Mikrotron eosens TS3, Руководство пользователя, Страница: 103 / 120

TS3 User Manual
page 94
Figure 10-3: Histogram: Linear Gradient
If we view the same mono-tonal content used in Figure 10-2 on a color camera, we get the more
complex histograms seen in Figure 10-3.
Assuming that the mono-tonal image is a neutral gray target, the histogram on the left sh
o ws what you
would expect if the camera’s color balance is shi ft ed a bit toward the red, as it might be under tungsten
illumination. In this ase 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 Settin
g
Professional Digital photographers are very sophisti c ated in their use of histograms. While the use of
histograms as a tool for fine-tuning lig h t and color content for arti s tic purposes is b e yond 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 identi f y 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 j
us t to see if you
are truly at Min: 0
4. Avoid Max: 255. Once an area reaches saturation (255), it is g ame-over in terms of image
processing. In color images this may even result in some color shifting as one c olor may
easily saturate before the others.
the same value. In practice, be c ause it is unlikely we will be able to image a perfectly fl a t field, the
histogram of a mono-tonal image will be a bell curve. The histogram shown to the right in Figure 10-2
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>.