Specifications

This darkening effect is caused by the auto iris in the camera. When it sees
the bright hot refection or headlights, the iris in the camera will close down the
opening in the lens. An auto-iris takes the light that it sees and tries to determine
how to set the opening in the lens for the best picture illumination. It can use a
peek measure of the light level entering the lens, or when you engage the “Back
Light” feature it will take an average of the light ignoring the hot spots.
Unfortunately the back light feature in most cameras will not handle headlights or
windshield flashes and only reduce the problem somewhat.
A CLUE.
A clue to the answer comes from the relatively good observations made
during daylight. During the day most of a cameras received light comes from
defused light and even if a car has its headlights on the camera hardly notices it,
because the energy from the headlights are just a small portion of the light
entering the lens.
HOW TO MAKE IT WORK.
To reduce the blinding effect of reflected light sources you can install a light
source pointing directly at the car from the direction of the camera to where you
wish to read the license plate. The light source must be bright enough to
overwhelm the headlights on the car and stop the cameras auto iris from closing
down due to the increase in direct light. To avoid blinding the driver of the car, you
must use an infra red light source. People cannot see light in the infra red
spectrum, but cameras can. In fact most CCTV cameras are very sensitive to the
infra red light spectrum and will work quite well. Infra red light sources are
available from the same manufacturers that you buy your cameras from.
Try a test the next time you have a license plate reading camera. Arrange
the camera in the normal way, and see what happens when the cars lights are
turned on. Notice the darkening of the picture on your monitor. Then point a
regular bright white spot light at the car from the direction of the camera. Notice
that the scene gets brighter but also the headlights have less of an effect on the
iris of the camera and you can see more of the details of the car, with much less
blinding effect.
The main point here it to overwhelm the direct light coming from the car with
a source of light coming from the camera. If you do this, license plates can be
seen easily even under the worst conditions.
CCTV TRAINING MANUAL PAGE 9 OF 28