Metamer materials under D65 light source, differentiated by sodium arc lamp
In this article, we will illustrate the concept of metamerism, a natural phenomenon that can be easily simulated with Ocean.
Ocean Light Simulator is a full spectral tool, and samples every possible wavelength in the user-defined wavelength range. No discrete sampling of wavelength is done, removing the need for adjusting the grid size, and avoiding the risk of introducing bias in the colorimetry. Therefore, it is a very precise tool for doing color rendering evaluation.
Because the human eye converts spectral information to tristimulus signals, different spectra may be perceived with the same color. For instance, a bright orange light may have a spectrum with a peak around 590nm, such as for sodium lamp which emit almost exclusively between 589nm and 590nm. But orange may be also obtained by mixing red and green : two lights emitting at 550nm(green) and 650nm (red) will blend to orange. This is how orange is produced on CRT or LCD monitors : by lighting red and green subpixels.
Metamer surface colors
The light reflected by an object has a spectrum equal to the product of the illuminant spectrum by the surface reflection spectrum. As for light source, two objects having different reflection spectra may, under a given illuminant, reflect lights with different spectra, but the same color. Under this illuminant, they look similar. However, under a different illuminant, the product will change, and the objects may appear different.
Consequences to rendering and color prediction
An object cannot be accurately described by a tristimulus value, such as RGB color. It will only describe the object’s color under a given illuminant. To describe a surface independently of the illuminant, and render it accurately under different light sources, it is necessary to know the object reflection spectrum.
D65 light source
The following picture shows two spheres made of lambertian diffuse materials in a light box, lit by a D65 light source. The materials are very different, as shown on their reflection spectra. However, they have the same apparent spectra under the D65 light source : they are metamers.
Dayligh fluorescent light source
In this picture, only the light source was changed to a fluorescent tube light, measured with a spectrometer on a real light source. We can notice that the two spheres are slightly different : the right one is more red, while the left one is brighter. Fluorescent tubes have emission peak in their spectra, which can change significantly the color rendering of objects.
1500K candle light source
This third picture was rendered with a 1500K light source, similar to a woodfire flame. Surprisingly, the left sphere is now more red, while the right one is more orange. This shows that relative color differences between two objects may be inverted by changing the light source.
Na vapor light source
In this last picture, we used the emission spectra of a sodium vapor discharge lamp as the light source. The left sphere, whose material has a drop in reflectivity around 590nm, appears very dark, while the right sphere appears much brighter. All this by just changing the light source : on the four render pictures, all materials were the same.
Using a spectral renderer, predicting accurately the colorimetry of objects is possible. This requires to work only with spectral data for both surfaces and light sources. This way, very subtle effect such as metamer color differentiation may be visualized.