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Installing the VIS
5
VISCAM 1000 Systems
(a)
Single Lane or Dual lane considerations
Figure 3 shows a typical single lane configuration. However, with the VISCAM 1000’s higher 5Mpixel
resolution, it can now cover up to two lanes of traffic.
The VISCAM 1000’s varifocal (zoom) lens is adjusted during installation to create the following conditions:
Standard for US style of plates: 135 pixels across a 1 foot wide license plate placed 2 feet above the ground
level at the nominal trigger position (ground loop, etc.). With typical US lane coverage (14’), the VISCAM
1000 will yield ~180 pixels/plate. If maintaining the 135 pixels per plate, the lane coverage is now 18’.
Standard for European style of plates: 140 pixels across a 50 cm wide license plate placed 60 cm above the
ground level at the nominal trigger position (ground loop, etc.). The coverage is now up to 9 meters, which
can definitely cover two lanes.
(Note: this can differ for other regions or plate styles.) See Section 4 for set up instructions. This resolution
maximizes license plate reader, vehicle matcher, or other image processing techniques performance. The
highest performance from the subsystem is achieved when the variation in plate size is kept to within ±5%,
which typically means that the vehicle trigger accuracy should be within ±1 foot/30 cm (@ trigger plane
defined by client.) at all speeds. Low latency vehicle triggering is very important and should not be
overlooked. Please contact JAI for support on this issue if needed.
(b)
Camera tilt considerations
The requirement to freeze the motion of high-speed vehicles limits how steep or shallow the tilt angle of
the camera may be. For example, it is important to prevent the horizon from appearing in the image, and
thereby allowing the sun to blind the camera. For over lane installations, a camera tilt between 20° to 30°
is recommended, with 25° being considered the optimal angle. This angle of tilt is the best compromise
between minimizing visibility blockages caused by closely spaced vehicles and maximizing plate visibility
for plate mounts that are slightly recessed or tilted downwards.
(b) Asynchronous triggering considerations
An embedded vehicle detector is employed to cause the camera to capture an image at the precise
moment the vehicle is in the best position to image both the vehicle and its license plate. If an external
triggering device is used (i.e. ground loop) the delay between the time the vehicle passes the trigger
position on the road and when the trigger signal actually reaches the VISCAM 1000 must be kept to a
minimum to prevent high-speed vehicles from moving out of the area viewed by the camera before the
image is snapped.
(c) Camera height versus trigger distance considerations
It is critically important to select the correct distance between the camera and the location on the road
where the camera is triggered to capture an image. Minimizing the cost of installation is usually also an
important concern. This means that whenever possible, it is best to use existing structures or previously
installed elements. Contact JAI for support on setting/validating site geometry.