Combating Glare

November 10, 2016

I'm definitely showing my age when I recall the 1961 recording, "Does Your Chewing Gum Lose Its Flavour (On the Bedpost Overnight?)" by Lonnie Donegan. Donnegan and his music were a British staple before The Beatles and the "British Invasion." The Chewing Gum record reached fifth position on the Billboard Hot 100.

I mention the Chewing Gum record for its inclusion of the simple riddle, "If tin whistles are made of tin, what do they make foghorns out of?" While this illustrates the imprecision of the English language,[1] it also serves as a roundabout introduction to our topic of seeing things through a fog.

I've driven through dense fogs on several occasions, and it was not a pleasant experience. several fog idioms come to mind: Not seeing your hand in front of your face, fog so thick you could cut it with a knife, and fog as thick as pea soup. As we all know, fog is a mass of water droplets suspended in air at ground level; in other words, a cloud at the Earth's surface. Fog forms when the air temperature is very close to the dew point.

 This is a typical early morning landscape just west of Tikalon's location in Northern New Jersey.Fog over the Erlauf valley near Scheibbs, Austria. (Photo by Uoaei1, via Wikimedia Commons.)

It's fairly obvious that the optical properties of fog arise from light bouncing off the water droplets. This scattering is called Mie scattering, named after German physicist, Gustav Mie. Mie solved Maxwell's electromagnetic equations for a medium containing refractive transparent spheres, which is a good approximation to the water droplets in fog.

Source code for Mie scattering calculations is available online for nearly every computer language. One C language implementation that compiled for me can be found in ref. 2.[2] Using this program, I calculated the optical scattering from a water fog in air with the following properties:
Water refractive index = 1.33
Fog droplet diameter = 10 μm
Fog/air concentration = 0.00005 g/g

The calculated scattering coefficient at optical wavelengths for such a fog was calculated to be about 85%.

 Gustav Mie (1868-1957)While Mie scattering describes optical scattering from particles larger than a wavelength of light, Rayleigh scattering describes scattering from smaller particles.(Wikimedia Commons image, modified for artistic effect.)

My experience in driving through fog demonstrated to me the utility of a system for viewing things through fog, although it was not apparent how such a system could be made. Now, engineers at the California Institute of Technology (Pasadena, California) have devised a camera system that can image objects behind fog or other murky media by canceling out the glare.[3-4] Their method, called "coherence gated negation," uses destructive optical interference to accomplish this task.[3-4]

The Caltech team was lead by Changhuei Yang, a professor of electrical engineering, bioengineering, and medical engineering. Yang shared lead authorship of the paper describing the system with graduate student, Edward Zhou. Their coauthors included postdocs, Atsushi Shibukawa and Haowen Ruan, and graduate student, Joshua Brake.[4]

The Caltech system departs from the method of conventional anti-glare imaging systems in which the target optical signal is acquired in a short time window that rejects most of the glare when the target is illuminated by a pulse of light. The Caltech coherence gated negation method selectively cancels the scattered light instead.[3-4] This is done by destructive interference that combines one beam of light over another beam to cancel the unwanted signal.[4]

The illuminating laser beam is split into twin parallel beams, with one used to illuminate the target and the other to cancel the glare. The combination of these beams cancels most of the glare at a camera sensor.[4] To do this, the system must try a range of amplitude and phase values of a reference beam to get the best cancellation of glare. They found that they are able to suppress glare by an order of magnitude, even for a non-uniform optical wavefront.[3]

As a test, the research team placed text behind a millimeter thick block of glass beads suspended in a gel. This gel filter rendered the text completely illegible. Using their coherence gated negation method, they could increase the contrast of the text by a factor of about thirty (see figure). This produced a legible image.[4]

 An example of the Caltech system of using destructive interference to image through glare. The obscured word, "HI," is made legible. As can be seen, continued research on this technique is still required. (Still frames from an Edward Zhou/Caltech animation.)

While imaging through terrestrial fog is one potential application, there are others. This might be a noninvasive way to optically examine tissue under the skin, as in mammography, since the optical scattering of skin is similar to a dense fog.[4] While the image acquisition rate of the system needs to be improved, says Yang,
"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?"[4]

This research was funded by the National Institutes of Health, and the National Institute of Biomedical Imaging and Bioengineering, among others.[4]

References:

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