To support my attempt to demystify the baffling phenomenon of metamerism, and enhance our understanding of it, let us review the four types of metamerism described in the previous post, “Seeing the Light…Metamerism”. We can then move on from there to deconstruct some of the formal terms used in their definitions, specifically, spectral reflectance distribution, and spectral power distribution, two terms/phenomena that are inter-related, but not the same, and which play an integral part in 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 samples respond to different light sources in different ways, reflecting wavelengths of light (which is the color that our eye sees: we see the wavelengths of light that are NOT absorbed by an object…IE, the wavelengths of light that ARE reflected by an object) differently, they will be seen by our eyes as different colors.
Illuminant metamerism: Easily confused with sample metamerism, illuminant metamerism occurs when the spectral reflectance distributions of the two color samples (which, as mentioned above, is their response to light, characterized by the wavelengths that they primarily reflect) is identical, and the spectral power distributions of the various light sources under which they are seen, differ. When two identical samples are seen under different lights sources with differing spectral power distributions (the output of a light source, characterized by its relative strength at each wavelength), their color will appear to differ, even though as regards to their spectral reflectance distribution, they are exactly the same.
Thus, Sample metamerism occurs as a result of differences in the reflectivity (ability to reflect) of the color samples (objects/surfaces) themselves , and Illuminant metamerism occurs as a result of differences in the output, or strength of the wavelengths of the light source itself, under which we are viewing the color samples.
Indeed! But when we read the definitions carefully, and break them down so that we understand the terms and what they are referring to, we can begin to understand how light, color, and sight work together.
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 strength of the wavelengths of the light source only, not in the samples themselves.
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.
Now, let’s explore,and attempt to define both spectral reflectance distribution, and spectral power distribution, and their curves, and see if this process illuminates our understanding of ” magical” metamerism!
The spectral reflectance distribution of a sample (object or surface) has to do with its response to light…to the light wavelengths it reflects, as opposed to those it absorbs. The color that we see is composed of the wavelengths (which compose white light) that are reflected…not absorbed by a surface. In essence, what we see is really not a “tangible” part of the object or surface we are looking at. We do not see absorbed wavelengths…they are no longer visible. We see the wavelengths that are reflected. The spectral reflectance distribution is a quality, or ability of an OBJECT or SURFACE…not of a light source. Light sources, whether natural or artificial, provide light, they do not reflect it.
The reflectance spectrum or spectral reflectance curve is the plot of reflectance as a function of wavelength, which can be charted graphically.
Spectral power distribution is how much power a light source puts out, “characterized” by its relative strength wavelength to wavelength.
It is a quality, or ability of a LIGHT SOURCE…not an object or surface. As stated above, objects or surfaces absorb and reflect light. They do not provide light. Light sources provide light that is then reflected or absorbed by an object/surface.
A spectral power distribution curve ” provides the user with a visual profile of the color characteristics of a light source. They show the radiant power emitted by the source at each wavelength or band of wavelengths over the visible region….” –http://www.gelighting.com/na/business_lighting/spectral_power_distribution_curves/
Whew! Is this…physics?
I know that I have repeated some of this information several times.
I have found that only by repetition, reading and rereading, sometimes even out loud, can I begin to wrap my mind around these concepts. I hope these efforts help YOU understand metamerism and its inherent phenomena, and that your color practice, consultation, choices, play and enjoyment is enriched.
Happy coloring! Try it outside the lines…maybe you will make a colorFULL discovery!