DCamProf - a new camera profiling tool

[Iliah]

> that is still several $Ks...
Yes, but it has no useless bells and whistles; while for a regular user i1Pro is ok in transmissive setups.

[ah693973]

I have been involved in modifying a drum scanner (Howtek) in a project over at the large format forum. I started out alone with better PMTs and amps and then worked with others who generously helped to fix some of the issues that came up that were out of my "comfort zone". One of the guys I was working with is using a RGB led as his source now instead of the halogen lamp that was originally used. He is reporting good results and is technically sharp enough that I believe him.

Of course, a bayer array and a drum scanner really work on the same principle, its just that one is two dimensional and one is a point. This is what led me to believe that a more direct approach might work.

Andy

[Iliah]

The difference is that film spectral transmission allows for relatively narrow spectrum light sources (see colour separation filters), while the response of a camera sensor does not.

[AlterEgo]

> that is still several $Ks...
Yes, but it has no useless bells and whistles; while for a regular user i1Pro is ok in transmissive setups.

so Torger can still find a use for his solux lamp by sticking gel filters along with a filter to raise CCT up on top of it and using his colormunki to measure the light directly and then shooting it directly with a camera in M mode to keep exposure the same... or alternatively find a way to mount a gel in front of styrofoam and measure the illuminated styrofoam (tele mode with colormunki) and then shoot it too, gel by gel... flare, etc will not matter much there too... now you just need to find a minimal set of gels to get results

[Alexey.Danilchenko]

Using transmissive targets is interesting though. In Image Engineering's CamSpecs product they use transmissive targets with several narrow bands to measure the camera's SSF. And I think if one is going to make some elaborate DIY project with hardware stuff, I think one should try to look into measuring SSF.

It would be less expensive to build proper monochromator setup to do that. I have completed mine (it needs to be calibrated) and overall it cost less than the 6500K light you were looking for. Used monochromator (from manufacturing like Luxtron/Lumasense/Xinix - like this one for example), used halogen light source with fiber optic feed (plenty on eBay) and used integrating sphere (even a simple one like this will do). Also a way to measure illumination level in a sphere - arduino + photodiode + 30mins of coding. All of that put together on a bench in a dark room gives a decent enough setup to measure SSF.

[torger]

It would be less expensive to build proper monochromator setup to do that.

That's super-cool, I'd love to see some photos when you have it all working!

[Iliah]

> It would be less expensive to build proper monochromator setup to do that.

I doubt "less expensive" part (unless we are chasing D65, which even an advanced user will not be doing, and which is unnecessary as obtaining spectral response is a much better way to do things), but much more precise, much more stable, and much more convenient if a lot of cameras and lenses are to be characterized.

Light spheres are easy DIY project.

[Alexey.Danilchenko]

That's super-cool, I'd love to see some photos when you have it all working!

Sure - I'll make the setup more permanent first and will do a calibration run (with spectrophotometer and photodiode measurements to make a correlation table). I have not got much spare time lately so it all goes a bit slow at the moment. At the beginning of the week I completed light source and put it all together on a table in a dark room for a quick run - walked through the spectrum with i1Pro measuring every 10nm (thanks Graeme for Android version to make it really convenient with the phone) and  all seems to work fine.

[Alexey.Danilchenko]

> It would be less expensive to build proper monochromator setup to do that.

I doubt "less expensive" part (unless we are chasing D65, which even an advanced user will not be doing, and which is unnecessary as obtaining spectral response is a much better way to do things), but much more precise, much more stable, and much more convenient if a lot of cameras and lenses are to be characterized.

I meant less expensive when constructed from used parts off eBay - getting all parts shiny new is prohibitively expensive for home enthusiast. My setup was half the price of new Colorchecker SG in total.

Light spheres are easy DIY project.

I did construct a sphere using paint and was about to attempt coating it with barium sulfate when a cheap one came along made with spectralon.

[Iliah]

>My setup was half the price of new Colorchecker SG in total.
That is what I would expect But wait for calibration before celebrating
All in all, you know I was suggesting this setup for quite some time.

[Alexey.Danilchenko]

>My setup was half the price of new Colorchecker SG in total.
That is what I would expect But wait for calibration before celebrating
All in all, you know I was suggesting this setup for quite some time.

I am far from celebrating - the real experiments are only just beginning...

[torger]

As many seems to be reading this thread it may be worth mentioning that spending lots of money to get the perfect D65 simulator "just because" it's not a wise thing to do.

When you make dual-illuminant DNG profiles for general use the defacto standard (=Adobe standard) is indeed to make them for the StdA and D65 pair. This means that in order for the EXIF lightsource tags to match the truth you need to profile under those illuminants. However, they don't really need to match. The only thing they're used for is when deciding the matrix mix, ie if the temp is 4700K 50% of the 2850K StdA matrix will be mixed with 50% of the 6500K D65 matrix. To know the mix you need to know the temperature and to now the temperature you need to know the mixed matrix, a hen and egg problem solved by an iterative loop. But only after mixing the CM is used to derive the actual temperature/tint that is shown in the white balance tool.

To summarize, you can mismatch the actual calibration illuminant and its tag with quite wide margin before it will have any real effect. Therefore it's perfectly feasible to use real daylight as "D65" even if you make a dual-illuminant profile. (On single illuminant profiles the EXIF lightsource tag has no effect at all. ICC profiles are always single illuminant.)

Having a simulator adds convenience though, especially if you make many profiles or experiment a lot. It would not need to be a D65 simulator though, you could have some other average daylight.

It's still some advantage to *know* what light that has been used as calibration illuminant, if not the spectra at least the temperature. This is needed to make the DNG profile estimate light temperatures properly. (The CAT is also affected by that, but you can of course enable color constant behavior)

I have a hunch that for dual-illuminant profiles it's an advantage to get a bit above 5000K to get some more distance from StdA. At D65 we have very limited options on artifical light sources with nice spectral properties though. The Liulabs expensive filtered halogen does not have much competition. I'm curious about the UV-pumped white LEDs though, seems like Yuji is about to release one. With those it seems to be that the higher temperature the worse spectrum, so I'm not so sure the 6500K will look that good, but probably better then a Solux with 80B filter.

[Iliah]

> I am far from celebrating
Not that far...
> - the real experiments are only just beginning...
That never ends

[AlterEgo]

> contrast using XYZ is something like log2(Y from white patch / Y from black patch), right ?
Contrast is Y1/Y2; but if you want it to convert to photographic stops, log2(Y1/Y2) or log2(Y1)-log2(Y2). For counter-flare splines, I use just contrast, not log2

I have a question... I flatfielded rawdigger samples from a CC24PP shot and I am getting the following (no WB, no scaling, no gamma/gamma = 1) :


19   D1   5442.64    13193.07    8222.41
20   D2   3414.98    8434.33     5383.07
21   D3   2123.34    5291.81     3394.74
22   D4   1127.10    2778.43     1783.99
23   D5   515.34     1304.47     855.50
24   D6   179.69     439.28      287.96

so contrast is (in stops) :

R = log2( 5442.64 / 179.69 ) = 4.920724585
G = log2( 13193.07 / 439.28 ) = 4.908495689
B = log2( 8222.41 / 287.96 ) = 4.835620983



CC24PP spectral data converted to XYZ / Y :

D6 = 3.128
D1 = 92.588

log2 (92.588/3.128) = 4.887512805

comparing with flatfielded RD it is too good to be true, so where is the error might be then ?

FFD data :


19   D1   0.997140   0.999829   0.999334   0.998824
20   D2   0.997535   0.999202   0.999851   0.999649
21   D3   0.997811   1.000000   0.999594   1.000000
22   D4   1.000000   0.999567   1.000000   0.999630
23   D5   0.994782   0.993781   0.993994   0.996559
24   D6   0.975756   0.977245   0.975554   0.977036


unflatfielded samples


19   D1   5427.07   13184.19   8216.93
20   D2   3406.56   8429.48   5382.27
21   D3   2118.70   5291.81   3393.36
22   D4   1127.10   2777.32   1783.99
23   D5   512.65   1298.17   850.37
24   D6   175.34   429.24   280.92



scene is :

[torger]

A quick question to any Yuji high CRI LED owners out there, it would be nice if you could post a spectrum measurement, for example of the 5600K. The "marketing" spectrums shown on web sites often make things look better than they really are...

[Iliah]

Frankly, I do not see much marketing looking at the 5600K SPD on page 7 of http://www.yujiintl.com/files/high-cri-led-datasheets/VX5730-YUJI.pdf
It is not pretty for the purpose of D-series simulation for profiling. Visual comparison with a D-series SPD is not very telling, it is close to what Dr.Fairchild calls "A Metric for the Aesthetic". Kino-Flo Celeb lights are made of Yuji LEDs. An independent and reputable study of different light sources (including Celebs http://www.gtc.org.uk/media/fm/TLCI_charts_April_2015/Kino_Flo.zip ) by Alan Roberts (former BBC research engineer) is at http://www.gtc.org.uk/tlci-results/tlci-results-new-format.aspx (SPDs are presented as TIFFs, you need to download files to see those).

[Iliah]

CC Classic was designed to cover 5 stops, flare and glare at this dynamic range is negligible. Try underexposing it by 2-3 stops and see how the error increases.

[torger]

I've discovered some silly bugs in v0.7.2, CAT is not run when it should and opposite, and flatfield not working properly. I will soon release a patch to fix those. I'll do a little bit more testing first.

[AlterEgo]

CC Classic was designed to cover 5 stops, flare and glare at this dynamic range is negligible. Try underexposing it by 2-3 stops and see how the error increases.

but it did not much, took EV-3 (vs the raw in the post above) exposed raw from the same series and

19   D1   673.49   1642.2   1017
20   D2   423.69   1044.29   666.82
21   D3   263.83   654.65   421.1
22   D4   140.29   344.66   221.82
23   D5   64.214   163.55   107.25
24   D6   22.477   55.471   36.186


R = log2( 673.49 / 22.477 ) = 4.905135127 vs 4.920724585  
G = log2( 1642.2 / 55.471 ) = 4.887752297 vs 4.908495689
B = log2( 1017 / 36.186 ) = 4.812744228 vs 4.835620983

drop by 0.02

and still strange vs measured target log2 (92.588/3.128) = 4.887512805

[Iliah]

but it did not much, took EV-3 (vs the raw in the post above) exposed raw from the same series and

19   D1   673.49   1642.2   1017
20   D2   423.69   1044.29   666.82
21   D3   263.83   654.65   421.1
22   D4   140.29   344.66   221.82
23   D5   64.214   163.55   107.25
24   D6   22.477   55.471   36.186


R = log2( 673.49 / 22.477 ) = 4.905135127 vs 4.920724585  
G = log2( 1642.2 / 55.471 ) = 4.887752297 vs 4.908495689
B = log2( 1017 / 36.186 ) = 4.812744228 vs 4.835620983

drop by 0.02

and still strange vs measured target log2 (92.588/3.128) = 4.887512805

Nope, not that way

8 EV - log2(13193.07/55.471) for G - about 0.11 EV

Always disliked the practice of shooting targets on complex backgrounds instead of dark grey and black; having foreign objects in the scene (even if they are not showing in the frame); semi-gloss or gloss targets (SG has 5.5 stops between the "same" 2 patches, and that already breaks IR shots), shallow hoods or no hoods...