A reflectance curve translates the wavelengths of light that we see as a colored surface, into a graph showing the colored wavelengths that are reflected when the surface is illuminated by white light. Each wavelength is measured in nanometers (nm) by a spectro-photometer that is a color measurement device, sort of like a scanner. The line graph is generated by computer software.
Reflectance curves show the mixing potential of each pigment. They confirm a color’s bias or leaning towards warm or cool. It’s necessary to know a color’s bias for successful color mixing. Becoming familiar with the reflectance curves for each paint takes the guesswork out of color mixing. They forever change our understanding of color, paints and color mixing, and get us past the uninformed color jargon surrounding artists’ paints.
I included reflectance curves in my books, “Color Right from the Start” and “Hilary Page’s Guide to Watercolor Paints.” That’s how I became familiar with them.
To make the curves, I first painted two 2×2″ squares of each pigment – one light and the other darker, as shown above. I sent the pairs (sometimes three shades) to the paint manufacturers Winsor & Newton, Daniel Smith Inc., and Holbein where their spectrophotometers converted the reflected wavelengths of their paints into line graphs that I transposed so all were the same scale. I next combined the light and dark curves. Within the graph I painted the colors to match my 2×2″ samples. It was a mass of work! These days I could just drop them into the graph using Photoshop! Finally, I painted a strip of the spectral colors over each graph in the appropriate place according to the wavelength.
The reflectance curves are taken from my book “Hilary Page’s Guide to Watercolor Paints.”
The curve for Cadmium Red PR108 is flat under the purple and blue wavelengths and so cannot be mixed with any color to mix purples. Most painters know this! This is what made Cadmium Red a teaching curve for me from which I deduced how paint reflectance curves work regarding color and color mixing. To mix colorful, saturated colors two pigments have to reflect common wavelengths i.e. the graph of each must rise ( even a bit) “under” the same colored wavelengths on each others curves.
Study the sequence of Curves and Captions below to grasp how Reflectance Curves work.
PRIMARY COLORS Saturated colors – warm lemon, magenta and blue – reflect across the broadest range of wavelengths and thus will mix to produce the greatest number of so called “secondary” colors. SECONDARY COLORS – Saturated color- green, orange, purple. They may be mixed from two primaries. NEUTRAL COLORS are dull colors that have flat curves and limited mixing potential. I include them for comparison. They’re sometimes called TERTIARIES since they may be mixed from three primaries.