Re: Neutral grey under different lighting
Re: Neutral grey under different lighting
- Subject: Re: Neutral grey under different lighting
- From: "Fleisher, Ken" <email@hidden>
- Date: Tue, 04 Sep 2007 11:17:02 -0400
- Thread-topic: Neutral grey under different lighting
On 9/4/07 9:21 AM, "Graeme Gill" <email@hidden> wrote:
> Fleisher, Ken wrote:
> I can't agree with that. Metamers are two different spectral distributions
> that appear identical. It is simple logic therefore that two spectral
> distributions that do not appear identical, are not metamers.
Two spectral distributions that do not appear identical are not metamers
only if they never appear identical. If there is an illuminant that can
cause them to appear the same, then they are metamers regardless of whether
you are currently viewing them under that condition. It refers to a property
of the objects (SPD) which doesn't change based on viewing illuminant.
>> That is,
>> they must match under one illuminant but not another.
>
> I don't think this has anything directly to do with the definition of
> metamerism.
Sure it does. If they produce the same XYZ under an illuminant and have
different SPDs, then by implication they cannot match under every reference
illuminant.
> It's the spectrums that are the metameric stimuli, not the reflective sample
> colors though.
I don't think that's correct. Metamers are objects that produce the same
tristimulus values when viewed under a reference illuminant for a particular
standard observer. Metamers are the objects, not the resulting spectra. You
cannot have two illuminants that are metamers. Refer back to the CIE
definition that Roger posted:
> CIE Publication 15.2-1986, Section 5.2, page 35, describes metamerism in the
> following way : "Two specimens having identical tristimulus values, XYZ, for
> a given reference illuminant and reference observer are metameric if their
> spectral radiance distributions differ within the visible spectrum".
I will concede that you can have two illuminants that have different SPDs
but still produce the same tristimulus values, say two D50 simulators, and
that this "could" be interpreted as a metameric pair. But in my opinion they
are not a metameric pair because there is no reference illuminant and the
term "specimens" to me suggests that we are referring to objects, not light
sources.
> Again, I can't agree. Two reflective sample colors may be potential
> sources of metameric colors if illuminated with the correct illumination.
> Under other illumination they are not metamers, since their colors appear
> to be different (this is from the basic definition of metamerism).
But if they are metamers, then they are always metamers. The spectral
properties of the two objects don't change just because you view them under
a different illuminant (we are ignoring any effects of fluorescence, of
course). If they will produce the same XYZ under D50 but not A, then just
because you are viewing it under illuminant A doesn't mean they no longer
will produce the same XYZ under D50. They are still a metameric pair. The
property is constant.
>> Metameric failure is simply when a metameric pair does not produce a match
>> under a particular viewing condition--I've never heard of it described in
>> relation to whether it is "intended" to produce a match or not, it simply
>> doesn't.
>
> Talking about "metameric failure" doesn't have any meaning otherwise. It's
> of little significance that two colors look different (they aren't metamers),
> it's only of interest if they are intended to be metamers, and are not so
> due to some failure in the source of the colors.
It's of interest if they "are" metamers--How different will they appear
under a different illuminant. If they are not metamers, then they will never
produce a match. I think this goes directly to our different understanding
of the term metamer.
--
Ken Fleisher
Photographer
Imaging & Visual Services
National Gallery of Art
Washington, D.C.
Phone: (202) 712-7471
email@hidden
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