Generating a Kodachrome profile from an IT8 target

[guyburns]

Stray Light Test
An IT8 target was scanned with the slide hole covered and uncovered, with sRGB and Gamma 1.0, to test the effect on the scan of stray light. The scans for each gamma were placed into the one image, resulting in two layers (covered and uncovered) for each of two images (sRGB and Gamma 1.0).

Luminance was measured by selecting a small area in the middle of GS19-22, then applying Filter > Blur > Average for each layer (layer must be selected and visible when applying filter, but not necessarily on top) to ensure exactly the same area was covered. Then using Info > Eye Dropper > Actual RGB > 16-bit, the 16-bit values of the averaged area were entered into the formula: (0.2126R + 0.7152G + 0.0722B)/327.68 to convert the RGB values to relative luminance as a percentage. The coefficients used are those for sRGB as I did not know the values for the Coolscan colour space.

The results are shown in the attachment. For each GS patch, the luminance value for the "uncovered" scanner is given, followed by the change in luminance when the slide hole was covered. Blocking the slide hole has a measureable effect on lightening the densest black patches. Moral: cover the slide hole.

[guyburns]

Yellow Wash Test
Targets were scanned at Gamma 1.0 and Gamma 1.8 with ICE off, scanned with either the POS or Kodachrome setting to see if the yellow wash of Kodachrome bleaches out the highest densities.

Measuring luminance was done by selecting a small area in the middle of GS19-22, then Filter > Blur > Average for each layer (layer must be selected and visible when applying filter, but not necessarily on top) to ensure exactly the same area was covered. Then using Info > Eye Dropper > Actual RGB > 16-bit, the 16-bit values of the averaged area were entered into the formula: (0.2126R + 0.7152G + 0.0722B) / 327.68 to convert the RGB values to relative luminance as a percentage. The coefficients are those for sRGB as I did not know the values for the Coolscan colour space.

See attachment for results. There is an approximately linear relationship of ~10:1 between the values for the Positive setting for the two gammas. A similar relationship exists for the Kodachrome setting. This proves there is no bleaching of high density patches caused by the yellow wash of the Kodachrome setting, as I mistakenly assumed when measuring with Photoshop's dopey Histogram > Average. i.e. both sets of figures should be suitable for profiling as they show a good spread between each patch.

So it's not stray light or yellow-wash that are causing inaccurate profiling of the high density GS patches. The finger is turning towards Argyll or the Coca interface. But it's not pointing directly just yet.

[crames]

sRGB warning!

Converting to sRGB damages to the darkest patches, and throws a number of color patches out of gamut.

Compare converting to ProphotoRGB and adjust the white point in Levels to show details in GS20+.

Also, if you examine individual color channels after converting to sRGB, you can see which colors are going out of gamut - they are the patches that go black in one or more channels. The black channels are channels that would have to have negative values to represent the color in sRGB, but have been clipped to zero. AdobeRGB is almost as bad.

It seems that it would be best to convert from the scanner profile to ProphotoRGB for all editing/correction (all the while in 16-bit) and delay conversion to sRGB until the very end, after all adjustments to lighten the dark tones have been made.

attached:
1 crop converted from scanner profile to sRGB plus levels adjustment
2 crop converted from scanner profile to ProphotoRGB plus levels adjustment
3 red channel after conversion from scanner to sRGB
4 green channel after conversion to sRGB

[crames]

attached:
5 blue channel after conversion to sRGB

[guyburns]

You beat me to this one. Yesterday when I was trying to compare L values of the the darkest patches after profiling, I was having difficulty converting to other colour spaces and retaining the same L values. I was going to ask you about it. Thanks for the images. Will check them out in detail later on.

Q1: When I finally decide on a scanning method, would you be able to generate what you think is the best possible profile from my IT8 scan using LPROF, and then generate error reports for your profile and my profile? I want to compare LPROF and Coca/Argyll. I would upload the scan at 2000 dpi, 16-bit Tiff with embedded profile.

Q2. Any idea why the IT8 designers put that white line between GS22 in GS23? It's in the worst possible place – bright white next to the darkest grays.

Q3. I've asked the lads over at the Photoshop forum about this next one, but nothing useful came of it. After a bit of a lead in about scanning IT8 targets, I said:

"I've been working for years with my screen set to a certain profile (My Books) set to colour temperature D50 and gamma 1.8 from memory, that quite accurately mimics what I see from a Xerox iGen printer with what I see on screen. Now, however, the only destination for my images will be my monitor at home, friend's High-definition TVs, or the digital projector at the local cinema, which most likely all have an image space of Rec. 709 or its virtual equivalent, sRGB, both of which use a gamma of 2.2 (or thereabouts) and D65. So I changed my monitor setting to sRGB with the result that the images that I have edited in PS (with monitor set to My Books) now look different. And I can't figure out how to make images in PS with monitor set to sRGB, look the same as when monitor is set to My Books."

Is it possible to mimic a monitor space from within another monitor space? I've tried soft-proofing, applying and converting to profiles, nothing seems to give the same look within sRGB as when looking at the image in My Books, which give a yellow glow to the images, a bit like when viewing a slide.

[crames]

Q1: When I finally decide on a scanning method, would you be able to generate what you think is the best possible profile from my IT8 scan using LPROF, and then generate error reports for your profile and my profile? I want to compare LPROF and Coca/Argyll. I would upload the scan at 2000 dpi, 16-bit Tiff with embedded profile.

Yes. Why not scan at the 4000dpi maximum?

There are some other profilers that I can try as well, such as SCARSE, Argyll (full-blown command line), inCamera. If there are others following this thread, maybe they can jump in with some others. Let's make it a shoot-out.

There's also Silverfast and Vuescan. Did you see Mark Segal's review of the Plustek 7600 here on LuLa, where he carefully evaluates Silverfast profiles for Kodachrome and Fujichrome on several scanners, including the Nikon 5000?

Q2. Any idea why the IT8 designers put that white line between GS22 in GS23? It's in the worst possible place – bright white next to the darkest grays.

Don't know. I agree, it's a big problem.

Q3. I've asked the lads over at the Photoshop forum about this next one, but nothing useful came of it. After a bit of a lead in about scanning IT8 targets, I said:

"I've been working for years with my screen set to a certain profile (My Books) set to colour temperature D50 and gamma 1.8 from memory, that quite accurately mimics what I see from a Xerox iGen printer with what I see on screen. Now, however, the only destination for my images will be my monitor at home, friend's High-definition TVs, or the digital projector at the local cinema, which most likely all have an image space of Rec. 709 or its virtual equivalent, sRGB, both of which use a gamma of 2.2 (or thereabouts) and D65. So I changed my monitor setting to sRGB with the result that the images that I have edited in PS (with monitor set to My Books) now look different. And I can't figure out how to make images in PS with monitor set to sRGB, look the same as when monitor is set to My Books."

Is it possible to mimic a monitor space from within another monitor space? I've tried soft-proofing, applying and converting to profiles, nothing seems to give the same look within sRGB as when looking at the image in My Books, which give a yellow glow to the images, a bit like when viewing a slide.

You might want to spin that question off into a separate thread. Did you try converting the My Books profile to sRGB using Absolute intent? That should preserve the relatively yellow D50 white within the D65 target space.

Earlier I provided a profile for your scanner that has a tungsten white point. The idea is similar - when you convert using Absolute intent the tungsten yellowness is carried over to the target color space. It could turn your digital projector into a tungsten-bulb slide projector. Did you try it?

[guyburns]

Yes. Why not scan at the 4000dpi maximum?

Coca doesn't accept 4000 dpi files, at least on my setup. Plus they would be 100 MB files. Time consuming to upload, but possible.

Did you see Mark Segal's review of the Plustek 7600 here on LuLa, where he carefully evaluates Silverfast profiles for Kodachrome and Fujichrome on several scanners, including the Nikon 5000?

I just had a quick look. My own experience with the 7600's smaller brother, the 7400 (I think), was not something you'd write home to mum about. I'm going to borrow it again when I finish with the Kodachrome, and put it through it paces. I will say though, that the 7400 looks and feels cheap and plasticy next to the Nikon.

[re white line] Don't know. I agree, it's a big problem.

I was thinking of drawing over it with a felt pen, but the line is only 0.1 mm wide. Could be tricky.

I provided a profile for your scanner that has a tungsten white point. The idea is similar - when you convert using Absolute intent the tungsten yellowness is carried over to the target color space. It could turn your digital projector into a tungsten-bulb slide projector. Did you try it?

Tried it, and had a horrible feeling it is an accurate representation of how slides look compared to digital – washed out, fuzzy, and dim. What I'm going to do is to get a scanned image on screen, and project the same image onto a mini slide-screen I've built which sits on the desk next to my monitor. Then I'll take a photo of both, side by side, and post.

I have already done a similar test with a mate's 46" Bravia screen showing a scanned slide, and next to it was the same slide projected at the same size. 3.5 stops difference in brightness (Bravia's favour), with most slides looking better on the Bravia, though a few projected images had the edge. And this was before profiling, and with minimal editing. I'll be repeating the tests soon, and expect that digital versions of the slides will look so-significantly better that I may never show slides again.


Shoot Out
Might be useful, but from what I've learned so far, there is so much optimising that has to be done, you may never be able to fairly compare various devices – unless each has been exhaustively optimised. I was hoping to put together a PDF, The Art and Science of Scanning Kodachrome, that would work for any setup, but I'm not sure that will be possible now. I'll still put it together, but the title might have to change. Something like Kodachrome and Black Magic – How Scanning and Satanic Practices Intertwine. Forget the science, it mostly black magic in my experience.

Another horrible Feeling
After all this time spent profiling, I find I'm ending up with mediocre images that require significant editing in PS – though I do have a technique that is semi-automatic and only takes 5-10 minutes per slide. I look at the editing required and I think: Why bother profiling? So that's another set of tests: to compare the same editing approach to profiled and unprofiled images. The former had better give better results, or I'll start practising satanic rites on my IT8 targets – starting with surgery to remove that white streak.

[guyburns]

Optimum Scan Gamma Tests
I scanned a Kodachrome IT8 target at Gamma 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 and sRGB (Nikon), then generated a profile using Coca. The profiles were then applied to the respective scans and the L* values measured for these patches: GS0, GS5, GS10, GS15, GS19, GS20, GS21 and GS22. The measurement was done on a small section within each patch (~25% of the area), each section being identically placed and sized for each scan.

The difference between the actual value of each patch (taken from the IT8 data file) and the measured value was calculated and then graphed: GS values horizontally and absolute L* difference vertically. See attachment.

Optimum Gamma
The optimum scan gamma for my setup when using a Coolscan V ED and profiled with Coca is about gamma 1.6.

The graph clearly shows that the accuracy of Argyll's fitting of the RGB data to the L* data depends on the shape of the data it is presented with. The closeness of the modeling alters as the input numbers alter their shape. The wild card in the whole process is that the GS23 patch is being affected by the white streak on its GS22 side – the streak throws flare across both patches causing them to be lighter than they should be, with greater effect on GS23 because of the direction the scanner scans (from left to right for the Coolscan). The flare would have a strong impact on curve fitting at high densities, because GS23 is scanned as being lighter than it should be – and lighter than GS22 (it should be darker). This lightening, together with inaccuracies because of the curve fitting process, throws the profiling into maximum error in the darkest sections – just where minimum error is required to accurately reproduce shadow detail.

Next step - faking a GS23 patch
1. I need to determine the maximum density that the Coolscan can measure by scanning a dense black from another slide, to make sure it is the same (or darker) than GS23.

2. When I have established that the density of GS23 is within the Coolscan's capabilities, I can alter the Lab value of the scanned GS23 patch in Photoshop so that it approximates the real figure.

3. Then get rid of some of the flare on the GS22 side of the white streak (again in PS), and reprofile. This should allow the profile to more accurately model the densest blacks because there won't be the unexpected density reversal of GS22 and GS23.

[crames]

Tried it, and had a horrible feeling it is an accurate representation of how slides look compared to digital – washed out, fuzzy, and dim. What I'm going to do is to get a scanned image on screen, and project the same image onto a mini slide-screen I've built which sits on the desk next to my monitor. Then I'll take a photo of both, side by side, and post.

Did you notice that (for any of the profiles) the image is dimmer with Absolute intent, compared to Relative? Absolute gives the actual Lab numbers of the target. Relative involves remapping the gray-scale to the white and black of the Profile Connection Space. So the Relative intent, which is the one normally used in practice, results in a stretching of the gray-scale, causing the Lab numbers to depart from the target values. Seems to have the largest affect in the lowest densities, but the high-densities are also affected. This is according to ICC recommendations. Another potential pitfall in the measurement process.

Shoot Out
Might be useful, but from what I've learned so far, there is so much optimising that has to be done, you may never be able to fairly compare various devices – unless each has been exhaustively optimised. I was hoping to put together a PDF, The Art and Science of Scanning Kodachrome, that would work for any setup, but I'm not sure that will be possible now. I'll still put it together, but the title might have to change. Something like Kodachrome and Black Magic – How Scanning and Satanic Practices Intertwine. Forget the science, it mostly black magic in my experience.

Not a shootout among devices, just among the profilers, using the same target scans.

Another horrible Feeling
After all this time spent profiling, I find I'm ending up with mediocre images that require significant editing in PS – though I do have a technique that is semi-automatic and only takes 5-10 minutes per slide. I look at the editing required and I think: Why bother profiling? So that's another set of tests: to compare the same editing approach to profiled and unprofiled images. The former had better give better results, or I'll start practising satanic rites on my IT8 targets – starting with surgery to remove that white streak.

Hopefully all the black magic will at least result in better shadow detail, but that remains to be seen!

[crames]

Next step - faking a GS23 patch
1. I need to determine the maximum density that the Coolscan can measure by scanning a dense black from another slide, to make sure it is the same (or darker) than GS23.

2. When I have established that the density of GS23 is within the Coolscan's capabilities, I can alter the Lab value of the scanned GS23 patch in Photoshop so that it approximates the real figure.

3. Then get rid of some of the flare on the GS22 side of the white streak (again in PS), and reprofile. This should allow the profile to more accurately model the densest blacks because there won't be the unexpected density reversal of GS22 and GS23.

I agree. That looks like a good plan.

[crames]

I had some time to kill so I tried the measurement modification trick to see how it can affect profile accuracy.

I decided to use the Argyll command line tools, in order to have the possibility of modifying the measurements before generating the profile, and generating error reports. Coca doesn't allow these things.

1. First I generated an Argyll TI3 measurement file from the target scan and Q60 reference file:
scanin -v "kodachrome it8 gamma 1.0 .tif" it8.cht K3199910.Q60

2. I took the measurements into Excel and played around with linearizing the RGB numbers of the GS patches against the linear XYZ data of the Q60 reference. If you examine the first Excel graph below you can see that it is necessary to modify more than just the GS23 (DMAX). I ended up fitting a linear curve to GS00 through GS12, and used the slopes of the curves to generate new RGB numbers for GS12 through GS23 from the target XYZs, as follows:

Red = X * 1.1648
Green = Y * 1.0487
Blue = Z * 1.5226

Note that the Argyll scales the RGB measurements to the range 0..100.

The second Excel chart below shows the linearized curves.

3. The linearized RGB numbers were inserted into the TI3 measurement file.

4. A high quality profile was generated:
colprof -v -D"KodaChrome GB G1 Nikon MOD" -qh -ax "kodachrome it8 gamma 1.0 MOD"

5. Error report generated:
profcheck -v2 "kodachrome it8 gamma 1.0 MOD.ti3" "kodachrome it8 gamma 1.0 MOD.icm" > kcit8g1MOD_err.txt

The errors for all patches are very good - peak err = 3.093032, avg err = 0.217543. The gray-scale errors are much lower with the modified measurements. Of course, these are the errors of matching phony data, but the hope is that the phony data looks more like the shadow regions of real slides, with less flare than the Q60 target.

Here are the Lab DE errors of the GS patches:
[Lab Error DE] Patch No.
[0.151678] GS00:
[0.060774] GS01:
[0.007665] GS02:
[0.024161] GS03:
[0.010669] GS04:
[0.009631] GS05:
[0.008370] GS06:
[0.003975] GS07:
[0.049782] GS08:
[0.013283] GS09:
[0.030466] GS10:
[0.022639] GS11:
[0.305744] GS12:
[0.042647] GS13:
[0.283512] GS14:
[0.379665] GS15:
[0.492750] GS16:
[1.368748] GS17:
[0.874377] GS18:
[0.885278] GS19:
[0.889087] GS20:
[0.729789] GS21:
[0.438823] GS22:
[0.562788] GS23:

The proof will be in the viewing - here is the MOD profile for you to test with your images.

Here is the  modified IT8 measurement file.

Here is the error file.

[crames]

Guy,

Despite the favorable error numbers, I am not seeing any visible improvement. The gray-scale looks just as smooth with any profile, as long as the scan is 16-bit.

I have only your 16-bit IT8 target scan to look at, and you must admit it's slightly boring. It would be nice to test profiles with your Example 5 scan, or other image in 14/16-bit, as it looks like there might be a huge amount of recoverable shadow detail in the very-lowest bit-depths that could be affected by these profiles. I tried the MOD profile on my 12-bit Sprintscan scans, but can't make out a difference within their 12-bit range.

At this point it's still not clear to me what visible improvement you're shooting for or is even possible.

[guyburns]

I have discovered yet another anomoly in PS (the reason I posted, then deleted, then reposted). When viewing images at 50% in PS on my system, under certain circumstances the blacks in images look significantly different, and the Lab values are reported incorrectly. I have just averaged a small square in GS20 (Gamma 1.0) and measured it at 1635. I get a different figure when I zoom out. Anyway, now that I am aware of it, I can proceed.

I have uploaded three of my recent IT8 scans at gammas 1.0, 1.2, 1.6 with the relevant profile applied. On my system, the only change I can see as I tab from one to the other, is in the GS17-23 area. All other colours look identical, not all for these three images, but for all my IT8 scans. Do you see any difference in GS17-23 as you tab from one to the next? I certainly do on my system. As I tab from Gamma 1.6 -> 1.4 -> 1.0 the blacks become lighter and a haze is cast over them – the same effect I see in the darkest area of some slides.

If GS20-23 look different on your monitor, we've got a problem – we're not seeing the same thing. Download the files (IT8 Gamma 1.0 to 1.6.zip) from: http://www.mediafire.com/?121gpv6pt7rjj6j

I'll upload some of my reference scans today – crops to areas with lots of blacks.

[crames]

Do you see any difference in GS17-23 as you tab from one to the next? I certainly do on my system. As I tab from Gamma 1.6 -> 1.4 -> 1.0 the blacks become lighter and a haze is cast over them – the same effect I see in the darkest area of some slides.

Yes, I see the blacks become lighter just as you have described.

Strangely enough, after applying 1.5 on the middle slider in Levels to lighten those patches, the effect is reversed - the blacks become darker as I tab from Gamma 1.6 -> 1.4 -> 1.0. I also see that the higher scan gammas show more noise.

But if the 1.5 gamma adjustment is done with Image/Adjustment/Exposure, the lightness is more similar among the scans and doesn't change very much while tabbing - but I can still see the increased noise at Gamma 1.2 and 1.6.

It also seems to matter whether the image has been converted to another color space or not.

Weird.

[guyburns]

I have uploaded 4 sets of reference images to: http://www.mediafire.com/?35wnw6cxd7vm1

Each folder contains four versions of a 16-bit image: G1.0, G 1.5, G1.8, all cropped the same at 4000 dpi; and a 1000 dpi full version so you can see the whole picture. The Gamma 1.8 versions have several curves attached, so you can see my efforts at editing. There is also an explanatory PDF.

Happy shadow looking!

QUES
I want to check if sRGB is the same as Nikon sRGB (a profile I can scan into if I decide to do so). In sRGB I generated 6 LAB patches:

• 1,1,1
• 2,2,2
• 3,3,3
• 4,4,4
• 8,8,8
• 12,12,12

I converted the sRGB to Nikon sRGB and then compared the RGB values. They were the same to within about 2 or 3 RGB units (16-bit). That means Nikon sRGB and sRGB are identical, doesn't it?

[guyburns]

Thanks Cliff, for the Lab script. Now that I can generate low L* values, I can insert a fake GS23. I'm wary of playing around with the target scan, but I really think that GS23 and to a lesser extent GS22, have been compromised by the white strip. I want to try and determine a suitable value for GS23 by extrapolating from the other patches. Here is a list of IT8 "L" values in pairs, the first is the IT8 value, the second is the measured value from the unprofiled scanned slide. The first figure in each case is for GS11, then GS15, and GS18-23.

Gamma 1.0
39.86, 42.79
20.67, 21.17
10.45, 10.1
7.29, 7.18
4.27, 5.10
2.80, 4.34
1.07, 3.75
0.51, 3.82

Gamma 1.6
39.86, 59.84
20.67, 40.86
10.45, 29.57
7.29, 26.21
4.27, 23.21
2.80, 21.84
1.07, 20.17
0.51, 20.41

I don't have software to correlate numbers, so I use this calculator: http://www.arachnoid.com/polysolve/index.html. When the Gamma 1 numbers are pasted into that calculator with linear fit, you can clearly see that the lowest two points, GS22 and GS23, are too high. Correlation for the linear fit is 0.989. Pretty good. So it looks like I should alter both GS22 and 23 downwards.

However, when the Gamma 1.6 figures are inserted, only the lowest point, GS23, is off by any significant amount. Correlation is extremely good at 0.999, suggesting that only very minor correction is needed.

The graphs seem to indicate for Gamma 1.0, both GS 22 and 23 should be lowered – as I would expect because of that white stripe. But for Gamma 1.6, the graph indicates that only GS23 needs adjusting. My guess is that lowering both to suit Gamma 1.0 may improve the profile for Gamma 1.0, but would it be as good an improvement as a slight lowering of only GS23 to bring it into line for Gamma 1.6?

I'm betting on the latter, as it only involves one slight modification to one patch. And I'm betting that as long as GS23 is lower than GS22, it won't make much difference what value it is. I don't think any of this will make much difference anyway, but it's a good way of learning about profiling.

If you think it is worthwhile trying both methods for the profile shootout, I'll give it a go. Looking at the graphs, how much lowering do you reckon? My idea is to remove GS23 from the graph, then insert its "X" value into the calculator (0.51), and see what "Y" value pops out – which will become the new L value for my fake GS23.

[crames]

I have uploaded 4 sets of reference images to: http://www.mediafire.com/?35wnw6cxd7vm1
...
Happy shadow looking!

Thanks! A lot of stuff to look at. It's a holiday weekend here so I won't be able to jump on it right away.

Looking quickly, #5 seems to have a herringbone noise pattern in it for some reason.

QUES
That means Nikon sRGB and sRGB are identical, doesn't it?

I'm not sure. I read on the Web that the Nikon profile might have a different sized TRC lookup table or something. Not sure of the significance. The ICC has a Profile Inspector that lets you see all the parts inside profiles if you want to compare them.

[crames]

I'm betting on the latter, as it only involves one slight modification to one patch. And I'm betting that as long as GS23 is lower than GS22, it won't make much difference what value it is. I don't think any of this will make much difference anyway, but it's a good way of learning about profiling.

If you think it is worthwhile trying both methods for the profile shootout, I'll give it a go. Looking at the graphs, how much lowering do you reckon? My idea is to remove GS23 from the graph, then insert its "X" value into the calculator (0.51), and see what "Y" value pops out – which will become the new L value for my fake GS23.

Well, it can't hurt to try....

My experiment with Argyll described above resulted in a profile ("MOD") that, despite having smaller errors,  introduces a bit of a red cast to the shadows. I think this is the opposite of what is wanted - if anything Kodachrome should have less red (higher red densities) in the shadows, if we're capturing exactly what's on the film. So far, the tint of the shadows is the main thing I am seeing that varies among the profiles. So not only do you want to eliminate the upward hook from flare at the bottom of the range, but also avoid a color cast by having a certain ratio among the channels in that range.

[crames]

Guy, your reference scans and pdf have been very interesting. And by the way I like your writing style - are you a professional?

I think the quality problems you describe are mainly caused by editing the scans while still assigned to the scanner profile space. You should convert to a standard color work space such as Prophoto RGB before doing any editing.

The problem is that, until you convert to another space, the profile is in effect floating and hasn't permanently corrected the scanned colors. By editing in this state, you are changing the numbers that are sent as input to the profile. The result is that the changed RGBs get indexed into inappropriate parts of the look-up tables and whatnot within the profile.

By converting to a standard work space, the original scan values are corrected by the profile as they should be. After that any edits will be based on the corrected colors.

It's the difference between:

scan - edit - apply profile

and

scan - apply profile - edit

So we have been evaluating profiles quite differently! I always convert to a working space (either Prophoto RGB or a linear variant of it) before anything else.

[guyburns]

So we have been evaluating profiles quite differently! I always convert to a working space (either Prophoto RGB or a linear variant of it) before anything else.

We must be inhabiting the same ether space and unconsciously communicating. Last night I was thinking about what happens when you change colour numbers in the profile space and thought: colours must be moving away from what the profile is trying to correct, but couldn't quite convince myself that it was true. Now I know it is. Yet another trap for a novice Kodachrome scanner – don't edit in the profile space, even when testing. It will be interesting to see if editing in another space makes much difference to the shadows.


Today's Testing – Checking the Target itself
I want to find out how internally consistent my GS patches are. I'm going to do that by:

1. Generating a selection the size of a GS patch.
2. Move the selection over GS0, average it, and measure the L*.
3. Step Back, scale the selection to 80%, average, and measure L*
4. Repeat for 60%, 40%, 20%.
5. Move to GS1 … GS23 and repeat.

By doing the above, I'll know how internally consistent each patch is and whether it is being affected by boundary effects. I have a suspicion that the darker patches (say GS18 and up) are being compromised in their L* values (by lighter areas that surround them) either when manufactured or when scanned. A similar effect to what was discussed in another thread (when I asked about Hunt's "ideal" DH characteristic for a slide) in which I drew a graph of the effect of ambient light on a slide when projected – a tiny amount of ambient light has a large effect on the dark areas, but hardly any effect on the brighter areas.

QUES
Any idea how profilers average a patch? How much of the patch do they average? Do they discard values exceeding certain limits? Do they correct the average if the histogram shows a bias (it should be bell-shaped, I assume)?

It seems to me that to obtain an optimum profile, you should precondition the RGB data and the darker IT8 patches.