How Long Does It Take For A Rolling Shutter To Completely 'ROll'?

Any camera manufacturers around to answer this?

The question grew out of a discussion of the benefits of global vs rolling shutter in our License Plate Recognition Axis App Tested.

A rolling shutter does not expose the entire frame at once. It 'rolls' across pixels / rows. The question is: How much time does it take to 'roll' across an entire frame? Does it vary? Can it be speed up? What is the fastest it can roll?


John,

here is a useful knowledge base article.

But to answer your question, the "roll" is directly related to the frame rate, so the higher the frame rate, the faster the "roll".

The difference in when the first row of the image is exposed vs. the time the last row is exposed is roughly 1 second divided by the frame rate.

Vlad

Vlad,

Thanks. So let's say we have a 1/2000s shutter speed. How long does it take the rolling shutter camera to 'roll' the entire frame?

John,

unfortunately shutter speed does not have an impact on the "roll".

Shutter speed is the duration that light is collected for, while the "roll" depends on the speed with which you read out the imager which is directly related to the frame rate.

Of course, the maximum shutter time can't be longer than 1s divided by frame rate, but shutter times can be quite short and have no impact on the frame rate and therefore no impact on the "roll".

I hope that this makes sense.

Vlad

Ok, then what is the roll time for a 30fps camera? Does this vary by sensor or is it configurable?

John,

the "roll" at 30FPS would be about 33ms (1s divided by frame rate). The faster the sensor the smaller the roll, so if the sensor is running at 120FPS the the "roll" is about 8ms.

Note that there are security cameras out there where the sensor runs at a particular frame rate, but is not sending every image. If you want to reduce the effects of the rolling shutter, you should select the highest frame rate possible and adjust the "broadcast" frame rate according to your needs.

Of course, the best is to use a camera with global shutter and have triggering capability, so that you get the vehicle exactly where you need it and not worry about rolling shutter. Monochrome cameras also provide higher resolution images, as you don't have to worry about the resolution loss due to the Bayer pattern used to get color.

I could probably do a full course on how to use machine vision cameras in traffic, biometrics and other security applications :)

Regards,

Vlad