A guide to avoiding motion blur with LPR cameras

April 04, 2017

LPR Legends and Myths

Shutter time, shutter speed, exposure time are the key concepts for this topic. But how do you choose the right setup for your LPR system?

The first thing I learned in photography is that for the best image quality you need longer exposure time. It was followed closely by the second lesson: long exposure time isn’t the right choice when the object is moving fast. So as the next step it was time to play with the parameters on a trial-and-error basis to find the right balance.

The only easy situation there is when we have the right amount of ambient light or a powerful light source beside the camera. In these cases even a short time is enough for the sensor to collect sufficient light reflected from the photo objects. If the environment is darker, the sensor needs longer time to collect the same amount of light. If the time required for the sensor is too long, the rapid movement of the vehicle causes motion blur.

So we come down to the parameter – that we also want to tweak a bit – called shutter speed or shutter time. In practice, these two have the same meaning. In photography shutter speed is used and allotted in fractions of a second. For example: 1/1000s shutter speed means that the light reaches the sensor for 1/1000 seconds (1 ms). Sometimes shutter time is also used as a value given in microseconds. In the latter measurement system, 1/1000 seconds means 1000 us. For calculations, you will most probably find using shutter time easier.

You will also see that for highway LPR projects, the shutter speed needs to be really fast compared to usual situations in regular photography, even sports events. The following tables provide some reference values. Note that the “best value” always depends on the circumstances including the quality of the sensor as well as the angle of the camera axis and the speed vector of the vehicle.