The Color Purple, and What We Can Learn From It to Print Better Images
Long reserved for emperors and members of the aristocracy, the color purple is relatively rare in nature. There's a good reason for its rarity, and depending on how you look at it, purple isn't even really a color at all. Allow me to explain.
We see an object's color based on the wavelength of light it reflects. An object that reflects green light and absorbs other wavelengths looks green. One that reflects only red light appears to us as red. A black object absorbs all wavelengths of visible light, reflecting none. This goes a long way toward explaining why black objects get so hot if you leave them out in the sun. All that absorbed light has to go somewhere. What isn't reflected as light gets re-radiated as heat.
That much makes sense. But what wavelength is purple? Any grade school kid will tell you that purple falls between red and deep blue or violet. But there is no such wavelength. Unlike the color wheel this same grade school kid is familiar with, wavelengths of light don't wrap back around from one end of the spectrum to the other. Colors don't form a wheel; they lie in a straight line continuum from deep blue or ultraviolet at the low end at around 400 nanometers, to red blending into infrared at around 700 nanometers. Never mind how small a nanometer actually is but suffice it to say that it's tiny.
But no matter how special a nanometer is, it has to obey the standard rules of mathematics. If purple lies between red and violet, then we have a problem. There is no wavelength both greater than 700 and less than 400. Going up in wavelength from red we get to infrared and ultimately to microwaves and radio waves. Decreasing wavelengths from blue and ultraviolet we reach the territory of x-rays and gamma rays. What happened to that nice pure purple treasured by the aristocracy? We only see a small slice of the electromagnetic spectrum, and nowhere in there is a wavelength we see as purple.
If objects are limited to reflecting a single pure wavelength from that spectrum, we are left with the conclusion that "purple" simply doesn't exist. No wonder it's so rare. And yet we all know that purple does exist, even if we don't encounter it as often as other colors of the rainbow.
The answer obviously lies in the fact that objects aren't limited to reflecting just a single wavelength. Indeed, white objects reflect all wavelengths of light more or less equally, which is why a white car parked out in the sun all day doesn't get as hot as a black one does. If it reflects all light, there's little absorbed light to re-radiate as heat. Unlike a spectrophotometer, our eyes can't see how much of each wavelength is being reflected back to us. The color we see is the combination of all the wavelengths reflected by an object.
An object that reflects only blue and red light will appear to our eyes as purple. Just like when mixing paints, an equal amount of blue and red will give us purple. Since these two wavelengths are at opposite ends of the visible spectrum, its uncommon for an object to reflect both without at least some of the wavelengths in between. If an object reflects every wavelength to one degree or another, we'd end up seeing it as gray or perhaps brown depending on how uneven the admixture of colors is. But purple takes a special kind of object.
The first purple dyes were derived from particular kinds of shellfish. It would take the extract from thousands and thousands of such shellfish to get enough eye to make one robe for the emperor. Such was the state of color technology back then. Ordinary people had no access to purple. Only someone able to command a large labor force could achieve such a feat. Today, modern chemistry makes purple and every other color available to all.
But even today, many cameras have a hard time accurately recording purple. Without an even sensitivity to light at both ends of the visible spectrum, it's easy to end up with an uneven balance and thus a color that veers more towards red or blue rather than right down the middle. Printers too have a hard time reproducing such mixed hues.
The more intricate the frequency distribution of what wavelengths something reflects versus absorbs, the harder it is to correctly render. There's a phenomenon known as metamerism that can make color reproduction notoriously difficult. Two objects that appear to be the same color under one light source but seem to differ in color when viewed under a different light source suffer from a metameric variance. That may sound complicated, but it leads directly from our discussion here of all things purple.
Objects rarely exhibit a narrow-banded response to light. Even if an object looks to reflect a pure spectral hue at just a single wavelength of perhaps green or blue, it likely reflects at least some other wavelengths at least to some degree. In fact, some objects that look perhaps pure green may reflect no green light at all and instead consist of equal amounts of blue and yellow light with no green in between. Since our eyes see the combination of what gets reflected rather than the actual spectral breakdown of wavelengths, we see pure green as pretty much the same color as blue plus yellow in equal proportions. Both average out to "green" so long as we are viewing these two under a white or neutral light source.
But if the light source itself isn't neutral, then what we see changes. Warm light contains more yellow wavelengths than blue, causing a color shift to how we see things. And that shift will be more pronounced for an object relying on "blue plus yellow equals green" than it will for one that truly does reflect green. The appearance of the truly green object will shift, but the one relying on equal amounts of blue and yellow will shift more. More yellow shining on it will result in more yellow being reflected. Less blue in the light source will mean less blue reflected. The same is true in reverse for a cooler light source that tends more toward the blue side of the spectrum.
So these two objects may look equally "green" under white light but differ significantly in apparent color in either warm or cool light. This is metamerism. Car manufacturers know all about metamerism and intentionally paint some cars in a way that make their color hard to pin down under varying lighting conditions, all so your eye is more attracted to them than to the competition.
Printer ink manufacturers view this more as a problem than a feature though and take great pains in their attempts to avoid the problem. Epson and others work hard to get their printers to use ink color combinations calculated to have the lowest potential metameric shift possible for any given hue. They microencapsulate the ink droplets so they scatter light evenly.
Thankfully, the affordability of such printers has come down sufficiently in recent years that you don't have to be an emperor or a member of the aristocracy to afford a good printer. If you haven't looked at the state of printer technology today, you owe it to yourself to do so. Your images, and not just the purple ones, will thank you.