Milestone Analytics 2.2 Dacolian LPR - Manual (Page 38)

Milestone XProtect Analytics 2.2 – Dacolian LPR; Administrator’s Manual
www.milestonesys.com Page 38 Camera Tips & Tricks
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, the use of compression, or similar can make it difficult to determine
what the colors of a license plate’s characters and background are (see also Physical Environment
on page 39 and Undesired Camera Features on page 42).
Lens & Shutter Speed
When configuring cameras’ lenses and shutter speeds for license plate recognition, bear in mind
the following:
 Focus: Always make sure the license plate is in focus when images are to be used for
license plate recognition.
 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 ND filters or—when the camera
supports manual configuration of the shutter time—the shutter time can be set to very
short.
o ND Filters (a.k.a. Neutral Density 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 also Physical Environment on
page 39).
 Infrared: If using an infrared light source, note 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 while recorded for license plate recognition,
cameras’ shutter time should be short enough to avoid motion blur. A rule of thumb for
determining the longest suitable shutter time is:
o Vehicle speed in km/h: Shutter time in seconds = 1 second / (11 × max vehicle
speed in kilometers per hour)
o Vehicle speed in mph: Shutter time in seconds = 1 second / (18 × max vehicle
speed in miles per hour)
where / denotes “divided by” and × denotes “multiplied by.”
The following table provides guidelines for recommended camera shutter speeds at
different vehicle speeds: