Spectro #'s vs PS #'s
Spectro #'s vs PS #'s
- Subject: Spectro #'s vs PS #'s
- From: Busher Jr Richard C <email@hidden>
- Date: Mon, 21 Apr 2008 21:23:52 -0700
Let's assume we are working in a specific output device color space,
say US Sheetfed Coated v2. We create a number of color patches in PS
(CS3). One of the patches is Lab = 000 (black) (= 95, 85, 85, 85 cmyk
in this color space). One of the patches is Lab = 100,0,0 (white) =
0,0,0,0 cmyk. We arbitrarily create a number of additional color
patches in cmyk, and PS tells us what the Lab values are for these
colors (the device independent colors).
I now print this file to my proofer. I do this by converting the cmyk
file (US Sheetfed Coated v2) to the color space of my proofer (Epson
9800, K3 inks, Epson Standard Proofing Paper, ColorBurst rip profiled
for ink and paper) using relative intent and black point compensation.
The objective is to print this file in a manner that represents what
this file will look like when printed on a 4-color printing press
that has been adjusted to the US Sheetfed Coated v2 standard. In
other words, I'm hoping to create a proof that is a match for the
press sheet if the press is suitably calibrated.
Now, in reality, Lab = 100,0,0 is not attainable. The press sheet
(and the proof sheet) is never color neutral. And an L* = 100 is not
attainable. In the same vein, L* = 0 is not attainable with ink on
paper, even though ab = 0,0 is attainable. Generally, L* = 95 is
reasonable for white, and L* = 8 or so is reasonable for black.
My first goal is to figure out how to adjust the file L* values of
the PS file so that I can analyze the measured results, i.e compare
them to the measured results from my spectrophotometer. My ultimate
goal is to come up with a method to verify that my proof is an
accurate representation of what I should get from the final output
device.
What I have been getting is proof results that are remarkable close
to the theoretical PS Lab numbers in the a and b values, except, of
course, for L values close to white. If I factor in the paper white
ab values the results are basically right on. However the measured L*
values are somewhat off. I'd like to create a curve adjustment to the
L* channel in PS that creates an accurate model of what I should
reasonable expect to see on the proof, and on the press sheet, etc.
Is a simple raising of the L* = 0 to an L* = x, and a simple lowering
of L* = 100 to L* = y, resulting in a new straight line curve,
appropriate?
Dick Busher
Cosgrove Editions
7042 20th Place NE
Seattle WA 98115
206-524-6726
888-507-7375
email@hidden
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