Monday, November 28, 2011

Photographic Purity– (Beyond B&W: Color)

Black and White photography transitioned to color imaging before most people alive today can personally remember. Bill Watterson, in his Calvin and Hobbes creation (probably the high-water mark of daily cartoon strips), probed this technological shift with an amusing and thought-provoking perspective. Out of respect for Watterson's copyrights I won't provide a direct link here to this particular strip, but you can find it out there if you search for it.

When the ability to capture "true color" images arrived, this new tool was widely seen as nearly miraculous and bringing the viewer ever closer to a "true" recreation of the original scene. This shift from B/W to color was almost (but not quite) as profound as the shift from painted images to photographs had been most of a century earlier. Instead of recording only values (light/dark), this new capacity could acquire the various hues and saturation levels in a scene as well. For nearly the first century of photography, paintings always had the upper hand when it came to coloration– photographs just could not compete in this arena. Now, with color added to the mix, the sky was nearly the limit. In fact, the sky no longer had to suffer the dreary limit of endless stretches of gloomy gray, but could now burst forth with the beauty of its truly blue self.

EARTH AND WATER INTERWOVEN, I, © Bill Brockmeier, all rights reserved
Of course this ability to record the colors apparent in a scene has to be closer to the reality of the original...or does it? It would be instructive here to ask a simple question: "What is color, anyway?" It might come as a surprise to find out that color is not actually a physical parameter, but, in fact, is simply a human perception, a construct of the mind.

While one might say that color is just a matter of the wavelength of the light, this is stripping down the truth of color far too simplistically. There are actual, perceivable colors that don't even exist on the complete spectrum of "all the wavelengths" that are spread out from white light as it passes through a prism. And any specific, perceivable color can actually be created from an infinite number of possible combinations of different other colors. Even more amazing, it is possible for there to be two things that appear exactly the same color under one lighting condition, and yet appear to be dramatically different in color from each other in a different lighting condition (metamerism).

Here is a short list of conditions that can substantially affect the perceived color of an object:
  • color of the light illuminating the object
  • brightness of the illumination
  • color of the object's environment/surroundings
  • colors and intensity of light viewed previous to viewing the object
  • color characteristics of any transmitting medium between the object and viewer

Most probably know that it is in the retina, the light sensitive back surface of the eye, that color is initiated when photons are preferentially absorbed by the different cone (color) receptors. While this is true, this is only the beginning (the barest beginning) of color perception. This is no simple RGB (red/green/blue) modeling of the light that is intercepted, as in a camera. The neural network of the retina takes this simple color "information" and already begins massaging it and reacting to it, before it sends it on to various locations in the brain, which further process these signals into a whole host of responses and outputs, gaining even emotional and visceral components along the way.

While the truth of color perception is highly complex and cannot be quantified unambiguously, what about the "simple" RGB (trichromatic) model used by photography? It turns out that even if we restrict ourselves to discussing this stripped-down technological version of "colors" it is a complex issue. First, there is the camera that takes the complex mix of light and turns it into an array of values that represent the "original colors." Of course, these values are necessarily compared to some "reference white" (would that be an incandescent lamp? a fluorescent lamp? or maybe natural sunlight...and at what time of day?).

The numerical values that are produced by this method are also limited in various ways. First, there is the sensitivity/detection function of the three separate color channels of the camera (basically, the filter transmission curves of the camera's detectors). Then, there is the dynamic range of the values (lightest to darkest) and the size of the steps from one value to the very next. There is also the issue of color gamut– how much of the totally perceivable color "real estate" can be covered by these numerical representations?

Finally, when these numerical values representing the colors in an image have been determined, how will the image be reproduced so it can be seen? Here, the loop is closed by somehow converting the values back into color representations either emitted by the light of a digital monitor, or by the density of different colored ink droplets on paper media. In both these cases, the representations themselves convolve the dynamic range and color gamut limitations as mentioned previously for the camera. On top of that, there are now even more considerations that effect the final colors– things like color gamma in monitors; and for printed images, dot gain, media brightness and whitepoint, and illumination whitepoint, color rendering index, and brightness. And if that wasn't enough, high-end photographic printers now boast of as many as ten or twelve different ink colors to represent those three original RGB colors. As you can see this is a highly complicated issue, and it is an amazing thing that the whole system works as well as it does.

It's clear that there really is no such thing as photographic "purity" when it comes to color. Attempting the photographic reproduction of the "original colors" of a scene may (or may not) be a worthy goal to strive for, but it is simply not possible today (and really never will be). And that's OK. After all, this is an art, and van Gogh did not mix his colors like Gauguin did.

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