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Author Topic: How to get the minimum L* value from an ICC profile ?  (Read 1916 times)

PhR

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How to get the minimum L* value from an ICC profile ?
« on: September 10, 2021, 09:30:13 am »

I find very useful to know the minimum L* value that a paper can reproduce when editing an image that is quite dark. This helps preserving differentiation in the dark tones on a mat paper and not having them crushed by color management on the print.

I use to measure this with a spectro. on a black patch. But I wonder if we can extract this information from the ICC profile of the printer/paper directly ?

Thanks

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MichaelKoerner

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #1 on: September 10, 2021, 10:29:19 am »

Just a thought experiment (I'm currently not at the office and can't test by myself), but measuring a 100% black patch in softproof mode of Photoshop with the pipette set to LAB mode should show the requested value. Let me know if that worked...

PhR

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #2 on: September 10, 2021, 11:24:45 am »

Thanks for the suggestion. But the pipette shows the Lab values of the unproofed file.
I also try to convert to a printer profile without success.
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JRSmit

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #3 on: September 10, 2021, 11:25:34 am »

I find very useful to know the minimum L* value that a paper can reproduce when editing an image that is quite dark. This helps preserving differentiation in the dark tones on a mat paper and not having them crushed by color management on the print.

I use to measure this with a spectro. on a black patch. But I wonder if we can extract this information from the ICC profile of the printer/paper directly ?

Thanks
Interesting question. If you have an icc viewing tool you can find a parameter for black point.
But how does the black point value tell you somesthing to about diiferentiation in dark tones? (Tonal separation)
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Rand47

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #4 on: September 10, 2021, 11:42:41 am »

I find very useful to know the minimum L* value that a paper can reproduce when editing an image that is quite dark. This helps preserving differentiation in the dark tones on a mat paper and not having them crushed by color management on the print.

I use to measure this with a spectro. on a black patch. But I wonder if we can extract this information from the ICC profile of the printer/paper directly ?

Thanks

I go about this another way.  I print from Lightroom Classic, and its print module is a helpful tool for my workflow.  You can hover your cursor over the histogram in LrC and right click.  There’s an option for “show lab values.”  If you select that, the cursor hovering over the image will show lab values rather than the mostly useless % RGB values.

This way I can turn on soft proofing, create a proof copy.  I can then uncheck “soft proof” and read the L values in my dark areas.  A simple test strip image of dark values with detail (that I desire to be able to see in the print) will show me what minimum L value I can use for that area (sort of “once and done” for any given paper) I can use and still retain shadow detail.

I learned this from using Charlie Cramer’s L value test strip and then doing something similar w/ LrC.   Typically, on good papers (e.g. Canson Platine Fibre Rag) I can see detail at L 6, with my Epson SC P7570.   But each paper varies.

Rand
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Re: How to get the minimum L* value from an ICC profile ?
« Reply #5 on: September 10, 2021, 12:21:00 pm »

I go about this another way.  I print from Lightroom Classic, and its print module is a helpful tool for my workflow.  You can hover your cursor over the histogram in LrC and right click.  There’s an option for “show lab values.”  If you select that, the cursor hovering over the image will show lab values rather than the mostly useless % RGB values.
Not for the soft proof or output, Lab is shown for Melisa RGB. In Develop module.
You want Lab values for an output profile, you have to convert to that profile after selecting a desired rendering intent and read those values. Which is doable in Photoshop.
Nothing useless about RGB values in percentages or based on encoding (0-255 which isn't happening inside LR). Just as miles and kilometers differ but are not useless, just different.
It helps to know what the values are based upon, Lab or otherwise and use them appropriately.   
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MHMG

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #6 on: September 10, 2021, 04:14:04 pm »

I find very useful to know the minimum L* value that a paper can reproduce when editing an image that is quite dark. This helps preserving differentiation in the dark tones on a mat paper and not having them crushed by color management on the print.

I use to measure this with a spectro. on a black patch. But I wonder if we can extract this information from the ICC profile of the printer/paper directly ?

Thanks

It can be done in a two step process in Photoshop. But first set LAB as one of the readout values in the info tool (you will use the info tool to check your values).

1) Under Edit>convert to profile,  convert the chosen source image (an RGB image in sRGB, aRGB, Prophoto, etc.) to your chosen destination profile (the printer/ink/media ICC Profile you wish to use) using the same rendering intent you would normally choose (i.e. perceptual, recoil w/bpc, etc). Note: make sure your source image has a black patch area, e.g., can add an easy-to-locate pure black (RGB = 0,0,0) patch area to the source image if it doesn't have one already. Also, if you also want to determine media whitepoint color, add another patch area to the source image filled with pure white (RGB = 255,255,255). After conversion to the ICC Printer profile, the PS info tool is still going to show source image Lab values, so black will still read 0 L* and white will be 100 L* in the info tool after this initial conversion.

2) Use the Edit>convert to profile menu setting again, but now choose LAB color as your "destination profile". It's a choice in the dialog box that will appear. And finally, set your rendering intent to ABSOLUTE rendering for this 2nd conversion step. That's essential for this method to work.

After step 2, you will have an image in LAB color mode with the absolute color values predicted by your chosen printer profile's internal tables and the CMM of your operating system. Media whitepoint of the paper will be found by using the info tool eyedropper hovering over the white patch in the image, and media Dmax (L*min) can be seen the same way by hovering over the black patch in the image. Also, as shown in the LAB readout setting of the info tool, all other colors in this two-step converted image will also be predictions of what you would expect to measure on the print with a spectrophotometer.

This two step process will give you a final image appearance that looks exactly like what you see visually when you set up soft proofing correctly in PS with "simulate paper color" checked , except it's the only way I know how to trick the info tool into displaying the actual Proofcolor values. PS has an option to let the into tool show the proof colors, but it's antiquated and reading out from the CMYK working space setup in the color setting menu, not the ICC printer profile chosen in the custom proof menu. I wish Adobe would update this Proofcolor readout option so that it works correctly. Until then, the two-step approach described above is the work around.

cheers,
Mark
http://www.aardenburg-imaging.com

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MfAlab

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #7 on: September 10, 2021, 11:51:38 pm »

You can get the black point data from ICC profiles. Normally, it will be a set of XYZ values, but you can simply calculate to L*. International Color Consortium website has a free software could do that if you don't have ICC edit software. https://www.color.org/profileinspector.xalter

Open a profile in ICC Profile Inspector, you will see a tab "bkpt", that's the black point. Double click, the software will show the XYZ values of black point. Just calculate L* from the Y value and you're done. Or use any online calculator like Bruce Lindbloom: http://www.brucelindbloom.com/index.html?ColorCalculator.html

Additional: You can get white point by the same way. "wtpt" tab is the paper white point.
« Last Edit: September 10, 2021, 11:59:18 pm by MfAlab »
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JRSmit

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #8 on: September 11, 2021, 02:10:31 am »

I find very useful to know the minimum L* value that a paper can reproduce when editing an image that is quite dark. This helps preserving differentiation in the dark tones on a mat paper and not having them crushed by color management on the print.

I use to measure this with a spectro. on a black patch. But I wonder if we can extract this information from the ICC profile of the printer/paper directly ?

Thanks
I think the real question is preserving diiferentiation in the dark tones.  And for that i use softproofing.
Also when making a paper profile i take care of the tonal separation in the bottom part of the profile.
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MHMG

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #9 on: September 11, 2021, 11:06:33 am »

I think the real question is preserving diiferentiation in the dark tones.  And for that i use softproofing.
Also when making a paper profile i take care of the tonal separation in the bottom part of the profile.

"preserving differentiation in the dark tones" is a good point to chime in on one other wrinkle in this ICC profile-predicted L* min discussion, namely that I'm finding more and more "M3 illuminant" built ICC Profiles are being quietly injected into the population of vendor supplied profiles for their media, especially with regard to matte media. IMO, venders who adopt the use of M3 to open up the shadow details in the tone reproduction curve (TRC) of the profile ought to alert the enduser to their M3 decision, perhaps most easily accomplished by adding "_M3" in the filename. None do at this time, AFAIK.

Although the polarized light M3 approach does alter the forward transform LUTs in a way advocates of M3 think results in a better TRC (more open shadow detail for matte media),  M3 built-profiles also contain totally unrealistic LAB data for normal print viewing conditions such that soft proofing is no longer accurate and thus L*min predicted values as well. If you use the PS two-step process I outlined earlier in this discussion or inspect the tags as per Mfalab's approach, you will find matte media with L* min values well below 5 whereas M0,M1,or M2 built profiles typically result in matte black L* readings around 15-18 for fineart matte papers with pigmented ink sets, sometimes but not often as low as 9-10 for dye based inks, but never below 5 like M3 profiles predict!

Lastly, I agree with JRSMIT. If you want an ICC profile that delivers more open shadow tones in a chosen matte paper like that produced with M3 data, it can also be accomplished in profiling software like i1Profiler but still using standard M0,M1, or M2 measurements. Thus, normal softproofing behavior is retained while providing very comparable TRCs to match an M3 built profile. i1Profiler has a custom "contrast" setting which will accomplish this task. I found that by setting the contrast slider to approximately -20 (lower contrast) and adding a slight "saturation" boost with the saturation slider (e.g. +20), I get a very very close match to an M3 built profile for the same printer/ink/media combination. I recently used these custom settings in i1Profiler to build a custom profile for my Canon Pro-1000 printer and the new Cifa BFK Rives White fineart matte media using M0 measurments. I also built another one with M3 measurements and the default i1Profiler settings for a direct comparison of the TRCs. Both M0 with the custom i1P settings and M3 measurements with i1P default settings produced a near perfect match in the TRC measured and plotted output on the BFK Rives White media.

cheers,
Mark
http://www.aardenburg-imaging.com
« Last Edit: September 11, 2021, 09:10:41 pm by MHMG »
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PhR

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #10 on: September 12, 2021, 02:20:47 am »

Thank you very much Mark for these relevant and clear explanations (as always) and Kang-Wei for the way to get the black point XYZ. I had Profile Inspector installed but never tought of double-clicking on the tags. :-/ I tried with one of my favorites uncoated paper, the "Velin d'Arches" on a picture containing a black and a white patch. Mark method gave me L* = 30 for the black with corresponds to the L* = 30.44 obtained  from the X = 0.06279, Y = 0.06419, Z = 0.04767 given by Profile Inspector in the profile I generated.

To answer JRSmith, when a large range of black tones in missing in the destination gamut, I find conventient to hoover over certain critical areas and see the L* values of that way below the min L* to check if something will be loss on the print.

I don't want a method that would push up all the blacks so that they all "enter" the destination gamut because this gives flat images in my own experience.  There are areas where the black tones crushing creates a perception of darkness and others where it creates a perception of being blocked (i.e. large areas that becomes flat). Of course softproofing helps to see this as well.

Thanks again for your precise answers.

Cheers

Philippe
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MHMG

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #11 on: September 12, 2021, 10:23:56 am »

Thank you very much Mark for these relevant and clear explanations (as always) and Kang-Wei for the way to get the black point XYZ. I had Profile Inspector installed but never tought of double-clicking on the tags. :-/ I tried with one of my favorites uncoated paper, the "Velin d'Arches" on a picture containing a black and a white patch. Mark method gave me L* = 30 for the black with corresponds to the L* = 30.44 obtained  from the X = 0.06279, Y = 0.06419, Z = 0.04767 given by Profile Inspector in the profile I generated.

To answer JRSmith, when a large range of black tones in missing in the destination gamut, I find conventient to hoover over certain critical areas and see the L* values of that way below the min L* to check if something will be loss on the print.

I don't want a method that would push up all the blacks so that they all "enter" the destination gamut because this gives flat images in my own experience.  There are areas where the black tones crushing creates a perception of darkness and others where it creates a perception of being blocked (i.e. large areas that becomes flat). Of course softproofing helps to see this as well.

Thanks again for your precise answers.

Cheers

Philippe

Glad both methods worked for you. Aqueous inkjet printing on uncoated papers typically produces L* min values in the 27-32 range, so your finding of 30 for "Velin d'Arches" is in good agreement with my own observations for uncoated media over the years. Uncoated papers require much more skill to achieve a lively print than when printing on media purposely coated for inkjet printers, but they can be truly beautiful. One of my favorite pieces in our Aardenburg Archives is a custom color print on Arches Cold press produced by my friend, artist, and exceptionally skilled printmaker Tyler Boley. :)

An approach I use for media with high L*min values is to set the profile rendering intent to recol but leave black point compensation unchecked. Then I immediately begin with an adjustment curve in PS. I set the blend mode to "luminosity" and then I lift RGB 000 on the curve upwards until the info tool tells me L* output is now at the known L* min of the paper (for your printer and "Velin d'Arches" the L* readout would thus match 30). This step initially results in a linear curve lifting of all the tones similar to what black point compensation does, but then I custom shape the curve, adding my own defined toe and shoulder to the curve shape which then achieves an optimal "global" blackpoint compensation curve for that specific image. That said, high L*min papers will always need additional curve layers and masks to achieve the most lively image on the print with highlights, midtones, and shadows where they are most visually appealing for that print. So, I agree with you entirely,  it's good to know what L* min for the printer/ink/media process really is. I always publish L* min (also Dmax) values in the Aardenburg light fastness test reports for that reason.

cheers,
Mark
http://www.aardenburg-imaging.com
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Paul_Roark

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #12 on: September 12, 2021, 06:42:38 pm »

For those into dedicated black and white inksets (like me), you can set up the system with 2 MK positions and obtain a dmax Lab L below 20 on Arches uncoated watercolor paper.  One QuadToneRip profile/test of mine on Arches Hot Press, Bright White, 140 shows Lab L = 18.8.  On opposing walls of my office, an Arches (full sheet) print (under museum glass) and a satin (high dmax, PK), un-glazed (but sprayed with Print Shield) print that both have large areas of black look essentially equal in their "blackness."   

This is the raw data from my spectro read (Lab L, A & B, from paper white to 100% black):

#Notes Pre-linearization LAB:
#Notes
#Notes 94.74   0.75   0.75   
#Notes 91.88   0.82   1.8   
#Notes 88.19   1.21   2.55   
#Notes 84.38   1.19   3.41   
#Notes 80.92   1.46   3.89   
#Notes 77.23   1.64   4.01   
#Notes 73.45   1.56   4.29   
#Notes 69.45   1.63   4.53   
#Notes 65.95   1.72   4.54   
#Notes 61.95   1.75   4.49   
#Notes 57.49   1.78   4.57   
#Notes 53.19   1.7   4.46   
#Notes 50.4   1.71   4.17   
#Notes 48.06   1.81   3.94   
#Notes 46.09   1.78   3.75   
#Notes 42.41   1.4   3.14   
#Notes 38.48   1.57   2.46   
#Notes 32.75   1.16   1.88   
#Notes 27.42   1.11   1.43   
#Notes 23.31   1.36   1.39   
#Notes 18.82   1.36   0.15   
PRINTER=Quad7800
CURVE_NAME=GCVT-Arches-Warm-T

Note that this is a 100% carbon pigment print -- no color pigments at all were used in the profile.  Yet, I was able to hold the Lab delta B to less than 4. 

The inkset used is described at https://www.paulroark.com/BW-Info/7800-Glossy-Carbon-Variable-Tone-2016.pdf .  It is for glossy or matte paper.  QTR or similar rip must be used.

Note that while I am a huge fan of 100% carbon pigments on an un-coated Arches watercolor paper, my purchasers through Gallery Los Olivos almost never have any interest in the technical details.  It's simply the image that sells.  As such, most of my sales are un-glazed satin paper (Red River Ultra Pro Satin),  that are dry mounted and framed without glazing.

Paul
www.PaulRoark.com

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PhR

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #13 on: September 13, 2021, 03:25:36 pm »

On the "Arches Satiné 300g" (violet pocket) I get a L* min = 22.3 on my 3880 - profile made with i1 Studio. But, in my opinion, printing on uncoated papers is also to get rid of the best white/best dMax performance race and to choose paper mostly for its surface rendition. Even a slight motling can be very nice on some pictures. The paper has room to speak. I found that one some coated papers (some Canson for instance) the thickness of the coating destroy the original paper surface aspect. It hasn't any longer much to do with the original paper when you know it. It remains the name, however.

Thanks for your description of image editing in Relative mode w/o black point compensation, Mark, I will give it a try.

I also observed that, on some profiles, there is no "bkpt" tag. I read that in ICC specifications it is optional, so there is no garantee to have it.

Philippe



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MHMG

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #14 on: September 13, 2021, 08:52:28 pm »

On the "Arches Satiné 300g" (violet pocket) I get a L* min = 22.3 on my 3880 - profile made with i1 Studio. But, in my opinion, printing on uncoated papers is also to get rid of the best white/best dMax performance race and to choose paper mostly for its surface rendition. Even a slight motling can be very nice on some pictures. The paper has room to speak. I found that one some coated papers (some Canson for instance) the thickness of the coating destroy the original paper surface aspect. It hasn't any longer much to do with the original paper when you know it. It remains the name, however.

Thanks for your description of image editing in Relative mode w/o black point compensation, Mark, I will give it a try.

I also observed that, on some profiles, there is no "bkpt" tag. I read that in ICC specifications it is optional, so there is no garantee to have it.

Philippe

I have also run into some profiles that don't contain the bkpt tag when I use BabelColor's Patchtool to inspect ICC profiles. Patchtool is very powerful for evaluating, editing, rearranging, and manipulating color patches and thus a key part of my print testing workflow, and it can also show the description tags and calculate profile gamut volumes. When the bkpt tag isn't available, the two step PS method I described earlier in this thread still works!

cheers,
Mark
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Ernst Dinkla

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #15 on: September 15, 2021, 09:04:39 am »

For those into dedicated black and white inksets (like me), you can set up the system with 2 MK positions and obtain a dmax Lab L below 20 on Arches uncoated watercolor paper.  One QuadToneRip profile/test of mine on Arches Hot Press, Bright White, 140 shows Lab L = 18.8.  On opposing walls of my office, an Arches (full sheet) print (under museum glass) and a satin (high dmax, PK), un-glazed (but sprayed with Print Shield) print that both have large areas of black look essentially equal in their "blackness."   

This is the raw data from my spectro read (Lab L, A & B, from paper white to 100% black):

#Notes Pre-linearization LAB:
#Notes
#Notes 94.74   0.75   0.75   
#Notes 91.88   0.82   1.8   
#Notes 88.19   1.21   2.55   
#Notes 84.38   1.19   3.41   
#Notes 80.92   1.46   3.89   
#Notes 77.23   1.64   4.01   
#Notes 73.45   1.56   4.29   
#Notes 69.45   1.63   4.53   
#Notes 65.95   1.72   4.54   
#Notes 61.95   1.75   4.49   
#Notes 57.49   1.78   4.57   
#Notes 53.19   1.7   4.46   
#Notes 50.4   1.71   4.17   
#Notes 48.06   1.81   3.94   
#Notes 46.09   1.78   3.75   
#Notes 42.41   1.4   3.14   
#Notes 38.48   1.57   2.46   
#Notes 32.75   1.16   1.88   
#Notes 27.42   1.11   1.43   
#Notes 23.31   1.36   1.39   
#Notes 18.82   1.36   0.15   
PRINTER=Quad7800
CURVE_NAME=GCVT-Arches-Warm-T

Note that this is a 100% carbon pigment print -- no color pigments at all were used in the profile.  Yet, I was able to hold the Lab delta B to less than 4. 

The inkset used is described at https://www.paulroark.com/BW-Info/7800-Glossy-Carbon-Variable-Tone-2016.pdf .  It is for glossy or matte paper.  QTR or similar rip must be used.

Note that while I am a huge fan of 100% carbon pigments on an un-coated Arches watercolor paper, my purchasers through Gallery Los Olivos almost never have any interest in the technical details.  It's simply the image that sells.  As such, most of my sales are un-glazed satin paper (Red River Ultra Pro Satin),  that are dry mounted and framed without glazing.

Paul
www.PaulRoark.com

I recall the discussion why two MK positions in a ink set could create that effect. My theory was that it was not the increased density (which overdone can result in a reflecting sheen on some papers) but a more homogene distribution of the black ink. Minimum L measurements are easily compromised when tiny spots still reflect more light than the surrounding area.

Met vriendelijke groet, Ernst Dinkla

https://www.pigment-print.com/spectralplots/spectrumviz_1.htm

"For every complex problem there is an answer that is clear, simple, and wrong." H. L. Mencken
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MfAlab

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #16 on: April 19, 2022, 01:36:33 am »

I also observed that, on some profiles, there is no "bkpt" tag. I read that in ICC specifications it is optional, so there is no garantee to have it.

Philippe

Sorry for the archaeological reply. But I think it's better to put the information together.

"bkpt" tag is not recommend to use since ICC V4, so many software generates ICCs without it. But most OEM ICCs still have it.

The PS two step method is convenient, but cannot get accurate numbers. To get a 8 bit/256 step number, you can use curve tool to set a control point of the black on converted image. Then use press "down arrow" on keyboard, the click count till the control point be set to 0 is the 8 bit L* number. The 8 bit L* number could be easily transferred to 0~100 by /255*100.

It's a better way to get accurate L* from an ICC profile.
1. Open the ICC file using Profile Inspector
2. Double click the "A2B1" tag
3. Choose CLUT tab
4. Make sure all channels on 0
5. First value on the right-up side is L*min (16 bit)

Or you can use another software ProfileDump to show L* value directly.
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NAwlins_Contrarian

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #17 on: April 23, 2022, 02:04:06 pm »

[T]he minimum L* value that a paper can reproduce ... I wonder if we can extract this information from the ICC profile of the printer/paper directly?
Maybe, in a way, and I'm interested to learn what the experts here think of my method. Basically, I upload the profile to the ICC View website, then have it plot a 3D color space with the profile selected for both Color space 1 and Color space 2, and I have it plot a spot color, usually starting at L* = 3, a* = 0, b* = 0, or something like that. Then I free rotate and zoom as necessary, and adjust the spot color (which you can do while viewing the plot) and visually estimate values like L* min; L* min at a* = 0, b* = 0; and a* and b* at *L min.

As an example, I've uploaded a couple of plots for my ColorMunki profile of Red River Palo Duro Softgloss Rag in my little Epson. I realize there is some potential for parallax error or simply seeing wrong, but I think all values are at worst +/- 2, and with care and practice I suspect it's more like +/- 1. It appears to me that these plots show approximately L* min = 5; L* min at a* = 0, b* = 0 is approximately 9; and at L* min, a* = 1 and b* = -8.

By all means, I'd really like to hear your thoughts on this method, both for its own sake, and because it involves a readily-available free tool.
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Re: How to get the minimum L* value from an ICC profile ?
« Reply #18 on: April 23, 2022, 05:29:52 pm »

a better way to get accurate L* from an ICC profile.
1. Open the ICC file using Profile Inspector
2. Double click the "A2B1" tag
3. Choose CLUT tab
4. Make sure all channels on 0
5. First value on the right-up side is L*min (16 bit)

Thanks, I was very interested to read about this. Note that you don't have to divide by 65535 and multiply by 100 to get a 0-100 scale. In the right pane, under Values, to the right of the top box, it shows you the L* value on a 0-100 scale, rounded to the nearest 1.

Exploring using this method, I decided to inspect some of my own ColorMunki-made profiles, and compare the data there to my observations made using ICC View 3D gamut plots, where I'd previously estimated that my visual readings were precise within +/- 1 or 2.
Canson Rag Photographique 210 - 11, versus 10 observed
Canson Arches 88 - 13, same as observed
Epson  Legacy Baryta - 10, same as observed
Epson  Legacy Etching - 19, versus 14 observed
Epson  Legacy Fibre - 12, versus 10 observed
Epson  Premium SemiGloss - 3, versus 2 observed
Epson  Ultra Premium Glossy - 4, versus 2 observed
Hahnemühle Photo Gloss Baryta - 8, versus 7 observed
Hahnemühle Photo Rag Pearl - 4, versus 2 observed
Red River Palo Duro Softgloss Rag - 6, versus 5 observed
Red River Pecos River gloss - 7 , versus 5 observed
Obviously 6 of the 11 are either the same or off by 1, and 4 are off by 2; but one is off by 5! I rechecked Epson Legacy Etching, the visual implication of L* min = 14 is clear (shown below), while Profile Inspector, using the method above, clearly shows it = 19. Also, that my observed values were often lower but never higher suggests to me that a parallax-type error may significantly affect the visual inspection.

At this point, a question: my observed L* min values stated above are all absolute minimums, not minimum at a* = 0, b* = 0. Is there some rule or convention that when reporting L* min, by default that means at a* = 0, b* = 0, even if a lower L* is achievable at a non-neutral color? With my little Epson, the absolute L* minimums are all fairly close to a = 0, but the b values appear to be between -4 and -9, i.e., apparently the 'black' ink prints somewhat blue.

Also, there's an apparent oddity: in the A2B1 CLUT tab, when you drag the Ch0, Ch1, and Ch2 sliders from 0 to maximum, every single profile shows 65280, which corresponds to L* max = 99.6, which is certainly not the white point of all these papers. If it were 65535, then I'd say, 'Well, this is something else, it doesn't tell us L* max.' But 65280?

My learning and exploration continues--and any and all of your comments and suggestions are much appreciated.
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digitaldog

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Re: How to get the minimum L* value from an ICC profile ?
« Reply #19 on: April 23, 2022, 06:31:54 pm »

Accurate L* from an ICC profile; what the profile reports and what the profile and printer produce (which itself must be measured) can differ by quite a deltaE.
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