IL3 User’s Guide
11S-3002B
page 61
Figure 7-3: Histogram: Linear Gradient
If we view the same mono-tonal content used in Figure 7-2 on a color camera, we get the more
complex histograms seen in Figure 7-3.
Assuming that the mono-tonal image is a neutral gray target, the histogram on the left shows what
you would expect if the camera’s color balance is shifted a bit toward the red, as it might be under
tungsten illumination. In this case the color channels are easily visible as they are separated. The
histogram on the right shows what you would expect if the camera’s color balance is a good match.
Histograms and Exposure Settings
Professional Digital photographers are very sophisticated in their use of histograms. While the use of
histograms as a tool for fine-tuning light and color content for artistic purposes is beyond the scope of
this App Note, there are a couple of simple rules to follow that will be helpful.
1. The best images contain a balance of highlights and lowlights. To this end it is good to strive for a
distributed grouping of peaks across the histogram.
2. Try to identify the peaks for any objects of interest in the scene. Make sure that those objects are
not in danger of exceeding the camera’s dynamic range--that they are not close in value to either
0 or 255.
3. Avoid Min: 0. If the minimum pixel level is 0, you know that some pixels are registering no light at
all. If there are some very dark areas in the scene, you might expect this and accept it. But if the
scene is more uniform and you need to be able to bring out details in the dark areas, avoid Min:
0, as no amount of gamma or brightness adjustment will help. If you are recording in 10-bit mode
and viewing the upper 8-bits, you may wish to view mid- and low-8 bit settings just to see if you
are truly at Min: 0
4. Avoid Max: 255. Once an area reaches saturation (255), it is game-over in terms of image
processing. In color images this may even result in some color shifting as one color may easily
saturate before the others.
histogram of a mono-tonal image will be a bell curve. The histogram shown to the right in Figure 7-2
is in the format used on the TS3. It shows minimum, maximum and peak values for the image. The
peak value also includes the number of pixels exactly at the peak value: Peak: <# of pixels>@<pixel
value>.
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