If someone asks you how can you see an object around a blind corner, the most spontaneous reply that you could come up with would be “with the help of a mirror!”. Well, a group of scientists led by Genevieve Gariepy has taken a similar approach as well.
The working of a mirror: the shiny surface of a mirror reflects scattered light from an object/light source at a well-defined angle towards our eye. This is because that light scattered from different points on the object, in the case of a mirror, is reflected at the same angle, and so, our eye sees a clear image of the object. In contrast to this, in the case of a non-reflective surface light gets scattered randomly in all directions, and hence, creates no clear image.
Scientists are inspired from the same. What they did – they developed a state-of-the-art detector which, with smart data processing techniques, can turn walls and floors into a “virtual mirror”, thus being able to locate and track moving objects, even one that are out of direct line of sight. The laser range-finding technology further helps assist in the same.
So, how does the laser technology assisted camera work?
LASER stands for Light Amplification by Stimulated Emission of Radiation. The type of laser we are talking about over here is mainly infrared, semiconductor, GaAs laser diode. The generated light energy has a wavelength of approximately 900 nanometers, with a beam divergence of 3 milliradians – equal to a beam width of roughly 0.3 m at 100 m or likewise, 3 ft at 1000 ft. Laser Technology helps to calculate the distance by measuring the time of flight of very short pulses of infrared light.This, however, differs from the traditional surveying instrument method of measuring phase shifts by comparing the incoming wavelength with the phase of the outgoing light. Any solid object will reflect back a certain percentage of the emitted light energy. This only needs to be a small percentage for the sensitive detector to pick it up. The time it takes a laser pulse to travel to the target and back with a precision, is thus measured by a crystal-controlled time base. Knowing the constant speed of light, it is then the distance traveled is then easily calculated. Also, For increased accuracy, the laser process as many as sixty pulses in a single measurement period. Target acquisition times range from 0.3 to 0.7 seconds. Sophisticated accuracy validation algorithms are in place to ensure a reliable reading. LTI lasers are completely eye-safe, meeting FDA Class 1 specifications.
A laser pulse is made to strike on the floor, which scatters in all directions. A small fraction of the laser light strikes the object, and the backscattered light is recorded on a patch of floor, which acts as the “virtual mirror”. This is explained in the image given below. We know that the speed of light is constant and is known to us(3*10^8m/s). Thus, by measuring the time interval between the start of the laser pulse and the scattered light reaching the patch of floor, the position of the object can be calculated.
Also Read: MIT Researchers Use Wi-Fi To See People Through Walls
Also, why the timing measurement needs to be accurate, is because of the fact that light levels that must be detected are extremely low. To overcome this obstacle demands the use advanced laser and detector technology. The laser pulses used for the timing measurement are just ten femtoseconds (100,000 billionths of a second, or 10-15) Long. Also, each pixel in the ultra-sensitive “camera” (known as a single-pixel avalanche diode array) used to image the patch of the floor is essentially an ultra fast stopwatch that records the arrival time of the scattered light pulse to within a few hundred billionths of a second.
Besides, light scattered from the object of interest reaches the virtual mirror of the floor, but the problem lies in the fact that light scattered from every other object in the vicinity does the same. Hence, it becomes essential that the two be separated, the “signal” of the hidden object from the background noise of everything else.So how is that achieved? Well, the logic applied here is that the hidden object the device is trying to detect is mobile, while other nearby objects are not. Because the moving object generates a signal in the virtual mirror that changes with time, it can be filtered from the constant background signal produced by the stationary objects of the surroundings.
The final stage being the timing measurement for scattered light arriving at a single point on the virtual mirror. This is recorded by a single pixel in the detector. A similar time delay could result from objects located at any number of different positions located an appropriate distance from the virtual mirror. While the timing data from a single pixel only locates the object to a range of positions, the range is different for each pixel. However, it turns out that there is only a single position at which the timing condition is satisfied simultaneously for all pixels, and this allows the object to be unambiguously identified from the background signals.
Take a look at the video below:
What’s more is that the prototype camera system allows the object’s position behind the wall to be localized within a centimeter or two. Also, the camera makes measurements every few seconds and hence can detect the speed of a moving object. Over the former method that demanded long data processing times, the new method can track moving objects in real time.
While currently, it’s limited to locating objects up to 60cm away from the virtual mirror on the floor, scientists are optimistic of extending the same to approximately ten meters, as well as to more closely detect the shapes of hidden objects as well as their positions. The future applications seem to be promising in areas such as surveillance or for security purposes.
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Source: fossbytes
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