Illuminant sensitivity and the Epson 2000P (abbreviated)
Illuminant sensitivity and the Epson 2000P (abbreviated)
- Subject: Illuminant sensitivity and the Epson 2000P (abbreviated)
- From: "Bill Kress" <email@hidden>
- Date: Tue, 9 Jan 2001 19:19:34 -0800
< Previous postings of an expanded version to other discussion groups have
gotten some very interesting feedback. Comments please. >
I recently acquired an Epson 2000P printer and noticed a surprising amount
of illuminant sensitivity with my prints. I have done some investigation to
understand reasons behind this phenomenon and see if modifications to color
profiles might help mask this effect.
A 2000P monochrome or a near-neutral color image, which has excellent
rendering under tungsten illumination, will appear greenish under D65. The
Premium Semigloss Photo (S041331) media is somewhat worse than the Archival
Matte (S041341). Additionally, yellows can appear excessively greenish
under cold illuminants.
Procedure
Spectral reflectance (380-730 nm) measurements were made of photographic
(IT8), color laser (Xerox, HP) and 2000P Matte and Semigloss test targets.
I also "painted" colorant from a used cartridge onto the two print media.
Calculations of CIELAB coordinates were made for 4 illuminants: Illuminant
A, D50, D65 and 10000 K.
Initial Conclusions
* Spectral density plots of equal L* neutrals from photographic, laser and
Epson 2000P illustrate that the colorant set of the Epson 2000P could
easily be metameric with other colorants.
* Color laser neutrals have minimal illuminant sensitivity because of their
spectrally flat black colorant and the use of a high UCR (which minimizes or
eliminates CMY composite neutral) with many color rendering modes.
* Photographic print neutrals are CMY composites but their sensitivity to
illuminants is significantly smaller than the Epson 2000P and, therefore,
not a problem.
* The Epson 2000P has a (approximately) spectrally flat black colorant which
has minimal illuminant sensitivity. But, the black is not used throughout
the entire neutral scale.
* The 2000P grayscale neutrals are composite (CMY) for lighter areas of the
tone scale, transitioning into high K usage at higher densities. Epson has
implemented different black replacement (UCR) strategies for Matte and
Semigloss media, with Matte being more aggressive, causing Semigloss
neutrals to be slightly more illuminant sensitive. The highest densities
(R=G=B=0) for both media are approximately spectrally flat.
* For 2000P neutrals and near neutrals, the CIELAB deltaE (RMS) from
Illuminant A to D65 is about 7.5 for Matte and 12 for Semigloss (at L*
values between about 40 and 60), and the direction of change is towards
green for colder illuminants. Subjectively, bias in the green-magenta
direction is very noticeable with certain image content, and I consider this
degree of illuminant sensitivity too large for optimum digital artwork.
* The 2000P yellow colorant peaks at 415-425 nm (photographic is 450 nm),
and is more greenish than yellow colorants from other marking technologies.
This greenish yellow shifts further towards the green with colder
illuminants.
* Compared with photographic or laser colorants, the dye peak of the 2000P
magenta colorant is sharper and shifted to higher wavelengths.
* A metamer sensitivity analysis showed that the resulting spectral "hole"
in the 475 to 525 nm region (between yellow and magenta dyes) and, to a
lesser extent, the yellow dye density modulation in the 400-415 nm region,
are the primary reasons for illuminant sensitivity of the Epson 2000P
colorant set.
* For the 2000P, acceptable gray scale, near neutral and yellow rendering
can be obtained for any specific illuminant. However, when viewed under
different illuminants, the color relationships can be quite different from
those intended by the digital artist.
* Other than yellows, changes with 2000P non-neutral colors with illuminant
color temperature are not much more objectionable than other marking
technologies.
* I've had some success in partially masking illuminant sensitivity by doing
the following:
o Profiles are tweaked to shift yellows to be more orange.
o I balance color prints slightly warm and for a D50 illuminant.
o For monochrome, bias towards sepia or blue.
Summary
As a printer of digital artwork, I had waited for an archival, reasonably
priced, color photo-quality, archival ink jet printer and hoped that the
2000P would satisfy that need. However, Epson's choice of a very illuminant
sensitive colorant set has put constraints on 2000P usage for
balance-sensitive images that will be viewed under different illuminants.
There are many applications that this illuminant sensitivity will not be a
problem, but there are many applications where the Epson 2000P colorant set
is inappropriate and will produce unintended renderings.
Hopefully, Epson can modify their yellow and magenta colorants in the
future, alleviating this problem.
I'd be glad to have further discussions and answer any questions, either
on-line or off-line.
Bill Kress, Mission Viejo, CA 92692 email@hidden 01-05-01