Chances are, you take color for granted. Whether or not you give it much thought, color is key to distinguishing your surroundings. It helps you identify fire, brown recluse spiders, and the right resistor for the job.
In the spotlight this week is a 1950s educational film called “This is Color“. It also happens to be a delightful time capsule of consumer packaging from the atomic age. This film was made by the Interchemical Corporation, an industrial research lab and manufacturer of printing inks. As the narrator explains, consistent replication of pigments is an essential part of mass production. In order to conjure a particular pigment in the first place, one must first understand the nature of color and the physical properties of visible light.
Each color that makes up the spectrum of visible rays has a particular wavelength. The five principal colors—red, yellow, green, blue, and violet—make possible thousands of shades and hues, but are only a small slice of the electromagnetic spectrum.
When light encounters a transparent material more dense than air, such as water or glass, it has to change direction and is bent by the surface. This is known as refraction. A straw placed in a glass of water will appear bent below the surface because the air and the water have different refractive indices. That is, the air and water will bend or refract different percentages of the light that permeates them.
When light encounters a smooth, solid surface, reflection is focused in one direction. A rough surface reflects light in all directions because the light hits so many different angles. This phenomenon is the basis for manufacturing knowledge about printing inks, lacquers, dyes, and chemical coatings. This is demonstrated with a coating made from linseed oil and glass that has been ground into a fine powder. Here, the linseed oil acts as a vehicle, and the glass as a pigment of the most basic form: fine particles of an insoluble substance suspended in the linseed oil. The refractive index (RI) of the linseed oil is different from that of air, so the light that permeates it is partially reflected.
If the RI of the pigment is the same as the vehicle, light will pass through to the surface begin coated and there will be no reflection. The coating is transparent. If the surface is white, nearly all the light will be reflected back through the coating. A black surface will reflect almost none of the light.
However, if the RI of the pigment differs from the RI of the vehicle, reflection occurs at each particle of the pigment. If it is thick enough, enough reflection occurs that the surface color will not be visible. The coating is opaque.
To reveal the relationship of colored pigments and light, we are shown the effect of several colors of glass. Each color of glass has all of the spectral colors inherent in it, but only certain colors are passed through while the others are absorbed. This experiment makes it possible to create graphs of the wavelengths that each spectral color transmits.
Using a device called a spectrophotometer, which measures the amount of light that is reflected or absorbed by a material at all points in the spectrum, the fine folks at Interchemical Corporation can nail down the exact green they need for a package both initially and for every production run thereafter. We are now equipped to understand the subtractive process of printing the color green: yellow and blue do not make it, but instead leave it. Really, though, they allow it to be transmitted.
Green occurs in the overlap of the reflectance curves of yellow and blue. This is another way of visualizing the subtractive process; we can see from the graphs that blue will absorb the red and yellow wavelengths. Yellow will absorb the blue and violet wavelengths, which only leaves green.
There is another way to invoke green, and it is used in color television screens. A checkerboard distribution of yellow and blue squares will, from a distance, blend together within the human eye. This is known as additive color mixing. This average of the reflectance curves for yellow and blue produce a certain shade of gray, which can be visualized with a Maxwell disc. Additive color mixture is not a property of light, but instead takes advantage of the way that the eye detects color.
Whether you’re squinting at a resistor or a spider, the important thing to remember is that color is not an intrinsic property of physical objects, it’s an intrinsic property of the visible light that strikes them.
Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.