Non-neutral grays with R1800?

[Bruce Watson]

That's how I understand colour constancy as well: the degree to which a colour appears to be the same under different lighting conditions. If it doesn't look the same, it exhibits colour inconstancy. So if something looks neutral in one type of illumination and green in another, it can be said to exhibit colour inconstancy.

Arguing definitions can be fun, but I doubt it's helping the OP. Still, since we are here...

I don't think we are in agreement here. I said that I recall that color constancy is a property of eye/brain perception. You are saying that it's a property of the object. They aren't at all the same thing.

Color constancy is the property of color perception that keeps the world from looking like a kaleidoscope. Without it, we would see a landscape of constantly changing colors. Every changing lighting condition would result in color shifts. Stepping out from the shadow of a building into sunlight would be a disorienting experience. One of the theories about how the eye/brain system accomplishes color constancy postulates the existence of a "black channel" that's actually somewhat separate from the rest of the visual processing wetware. IOW, the eye/brain system does not see in RGB, but in RGBK. Interesting ongoing research.

My understanding of metamerism is that defined by the CIE in the CIE International Lighting Vocabulary: spectrally different colour stimuli that have the same tristimulus values. The corresponding property is called metamerism. (ie two different materials that appear to be the same colour although they don't have the same spectral characteristics. The entire field of colour reproduction depends on metamerism.) For example a painted wall and a photograph of the wall that has been colour matched to the wall can be said to be an example of metamerism. Under the CIE definition, the greater the degree of metamerism the more they look alike under different illuminants. What you call metamerism, others would call metameric failure.

One of the joys of language, particularly English, is that individual terms often come to have many meanings. This is particularly true of languages used over wide geographic areas.

In this case, I am using the term as you say. The definition I was taught for metamerism is that it is the property of an object to change color under different lighting -- the object is said to exhibit metamerism. For better or worse, that's what I was taught.

I have a failed ink experiment that provides a neutral B&W print only under 3200K halogen lighting. It's been years since I looked at it, but IIRC, with a 5000K fluorescent it turns green, while in direct sunlight it turns a striking blue. Metamerism or metameric failure, it's still bad ink, and an unacceptable print.

[Helen Bach]

"The definition I was taught for metamerism is that it is the property of an object to change color under different lighting..."

Now that is what I would call colour inconstancy, definitely not what I would call metamerism.

The CIE definition of metamerism is that metamers look the same, not different. You appear to be using it to denote difference. Metamerism is not a property of one object, it is a property of at least two objects, according to the CIE. In this case one of the objects seems to be a hypothetical, perfectly neutral, object.

Well, I knew that there were two different and opposite meanings in general use for metamerism. Now I have discovered that there are two different meanings for colour constancy as well. It's a good job that this is an inconsequential issue in the grand scheme of things, but it's probably worth bearing in mind that the two terms appear to have no internationally accepted consistent meaning.

Best,
Helen