Milestone LPR 1.0 - Manual (Page 35)

Milestone XProtect
®
LPR 1.0 Administrator's Manual
www.milestonesys.com 35 About LPR cameras
o Smear is an effect leading to unwanted light vertical lines in images; it is frequently linked
to slight imperfections in cameras’ charge-coupled device (CCD) imagers (the sensors
used to digitally create the images). In general, CCD imagers with large surfaces are less
sensitive to smear than CCD imagers with small surfaces. Cameras with complementary
metal
–oxide–semiconductor (CMOS) imagers (a different type of sensors, with a higher
noise immunity) are less sensitive to smear than cameras with CCD imagers.
License plate image with smear

Little light: Too little environmental or external lighting can lead to underexposure.
o Underexposure is when images are exposed to too little light, resulting in a dark image
with hardly any contrast. When auto-gain (see "Unwanted camera features" on page 37)
cannot be disabled or when you are not able to configure a maximum allowed shutter time
(see "Lens and shutter speed" on page 33) for capturing moving vehicles, too little light will
initially lead to gain noise and motion blur in the images, and ultimately to underexposure.
Underexposure can be avoided by using sufficient external lighting and/or by using a
camera which has sufficient sensitivity in low-light environments without using gain. In
general, cameras which have CCD imagers with large surfaces are more light-sensitive
than cameras which have CCD imagers with small surfaces.

Infrared: Another popular way of overcoming difficult environmental lighting conditions is to
use artificial infrared lighting combined with an infrared-sensitive camera with an infrared pass
filter. Retro-reflective license plates are particularly suitable for use with infrared lighting.
o Retro-reflectivity is achieved by covering surfaces with a special reflective material which
sends a large portion of the light from a light source straight back along the path it came
from. Retro-reflective objects appear to shine much more brightly than other objects; at
night they can thus be seen clearly from considerable distances. Retro-reflectivity is
frequently used for road signs, and is also used for different types of license plates.

Weather, etc.: Snow, very bright sunlight, etc. may require special configuration of cameras.

Plate condition: Vehicles may have damaged license plates. License plates may occasionally
have been damaged deliberately in an attempt to avoid recognition. Vehicles may also simply
have dirty license plates.

Camera condition: Camera lenses may accumulate dirt over time.
Physical positioning of cameras
When mounting cameras for LPR, note the following: