Metamerism, commonly defined as the optical phenomenon of colors seeming to match under one light source, yet appearing different under another, is fascinating, and perplexing. Why and how does it occur?
Well, the answer, from my research, can get pretty complex…involving such things as CRI, or color rendering index, SPD, or spectral power distribution, spectral reflectance distributions and reflectance curves.
I became interested in metamerism afresh after viewing a webinar on the subject created by the paint company Sherwin-Williams. I must admit, I watched and listened to this recorded webinar several times, took notes, and then watched/listened to it again. Light in tone, with fun images, and plenty of humor, the webinar contained information that has taken me time, focus, and further research to even begin to understand.
Because I find the subject so complex, I have decided to devote a series of Artissima blogs posts to metamerism, and attempt to break it down into comprehensible chunks…siting examples and resources along the way which I hope will be helpful.
Color involves light, the object illuminated, and the observer of the illuminated object. As color is a function of light, very simply put, the color that we (the observer) see is reflected light waves. In essence: “Visible light is made of seven wavelength groups. When light hits objects, some of the wavelengths are absorbed and some are reflected, depending on the materials in the object. The reflected wavelengths are what we perceive as the object’s color.” –http://www.devx.com/projectcool/Article/19954/ Put another way; “Objects affect light by selectively reflecting or absorbing light of different wavelengths. So an object that absorbs most blue wavelengths and reflects most red wavelengths will usually appear red to our eyes. The actual color it appears to us is dependent on the spectral composition of the light reflecting off the object.” --http://photoshopnews.com/2005/04/20/metamerism-%E2%80%93-friend-or-foe/
Let’s look at four kinds of metamerism…
Sample metamerism: What we think of as “metamerism” is actually one type, sample. When two color samples appear to match under a particular light source but do not match under a different light source, this is called “sample metamerism.” Sample metamerism has to do with differences in each object’s spectral reflectance distribution, or its response to light, characterized by the wavelengths that it primarily reflects. When the spectral reflectance distributions of the two samples (the objects themselves) differ, the color of each will look different in different lights.
Illuminant metamerism: Easily confused with sample metamerism, illuminant metamerism occurs when the spectral reflectance distributions of the two color samples observed are identical. These identical samples are seen under different lights sources with differing spectral power distributions (SPD, or, the output of a light source, characterized by its relative strength at each wavelength)
Thus, Sample Metamerism occurs as a result of differences in the reflectivity of the color samples themselves, and Illuminant Metamerism occurs as a result of differences in the output of the light source itself, under which we are viewing the color samples. (Confusing…I think so…but with careful study of the differences, they can become clear)
Illuminant metamerism is not often seen, unless the observer use a light box to see identical samples illuminated by both light sources separately, yet simultaneously. Again, this type of metamerism is created by differences in the light source only, not in the samples themselves.
The complex phenomena of SPD/Spectral Power Distribution, Spectral Reflectance Distribution, and Spectral Distribution Curves will be discussed in greater depth in a subsequent post on Metamerism. It can take fortitude to keep them all straight!
Observer metamerism: Do we all see color differently? It is commonly agreed upon that we do, assuming that each individual possesses adequate color matching aptitude. Observer metamerism can occur because of differences in color vision from one person to the next. Again the process of seeing is complex, but for the purposes of this post, suffice it to say that the physical act of seeing, what happens in each person’s eyes and brain when they see an object in whatever light, can alter their color perception. A common source of observer metamerism is color blindness, but it occurs with the “normal-seeing” as well. In the case of observer metamerism, two lights or surfaces may be a color match for one observer but not for another.
Geometric metamerism: The angle, distance or light position from which identical colors are viewed may change the color that we see. The distance between a woman’s eyes is, on average, slightly less than a man’s. This slightly different angle of stereoscopic viewpoint may be why men and women have been known to perceive colors differently! Most of us have probably had the experience of two samples appearing to match when viewed from one angle, but then not matching when viewed from another angle. Examples would be the color variations that appear in pearlescent auto finishes or “metallic” paper. This may be something to think about when using specialty finishes in interiors, fashion and works of art.
I hope this post on Metamerism has lit a fire in your belly to know more about it. I know will be continuing my research, and delving further into the subject to help demystify it for both YOU and me.
The subject of color, light, and the relationship between the two is so vast, multifaceted and complex, that it will never be demystified completely. Thus we have a lifetime of color mystery and magic to look forward to. We can join in the efforts of fearless color explorers through the ages, and add our own special hue…ah, I mean, view (!), to their findings, while enhancing our own knowledge base and experience.