RE: =Color constancy and metamerism
RE: =Color constancy and metamerism
- Subject: RE: =Color constancy and metamerism
- From: "Mark Rice" <email@hidden>
- Date: Tue, 20 Sep 2005 16:53:52 -0400
I don't think so - metamerism can occur with a single sample.
In the seventies, when I worked for a very high priced photographer, he was
always asking me questions about why film behaved in certain non-intuitive
fashions. So I studied photographic theory, and in particular, I read "The
Theory of the Photographic Process" by C.E.K. Mees, a former Kodak color
scientist. It was written in 1942, and is about 3000 pages long. It has an
extensive section on metamerism, with pages of exotic equations. I copied
the pages of equations and blew them up to 30"x40" prints and hung them on
the wall. When a client would ask why the photo didn't match the product, I
simply pointed to the equations on the wall!
But seriously, the prime example that Mees used of metamerism is the olive
dress example. This discussion did not involve the usual pair of object -
no, I take that back. The pair of objects was the original green dress and
the photo of the dress. The dress appears green to the eye, but it appears
brown on the film. This occurs because the spectral response curves of the
average eye (different peoples eyes can see different objects as different
colors too, of course - color blindness is an extreme example) don't match
the spectral response curves of the film, or of the digital chip. Mees used
this a primary example of metamerism, and it is a very common problem for
photographers, especially when photographing fabrics. We only use 3 primary
colors in most cases, but one could use 10 primary colors and achieve
greater color accuracy (a spectrophotographic camera, in effect!)
By the way, Edwin Land, inventor of Polaroid, once had an article in
Scientific American, where he demonstrated that by using two primaries - two
narrow cut green filters at 520 nm and 540 nm - he was able to produce a
full color picture! It was somewhat desaturated, but it did indeed contain
all the colors of the original subject!
In a similar vein, have you ever wondered why photographers use RGB for
their primary colors, and artists use Red, Yellow and Blue? When I was in
college, I went to my photography professor and asked him, and he basically
told me that it was because artists didn't know what they were doing. When I
asked an art professor, he said the same thing about photographers. I found
the answer in Mees's book. Yellow matches the peak sensitivity of the
"beta" band of the eye, and works better when mixing pigments. Green is not
at the peak sensitivity of the "beta" band (RGB are referred to as the
alpha, beta and gamma receptors in the eye), but it has the least overlap
with the alpha and the gamma bands, therefore producing more saturation, and
less metamerism!
Well! My wife always tells me that if she asks me for the time, I tell her
how to build a watch, and then throw in the history of Switzerland as well.
Sorry if I rambled.
Mark
www.zero1inc.com
email@hidden
-----Original Message-----
From: Marco Ugolini [mailto:email@hidden]
Sent: Tuesday, September 20, 2005 2:31 PM
To: Mark Rice
Subject: Re: Color constancy and metamerism
Mark,
You may be correct, but "metamerism" isn't the right word either. Color
constancy is a feature of human vision, true. Then perhaps it's more
accurate to say that a specific printed sample FAILS the color constancy
test if its appearance changes with the illuminant.
It seems clear to me that metamerism can only take place when 2 (or possibly
more) samples are compared under ONE illuminant. But what people refer to
when using "metamerism" in regard to inkjet prints is that ONE print changes
appearance under TWO OR MORE illuminants.
Clearly not the same thing.
It's complex and I don't pretend to understand it fully. I'm just pointing
out that the use of the word appears to be inappropriate.
Ciao.
Marco
--------------
> From: Mark Rice <email@hidden>
> Date: Tue, 20 Sep 2005 12:26:09 -0400
> To: Marco Ugolini <email@hidden>
> Subject: =Color constancy and metamerism
>
>
>
> Marco, I am no color scientist either, but I think you have it
> backwards here. Color constancy is a characteristic of human beings,
> not of ink jet prints. Color constancy means that our eyes and brain
> adjust to the existing illumination to see whites as white, no matter
> what the surrounding illumination (within reason). Ink jet prints cannot
make this adjustment.
>
> Metamerism means that two different sample areas on the ink jet print
> may appear identical under one lighting condition, but appear
> different under a different lighting condition. A classic example of
> this is my Epson 2200 simulating B&W prints - I have achieved near
> perfect neutral appearing prints under 5000K lighting, but under cool
> white deluxe, the prints appear slightly sepia. That is metamerism.
> Only an object can have metameric qualities - human being cannot be
metameric.
>
> A classic reason for metamerism is this: take a single wavelength of
> light, say 530 nm green and illuminate an object under it. Then take a
> series of wavelengths (representing the spikiness that one often gets
> from fluorescent
> illumination) that average out to equal 530 nm and illuminate the object.
> Many object will appear to be different color under the different
> light sources, although the color temperature and lighting "color" is the
same.
>
> Another classic metamerism example that many photographers have torn
> their hair out over is this: an "olive green" dress will appear green
> to most observers, but many times it will appear brown to many films
> (if anyone is still using film). That's why us retouchers were invented!
>
> Mark Rice
> www.zero1inc.com
>
>
>
>
>> From: Marco Ugolini <email@hidden>
>
>> Subject: Metamerism vs Color Constancy
>
>> To: ColorSync Users Mailing List <email@hidden>
>
>> Message-ID: <BF54D55F.5428%email@hidden>
>
>> Content-Type: text/plain; charset="US-ASCII"
>
>> I premise this message by saying that I am NOT a color scientist
>> (though I
>
>> wish I was sharp enough to be one): just a guy who has been dabbling
>> for a
>
>> few years in matters that have scientific ramifications, which he is
>> still
>
>> trying to sort out for himself.
>
>> So, there went the introduction.
>
>> Now to what I wanted to ask: Metamerism vs Color Constancy.
>
>> First, a definition of the terms (taken from the great web site
>
> <http://www.answers.com <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]
>
> ------
>
>> 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").
>
> I>f 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?
>
>> Thank you.
>
> --------------
>
>> Marco Ugolini
>
>> Mill Valley, CA
>
>
>
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