Rolling CMOS Shutters and Curved Wiper Blades

Ben Mesander

One time last winter I shot a photo with my camera phone out the windshield of my car, and got a strange image with curved wiper blades:

Blizzard

No, my wipers don’t look like this!

I’ve been meaning to track down the reason why this happened. It is clear that cellphone cameras don’t usually use mechanical shutters—so what is going on in this picture?

It turns out there are two commonly used sensor types for digital camera applications: CCD and CMOS. CMOS sensors often have what is called a rolling shutter. With this technology, a row or column of pixels at a time is activated in the sensor “rolled” across the image. When the time required for the row or column of active pixels is comparable to the motion in the frame, bizarre effects occur similar to the old analog technique of slit scan photography. Low-light conditions, like the one in which my wiper blades showed up curved, cause the electronic CMOS shutter to roll more slowly, thus lengthening the exposure.

CMOS sensors are one of two popular options for consumer cameras, the other one being CCD sensors. I found a good article here on the imaging tradeoffs for these sensors.

Finally, a similar effect can be seen in video camera sensors that use rolling shutters. Here’s one I think is particularly cool—this was evidently recorded (by a guy named “mikelo”) using a Nokia N95 cellphone:

Ben Mesander has more than 18 years of experience leading software development teams and implementing software. His strengths include Linux, C, C++, numerical methods, control systems and digital signal processing. His experience includes embedded software, scientific software and enterprise software development environments.

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