CHEETAH Hardware User’s Manual
Imperx, Inc.
Rev. 6.2
6421 Congress Ave.
7/7/2015
Boca Raton, FL 33487
+1 (561) 989-0006
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exposure period for the bright areas of the image. If more contrast is needed in these
brighter regions of the image, try increasing P1. View a histogram of the output to get a
sense of how changes in P1 affect the output.
In a scene with several different intensity levels, two knee points can be helpful. If two
knee points (3 intensity slopes) are required, then both E1 and E2 (E1 is the exposure
period for bright pixels and E2 is the exposure period for the Very Bright pixels.) must
be used. Below is an initial setting for applications that have several areas of varying
intensities.
1)
Set the overall camera exposure period so areas in the darkest regions of the image
have contrast and the bright areas of the image are overexposed.
2)
Set Texp - E1 to 10% of the overall exposure period and Set Texp - E2 to 1% of the
overall exposure period
3)
Set P1 to 40%, P2 to 70% and P3 to 100%.
Extended dynamic range images will look ‘flat’ and ‘dark’, because the bright areas of
the image have been compressed into the camera’s output range. Viewing the histogram
of the output is a very useful tool for seeing the impact of changes to P1, P2 and P3
settings. The LUT function can be used to stretch the image and increase the image
‘brightness’, if needed, keeping in mind that data is not linear and is partitioned into
different exposure ranges.
2.11.4 WDR at maximum frame rates (image artifact prevention)
To provide the highest frame rates, the exposure time and readout time overlap- meaning
while Frame 1 is being readout out, Frame 2 is being exposed. The exposure period for
Frame 2 is always positioned at the end of the Frame 1 and any additional WDR exposures
(E1, E2 and E3) are positioned at the very end of the overall exposure time.
When the application demands maximum frame rate, then the exposure period of Frame 2
overlaps with the readout of Frame 1. (For lower frame rate applications, the exposure for
Frame 2 will occur after Frame 1 is readout (non-overlapping)). An image artifact (a faint
horizontal line) may be seen at the beginning of the WDR (E1, E2 and E3) exposures,
if
the
WDR exposures for Frame 2 overlap with the Frame 1 readout time. At maximum frame
rate, these artifacts typically occur very near the bottom of the image.
For example, suppose the camera is providing a system constrained maximum frame rate of
50 fps (20,000 micro-second frame time). Let’s also suppose that the exposure time (Texp)
is set to 5,000 micro-seconds and two additional WDR exposures (E1 and E2) are used. E1
is set to 4,000 micro-seconds (a WDR exposure of 1,000 micro-seconds) and E2 is set to
4,900 micro-seconds (a 2
nd
WDR exposure of 100 micro-seconds). Let’s also assume a full
4000 x 3000 resolution image is readout from the C4080 camera and Frame 1 begins
reading out at time equals 0 seconds. In this example, the exposure for Frame 2 begins 15,
