Milestone LPR 1.0 - Manual (Page 37)

Milestone XProtect
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LPR 1.0 Administrator's Manual
www.milestonesys.com 37 About LPR cameras
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For single line European style plates (plate width 52 cm; stroke width around 1 cm) this
typically means that the plate width in the image must be at least 170 pixels.
When vehicles are moving when recorded, and an interlaced camera is used, only a half of image can
be used (only the even lines). This means that the resolution requirements almost must be doubled.
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For single line US plates (plate width 30.5 cm; stroke width around 0.7 cm) this typically means
that the plate width in the image must be at least 215 pixels.

For single line European style plates (plate width 52 cm; stroke width around 1 cm) this
typically means that the plate width in the image must be at least 280 pixels.
What is interlacing? Interlacing is a method determining how an image is refreshed when shown on
a screen. With interlacing, the image is refreshed by first scanning every other line in the image, then
scanning every opposite line, and so forth. This allows for a faster refresh rate because less
information must be processed during each scan. However, in some situations, interlacing may cause
flickerin
g, or the changes in only half of the image’s lines for each scan may be noticeable.
Interlaced image of a moving car
Unwanted camera features
When configuring cameras for LPR, note the following:

Automatic gain adjustment: One of the most common types of image interference caused by
cameras is gain noise.
o Gain is basically the way in which a camera takes a picture of a scene and distributes light
into it. If light is not distributed optimally in the image, the result is gain noise.
Controlling gain requires that complex algorithms are applied, and many cameras have
features for automatically adjusting gain. Unfortunately, such features are not always
helpful for cameras used for LPR. It is therefore recommended that you configure your
cameras’ auto-gain functionality to be as low as possible. Alternatively, disable the
cameras’s auto-gain functionality altogether.
License plate image with gain noise
Tip: In dark environments, gain noise can also be avoided when your external lighting is
sufficient.
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Automatic enhancement: Some cameras use contour, edge or contrast enhancement
algorithms to make images look better to the human eye. However, such algorithms can