Datacolor User to User Support Group.

Hi,

Thanks again for the follow up.

First, the easy question: What Printer?  This is an Epson R1800 with 
an InkRepublic Continuous Ink System and Image Specialists inks. 
Although it has two black inks, Photo Black and Matte Black, I think 
it only uses one ink at a time depending on the paper/media setting. 
So, I guess that makes it a "one black" printer. It has Cyan, Yellow, 
Magenta, Red and Blue for the colors. No more Photo Cyan or Photo 
Magenta.

Now for the embarassing part. For some unknown reason the engineer in 
me decided to reproduce the process.  I went back to the original 729 
patch plus extended grays data set and printed the test chart with no 
correction. Measured gray was similar to the first (48.7, 3.24,-.01 
vs 49.45, 4.1 -.06) but when I applied the correction of +2, +2, -4 I 
got a gray reading of 48.53, 2.88, 1.87--pretty far from neutral 
gray.  So, I went through the process again ( 18 iterations) and 
wound up with corrections of -5, +4, -2 and a measured gray of 48.62, 
0.21, -.32. I have not yet printed full test imges with this new 
profile.

I can not figure out where I messed up. I thought I took meticulous 
notes on each change.  Maybe I mixed up the original data set ( I 
have data sets for Ilford Pearl as well as Epson Glossy); maybe I 
left out extended grays, I just don't know. But, the first profile 
that I can't reproduce, makes very nice prints. The second iteration 
seems more intuitively correct since the red is decreased as opposed 
to increased in the first iteration.

One observation from this process--the measurements change with time 
and location. The largest variations come just after the chart is 
printed; a drying effect I guess.  But, I do notice a change 
overnight. One example: 47.57, 4.39, 0.54 went to 48.77, 3.69, 1.60.  
Do you have a feel for what acceptable measurement variation is?  
Your suggestion to back up the measurement charts with extra paper is 
very good. I notice a variation if I do not back up the paper. I also 
notice some variation within the measurement patch, I guess that's 
the printer/ink.

I tried the PhotoShop ACV curve correction process but did not have 
good luck. I used the R channel since the image had a pinkish cast 
but the error was so slight a small adjustment fixes the SoftProof 
but the print is not correct. I tried assigning the printer profile 
to the test image and then "auto correct" with the curves layer but 
that's disastrous. I guess that's why I asked about a LAB correction 
option in Spyder.  BTW, I find the SpyderProof "exagerates" the 
errors/correction compared to PhotoShop or QImage soft Proof.

Sorry for the long post but, in spite of the effort involved, it is 
still fun.  Just wish I knew more.

Thanks,

Bob Schoner

--- In colorvision_group@yahoogroups.com, CDTobie@... wrote:
>
> 
> In a message dated 5/11/08 9:04:04 PM, rschoner@... writes:
> 
> 
> > My problem seems to be that I have difficulty translating the LAB
> > values to adjustments for the RGB sliders.
> > 
> There aren't really 'RGB' sliders, only color sliders (which, 
admittedly are 
> not labeled as 'positive a star' or the like <G>)
> 
> >  This gets compounded by
> > changes in one color affect the other colors.
> > 
> Thats the way life works... 
> 
> 
> 
> >   For example I profiled
> > Epson Glossy paper with Image Specialists Ink. The original 
reading
> > for the gray patch was  LAB 49.45, 4.10, -0.06 or RGB of 115.6.
> > 105.8, 108.28. SInce this had a pinkish cast I started reducing 
the
> > cyan/red slider.  After several iterations and a complete re-
start I
> > wound up with slider adjustments of Cyan/Red +2, Magenta/Green +2,
> > Yellow/Blue -4.  This gave measured values of LAB 47.93, 0.26, -
0.07
> > and RGB 104.7, 104.2 104.4 essentially neutral and confirmed by
> > different prints; colors seem to be very close, if not right on.  
At
> > this point I am happy.
> > 
> > It may not be possible, but maybe the Spyder software could have 
LAB
> > correction sliders as well as the conventional RGB sliders.
> > 
> Unfortunately people would take them too literally, and expect 
units of the 
> exact size of Lab value units. We can't know exactly how much 
change, in Lab 
> units, an adjustment will make, as that will vary with factors 
beyond our 
> control. You are the first to ask for this, what I hear more often 
is a request to 
> make our units line up exactly with Photoshop values... again not 
really 
> practical for similar reasons. So the recommendation is: if you 
want to use 
> Photoshop RGB units, or see your adjustments visually on screen, 
build a Photoshop RGB 
> curve set and once that is doing what you want, import it into 
Spyder3Print 
> and apply to your profile.
> 
> You never mention what printer you are using, but its unusual with 
a 'two 
> grays' printer to have results after profiling that are as far off 
neutral as 4 
> a* units. Is this by any chance a one gray, or no grays, printer?
> 
> C. David Tobie
> WW Product Technology Manager
> Digital Imaging & Home Theater
> Datacolor
> CDTobie@...
> www.datacolor.com/Spyder3
> 
> 
> 
> 
> 
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