Re: Neutral grey under different lighting
Re: Neutral grey under different lighting
- Subject: Re: Neutral grey under different lighting
- From: Marco Ugolini <email@hidden>
- Date: Wed, 05 Sep 2007 21:50:27 -0700
- Thread-topic: Neutral grey under different lighting
In a message dated 9/4/07 8:17 AM, Fleisher, Ken wrote:
> 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.
Sounds right to me. I would add, for added precision: "Metamers are the
objects when viewed under a given illuminant that causes them to appear of
the same color."
By definition, the spectral power distribution of an object is a constant.
It's the *interaction* of the two objects' separate SPDs *with a specific
illuminant* that produces two sets of tristimulus values, which in turn may
match one another (hence the "metameric match").
> You cannot have two illuminants that are metamers.
Correct. You must have an observer, an object (two objects, in metameric
matches) and an illuminant in each case. None of these three elements
suffices in isolation.
> 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.
Right. But saying that a pair is metameric, and not specifying *exactly
which* illuminant is needed to effect a match, is confusing. Specially if,
at the time that this description of them as a metameric pair is being
offered, they are being viewed with an illuminant under which they do *not*
match.
Marco Ugolini
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