Photosensor modules are compact light detectors with built-in power supplies. They use photomultiplier tubes, avalanche photodiodes, PN photodiodes, or PIN photodiodes. Photomultiplier tubes (PMTs) are evacuated envelopes with a photocathode that emits electrons when exposed to light. These electrons are accelerated by a positive electrostatic field and fall upon a metal surface, where they emit secondary electrons that are accelerated again to generate more electrons at the next metal surface. The whole arrangement acts as a combination of a simple photocell with a high-gain amplifier in a self-contained unit. Avalanche diodes create hole-electrons from absorbed photons to achieve the avalanche multiplication of photocurrent. When the reverse-bias voltage nears the breakdown level, the hole-electron pairs collide with ions to create additional hole-electron pairs, thus achieving a signal gain. PN photodiodes consist of P-doped and N-doped semiconducting regions. They have a better, more linear response than photoconductors but are not suitable for measuring low light intensities. PIN diodes contain a large intrinsic region sandwiched between P-doped and N-doped semiconducting regions. The photons absorbed in this region create electron hole-pairs that are separated by an electric field, thus generating an electric current in a load circuit.
Selecting Photosensor Modules
Selecting photosensor modules requires an analysis of performance specifications, output types, photosensor sensitivity, photosensor noise, and photosensor features. Performance specifications include effective area diameter or length, effective area width, spectral response or wavelength range, supply voltage, and number of channels. Output types for photosensor modules include voltage, current, and photon counting. Depending on the output type, manufacturers specify voltage output sensitivity, current output sensitivity, or photon counting sensitivity. Noise equivalent power (NEP) is the power of incident light, at a specific wavelength, required to produce a signal equal to the noise. Highly sensitive devices do not introduce additional noise into the signal. Some photosensor modules are cooled to provide improved sensitivity. Others use transistor outline (TO) packages such as TO-5, TO-18, TO-39, and TO-99 for mounting on printed circuit boards (PCBs).
Photosensor modules are used in a variety of applications. Examples include florescence spectroscopy, molecular biology, semiconductor wafer inspection, and low light measurement detection. Photosensor modules are also used in optoelectronic converters, portable optical detection instruments, and medical diagnostic equipment. Specialized devices are used in immunoassay, gene sequencing, bacteria detection, oil well drilling, high energy physics, and the detection of nuclear materials.