Milestone LPR 1.0 - Manual (Page 33)

Milestone XProtect
®
LPR 1.0 Administrator's Manual
www.milestonesys.com 33 About LPR cameras
Contrast
When determining the right contrast for LPR, consider the difference in gray value (when images are
converted to 8-bit grayscale) between t
he license plate’s characters and the license plate’s
background color:
Good contrast
Acceptable contrast; recognition is still possible
Too little contrast; recognition is not possible
Pixels in an 8-bit grayscale image can have color values ranging from 0 to 255, where grayscale value
0 is absolute black and 255 is absolute white. When you convert your input image to an 8-bit grayscale
image, the minimum pixel color value difference between a pixel in the foreground and a pixel in the
background should be at least 15.
Note that "noise" in the image (see "Unwanted camera features" on page 37), the use of compression
(see "Unwanted camera features" on page 37), light, or similar can make it difficult to determine what
the colors of a license plate’s characters and background are.
Lens and shutter speed
When configuring cameras’ lenses and shutter speeds for LPR, note the following:

Focus: Always make sure the license plate is in focus.

Auto-iris: If using an auto-iris lens, always set the focus with the aperture as open as possible.
In order to make the aperture open, you can use neutral density (ND) filters or
—when the
camera supports manual configuration of the shutter time —the shutter time can be set to very
short.
o Neutral Density (ND) filters or (gray filters) basically reduce the amount of light coming
into a camera; effectively working as "sunglasses" for the camera. ND filters thus affect the
exposure of images (see "Physical positioning of cameras" on page 35).

Infrared: If using an infrared light source, focus may change when switching between visible
light and infrared light. You can avoid the change in focus by using an infrared compensated
lens, or by using an infrared pass filter. Note that when using an infrared pass filter, an infrared
light source is required
—also during daytime.

Vehicle speed: When vehicles are moving, cameras’ shutter time should be short enough to
avoid motion blur. A rule for determining the longest suitable shutter time is: