Tau 640 User’s Manual
3—Basic Operation of the Tau 640 and GUI
TAU-0640-00-10, version 110
June 2011
3-23
AGC Modes:
The Tau 640
provides five AGC algorithms for Image-Optimization:
Automatic
: This is the most sophisticated algorithm
and for most imaging situations, the best all-around
choice. This factory default along with the default
parameter settings should be used in general imaging
situations. In
Automatic
, image contrast and
brightness are optimized automatically as the scene
varies. This mode provides an AGC which is based on
a histogram-equalization algorithm. Controls for the
ITT Mean
(gray scale mid-point),
Max Gain
(AGC gain)
and
Plateau Value
are enabled.
The histogram equalization used in the automatic mode scales the 14-bit to 8-bit
transfer function based on the number of pixels in a bin. The more pixels in a bin, the
higher the gain. But the Plateau value is the pixels/bin limit when the transfer function
is maximized. Normally 250 is the plateau value for imaging cameras when more
contrast is desired.
This algorithm analyzes the scene content in real time and redistributes the dynamic
range of the scene. The goal of this redistribution is that every one of the 255 bins of
display dynamic range has an equal number of pixels in it. This method tends to give
better scene contrast under conditions where the scene statistics are bimodal (for
example, a hot object imaged in a cold background. It should be noted that the heat
range in a given scene is not divided evenly across the grey levels sent to be displayed.
Instead, the AGC algorithm analyzes the scene and distributes the dynamic range to
best preserve statistical scene content (populated regions of the histogram) and
display a redistributed scene representation.
Once Bright
: In this mode, the brightness will be set once when the mode is selected.
The brightness (level) is calculated as the mean of the current scene when the
Once
Bright
button is selected. The scene is mapped to the analog video using a linear
transfer function. Image contrast can be adjusted by the
Contrast
slider. This is the
only user adjustable parameter. Upon entry into the once bright mode, the currently-
stored value of contrast is applied (i.e. the power-on defaults or the last saved values).
Auto-Bright
: In this mode, the brightness (level) is calculated as the mean of the
current scene just as in
Once Bright
mode. The difference with
Auto-Bright
is that
the values selected for the start and end of the linear transfer function are
automatically updated in real-time, not only at the start of AGC mode selection. The
Brightness Bias
offsets the displayed image in intensity. Upon entry into the auto
bright mode, the currently-stored values of
Contrast
and
Brightness Bias
are applied
(i.e. the power-on defaults or the last saved values).
Manual
: In this mode, image
Contrast
(gain) and
Brightness
(level) are entered
completely manually via the sliders. The scene is mapped using a linear transfer
function. Upon entry into the manual mode, currently-stored values of brightness and
contrast are applied (i.e. the power-on defaults or the last saved values).
