminant E, because CIE CMFs integrate to 1; or is it the background mix of galaxies and stars in the telescope's field of view that I assume the observer is adapted to?

Thanks for any pointers,
Jack

« Last Edit: March 19, 2018, 04:30:28 AM by Jack Hogan »
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Czornyj

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Re: Accurate Astro Color - Adaptation
« Reply #1 on: March 19, 2018, 07:04:54 AM »

I doubt standard CIE CMF will be useful in such case.

Jack Hogan

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Re: Accurate Astro Color - Adaptation
« Reply #2 on: March 19, 2018, 07:37:19 AM »

Hi Marcin,

For the sake of argument, then, pretend that the absolute spectral irradiance from the nebula is the same as that reflected from the blue patch in a ColorChecker 24 illuminated by a star (the sun).  Say that same absolute spectral irradiance is in the field of view of the telescope in the situation I described.  How would you estimate its Adobe RGB coordinates then?

Jack
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opgr

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Re: Accurate Astro Color - Adaptation
« Reply #3 on: March 19, 2018, 02:52:46 PM »

Interesting because it might depend on the incidence of the light received from the object. That is, one would probably like to know the color relative to the sun, thus relative to the sun-atmosphere interaction, meaning the reference should be a normal daylight equivalent. However, the corresponding temperature likely depends on where the object is located relative to the horizon.
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GWGill

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Re: Accurate Astro Color - Adaptation
« Reply #4 on: March 19, 2018, 07:03:38 PM »

There is no light pollution so light from the telescope's narrow field of view strikes your eye and you perceive the wondrous colors of galaxies and nebulae within it.  With some effort and the proper tools you are able to estimate the spectral power distribution of a particular nebula.
Note that color perception depends on the light level. Natural night time viewing will be in the scotopic or mesopic range, so normal observer spectral sensitivities don't apply. In the scotopic range, vision is monochromatic.

So a key question is whether you are trying to reproduce color as we would naturally see it, or whether you are attempting something more synthetic - i.e. how would we see it if the light levels were in the photopic range ?

(You may find this website of interest. )
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Jack Hogan

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Re: Accurate Astro Color - Adaptation
« Reply #5 on: March 20, 2018, 03:47:49 AM »

Interesting because it might depend on the incidence of the light received from the object. That is, one would probably like to know the color relative to the sun, thus relative to the sun-atmosphere interaction, meaning the reference should be a normal daylight equivalent. However, the corresponding temperature likely depends on where the object is located relative to the horizon.

Hello Oscar, thanks for your thoughts.

In this case the sun per se would not be in the field of view but perhaps a number of sun-like stars and other faint background lighting might be (so called sky glow, what the Hubble would see).

I am thinking that if we knew the spectrum of three or more objects (e.g. nebulae or galaxies) in the field of view with 'orthogonal' colors and we captured them with a digital camera we could easily calculate a compromise color matrix from raw data to linear Adobe RGB using CIE CMFs to provide a reference.  CIE CMFs produce absolute values in XYZ with white point 'E', so I guess they assume that the observer is adapted to illuminant E - and that should be the starting adaptation to rotate to D65 for Adobe RGB.  Does this make sense?
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Jack Hogan

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Re: Accurate Astro Color - Adaptation
« Reply #6 on: March 20, 2018, 04:39:22 AM »

(You may find this website of interest. )

Hi Graeme,
Funny you should mention that: the color pages on that site are the reason I got thinking about the question in the OP in the first place.  They are wildly (did I say wildly? WILDLY!) wrong.

To calculate colors in Adobe RGB from spectrum he dot multiplies it with Stiles and Burch 1955 RGB CMFs (the ones with negative portions) and stores the result as-is in an Adobe RGB file.  No conversion to XYZ, no adaptation to D65, no projection to Adobe RGB, no gamma (!).   For instance, take a look at Figures 5-8 here, not a single one of those tones is displayed properly (colorimetrically).

It became apparent very quickly in this thread that Roger Clark does not know what he is doing as far as displaying 'true' color is concerned.  His relative pages are an embarrassment and should be corrected or taken down.  I offered to help him do them right but so far he has not taken me up on it (:-).

Note that color perception depends on the light level. Natural night time viewing will be in the scotopic or mesopic range, so normal observer spectral sensitivities don't apply. In the scotopic range, vision is monochromatic.   So a key question is whether you are trying to reproduce color as we would naturally see it, or whether you are attempting something more synthetic - i.e. how would we see it if the light levels were in the photopic range ?

Yes, that's something else to think about.  Since human observers actually do see colors of celestial objects through powerful telescopes I think we are at least in mesopic vision, if not full fledged photopic.  But for the sake of this question let's assume that the telescope is capable of collecting enough light (> a few cd/m^2) that the Observer is in the Photopic range where CIE CMFs apply.

So, when converting from spectrum to Adobe RGB, can I assume a starting adaptation of illuminant 'E' once CIE CMFs have landed me in XYZ?  Or do I have to take Observer adaptation to the background in the telescope's field of view into consideration?  After all, the original CMFs were derived by projecting absolute spectral irradiances (of which the nebula in question is an example), no?

Jack
« Last Edit: March 20, 2018, 05:06:17 AM by Jack Hogan »
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GWGill

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Re: Accurate Astro Color - Adaptation
« Reply #7 on: March 20, 2018, 07:55:40 AM »

It became apparent very quickly in this thread that Roger Clark does not know what he is doing as far as displaying 'true' color is concerned.  His relative pages are an embarrassment and should be corrected or taken down.  I offered to help him do them right but so far he has not taken me up on it (:-).
Ah - sad to hear, since the website claims to be "doing it right"!
Quote
So, when converting from spectrum to Adobe RGB, can I assume a starting adaptation of illuminant 'E' once CIE CMFs have landed me in XYZ?
Probably not, although it can be hard to figure out what the observer will take as white with unconventional scenes, and some conventional white points like D50 and E would be worth experimenting with in comparison with other approaches.
Quote
Or do I have to take Observer adaptation to the background in the telescope's field of view into consideration?
Worth a try.

It might also be worth following up some of Thor Olson's articles in the CIC proceedings:

High Dynamic Range Astronomical Imaging

The Colors of the Stars

The Colors of the Deep Sky
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opgr

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Re: Accurate Astro Color - Adaptation
« Reply #8 on: March 20, 2018, 12:27:08 PM »

I don't think adaptation in the usual sense is applicable in this case. I don't believe our perception adapts to background radation visible through the telescope, or at least we can ignore that. If we can also ignore atmospherics then perhaps it's more a question of what to consider white, and particularly what colortemperature. Then you could use temperature to blackbody spectrum conversion as the source illuminant.

Hence, you could take the sun as a reference for a white star (even though it is obviously not in the frame), and "adapt" all spectral data of other stars and objects using the sun's corresponding blackbody spectrum (T = ~5800 something) and convert to XYZ.

The more pressing problem likely is how to "ignore" the background radiation accumulated during an exposure. I remember having written a plugin to compensate for inverse fall-off, and vaguely recall it wasn't a simple subtraction. I'm sure though there must be some really useful and open solutions for that problem these days which may also help explain the best options for the illuminant problem.

Perhaps it turns out to be more of an easthatic problem than it is a scientific one? i.e. the nightsky may not actually be blue and the stars are perhaps mostly white, but we just prefer the sky darkblue and the stars yellowish...

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Czornyj

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Re: Accurate Astro Color - Adaptation
« Reply #9 on: March 20, 2018, 03:41:05 PM »

I don't think adaptation in the usual sense is applicable in this case. I don't believe our perception adapts to background radation visible through the telescope, or at least we can ignore that. If we can also ignore atmospherics then perhaps it's more a question of what to consider white, and particularly what colortemperature. Then you could use temperature to blackbody spectrum conversion as the source illuminant.

Hence, you could take the sun as a reference for a white star (even though it is obviously not in the frame), and "adapt" all spectral data of other stars and objects using the sun's corresponding blackbody spectrum (T = ~5800 something) and convert to XYZ.

The more pressing problem likely is how to "ignore" the background radiation accumulated during an exposure. I remember having written a plugin to compensate for inverse fall-off, and vaguely recall it wasn't a simple subtraction. I'm sure though there must be some really useful and open solutions for that problem these days which may also help explain the best options for the illuminant problem.

Perhaps it turns out to be more of an easthatic problem than it is a scientific one? i.e. the nightsky may not actually be blue and the stars are perhaps mostly white, but we just prefer the sky darkblue and the stars yellowish...

If adaptation in the usual sense is not applicable in such case, what can explain Kruithof curve/Purkinje effect? The stars are monochromatic (or "white") when their brightness is in scotopic range, the sky is darkblue when its brightness is in mesopic range.

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Re: Accurate Astro Color - Adaptation
« Reply #10 on: March 20, 2018, 06:25:10 PM »

If adaptation in the usual sense is not applicable in such case, what can explain Kruithof curve/Purkinje effect? The stars are monochromatic (or "white") when their brightness is in scotopic range, the sky is darkblue when its brightness is in mesopic range.

But that concerns adaptation to darkness, not adaptation to colordifferences of illuminants. I think the background in a telescope or perhaps even in a long exposure is simply not judged by our perception as a dominant illuminant color, even if brighter objects are available to trigger photopic vision.
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Jack Hogan

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Re: Accurate Astro Color - Adaptation
« Reply #11 on: March 21, 2018, 04:13:42 AM »

Ah - sad to hear, since the website claims to be "doing it right"!Probably not, although it can be hard to figure out what the observer will take as white with unconventional scenes, and some conventional white points like D50 and E would be worth experimenting with in comparison with other approaches.Worth a try.

It might also be worth following up some of Thor Olson's articles in the CIC proceedings:

High Dynamic Range Astronomical Imaging

The Colors of the Stars

The Colors of the Deep Sky

Thanks Graeme, very interesting articles, especially the last one.  Unfortunately he does not show his work in obtaining the final matrices, so I am not sure what his starting (white) point ended up being.
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Jack Hogan

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Re: Accurate Astro Color - Adaptation
« Reply #12 on: March 21, 2018, 04:16:37 AM »

I don't think adaptation in the usual sense is applicable in this case. I don't believe our perception adapts to background radation visible through the telescope, or at least we can ignore that. If we can also ignore atmospherics then perhaps it's more a question of what to consider white, and particularly what colortemperature. Then you could use temperature to blackbody spectrum conversion as the source illuminant.

Hence, you could take the sun as a reference for a white star (even though it is obviously not in the frame), and "adapt" all spectral data of other stars and objects using the sun's corresponding blackbody spectrum (T = ~5800 something) and convert to XYZ.

The more pressing problem likely is how to "ignore" the background radiation accumulated during an exposure. I remember having written a plugin to compensate for inverse fall-off, and vaguely recall it wasn't a simple subtraction. I'm sure though there must be some really useful and open solutions for that problem these days which may also help explain the best options for the illuminant problem.

Perhaps it turns out to be more of an easthatic problem than it is a scientific one? i.e. the nightsky may not actually be blue and the stars are perhaps mostly white, but we just prefer the sky darkblue and the stars yellowish...

All good points Oscar.
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Jack Hogan

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Accurate Dark Adapted Color
« Reply #13 on: March 21, 2018, 04:47:21 AM »

Allow me to approach the problem from a different direction:

Say I have been sitting in a dark room for a while, totally dark except for the power LED on the TV (say red, narrowband, around 700nm but within Adobe RGB's gamut).   I am dark adapted but I see the normally bright LED well and recognize its color as the 'correct' shade of red (correct in the sense that it looks to me to be about the same color as when the lights are on).

A) Now say I want to calculate the Adobe RGB coordinates of the LED directly from its spectrum.  I take the 700nm narrowband SPD and dot-multiply it with the CIE CMFs of choice.  Do I need to adapt to D65?  If so, from what white point?

B) Next I capture the dark scene with the LED in it with a DSLR.  The Adobe RGB value a linear rendering will produce will be within 1 dE2000 as long as the choice of WB/matrix is consistent. I simulated this for white points in the 4000-7000K range, each time recalculating the WB and Compromise Color Matrix as I normally do - minimizing a color error function to reference LAB values of a CC24, obtained via CIE CMFs.  So the camera is effectively being used as a colorimeter and it produces relatively constant, illuminant-independent, absolute values in Adobe RGB for the LED.  Will the values in A) and B) match? And am I leading the witness, by finding the CCM as I do?

Jack
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Czornyj

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Re: Accurate Astro Color - Adaptation
« Reply #14 on: March 21, 2018, 05:37:53 AM »

But that concerns adaptation to darkness, not adaptation to colordifferences of illuminants. I think the background in a telescope or perhaps even in a long exposure is simply not judged by our perception as a dominant illuminant color, even if brighter objects are available to trigger photopic vision.

Exactly, but the question is what's the state of chromatic adaptation when we're adapted to darkness? IMO the Purkinje effect may indicate, that it's not at D50-E range in case of such viewing condition. Furthermore, darker objects in the image will be desaturated or monochromatic, so when we use standard CMF the resulting image may be oversaturated in nearblacks and shadows.

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Re: Accurate Astro Color - Adaptation
« Reply #15 on: March 21, 2018, 06:06:30 AM »

Exactly, but the question is what's the state of chromatic adaptation when we're adapted to darkness? IMO the Purkinje effect may indicate, that it's not at D50-E range in case of such viewing condition. Furthermore, darker objects in the image will be desaturated or monochromatic, so when we use standard CMF the resulting image may be oversaturated in nearblacks and shadows.

I agree Marcin, but let's assume we are not interested in what else is in the room.  We just want to know what the coordinates should be in Adobe RGB to reproduce the LED's color 'accurately'.  Perhaps we should assume that the monitor displaying the captured LED is in a pitch dark room?  How do we go from absolute spectral irradiance to absolute XYZ values to the Adobe RGB file in that case?  If the camera can do it, why can't we calculate it empirically?
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Re: Accurate Astro Color - Adaptation
« Reply #16 on: March 21, 2018, 10:30:03 AM »

Exactly, but the question is what's the state of chromatic adaptation when we're adapted to darkness? IMO the Purkinje effect may indicate, that it's not at D50-E range in case of such viewing condition. Furthermore, darker objects in the image will be desaturated or monochromatic, so when we use standard CMF the resulting image may be oversaturated in nearblacks and shadows.

Perhaps under non-photopic vision our colorperception can be considered a system at rest in default state; i.e. no excitation = no adaptation so it might be at rest representing the overall average which probably is some kind of daylight equivalent. Saturation, especially in near-blacks, will certainly be interesting, first because we probably prefer saturated objects in this context, and second because we don't actually know what saturation would be deemed "normal" since we don't "normally" view the saturated version as you mentioned (other than artist's impressions of course)...
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Re: Accurate Dark Adapted Color
« Reply #17 on: March 21, 2018, 10:38:06 AM »

Allow me to approach the problem from a different direction:

Say I have been sitting in a dark room for a while, totally dark except for the power LED on the TV (say red, narrowband, around 700nm but within Adobe RGB's gamut).   I am dark adapted but I see the normally bright LED well and recognize its color as the 'correct' shade of red (correct in the sense that it looks to me to be about the same color as when the lights are on).

A) Now say I want to calculate the Adobe RGB coordinates of the LED directly from its spectrum.  I take the 700nm narrowband SPD and dot-multiply it with the CIE CMFs of choice.  Do I need to adapt to D65?  If so, from what white point?

B) Next I capture the dark scene with the LED in it with a DSLR.  The Adobe RGB value a linear rendering will produce will be within 1 dE2000 as long as the choice of WB/matrix is consistent. I simulated this for white points in the 4000-7000K range, each time recalculating the WB and Compromise Color Matrix as I normally do - minimizing a color error function to reference LAB values of a CC24, obtained via CIE CMFs.  So the camera is effectively being used as a colorimeter and it produces relatively constant, illuminant-independent, absolute values in Adobe RGB for the LED.  Will the values in A) and B) match? And am I leading the witness, by finding the CCM as I do?

Jack

I think in this case you can safely assume a daylight equivalent for the source, and it might as well be D65 to make it simple.

The horror of actually displaying the correct representation is however in the colormanagement details of your viewing system. If you don't properly prepare the result as source for the displaysystem, you still end up with skewed results.

What if it was a "white" LED? How would you want to represent that particular white LED in your destination space and on your monitor? Should it be R=G=B=1 in both spaces?
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Jack Hogan

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Re: Accurate Dark Adapted Color
« Reply #18 on: March 21, 2018, 11:23:35 AM »

I think in this case you can safely assume a daylight equivalent for the source, and it might as well be D65 to make it simple.

Thanks Oscar - good point about us having a 'resting' adaptation around daylight CCTs.  Do you see any counter indications to using 'E'?  That's what I have used in the past and at 5450K it seems to be right in the thick of things.


The horror of actually displaying the correct representation is however in the colormanagement details of your viewing system. If you don't properly prepare the result as source for the displaysystem, you still end up with skewed results.

What if it was a "white" LED? How would you want to represent that particular white LED in your destination space and on your monitor? Should it be R=G=B=1 in both spaces?

Tricky question.  What white?  Say 'E'.  It would define the white point, would it not?  Then it would be just a matter of rotating XYZ coordinates obtained the usual way to D65 and projecting to Adobe RGB (where R=G=B=1 for white).



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Re: Accurate Dark Adapted Color
« Reply #19 on: March 21, 2018, 01:02:46 PM »

Thanks Oscar - good point about us having a 'resting' adaptation around daylight CCTs.  Do you see any counter indications to using 'E'?  That's what I have used in the past and at 5450K it seems to be right in the thick of things.

I personally like E very much, especially for emissive cases as discussed here. For reflective cases it might be debatable.

I also wouldn't be surprised if the chromaticity coordinates of E are the resting state of adaptation, since it can be considered an optimal spot to adapt from. Mind you, I have no proof or research whatsoever to substantiate any of this.
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