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Your eyes are exquisite light detectors, determining the intensity, color and spatial distribution of the rays incident on them. The human retina has more "pixels" than a consumer digital camera, containing about six million color-sensing cone cells and more than 100 million of the rod cells responsible for vision in the dark. And eyes are highly sensitive: a dark-adapted rod cell can fire off a signal to the brain on absorbing a single particle of light, or photon, the smallest quantum unit of an electromagnetic wave. As few as six of these single-photon signals are required for your brain to perceive a flash. But eyes and commercial cameras are far from ideal for many tasks, because they can detect only those photons whose frequencies lie in the narrow visible range. Furthermore, their color capabilities do not involve a measurement of each photon's precise frequency.
Scientific and industrial photon detectors, in contrast, peer into the electromagnetic realms beyond that of visible light--into the low-frequency (long-wavelength, low-energy) world of infrared and microwaves and into the high-frequency regime of x-rays and gamma rays. Yet they too are limited in their abilities. In particular, for visible and longer wavelengths scientists have lacked a detector able to "see" an individual photon and discern its frequency, and thus its energy, with any accuracy. Determining the frequency of photons opens the door to a wealth of infor-mation about the matter that emitted the photons.