Milestone Analytics 2.2 Dacolian LPR Instrukcja Użytkownika Pobierz pdf (Strona 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:

1 2 ... 33 34 35 36 37 38 39 40 41 42 43 ... 48 49

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Milestone Analytics 2.2 Dacolian LPR Instrukcja Użytkownika Strona 38