Sunday, May 17, 2009

Temperature of Light - part 3 (An Introduction to Histograms)

An Introduction to Histograms
Before getting into the experiment showing the effect of color temperature on a camera's output, I want to touch on histograms. If you are familiar with histograms, please feel free to skip this little side excursion. If you are not familiar with histograms, then you will need this information to better interpret what follows.

A histogram is a graph that shows the distribution of pixels over for a range of illumination from none to full brightness. The x-axis shows illumination with none on the left and full brightness on the right. The y-axis shows the number of pixels at a particular level of illumination. If you see an area of the histogram with a peak at some point along the x-axis, then you know that the image contains a concentration of pixels with that level of brightness.

Digital cameras represent colored light using three channels of color, Red, Green, and Blue. Using combinations of these three primary colors, we can model virtually any color of visible light. There are other ways to model the color (CMY, HSB, Lab, etc.), but that is beyond the scope of this introductory discussion. The point is that there are three color channels and to get an idea of a light's color content, we need a way to view the information from all three channels separately. If we look at the combined luminance only, then we can make no judgment about color.

Take a look at the histograms in the image above. The black histogram on top represents only luminance. It tells us about the exposure, but contains nothing about color. Below it are three more histograms by color. These represent the luminance for each color channel. In this example, the peak that shows up in the middle indicates that middle gray is a dominant color. All three of the histograms have a similar shape, which tells us that there is a fairly even distribution of colors in the image.

Most subjects we photograph will have a fairly equal distribution of colors, so when looking at the resultant histograms they will have a very similar appearance. Any time there is a big difference between the RGB histograms, either the subject has some dominant color or the there is a problem with the white balance.

Histograms are an important tool for analyzing digital images and are one of the most used tools in the digital photographer's took kit. Much more can be said about the interpretation of a histogram and its use for calibrating exposure and post processing. I hope to touch on some of that in future posting, but for now we have enough understanding to continue with the discussion of light temperature.

Coming Next
In the next posting I will continue the series with a demonstration of what the camera sees when photographing under different light sources. Stay tuned...


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