Propeller Image Aliasing
Posted by ScalarMotion on March 15, 2009
Many people, who have tried taking a picture of a spinning airplane propeller with their cellphone cameras, have been surprised by the outcome. After all, they didn’t expect to see many ghostly propeller blades floating in the air without being totally detached to the airplane. Here is an example: a photograph of a rotating propeller taken by an iPhone.
If you click on the photograph, it’ll take you the forum where it was originally posted. Sure enough, you’ll also see the ususal cries of “photoshop”.
What really happens
The cameras on many cellphones are slow scanning digital cameras. When capturing an image, they do not expose all the pixels at the same time. Rather they expose and capture one row of the sensor before moving onto the next. What this means is that different rows (or columns, depending on the orientation of the sensor) of the image are captured at different times. And during this interval the blades move appreciably. The entire thing is a bit difficult to visualize. So I have created a MATLAB simulation to demonstrate this effect in a slow-motion video. In the video below: a sensor is capturing the photograph of a spinning propeller by exposing one row at a time.
Simulation parameters: Propeller speed = 1800 RPM. Camera shutter speed = 1/15 s.
Notice the similarities between the simulation result and the propeller picture above?!