Re: Metamerism vs Color Constancy
Re: Metamerism vs Color Constancy
- Subject: Re: Metamerism vs Color Constancy
- From: Robin Myers <email@hidden>
- Date: Tue, 20 Sep 2005 05:50:13 -0700
On Sep 19, 2005, at 8:51 PM, Marco Ugolini wrote:
<snip>
First, a definition of the terms (taken from the great web site
<http://www.answers.com>):
------
METAMERISM
Metamerism is a psychophysical phenomenon commonly defined
incompletely as
"two samples which match when illuminated by a particular light
source and
then do not match when illuminated by a different light source."
["Incomplete" because metamerism can also be subdivided into sample,
observer, illuminant, and geometric metamerism]
I disagree a bit with this definition. As far as I know, there has
not been a definition of "sample metamerism". There is observer,
illuminant and geometric metamerism and I have witnessed all three of
these phenomena. The CIE has defined Metamerism Indices for observer
and illuminant metamerism.
------
COLOR CONSTANCY:
Color constancy is a feature of the human color-perception system
which
ensures that the perceived color of objects remains relatively
constant
under varying illumination conditions.
------
I am bringing this up because my poor insufficiently scientifically-
trained
mind is trying to comprehend whether the term "metamerism" is
currently
being used improperly in color management circles.
When we refer to inkjet prints as being "metameric" (a common
buzzword these
days among the initiated to the growing sect of inkjet printing),
do we
actually mean, instead, that they lack color constancy?
Metamerism happens between two samples (in sample metamerism, at
least)
whose colors have different spectra. This spectral difference
creates a
match under one illuminant (called a metameric MATCH) and a
mismatch under
another illuminant (a metameric FAILURE). I have been unable to
retrieve the
etymology of the word "metameric," but it seems that it refers to
parts that
appear or ought to appear similar if not identical to one another
(as in the
segments of the body of an earthworm, which are called "metameres").
If that is so, then a print cannot, by itself, be metameric.
So, if we use the word "metameric" to mean that a print changes its
color
appearance under different illuminants, shouldn't we actually say
that IT
LACKS COLOR CONSTANCY instead?
Please tell me if I am wrong about this, but if I am correct could
we please
stop saying that inkjet prints are metameric, then?
Metamerism, when taken in the context of changing illumination, must
always be between a PAIR of samples, usually referring to individual
colors, not an entire print. However, not many color scientists will
give you too hard a time for referring to two prints and comparing
their match with the term metamerism. But when viewing a single
specimen, you are correct that the term "color constancy" applies
when the print remains constant, or more often the case, "color
inconstancy" when the print appears to change hue, tone,
colorfulness, or some combination of these.
The public follows the lead of the experts it follows and I have
heard many color management consultants misuse the term metamerism.
If we lead through correct terminology, the public will correct itself.
Robin Myers
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