D12 Ultra high speed cameras

Topic

An insect's wingbeat cycle occurs so rapidly—typically between ten and a thousand times per second—that only ultra-high-speed cameras can capture its full dynamics, revealing how the wing moves moment by moment throughout each stroke.

To capture the wingbeat cycle at all, the camera must satisfy the Nyquist criterion: the frame rate must be at least double the maximum frequency of the cycle being captured; below this minimum, the camera cannot distinguish the wing's actual movement from a different, apparent motion.

In practice, however, this Nyquist minimum is insufficient to observe the wingbeat cycle in the desired detail. Researchers use far more frames per complete cycle—between ten and one hundred—to resolve fine details such as pronation and supination: the rapid rotation the wing undergoes at the start and end of each stroke.

Increasing the frame rate to obtain more frames per cycle comes at a cost, however: the faster the camera captures frames, the less exposure time is available for each individual frame. This reduced exposure means fewer photons reach each image pixel, degrading image quality by lowering the signal-to-noise ratio.

Furthermore, exposure time is subject to an upper limit imposed by the wing's movement itself: if the exposure time is too long, the wing appears blurred because it has moved a significant distance while the camera was capturing the frame. For a wingtip moving at two meters per second—imaged at a scale of one hundred micrometers per pixel—the exposure time must be less than fifty microseconds to prevent the wing image from appearing blurred. Based on the wing's angular position extracted from each frame, it is possible to calculate—via numerical differentiation—the wing's angular velocity and angular acceleration throughout the entire wingbeat cycle. By combining this angular acceleration with the wing's moment of inertia, one can ultimately derive the aerodynamic and inertial torques acting on the wing during insect flight.

In practice, this same trade-off between capture speed and image quality defines the current capabilities of the most advanced ultra-high-speed cameras for studying the insect wingbeat cycle: they can achieve rates on the order of one million frames per second if resolution is sacrificed, or around one hundred thousand frames per second if a one-megapixel resolution is maintained, leaving the researcher facing the same dilemma between speed and detail that characterizes this entire measurement process.

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