Camera System Images Weak Targets by Negating Glare
PASADENA, Calif., Oct. 31, 2016 — Coherence gated negation (CGN) is a novel imaging method that uses destructive optical interference to suppress glare and allow imaging of a target that may be hidden behind a scattering medium such as fog or clouds. In contrast to conventional coherence gating methods, which “gate in” the target optical signal, CGN works by actively “gating out” the unwanted optical contributions.
Destructive interference cleans up an image to make text legible. Courtesy of Edward Zhou/Caltech.
Researchers at the California Institute of Technology (Caltech) created the device, which selectively cancels scattered light, leaving only the light that has been reflected or bounced off the target object. The device relies on destructive interference. It splits a laser into twin parallel beams, using one beam to illuminate a target and the other to cancel out the glare. Superimposing the light from each beam results in a cleaner image on a camera sensor.
“The idea that we can directly cancel glare is new,” said professor Changhuei Yang.
To test the device, researchers placed a line of text behind a one-mm thick block of glass beads suspended in a gel, rendering the text completely illegible. With CGN, they were able to suppress the glare intensity by a factor of 10 times using a permutation set of size 256. They demonstrated CGN’s ability to suppress glare over optical distances as short as several μm through the use of low coherence light sources such as super-luminescent diodes. Glare suppression was demonstrated on the length scale of 2 mm—a regime that conventional time-of-flight methods are presently unable to reach.
The researchers also showed that by suppressing glare and permitting all other optical signals to pass, CGN allows for the simultaneous imaging of objects at different distances. This is in contrast to conventional CG methods, which are good at imaging objects at a given distance and rejecting optical contributions before and after the chosen plane.
CGN shares the same roots as acoustic noise cancellation. Using a reference optical field of the same magnitude and opposite phase to destructively interfere with the glare component of a returning optical field, it nullifies the glare and its associated noise, thereby allowing the electronic detector to measure only the optical signal from the hidden target.
Experimental demonstration of CGN. (a) Experimental setup. AM, amplitude modulator; BS, beam splitter; CBS, cubic beam splitter; FP, fiber port; HWP, half-wave plate; L, lens; M, mirror; OBJ, objective lens; OS, optical shutter; P, polarizer; PM, phase modulator; PSMF, polarization-maintaining single mode fiber. (b) Image of the target without glare. (c) Image of the target with glare before CGN. (d) Image of the target after CGN. Courtesy of OSA, the Optical Society of America.
Currently, the CGN method can only be used to assist the imaging of amplitude objects. While the researchers do not see a straightforward way to extend CGN to enable phase imaging, they do not preclude the possibility of such developments in the future.
CGN has numerous potential uses, including for the satellite exploration of cloud-obscured planets like Venus. It also has potential biomedical applications, offering a noninvasive way to optically examine tissues under the skin.
"Optically, our skin behaves very similarly to a dense fog. CGN can be used to cancel the tissue glare and allow us to see through the skin," said researcher Edward Zhou.
The device may also someday help drivers navigate foggy roads, although the speed of the image resolution would need to be improved significantly.
"A very nice aspect of this method is that there is a fairly straightforward approach for increasing its speed by several orders of magnitude. Wouldn't it be nicer and safer if you can see the whole San Francisco bridge as you drive across it on a foggy day?" said Yang.
The research was published in Optica, a journal of OSA, the Optical Society of America (doi: 10.1364/OPTICA.3.001107).
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