What Are Pyroelectric Infrared Sensors?
Translated by Alexandra Igna (2019)
Basic structure and operation mode
The basic structure of a pyroelectric infrared sensor consists of a few essentials components, which are the sensitive element and the preamplifier. The critical elements are integrated in the sensor. The sensitive element consists of a thin pyroelectric chip with an electrode. An additional black layer may be present for enhanced absorption properties.
If the radiation flux ΦS (t) strikes the sensitive surface AS, it becomes the sensitive surface element absorbed with the degree of absorption α. This creates a pyroelectric Temperature change ∆T (t), which leads to a charge change ∆Q (t) on the electrodes. With the help of the preamplifier, this charge change is converted into the signal voltage uS´ (t) converted.
In addition to the signal voltage, there is also a noise voltage at the output of the preamplifier uR´ (t) available. This noise voltage has its cause in the noise sources of both the sensitive element as well as the preamplifier. It does not cause any minor radiation flow that can be detected by the sensor.
Sensor Parameters
The most important sensor parameters are the sensitivity SV, the noise equivalent radiation power NEP and the specific detectivity D *. They are used in sinusoidal processes in the steady-state and generally by the modulation frequency f, the wavelength λ and the sensor temperature T.
Pyroelectric Material
The pyroelectric takes on the actual transducer element in the pyroelectric radiation sensor in a central position. There are 32 existing crystal classes. Crystals with point group symmetry 1, m, 2, mm2, 3, 3m, 4, 4mm, 6 and 6mm are the so-called spontaneous polarization crystals that enable the occurrence of the pyroelectric effect.
Crystals parts of these crystal classes are called pyroelectrics and they are technically significant pyroelectrics, in which the spontaneous polarization can only occur in one direction in the crystal can (uniaxial pyroelectrics). These include the crystal classes 2, mm2, 3, 3m, 4,
4mm, 6 and 6mm. The direction of spontaneous polarization coincides with the polar axis of the crystal.
The following material parameters need to be used in pyroelectric radiation sensors:
- p pyroelectric coefficient
- εr dielectric constant
- tanδ dielectric loss
- cP´ volume-specific heat capacity
For high values of sensitivity SV and specific detectivity D*, the pyroelectric coefficient p are high. Also, the low values for the dielectric constant εr, dielectric loss tanδ and the volume-specific heat capacity cP´ that is required. For many years, the pyroelectric lithium tantalate (LiTaO3) has been used in proven pyroelectric sensors. It not only meets the requirements just mentioned the material parameters, but is also characterized by excellent Temperature stability and very good reproducibility of the properties of the sensor.
Equivalent Circuit Diagrams Of The Sensitive Element And The Preamplifier
The absorption of the radiation flow ΦS causes a change in temperature in the pyroelectric ∆T, which generally depends on the location in the pyroelectric. This location dependency can often times be neglected in good approximation so that the thermal behavior of the sensitive element in these cases by a simple analog electrical equivalent circuit diagram.
Normalized Noise Voltage
The non-correlating noise sources have the following preamplifier noise voltage components:
- uRnT ′ ~ noise voltage component caused by the Temperature noise source pRnT of the sensitive element
- uRnD ′ ~ noise voltage component caused by the tan δ noise source RnD i of the sensitive element
- uRnR ′ ~ noise voltage component caused by the thermal noise RnR i of the preamplifier input resistance reV
- uRnI ′ ~ noise voltage component caused by the Current noise RnV i of the preamplifier
- uRnU ′ ~ noise voltage component caused by the Voltage noise uRnV of the preamplifier
- uRnGK ′ ~ noise voltage component caused by the thermal noise RnGK i of the preamplifier negative feedback resistor RGK (only when the preamplifier is in current operation)
Specific Detectivity
From the definition equation of the specific detectivity and the basic equations the sensitivity and standardized noise voltage follow the from the individual noise voltage fractions of the specific fractions' detectivity:
- DT proportion of specific detectivity caused by the Temperature noise source pRnT of the sensitive element
- DD percentage of specific detectivity, caused by the tan δ noise source RnD i of the sensitive element
- DR proportion of specific detectivity caused by the thermal noise RnR i of the preamplifier input resistance reV
- DI proportion of specific detectivity caused by the Current noise RnV i of the preamplifier
- DU portion of the specific detectivity caused by the Voltage noise uRnV of the preamplifier
- DGK share of specific detectivity, caused by the thermal noise RnGK i of the preamplifier negative feedback resistor RGK (only when the preamplifier is in current operation)
Source:
@ DIAS Infrared GmbH, 2006
