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2.2.3.2.3 Automatic Gain
Automatic Gain Control automatically op-
timizes the image contrast and brightness
based on a histogram-equalization algo-
rithm. Values for the Plateau Value, ITT (Im-
age Transfer Table) Mean, and Max Gain
are user programmable.
Plateau Value:
The Plateau Value sets the limit for the number of pixels in each bin. When this number
is small, the Automatic AGC will approach a linear algorithm that preserves a linear mapping between the
14-bit and 8-bit data. The goal of the Automatic algorithm is to try and make each of the 255 bins have the
same number of pixels in it, which gives the highest possible dynamic range for the given scene. When
the plateau value is higher, the algorithm is more able to redistribute the data to achieve this goal. This
prevents wasted levels of grey on regions that have no scene content. The default Plateau Value is 250
(640 x 480) and 150 (320 x 240).
ITT Mean:
Changing the ITT Mean changes the midpoint of the ITT. It directly affects the brightness of the
image by changing the gray scale mid-point. Lowering the ITT Mean value will make the image darker. In-
creasing the ITT Mean will make the image much brighter. The default ITT Mean setting is 110 (640 x 480)
and 127 (320 x 240).
MAX Gain:
The Max Gain setting sets the upper limit of gain that the algorithm can use as it attempts to
stretch the data to fill the full 8-bit range. If the scene has a high level of contrast, it will use much less gain
than the maximum gain setting. The default value is 15 (640 x 480) and 8 (320 x 240).
AGC Adjustment tips:
Lowering the Plateau Value results in a somewhat darker image, while increasing the value will make the
image lighter in contrast.
Lowering the ITT Mean Value will noticeable darken the image, while increasing the value will make the
image significantly brighte
Gain will bring out details in a thermally low contrast scene, but will increase the image noise. Changes to
the Max Gain will only be noticeable when the scene requires active gain. For example, if the automatic
algorithms determine that the AGC level should be at 4, changing the Max Gain level from 5 to 10 will not
result in an image change for that scene. However, if the automatics determine that the active gain level
be at 12, changing the Max Gain value from 15 to 5 will have a very noticeable effect on the image in terms
of noise and detail.
AGC Presets:
For convenience, presets for several general AGC settings will be included in a drop down menu. These
AGC presets are intended to be used as a starting point to define the best possible AGC settings for a
particular scene.
The user is encouraged to compare various AGC settings modes to determine the
optimum settings for the scene being viewed.
•
Default/Outdoors: Plateau Value – 250 (640 x 480) and 150 (320 x 240); ITT Mean – 110 (640 x
480) and 127 (320 x 240); Max Gain – 15 (640 x 480) and 8 (320 x 240).
•
Low Scene Contrast (for both resolutions): Plateau Value – 150; ITT Mean – 110; Max Gain – 18.
•
Indoors (for both resolutions): Plateau Value – 100; ITT Mean – 120; Max Gain – 10.
