Color Reflectance Curves: The Finger Print of Art and Science

Color reflectance curves are a graphical representation of how a material reflects light at different wavelengths. These curves are used in a variety of fields, including art conservation, material science, and colorimetry.  We create reflectance curves by measuring a colored sample using a spectrophotometer.

When a spectral measurement is made, a color reflectance curve is plotted on a graph with the horizontal axis representing the wavelength of light (in the visible spectrum) and the vertical axis representing the percentage of light reflected by the material at that wavelength. The curve shows how the material's reflectance changes as the wavelength of light changes.

We also notice that when we bounce light off of a colored sample, we don’t get 100% reflectance of the light source off of the sample.  That is because pigments absorb light.  This science is noted in the reflectance curve.  Not even a single pigment titanium dioxide (white) colored sample will reveal 100% reflectance.

The shape of the curve is determined by the pigments used in the color formula.  If we change a pigment, we change the curve.  That makes the reflectance the “finger print” of the color that is in the sample.  The curve is unique to the color formula.  Change a pigment, we change the finger print.

Did you know that one of the key uses of color reflectance curves is in art conservation? When a painting or other work of art is created, the artist chooses pigments and other materials that will produce the desired colors. Over time, however, these pigments can degrade or fade, changing the appearance of the artwork. By analyzing the color reflectance curves of the original pigments, conservators can determine the original colors of the artwork and restore it to its original appearance.

Color reflectance curves are also used in material science to study the optical properties of different materials. By analyzing the curves for a variety of materials, scientists can understand how different materials interact with light and how they might be used in different applications. For example, a material with a high reflectance at certain wavelengths may be useful in solar cells, while a material with a low reflectance at certain wavelengths may be useful in protective coatings.

In colorimetry, color reflectance curves are used to specify and measure the colors of different surfaces. Colorimetry is the science of measuring color and color reflectance curves are a key tool in this field. They provide a way to specify the colors of different surfaces and to measure how closely a surface matches a particular color.  When a color supplier encounters a supply chain issue with a specific pigment, it is particularly challenging for the formulator to come up with an alternate formula (using available pigments) without disrupting the original reflectance curve.