# Zero Drift Infrared Radiation Thermometer Using Chopper Stabilised Pre-Amplifier
## 3 Results and Discussion
### Characterisation of IRTs against a Blackbody Source at Low Temperatures
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Source** & **Calibration RTD** & **Calibration Blackbody** & **Calibration Instrument** & **Digital Multimeter** \\
**Temperature \({}^{\circ}\)C** & **Uncertainty K** & **Uncertainty K** & **Uncertainty \%** \\ \hline
40 & 0.028 & 0.50 & 0.16 & 0.0085 \\
70 & 0.003 & 0.45 & 0.24 & 0.0085 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Contributions of calibration uncertainties to overall measurement uncertainty (\(k=2\)). Figure 3: (a) Output voltage as a function of blackbody temperature for zero-drift and precision IRTs. (b) \(ln(V_{o})\) as a function of 1/\(T\) for zero-drift and precision IRTs. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Source** & **Interpolation Error of Thermometer** & **Infrared Thermometer Voltage** \\  & **Mean Measurement K** & **Measurement Variability K** \\ \cline{2-5}
**Temperature \({}^{\circ}\)C** & **Zero-Drift IRT** & **Precision IRT** & **Zero-Drift IRT** & **Precision IRT** \\ \hline
40 & 2.3 & 6.9 & 1.0 & 9.5 \\
70 & 2.7 & 4.9 & 0.9 & 3.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Contributions of measurement uncertainties to overall measurement uncertainty (\(k=2\)). highlighting an additional benefit of the zero-drift IRT; it eases the calibration procedure. The limiting effective wavelengths for the IRTs were calculated to be 4.12 \(\upmu\)m and 3.71 \(\upmu\)m for the zero-drift and precision IRTs, respectively. Practical calibrations of the IRTs were used to determine interpolation curves. There was good agreement between the measured and modelled output voltages for the zero-drift IRT at each temperature within the range, with only the lowest temperature having a deviation that exceeded 10% of the measured voltage. The precision IRT afforded reasonable agreement, with larger deviations between modelled and measured output voltages than recorded for the zero-drift IRT. The difference between modelled and measured voltages for the precision IRT was 47.5% at 20 \({}^{\circ}\)C, reducing to circa 30% for temperatures above 40 \({}^{\circ}\)C. RMS noise was calculated from the calibrations for each IRT. The RMS noise of the zero-drift IRT was evaluated to be 3.5 K at 20 \({}^{\circ}\)C, reducing to less than 1.0 K for temperatures above 40 \({}^{\circ}\)C. The RMS noise of the precision IRT was evaluated to be 14.7 K at 20 \({}^{\circ}\)C, reducing to 5.0 K for temperatures above 40 \({}^{\circ}\)C.  
The results have demonstrated that the use of a chopper stabilised op-amp within the TIA circuit of a zero-drift IRT improves the quality of the temperature measurement for target objects at ambient room temperature and above. The zero-drift IRT incurred lower RMS noise compared to an equivalent precision op-amp based IRT. The zero-drift IRT performance could be improved further by deploying a higher value of transimpedance resistor within the feedback loop to achieve higher 'gain'. This would enable lower magnitude photocurrents to be amplified to measurable output voltages. In addition, further investigation of different chopper stabilised op-amp variants could be performed to establish the lower noise performance possible for the zero-drift IRT. By combining this with further instrumentation optimisation, such as additional filtering and averaging, a zero-drift IRT could be developed for measuring temperatures from artefacts such as Lithium ion cells, warm fluids or body temperature. Such an IRT should offer greater stability and be more robust against ambient temperature variations compared with an IRT that used a precision op-amp and could be achieved without photodetector cooling or optical chopping.