# Zero Drift Infrared Radiation Thermometer Using Chopper Stabilised Pre-Amplifier
## 1 Introduction  
Non-contact temperature measurements, acquired from processes using infrared radiation thermometers (IRTs), afford advantages over contact temperature measurements. These include amelioration of the susceptibility of thermocouple wires to degradation from exposure to the measurement conditions and the effects of contact upon the measurand and object [1, 2]. Thermocouples afford a wide range of temperature measurements, typically having the capability of continuous measurement from zero to many hundreds of degrees Celsius [1]; a capability that would need to be replicated by IRT based thermocouple replacements. IRTs that use mid-wave infrared (MWIR) detectors can be deployed in various applications that need the capability to measure lower temperatures, for example, measurement of temperatures below 100 \({}^{\circ}\)C. Mercury Cadmium Telluride (MCT) photon detectors have been used in low-temperature IRTs but require thermoelectric cooling to reduce their dark current and Johnson noise, whilst increasing their photosensitivity to enable the measurement to be acquired [3]. Cooling a photon detector increases the cost and size of IRTs, making their usage undesirable, particularly within size-constrained instrumentation [4]. Thermal detectors, such as thermopiles, can measure over this spectral range, however, they suffer from slow response time and low sensitivity compared to photon detectors [1]. Thermal detectors require high gain amplification to achieve output voltages that can be measured with benchtop instrumentation and can only represent, accurately, signals that change slowly, as demonstrated by [PERSON] et al. [5]. Both MCT and thermal detector technologies incur drift that increases the measurement uncertainty and introduces non-linearity. Variation of voltage offset with IRT ambient temperature is of particular concern when using uncooled detectors [6]. A mechanical chopper, or shutter, can be used to overcomedrift within IRTs to improve their stability [7, 8] but at the expense of increasing the overall instrument cost and size. In addition, IRTs incorporating mechanical chopping also need detectors with lower shunt capacitance. This enables them to keep up with the chopper which, presumably, is higher frequency than any measurement expected of the IRT. Typically, IRTs that use photodiodes also utilise a transimpedance amplifier (TIA) to provide a measurable voltage from the photocurrent generated. MWIR photodiodes generate small magnitude photocurrents when exposed to low radiant flux, which requires a high transimpedance to generate a measurable signal from the TIA. This dictates the need for op-amps with low noise and input voltage offset to be used in the TIA to afford the measurement of low temperatures with low uncertainty. Manufacturers offer 'precision' op-amps that have low magnitudes of input voltage offset, input current bias, noise and temperature-induced zero offset drift. Precision op-amps are often used in amplifier circuits to achieve precision measurements from low magnitude signals [9, 10, 11]. Similarly, low magnitude voltages to those arising from MWIR photodiodes but arising from physiological, micro-electro-mechanical-systems (MEMS) sensors and thermal detectors, have been demonstrated to be measurable using high gain amplifiers that had low noise and offset [5, 12, 13, 14, 15, 16]. An additional consideration is how the variation of the ambient temperature can lead to drift in the output voltage of typical TIA circuits used in IRTs, causing non-linearity in the IRT response and increased measurement uncertainty, which needs to be mitigated to allow low magnitude currents and voltages to be measured accurately [4, 17, 18]. These limitations in IRT characteristics have constrained the capability of MWIR thermometers to achieve low-temperature measurements with low uncertainties when using uncooled detectors, even with the use of a precision op-amp.