When light gets scattered as it passes through a translucent material, the emerging pattern of “speckle” looks as random as static on a television screen with no signal. But it isn't random. Because the light coming from one point of an object travels a path very similar to that of the light coming from an adjacent point, the speckle pattern from each looks very much the same, just shifted slightly. Because modern cameras can record hundreds of millions of pixels at a time, only a single exposure is needed to make the statistics work.
While this approach can reconstruct a scattered image, it has limitations. The object has to remain motionless and the scattering medium has to be constant. A new approach to memory effect imaging extracts a sequence of images from a single, long exposure using a coded aperture. As long as this pattern is known, scientists can computationally extract what the original image looked like.
The technique uses a sequence of coded apertures to stamp which light is coming from which moment in time. But because each image is collected on a single, long photographic exposure, the resulting speckle ends up even more of a jumbled mess than usual. Today's cameras have such precise resolution that there is still enough of a pattern to computationally tease them apart.
In an experiment, a simple sequence of four backlit letters appeared one after the other behind a coded aperture and a scattering material. The shutter of a 5.5-megapixel CCD camera was left open for more than a minute during the sequence to gather the images. While the best results were achieved with a 100-second exposure time, good results could still be obtained with much shorter exposure times. After only a few seconds of processing, the computer successfully returned the individual images of a D, U, K, and E from the sequence (see figure).
The researchers then showed the approach also works when the scattering medium is changed, and even when both the images and scattering mediums are changing. The best results were achieved when the letters appeared for 25 seconds each because the intensity of the backlight was not very high to begin with and was even further diminished by the coded aperture and scattering material. But with a more sensitive camera or a brighter source, the approach could be used to capture live-action images