Comparison Pyroelectric Detectors versus Thermopiles

Thermopiles  are an “array” of thermocouples made from dissimilar materials with differing Seebeck coefficients.  Each “couple” produces a voltage proportional to its temperature and the differences in the Seebeck coefficient of the couple materials. N and P type polysilicon are used as the couples in low-end thermopiles while Antimony and Bismuth are used in higher performing parts but at a higher cost, other materials such as Iron-Nickel are used as well.  Half of the “couples” are blackened and called “actives” the remainder are made reflective and positioned out of the FOV and called “compensators”. The signal polarity of the “compensator” couples is the opposite of the active couples which cancels the voltage from the local ambient temperature.

They are usually connected electrically in series to raise the total resistance which improves the signal to noise. These devices are truly differential sensors in that the output signal is proportional to the temperature difference between the active and the comp. This cancels changes in the temperature of the micro atmosphere inside the detector capsule. Although these are usually very slow effects the thermopiles would respond to these as they are DC devices so the differential connection mitigates this problem. 

The temperature rise in the active junction is a function of the IR energy received and the thermal inertia of the couple. Thermopiles are limited by the fact that the thermal mass of the sensor prevents it from changing temperature very quickly. This is governed by its thermal constant. The materials used, the substrate, the method of preparation and the gas sealed in the detector capsule all affect the thermal time constant. Typical thermal time constants for thermopiles range from 10 to 100 milliseconds and govern the rate of heat arriving and leaving the couples. Reducing the time constant will only reduce the output as it reduces the temperature rise and fall in the element.

Because of this fact thermopiles have excellent DC response (constant temperature) but this falls off very quickly so that at frequencies above the thermal time constant the output is reduced.  The main noise mechanism in thermopiles is the Johnson noise from its total resistance.