Infrared (IR) sensors can be broadly classified into two main categories based on their detection mechanism:
1. Thermal Detectors
These sensors operate by absorbing infrared radiation, which causes a change in the temperature of the detector element. This temperature change then leads to a measurable change in some physical property of the material (like voltage, resistance, or capacitance). They generally have a broad spectral response (sensitive to a wide range of IR wavelengths) but tend to have slower response times compared to quantum detectors.
- Pyroelectric Sensors: Detect changes in temperature via the pyroelectric effect, generating a voltage. This is the category pyroelectric sensors belong to.
- Thermopile Sensors: Consist of multiple thermocouples connected in series or parallel. Absorbed IR radiation creates a temperature difference, generating a voltage via the Seebeck effect.
- Bolometers/Microbolometers: Measure the change in electrical resistance of a material as its temperature changes due to absorbed IR radiation. Commonly used in thermal imaging cameras.
2. Quantum (or Photon) Detectors
Quantum detectors operate based on the direct interaction of incident infrared photons with the electrons within the semiconductor material of the detector. This interaction excites electrons, generating free charge carriers (electron-hole pairs) which are then detected as a current or voltage change. These detectors typically offer much faster response times and higher sensitivity than thermal detectors, but their spectral response is usually limited to specific wavelength ranges, and they often require cooling for optimal performance, especially for longer wavelengths.
- Photovoltaic Detectors (Photodiodes): Generate a voltage when photons create electron-hole pairs near a p-n junction.
- Photoconductive Detectors: Change their electrical resistance when absorbed photons generate free charge carriers.
- Photoemissive Detectors: Emit electrons from a photocathode material when struck by sufficiently energetic photons (less common in the typical IR range).
Therefore, the two fundamental types are Thermal detectors and Quantum detectors, distinguished by whether they respond to the heating effect of IR radiation or the quantum interaction of individual photons.