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.