Really strange problem. colormunki can't read very dark patches of matte black ink?

[xilvar]

I've had a colormunki for quite some time and today I decided to create a quadtone rip profile for the inkpress (sihl) baryta warm tone paper.

In photoshop I created a set of test strips which could be read in sweeps of the colormunki and printed it out in my preliminary profile.

When I read the strips back in using the colormunki (after waiting a few hours for drying) I found that the color munki readings do not make sense!

Basically the absolute darkest patch which is 100% ink of the matte black ink on my 2200 reads as being significantly brighter than the 98% black patch (which is actually somewhere around 65% black and mqaybe 15% light black)!

To my eye the darkest patch definitely looks darkest. I then took a photo of the two patches with them in the center of the frame. With photoshop I then averaged each of the areas of the two patches and the 100% patch does indeed show to be darker than the 98% patch.

So what gives? I've been bothered by the colormunki's odd behavior in the dark regions before but this is pretty strange! Is it possible that the colormunki simply cannot read patches of matte black ink correctly?

Here's a view of the photo of the test strip :

 

[RGBCMYK]

What happens if you either slow down or speed up the movement across the patch? I bet you are at the limit of the device. I was doing some profiles with the eye one a few months ago and it would not read the darkest patches on some matt paper with matt ink. These patches were the real dark brown patches. I just took it as beyond the limit of the device. I have never used my munki to try and read a single patch, have you tried it in this mode and see no difference?
--
http://www.christopherbroughton.com

 

[xilvar]

Yep! right after I scanned the strip I scanned those two patches again in spot mode with the same results...

What annoys me a lot is that it doesn't hit a limit and stop there. It appears to actually reverse.

I got an L value of 22.5 for the 100% patch
and an L value of 16.4 for the 98% patch!

 

[RGBCMYK]

I know this sounds strange but I wonder what would happen if you coated the paper and read the values again. Would rule out surface issues. It is could be something to do with the interaction of ink and the surface, what about brighteners, could you be getting some kind of flare, scatter. What about building a mask for reading the smaller areas. What happens if you read the paper on top of a black paper? I bet is has something to do with the surface and a interaction. What about UV fluorescence? Take it outside and look at the paper on the north side of a building without direct sunlight. Trying to get a lot of uv.
--
http://www.christopherbroughton.com

 

[xilvar]

Yeah I'll definitely try coating it and measuring again. Unfortunately its been pouring here in sf so I can't coat anything right now (I do it outside).

I've looked at the sheet uncoated in sunlight, by the light of my phillips daylight deluxe fluorescents, and various other uglier fluorescents. Under all those light sources the correct patch is darker.

 

[xilvar]

I finally had a chance to coat some of my test strips and remeasure them with the colormunki. The coating definitely helps the colormunki read a deeper black. It still isn't quite right but it now reads L=6.7 on the darkest patch instead of L=22 or so... Certainly an improvement.

Annoyingly the slightly lighter patch STILL reads as being darker though... at L=4.8 (instead of L=16.7)

Last night I finally bothered to look at the light and optical arrangment of the colormunki more carefully and I think I understand why the color munki inherently does a bad job with dark blacks in mixed matte/glossy prints. It probably isn't doing the greatest job in the world with pure matte prints either...

If you run the photo color picker and press the scan button while looking in the scanning hole you'll note there is exactly one white LED at a 45 degree angle to the paper surface which shines to illuminate the surface. On very glossy papers with for black almost all the LEd light will bounce off the paper and reflect off at another 45 degree angle which does not hit the sensor.

However, on a matte paper, or a matte black ink on ANY paper the Light will obviously scatter quite a bit from the non-glossy surface and a fair amount of the light will hit the sensor.

Thus, the higher the effective gloss differential of a print the worse the colormunki built profile is.

What makes this a problem is that I view most of my prints with diffuse light all around and expect them to be viewed that way by others too. Under those lighting conditions a glossy surface will reflect quite a bit more light back into the 'sensor' (the human eye).

I wonder now, given this understanding if ANY of the colorimeters are truly accurately measuring color on matte surfaces. I would hazard a guess that Dmax's calculated using colorimeters on matte surfaces are significantly under-rating the actual dmax. (or possibly overrating dmax on gloss)

 

[RGBCMYK]

I am sorry I didn't get back, been busy but I see you took up my idea of coating and I have always thought that might be an issue. It could also be like when you look along the axis of the paper and you see the paper tooth?. What about using a simple sheet of acetate or even a piece of plastic wrap? I have some clear film 8x10 sleeves that I have read through with a densitometer and they are practically clear. I wonder if it will change the scatter? What about if you attached a small diffuser over the lightsource. I know when the calibration is done it would take into account the change in intensity, maybe
--
http://www.christopherbroughton.com

 

[xilvar]

Yeah I completed an entire profile by coating each sheet and then optimized it for a particular photo. The results were vastly better in the dark regions than not coating.

I don't think putting glass or plastic over top will help because it won't alter what the surface looks like (unless its uv thats giving me this problem to begin with), but what I am going to try is to use oil or alcohol to press the sheets to glass. If THAT works then my real solution will be to buy some of the special drum scanner 'gel' which purportedly works really well for drum scanners yet still cleans up easily.

I'm not sure this will work for true matte papers with deep texture though. ie - water color papers. because it would be hard to fill all the texture in with liquid while pressed to glass AND the paper would probably start to absorb the gel.

I THINK it will work for the baryta papers I care the most about though.