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Author Topic: Does a raw file have a color space?  (Read 116147 times)
bjanes
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« Reply #200 on: January 31, 2008, 06:04:02 AM »
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All of them do unfortunately. That is why I used wavelengths that are rather far from IR.  Experimentally, it is very interesting to shoot rainbow formed by a prism (or formed some other controlled way - keeping in mind that AA filters sometimes have polarizing effect).
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Most all DSLRs have an IR cutoff filter which removes most light with λ > 700 nm. Some IR does get through and one can still take IR photos by using a filter which cuts off visible light and passes IR. Up to +10 EV exposure compensation is necessary with this approach, so the IR response normally would be pretty much limited to the shadows.

[a href=\"http://www.astrosurf.com/~buil/350d/350d.htm]Christian Buil[/url] shows data for a Canon 350d with and without the IR filter. Interestingly, IR response without the filter is not limited to the red channel.

Bill
« Last Edit: January 31, 2008, 06:04:48 AM by bjanes » Logged
Iliah
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« Reply #201 on: January 31, 2008, 10:01:26 AM »
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Most all DSLRs have an IR cutoff filter which removes most light with λ > 700 nm.

Lets look at the previous then.

"Noting that the sensor referred to in Iliah's post has a substantial response in the IR"

"All of them do unfortunately."

Emil just made a note why he would not use that sensor data sheet for his analysis; and went further using experimental data obtained from a fully assembled camera.

I'm talking about sensor, not dSLRs. It was clear from the post. There is no need to discuss IR in the context of the wavelengths suggested for analysis - those have nothing to do with IR at all. I mentioned that too.

As far as your double "most" - point your IR remote to the lens of a camera with a Sony sensor and see if you are getting purple blobs on the image. That may explain why external hot mirror filters from Schneider and Heliopan are still in high demand.
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papa v2.0
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« Reply #202 on: January 31, 2008, 01:00:33 PM »
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Christian Buil shows data for a Canon 350d with and without the IR filter. Interestingly, IR response without the filter is not limited to the red channel.

Bill
[a href=\"index.php?act=findpost&pid=171202\"][{POST_SNAPBACK}][/a]


you have to remember its the spectral response of the bayer filters that is being measured not the chip
so ir can be detected if the  green or blue etc filter is defective in the ir regions or uv
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bjanes
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« Reply #203 on: January 31, 2008, 01:35:46 PM »
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you have to remember its the spectral response of the bayer filters that is being measured not the chip
so ir can be detected if the  green or blue etc filter is defective in the ir regions or uv
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Papa,

I do not think that is correct. In the Stanford D70 measurements, the SRF was obtained by removing the lens from the camera and taking exposures with 35 center wavelengths from a monochromator, with wave lengths from 400 nm up to 730 nm in increments of 10 nm. The responses were recorded as NEFs and the raw data were read by MATLAB and the average RGB pixel values were determined within that program. I don't know if the data were derived from the raw data channels directly or if a demosaicing was performed, but as EJ Martin has pointed out, it makes no conceptual difference.

What they were measuring was the response of the silicon _AND_ the CFA filters _AND_ the IR filter. I don't know what methodology Christian Buhl used, but he noted: "The colored dye of the bleue and green pixels filters are also nearly transparent in the infrared.": For the green and blue pixels, this would mean that the measurement in the infrared is primarily that of the silicon response, not that of the filters.

From what Emil and Ihiah have contributed, it seems the the derivation of the camera XYZ response is relatively straightforward (accepting non-colorimetric limitations), but the real problem is in obtaining a white balanced image as I mentioned in my previous post. When one is converting between D50 and D65 spaces, the illuminants are relatively similar and well defined, whereas a digital camera has to deal with a wide range of illuminates with an unknown spectral frequency distribution.

Bill
« Last Edit: January 31, 2008, 09:39:53 PM by bjanes » Logged
papa v2.0
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« Reply #204 on: January 31, 2008, 03:58:37 PM »
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yes
 its the filter dyes that give the spectral curves.

heres a D70s spectral response I did on a bentham monochromator late last year and one from Image Engineering.
Mine is scaled to unity but you can see the "bumps" are in the right place.

Just finishing the spectral characterisation using a double grating configuration of the bentham from 350nm to 750nm.

I got my characterisation down to an average of 2.25 Delta E LAB on the macbeth 24 patch colour checker. Room for improvement!
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papa v2.0
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« Reply #205 on: January 31, 2008, 04:00:10 PM »
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heres the d70s i did
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ejmartin
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« Reply #206 on: January 31, 2008, 04:14:06 PM »
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yes
 its the filter dyes that give the spectral curves.

heres a D70s spectral response I did on a bentham monochromator late last year and one from Image Engineering.
Mine is scaled to unity but you can see the "bumps" are in the right place.

Just finishing the spectral characterisation using a double grating configuration of the bentham from 350nm to 750nm.

I got my characterisation down to an average of 2.25 Delta E LAB on the macbeth 24 patch colour checker. Room for improvement!
[a href=\"index.php?act=findpost&pid=171336\"][{POST_SNAPBACK}][/a]


Thanks for that!  Do you have or know of any similar characterization of Canon sensors?

BTW, are you sure that it's the filter dyes that give the spectral curves?  They should be only one factor.  I would have thought the response is the product of the transmissivity of the IR filter, the CFA color filter, and the response of the silicon.  Granted all color channels would be the same without the CFA, but the response curve would not be flat.
« Last Edit: January 31, 2008, 04:17:27 PM by ejmartin » Logged

emil
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« Reply #207 on: January 31, 2008, 04:20:36 PM »
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I dont have the spectral response of the chip without the filters of the IR

Its a sony chip in the D70 and D70s. Ill try to get hold of it.
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ejmartin
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« Reply #208 on: January 31, 2008, 05:18:26 PM »
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I dont have the spectral response of the chip without the filters of the IR

Its a sony chip in the D70 and D70s. Ill try to get hold of it.
[a href=\"index.php?act=findpost&pid=171341\"][{POST_SNAPBACK}][/a]

Sorry, I should have written Canon DSLR's.  I'm not interested in the chip w/o all that comes with it in an actual camera body.
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emil
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« Reply #209 on: January 31, 2008, 07:18:24 PM »
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How much can the intensity of the output of a monochromator vary at different wavelengths?
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Gabor
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« Reply #210 on: February 01, 2008, 05:43:47 AM »
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It changes over wave length with the brightest about 500nm.

should look something like this
« Last Edit: February 01, 2008, 05:46:01 AM by papa v2.0 » Logged
bjanes
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« Reply #211 on: February 01, 2008, 09:03:10 AM »
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yes its the filter dyes that give the spectral curves.

heres a D70s spectral response I did on a bentham monochromator late last year and one from Image Engineering.
Mine is scaled to unity but you can see the "bumps" are in the right place.

[{POST_SNAPBACK}][/a]

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BTW, are you sure that it's the filter dyes that give the spectral curves?  They should be only one factor.  I would have thought the response is the product of the transmissivity of the IR filter, the CFA color filter, and the response of the silicon.  Granted all color channels would be the same without the CFA, but the response curve would not be flat.
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Papa,

The sophistication of your measurements indicates that you are a high level color person, but your statement that the filter dyes determine the spectral response puzzles me. I'm no expert, but Emil's explanation seems more likely. Without any filters, all the channels would be the same but the response curve would not be linear across the wavelength spectrum.

The spectral response of a typical CCD is shown [a href=\"http://www.andor.com/learn/digital_cameras/?docid=315]here.[/url] To create an electron, the photon must reach the depletion layer of the chip. Below about 350 nm, photons are absorbed by the gate structures (the silicon has a low transmittance at this wavelength) or reflected. As the wavelength lengthens, more photons reach the depletion layer and produce electrons. In the example, response is greatest at about 600 nm (yellow-red). In the far infrared region, photons can pass through the depletion layer and not be detected.

Similar explanation and a nice Java Tutorial is presented on the Olympus Microscopy Site.

In the case of the D70 chip, the Stanford SRF was obtained with the CFA and IR filters in place. Papa's SRF is similar, but shows relatively more response at about 625 nm.

The output of the pixel is dependent on the photons reaching the depletion layer of the chip and the spectral response of the silicon, which varies markedly with wavelength. The filters shape the response by limiting what wavelengths fall on the pixel, but the spectral response of the silicon plays an important role.

Since the SRF is obtained by observation of an intact camera sensor, there is no way to determine the individual contributions of the filters or silicon other than by a controlled experiment where the responses are determined individually.

Bill
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papa v2.0
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« Reply #212 on: February 01, 2008, 12:57:45 PM »
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Hi bill
sorry must have mis-understood what you were talking about.
The underlying response of the chip is a factor that cant be changed (unless you use another chip!) but the filter pattern and dye set can be (albeit by the manufacturers only)!

So what i meant is its the filter sets that give the final spectral response particular to that camera.

Im just talking characterization of the camera as a whole.
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Panopeeper
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« Reply #213 on: February 01, 2008, 01:52:19 PM »
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It changes over wave length with the brightest about 500nm.

I guess you have incorporated this when you created the response graph, have not you?

Btw, how do you know the relative light intensity of the monochromator? Does it come with the documentation of the equipment? I guess it depends mainly on the light source, and perhaps on the quality of the grating.
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Gabor
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« Reply #214 on: February 01, 2008, 03:30:32 PM »
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had to measure it at each wavelenght interval with a spectroradiometer.
Stepped from 380 to 730 at 5nm intervals.

I think the last chart was the spd of the source at zero grating setting (white only) from the integrating sphere.
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bjanes
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« Reply #215 on: February 01, 2008, 03:45:43 PM »
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Hi bill
sorry must have mis-understood what you were talking about.
The underlying response of the chip is a factor that cant be changed (unless you use another chip!) but the filter pattern and dye set can be (albeit by the manufacturers only)!

So what i meant is its the filter sets that give the final spectral response particular to that camera.
[a href=\"index.php?act=findpost&pid=171571\"][{POST_SNAPBACK}][/a]

Papa,

Yes, it was apparently a misunderstanding that is now cleared up. EJMartin had a similar misunderstanding. BTW, you don't list your background in your profile, but from the sophistication of your equipment and analysis, I infer that you must be a color scientist?

Bill
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Iliah
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« Reply #216 on: February 01, 2008, 04:52:07 PM »
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Curves taken from dissolved dyes add nicely to typical silicon response, result is what you have from the sensor with stripped AA/IR filter. AA component adds polarization (usually circular, but not always) above all which can heavily interfere with measurements.
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mrgalleta
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« Reply #217 on: March 17, 2008, 08:18:56 AM »
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I dont know if this question is a bit offtopic, but Id like to know how could it be possible to make a conversion from the raw data to XYZ with the provided adobe matrices, I mean these ones:

DCRAW
/*
Thanks to Adobe for providing these excellent CAM -> XYZ matrices!
*/
{ "Canon EOS-1Ds Mark II", 0,
{ 6517,-602,-867,-8180,15926,2378,-1618,1771,7633 } },

Is it possible to obtain with this matrix an approximated measured XYZ value like the one from an spectroradiometer knowing the values of each element in the Bayer array ?

Thanks a lot.
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ejmartin
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« Reply #218 on: March 19, 2008, 03:11:51 PM »
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I dont know if this question is a bit offtopic, but Id like to know how could it be possible to make a conversion from the raw data to XYZ with the provided adobe matrices, I mean these ones:

DCRAW
/*
Thanks to Adobe for providing these excellent CAM -> XYZ matrices!
*/
{ "Canon EOS-1Ds Mark II", 0,
{ 6517,-602,-867,-8180,15926,2378,-1618,1771,7633 } },

Is it possible to obtain with this matrix an approximated measured XYZ value like the one from an spectroradiometer knowing the values of each element in the Bayer array ?

Thanks a lot.
[a href=\"index.php?act=findpost&pid=182094\"][{POST_SNAPBACK}][/a]

Well, it might if Adobe correctly color calibrated their raw converter; there is some doubt about that, given that many feel the need to run calibration scripts to get accurate color from ACR.

But yes, the matrix is very likely a best fit linear map from the camera's spectral response curves to those of the XYZ color matching functions (minimizing the difference between the mapped curves and the XYZ cmf's), along the lines that I posted above in this thread.
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emil
bjanes
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« Reply #219 on: March 20, 2008, 07:38:05 AM »
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Well, it might if Adobe correctly color calibrated their raw converter; there is some doubt about that, given that many feel the need to run calibration scripts to get accurate color from ACR.

But yes, the matrix is very likely a best fit linear map from the camera's spectral response curves to those of the XYZ color matching functions (minimizing the difference between the mapped curves and the XYZ cmf's), along the lines that I posted above in this thread.
[{POST_SNAPBACK}][/a]

The following links may help clarify some of the principles involved in converting from the camera space to CIE XYZ or the camera space directly to sRGB or some other working space. Of course, this implies that the camera does indeed have a space and largely negates statements to the contrary. Although the papers apply to Foveon sensors, which have 3 colors per photosite, they also should apply to Bayer sensors since, as Thomas Knoll pointed out, the missing color information in the Bayer sensor is filled in by demosaicing and interpolation early in the conversion process and the "monochrome" issue is a red herring.

As the first paper indicates, a set of color matching functions can be generated by finding a matrix that maps the spectral response into a tristimulus response using techniques such as least square matrix inversion. Depending on the error constraint goals, the matrix may yield either a balanced error or biased error, favoring some metamers over others.

[a href=\"http://billjanes1.home.comcast.net/~billjanes1/binary/KopieSensorChar.pdf]X3 Sensor Characteristics[/url]

Image Sensor with Layered Photodiodes

Bill
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