― 27 ―
ALC Maximum Gain Level
– This sets the maximum amount of gain the ALC will use to adjust the
camera exposure. The value is a normalized number between 0% and 100% gain. This will
essentially limit the maximum worst-case gain and hence limit the amount of noise in the images
when it becomes dark.
ALC ROI top/bottom/left/right
– these four settings specify the area from which the ALC calculates the
average measurements for the control loop. This area should be selected so it covers the background
(asphalt or concrete road) area where the vehicles will be located when the images are captured.
This will in most cases be close to the Vehicle Detection area. If the Vehicle Detector is in Rear Shot
mode then the ALC area should be put a bit higher than the detection area (since the vehicles are
captured when they are leaving the detection area). If the Vehicle Detector trigger line is being moved
up or down then the ALC ROI should be moved the same amount in the same direction.
4.8 High dynamic range (HDR) settings
The imaging sensor used in the Drive5 supports multi-slope integration. This makes it possible to
increase the dynamic range of the sensor, and helps avoid over-exposed license plates in challenging
light scenarios where the ALC control is unable to correctly assess the brightness of the license plate
and the vehicles when the images are captured. The challenging light scenarios are often caused by
shadows being cast onto the scene in a way where the ALC measures the average of the road
surface in shadow
– but where the vehicle will be in the direct sun and thereby inadvertently cause
the vehicle to become over-exposed!
However, be aware that multi-slope HDR reduces the contrast in the brighter parts of the image and
also imp
acts the Drive5’s ability to properly process the color of the scene. Images will tend to have a
pinkish hue in the brightest areas, which will be further exaggerated if white balancing is used.
Therefore, it is recommended that the High dynamic range remains disabled (default setting) unless
required by the lighting conditions. Furthermore, when enabled, users should set the controls to apply
only as much additional dynamic range to the image as needed to resolve the exposure issue.
The HDR settings are adjusted directly in EN Setup camera properties using Section 10
– “High
Dynamic Range Control.”
Figure 26. HDR settings in EN Setup
HDR Enable - Selects if the multi-slope integration is enabled or disabled. Default is Disable.
HDR Slope 1/2 Reset percentage - Select the multi-slope reset time for the second and third slope of
the multi-slope integration of the sensor. The reset time is set as a percentage of the total exposure
time so it will automatically be adjusted when the exposure time is changed by the ALC. The higher
the value, the later the reset, which in turn increases the dynamic range.
The multi-slope integration will change the sensor
’s response to the light as shown in the following
graphs:
