Lens and shutter speed
When configuring camera 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—if the camera supports manual configuration of the shutter time—the shutter time can be set to a very short time
- Neutral Density (ND) filters or gray filters basically reduce the amount of light coming into a camera. They work as "sunglasses" for the camera. ND filters affect the exposure of images (see Understanding camera exposure)
- 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. If you use 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 formula for calculating the longest suitable shutter time is:
- Vehicle speed in km/h: Shutter time in seconds = 1 second / (11 × max vehicle speed in kilometers per hour)
- 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 for different vehicle speeds:
|
Shutter time in seconds |
Maimum vehicle speed in kilometers per hour |
Maximum vehicle speed in miles per hour |
|---|---|---|
|
1/50 |
4 |
2 |
|
1/100 |
9 |
5 |
|
1/200 |
18 |
11 |
|
1/250 |
22 |
13 |
|
1/500 |
45 |
27 |
|
1/750 |
68 |
41 |
|
1/1000 |
90 |
55 |
|
1/1500 |
136 |
83 |
|
1/2000 |
181 |
111 |
|
1/3000 |
272 |
166 |
|
1/4000 |
363 |
222 |