Visibility of license plates.
When designing security television systems, the task of estimating the maximum size of the horizontal field of view at which manual or automatic recognition of a license plate is still possible often arises.
The probability of correct recognition of a car license plate depends on many factors that have an impact directly at the location where the video camera is installed:
installation height and angle of the video camera,
level and quality of illumination of the license plate,
condition of the license plate surface,
air quality (especially when using long-focus lenses), etc.
The quality of the image of a license plate displayed on a monitor screen is affected by the following parameters of the equipment used:
the resulting resolution of the video system (video camera, monitor, signal processing devices, etc.),
the quality of the lens used,
the scanning method used (interlaced or progressive scanning),
possible blockage of high frequencies by the transmission path, if analog signal transmission via cable is used.
If the recognition of vehicle license plates is carried out by an operator, then the quality of recognition in the security room is affected by:
the psychophysiological state of the operator (in particular, his age, vision, fatigue),
the level and quality of lighting in the security room,
the size of the monitor screen,
the distance from the monitor to the operator, etc.
The influence of the listed factors on the recognition result is difficult to evaluate analytically, therefore, to solve this problem, it may be recommended to use a special table developed by the Security Bridge:
http://security-bridge/download/licence_plates.pdf
This table allows you to estimate the maximum horizontal field of view of a video camera when it is positioned frontally relative to the center of a license plate, without going to a real object, without full-scale testing using real license plates.
It is clear that in reality the level of recognition of license plates will be worse due to the above factors at the location of the video camera. It should also be taken into account that the lens used to view the table will differ from the lens used at the site to view license plates.
However, with all this, this table allows you to take into account the influence of the equipment used, the physiological capabilities of the operator, the quality of the automatic license plate recognition device — and this is already a lot. In addition, with the help of this table, you can evaluate the quality of previously recorded and presented to the operator from the archive images of license plates.
To work with this table, you should download it from the site and print it out using a laser printer on a standard sheet of A4 paper (when printing, you should select zero margin values in the page parameters).
To use the table correctly, it must be positioned perpendicular to the axis of the video camera lens at such a distance that the reference marks (black triangles at the top and bottom of the table) coincide with the top and bottom edges of the video monitor screen. In this case, the license plate will be displayed on the monitor screen exactly as if it were installed on the car, if the horizontal field of view corresponds to that indicated on the left in the table.