I’ve been telling everybody about this particular paper presentation at Electronic Imaging, because I think the implications are pretty cool. It’s yet another presentation where electrical engineers and image processing folks are treating machine vision more like human vision than they have tended to do in the past. This phenomena isn’t exactly new — for example, retinex goes back over 35 years — but it’s a refreshing to see that we can learn from neuroscience at the same time that we are shaped by computers. Anyway, that’s enough Arthur C. Clarke for now.
Before I give you some scanty details about Hooman Mohseni‘s work into single-photon detectors, I’ll do what he did in his presentation and discuss rod cells, the eye’s own single-photon detectors.
The rod cells in your eye are almost perfect single-photon detectors. When a photon of light in the visible range strikes a rod cell, it causes a chemical reaction with rhodopsin, a photo pigment. The rhodopsin molecule changes shape, closing an aperture in the rod cell wall. This partially interrupts the flow of charged sodium ions into the rod cell, and an electrical charge builds up. The charge causes an impulse that is transmitted to the rest of the visual system, possibly causing the sensation of seeing light. There are several layers of rhodopsin in the cell, allowing a greater impulse when more photons strike it. (You can read more details about rhodopsin and its paramour retinal if you want.)
Each rod cell can detect a single photon event, and rods are photon number resolving, meaning that the visual system can use the impulse to distinguish whether one photon, two photons or twenty photons struck the rod cell. And the false count rate is very low, approximately one per 100 seconds. And all of this happens at low electricity levels. Pretty cool, huh?
Compare that to imaging sensors like CCDs, where at very low illumination levels noise overwhelms the actual signal. Even very accurate detectors that can detect single photons reliably (such as photo-multiplier tubes) require large amounts of voltage and usually can’t resolve the difference between one or two photons. Signal-to-noise ratios have been pretty bad so far, too.
Prof. Mohseni’s group at Northwestern has been using nanotechnology fabrication techniques to create a “focalized carrier augmented sensor (FOCUS)” which can transform a single photon event into an output of about 1,000 electrons, at very low voltages. Essentially they have produced something like a rod cell.
Their detector works really well, but nanofabrication is hard and slow. They can’t even image the small, tube-like sensors because they are too delicate; for example, atomic force microscopy — a very gentle technique — destroys the tubes. Nevertheless, this may be the future of things like night vision, positron-emission tomography, and so on.