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Shadows cast onto the plate may not be cast on different parts of the field of view making the
plate seem darker in the resulting image.
Auto Level Control (ALC) settings
The Drive5 supports real-time light sensing using the internal Auto Level Control (ALC) light sensing
functionality for exposure control of the camera. The ALC settings are found in the ADR Control
section of the EN Setup camera properties (Section 5). ADR Control is set to ALC by default and
should not be changed by the user. The specific ALC control settings are highlighted in Figure 25 and
are explained in the paragraphs that follow.
Note that the ADR Shutter Max setting is also used by ALC. It specifies the longest (slowest)
allowable shutter speed for auto exposure purposes. Recommended values range from 0 (30 µs) to
855 (1000 µs). Other ADR-related settings are for use with external light sensing equipment, which is
not supported by Drive5.
The settings specific to the ALC mode of operation are marked in the following figure.
Figure 25. ALC settings
Camera ADR Control
– The mode has to be set to ALC in order to enable the internal light sensing.
ADR Shutter Max
– See explanation on previous page.
ALC Target Value
– This is the “set-point” for the closed-loop control of the camera exposure. This
value represents the target average value for the ALC Region of Interest (ROI). It is a 10-bit value
which is then four times higher than the average value measured in the output images from the
Drive5 (which are all in 8-bit resolution). So if the ALC Target Value is set to 200 for instance, then
the average intensity of the ALC ROI in the output images will be around (200/4)=50. The ALC Target
Value should be kept between 150 - 250 in order to insure a proper brightness and contrast of the
resulting images from the camera. By keeping the value low there will be a larger “margin” so that
over-exposed license plates can be avoided.
ALC Damping Factor
– This setting controls the speed of the closed-loop control of the camera
exposure. It represents the percentage of change allowed by the control loop for each individual
frame. A low value (10-25) makes the ALC more stable and will give less fluctuation in the exposure.
A high value (75-90) will make the ALC very responsive but at the same time the brightness of the
output images will vary a lot depending on the brightness of the passing vehicles. It is recommended
to keep the ALC Damping Factor as low as 25% or even lower in order to prioritize stability over
responsiveness of the ALC.
