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Good camera position:
If there is no barrier, the installation position for the camera is
easier to choose. Nevertheless, the same principle applies:
any permanent or temporary concealment of the plate should
be avoided (other vehicles, pedestrians, vegetation etc...).
Natural illumination restrictions
Excepted indoor installations with constant illumination
(covered car parks), the lighting conditions of all other
installations vary considerably according to the time of day or
night, the weather conditions and the seasons.
The Sony camera has no automatic diaphragm but the
contrast of the image is automatically adjusted by changing
the speed of the shutter and the gain values. This means that
the camera will automatically adapt the contrast of the image
regarding the overall illuminating conditions so that the image
is neither under nor over exposed. The addition of an infrared
filter associated makes the filming less sensitive to variations
in illuminating conditions.
However the shutter speed adapts according to the global
illumination of the image, although it is the number plate
which is decisive for the performance of the system. As a
result, two types of situations can cause a loss of contrast of
the characters of the plate and a significant reduction in the
system performance:
• When an image is bright all over (white vehicle and/or a
bright background because of clear skies), but the plate is
in the shadows, the shutter speed increases and the plate
becomes strongly under exposed (black plate).
• When the sun reflects on the number plate and illuminates
the CCD image sensor of the camera but the image is not
otherwise over exposed, the shutter speed decreases and
the plate becomes over exposed (white plate).
In certain cases, the system does not work at all.
In order to avoid these effects which can happen
outdoors, the installer has one option:
• To choose a location for the camera and a filming axis
of which he is certain that at no time of the day or year
that any of the situations described can occur. The
shadow cast by a tree or buildings can also provide an
efficient protection.
• As a matter of fact, ideal installation is indoor.
For outdoor installation, the installer must unsure that
there is an infrared reflective barrier in the view of the
camera for the camera auto reinitialisation process which
is done each 30 minutes without any recognition done.
In addition to the barrier, the installer can use:
Bumps with infrared reflective paintings
-
Infrared reflective paintings on concrete
-
Markers usually called "Cats eyes"
-
Headlight illumination
restrictions
Another potential source of excessive illumination is from the
vehicles headlights which can cause the same sort of
performance problems in the same way as described above.
To avoid risk of over exposure by the car's headlights, Sony
recommends either placing the plate cameras at a height
where they are not in the headlights axis (without excessive
lateral angle, see chapter regarding lateral angles), or adding
an infrared filter to the lens in order to reduce the halo effect
around the headlamps (infrared filter of the type high pass
which cuts the light at a wavelength of 800 to 830
nanometres).
Physical protection restrictions
The risks of damage of the cameras can be related to theft or
vandalism, or to the carelessness of drivers of the vehicles
controlled.
The best method to avoid all damage is to install the camera
at a height, out of reach of passers-by, and away from the
path where the vehicles to be controlled are circulating (while
respecting the angle constraints in the section below).
Angle & Speed restrictions
Standard
&
Recommended
reliability guaranteed:
Vehicle stops at a barrier
-
The barrier has straights infrared reflective
-
The camera respect the angle restrictions
-
The camera respect the lens adjustments
-
The camera film both the barrier & the vehicle
-
Because cameras cannot generally be placed directly in front
of vehicles, cameras always have a lateral angle to the
plates. For security reasons, and to avoid direct illumination
of the camera's CCD image sensor by the headlights,
cameras also have a vertical angle with the plates.
installation,
99%
Page 16 / 42
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