Last night, the WashU Science-on-Tap lecture series restarted for 2016. The speaker was Dr. Lihong Wang and his talk was titled, “Compressed Ultrafast Photography”. He is the creator of the world’s fastest 2D receive-only camera that can capture light propagation at light speed, but he started his talk off much more slowly with Eadweard Muybridge’s galloping horse movie. At the Palo Alto racetrack in 1878, Muybridge took eleven cameras, each with a string attached to their shutter and then strung taught across the track. A galloping horse came riding by, firing in sequence each camera’s shot. I’ve downloaded the resulting animated GIF from Wiki. Interestingly, the Wachowski brothers would revisit this photographic technique in their Matrix movies with their so-called “bullet time” effect. Wiki describes this visual effect as detaching the time and space of the viewer from that of the subject. In many ways Dr. Wang ends up doing something similar.
What exactly does Wang and his team do? They make movies, like Muybridge and Wachowski. Their movies have frame-rate of 30-60 frames-per-second. Wang’s movies refresh between a billion and a trillion times per second. This is fast enough to watch light move across the screen. The speed of light is about 9” per nanosecond. The light source for his movies comes from a laser that is fired for just one trillionth of a second, a picosecond. It looks like a little red blob that in different movies is either reflected or refracted and in a third two blobs race each other, the faster one in air and the slower one in plastic, illustrating the different speeds of light that different media have. In his newest and best movie, he has sped up his game by a 1000 times. Here his pulse streaks across the screen and strikes a Phosphorescent screen that then begins to glow. This movie got him the cover article of Nature.
What I find most fascinating about Wang’s research is that he makes these movies with off-the-shelf equipment. His digital camera is similar to the one in your phone. The events that he is photographing occur so quickly that there is no time to get any information off the CCD chip that is your digital camera’s eye. So, the chip’s onboard memory size becomes the major limiting factor to what he can do. He invokes some rather fancy math to help alleviate this limitation. A CCD chip is a 2D array that is designed to record the x and y dimensions of an image. What Wang does is have his CCD record a single x-line of pixels from the image and then use the y-dimension to record the time history of that single x-line. What he gets is a 1D image and its time history. Then in true Muybridge and Wachowski fashion he gangs together a bunch of these CCDs, each one oriented to get a different y-line of the original image and the result is a movie that is traveling at light speed and a career traveling faster.