Re: Colormatch vs Adobe 98
Re: Colormatch vs Adobe 98
- Subject: Re: Colormatch vs Adobe 98
- From: Chris Murphy <email@hidden>
- Date: Fri, 1 Feb 2002 01:14:10 -0700
Don Hutcheson <email@hidden> writes:
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What I MEANT to say is "Three-and four-band colorimeters are normally
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'hardwired' to produce CIELab(D50) values and CANNOT produce true
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CIELab(D65) values correctly."
We should refrain from referring to D50 and D65 when it comes to monitors
because it is impossible to calibrate them to either one. D50 and D65 are
illuminants and have a specific spectral power distribution associated
with them. A monitor does not have the ability to produce such spectra.
We can say that we are trying to get a correlated color temperature of
xxxxK.
I don't think it's accurate to say that a colorimeter is hardwired to a
specific illuminant, as this would imply that you only get accurate
readings if a monitor's white point is at a certain correlated color
temperature. This is the first I've heard of such a suggestion. It would
be accurate to say that a colorimeter is dependent on the source spectral
power distribution however. A colorimeter designed for CRT's is
"expecting" a specific spectral power distribution (or assumes a specific
spectral power distribution) in the computation of CIEXYZ. Actually one
could argue that it's possible to produce different phosphor sets among
CRTs and introduce an error in measurement due to the changed SPD.
What I find curious is that I have yet to come across a CIELUV display
profile. The ICC specification I have (2001-04) only stipulates that
matrix profiles must use CIEXYZ; and that if LAB is used the profile
must be table based. Nothing is said for or against CIELUV display
profiles, but the specification does include a color space signature for
LUV.
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In spite of this, I would contend that twenty thousand old or incorrect
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booths don't invalidate the standard. Accurate or not, the assumption has to
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be that the AVERAGE Fred will be looking at my proof in a D50 booth.
Reality proves absolute assumption and adherance to the standard is moot
as well as absurd. For the time being we have little choice but to use
the excuse that D50 is the standard, so you are SOL if your viewing
conditions aren's D50. The fact of the matter is, most viewing conditions
are NO WHERE near D50 and never will be. What we need is the ability to
create output profiles based on illuminants other than D50.
There is a lot of utility in the packaging market, for example, where
tons of packaging is produced and proofed under warm fluorescent lamps
and cool fluorescent lamps (one is commonly used in grocery stores and
the other is used in the refrigerated sections of the grocery store - I
forget which). D50 is totally unhelpful and inapplicable in a lot of
situations and I think it's futile trying to force conformity but this is
another discussion.
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I have long held the same suspicion. But if so, there's much work to be done
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to improve the CIE's 'standard observer' model.
It's rare that a randomly selected observer will match the CIE Standard
Observer in any case. I think the variation is probably greater than the
attempts thus far at improving the model would solve.
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And although colorimeters
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may have a problem, a good spectrophotometer should theoretically not
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exhibit the same fault, so long as it reads well into IR and UV and has
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enough spectral resolution to accurately track the sharp spikes and troughs
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of each source.
Most of them are at 10nm resolution which arguably may not be enough
resolution for always accurately sampling CRT emission.
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My gut tells me we are both missing something here. It may be as simple as
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UV or IR, or perhaps it's the spikey spectral curve in the D50 standard -
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not at all the smooth locus you'd expect of a Planckian 5000K source.
Hmmm - I don't see much of a difference between a D50 SPD and the SPD for
a blackbody heated to 5000K. They are a bit different, but D50 isn't what
I would call spikey. Fluorescent light and CRT's I would consider spikey.
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As a side note, (new Photoshop 'feature'), I wish Photoshop had a 'Simulate
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Paper Color' function which would match the a* and b* but not attempt to
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match L*. The current Simulate Paper White function just unnecessarily
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darkens the screen. If this new option was available I would argue that it
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should be a fixed default, as it's the only logical way to soft proof
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anything accurately.
I agree with the intent but disagree with the solution. If L* is totally
ignored, then paper stocks that are dark won't be simulated - such as
newprint. I think the problem is that Photoshop universally reduces
luminoscity too much and this probably has something to do with the fact
we don't have luminoscity compensation in any of this stuff. L* is
relative - it's not an absolute measurement based on something concrete
like CD/m^2. I routinely compute monitor L* values (in LAB or LUV) in the
high 90's despite having variations of 80 CD/m^2 and 130CD/m^2 - and of
course no ICC profile has any idea what the luminoscity of my viewing
booth is - so how can it compensate? And the models we are using don't
compensate anyway. If you take a green leaf and add more and more light
to it, it doesn't just get brighter. More than anything it gets more
saturated - and none of the models we are dealing with predict this.
So I think it's just another example of how we long ago reached the
limits of CIE colorimetry.
Chris Murphy
Color Remedies (tm)
Boulder, CO
303-415-9932