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Light is made up of small packets of energy called photons, so small, in fact, that they are almost impossible to measure directly. The amount of energy that is deposited when one photon of wavelength <math>\lambda</math> is absorbed is equal to <math>h/\lambda</math>, where <math>h</math> is Planck's constant. For example, a candle puts out <math>5 * 10^{16}</math> photons per second.  The human eye is not able to detect individual photons; even a light pulse of one million visible photons would not be bright enough to be seen.  This is not a problem in everyday life because normal light levels are much higher than this.  However there are some situations where detection of very low levels of light is required.  Examples are viewing distant objects through a telescope, or imaging a tumor in the human body using a PET scanner.  Both of these applications require cameras with single-photon sensitivity.   

Light is made up of small packets of energy called photons, so small, in fact, that they are almost impossible to measure directly. The amount of energy that is deposited when one photon of wavelength <math>\lambda</math> is absorbed is equal to <math>h/\lambda</math>, where <math>h</math> is Planck's constant. For example, a candle puts out <math>5 * 10^{16}</math> photons per second.  The human eye is not able to detect individual photons; even a light pulse of one million visible photons would not be bright enough to be seen.  This is not a problem in everyday life because normal light levels are much higher than this.  However there are some situations where detection of very low levels of light is required.  Examples are viewing distant objects through a telescope, or imaging a tumor in the human body using a PET scanner.  Both of these applications require cameras with single-photon sensitivity.   

The standard technology for such detectors, originally developed for atomic and nuclear physics experiments, is based on vacuum tube technology.  Particle physics experiments have relied on such detectors for over 40 years.   

The standard technology for such detectors, originally developed for atomic and nuclear physics experiments, is based on the photomultiplier vacuum tube.  Particle physics experiments have relied on such detectors for over 40 years.   

Ever since the invention of the transistor, efforts have been made to create semiconductor-based photon detectors, but certain drawbacks have limited their use to a few niche applications.  Recently, however, progress has been made toward the goal of creating silicon-based detectors with single-photon sensitivity that can operate at room temperature.  These devices are called silicon photomultipliers.

Ever since the invention of the transistor, efforts have been made to create semiconductor-based photon detectors, but certain drawbacks have limited their use to a few niche applications.  Recently, however, progress has been made toward the goal of creating silicon-based detectors with single-photon sensitivity that can operate at room temperature.  These devices are called silicon photomultipliers.