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Introduction 13
IIT Camera System Manual
The concept of the IIT Camera is to exploit the retro-reflective properties of the vehicle license
plates. A retro-reflective surface returns the incoming light in the same direction it comes from.
The IIT camera is equipped with a number of emitting infrared diodes that are positioned evenly
across the camera lens. The light is frequency modulated. A beamsplitter is mounted in front of the
CCD for distribution of the incoming light to both the CCD and an array of optical fibers. The optical
fibers are connected to photodiodes on the I/O board.
Figure 3. Camera front with beamsplitter and fibers
Infrared modulated light is continuously transmitted from the camera’s IR illuminator. The reflected
signal caused by a license plate on a passing vehicle is correlated for correct modulation frequency,
phase, amplitude and the number of high samples. If the trigger criteria are validated as correct, a
trigger is generated and an image is grabbed.
The light sensor values – shutter, gain and pedestal that ensure optimal image acquisition properties
– are sampled from the reflected signal simultaneously.
Due to the optical inverting properties of the lens, the retro-reflective sensitive part of the picture
is positioned in front of the lower part of the CCD even though the fiber array is located in the top.
Figure 4. The sensitive region of the picture area
This illustration shows the sensitive region and the way the vehicles must pass it to make the system
function.