That light travels in straight lines, is a well-known fact of physics. This property of light prevents us from seen around corners, unless helped by a mirror. However, scientists have not allowed this limitation to prevent them from developing a far from average camera that can see around the average corner, without using x-rays or mirrors.
Genevieve Gariepy, along with his group of scientists, has developed a latest device with a detector that treats walls and floors as if they were virtual mirrors. Along with some clever data processing techniques, this amazing device has the power and is able to track moving objects that are out of its direct line of sight.
A mirror works by reflecting scattered light from an object. As the surface of a mirror is usually shiny, all the reflected light travels in a well-defined angle. As light scattered from different points on the object travel at the same angle even after reflection, the eye sees a clear image of the object. On the other hand, light reflected from a non-reflective surface is scattered randomly – preventing us from seeing a clear image.
At the University of Edinburgh and the Heriot-Watt University, researchers have found a way to tease out information on an object even when light from it is scattered randomly by a non-reflecting surface. They have published their method in Nature Photonics and it relies on the technology of laser range finding. This technology measures the distance of an object by the time it takes a light pulse to travel to the object, scatter and return to the detector.
In principle, the method of measurement is similar to range finding by ultrasonic technology. Only, instead of sound pulses, researchers use pulses of light. In practice, researchers bounce a laser pulse off the floor making it scatter. A tiny fraction of the laser light striking the object backscatters on the floor. The detector records the back scattered light from the virtual mirror spot, close to the initial spot where the laser strikes.
The speed of light is a constant and a known factor. Therefore, the device triangulates the position of the object by measuring the time interval from the start of the laser pulse to the scattered light reaching the patch on the floor. There are a few difficulties involved with this method.
The light levels the detector has to detect on the virtual spot are extremely low. Moreover, the timing measurement has to be accurate to within 500 nanoseconds or 500 billionths of a second. To overcome both these obstacles, researchers had to go in for some serious laser and detector technology. They had to use a laser pulse only ten femtoseconds long for the timing measurements.
The ultra-sensitive camera uses a one-pixel avalanche diode array called as SPAD to detect the image on the patch of the floor. The combination acts as an ultrafast stopwatch for recording the time the light pulse arrived after scattering. All this happens within a few billionths of a second.
It helps if the out of sight object the device is trying to locate is moving, while the nearby objects are not. The moving object then generates an image that changes with time and this can be filtered from the unvarying background.
