Best Practices for Body Temperature Measurement with Infrared Thermography: External Factors Affecting Accuracy

Abstract

Infrared thermographs (IRTs) are commonly used during disease pandemics to screen individuals with elevated body temperature (EBT). To address the limited research on external factors affecting IRT accuracy, we conducted benchtop measurements and computer simulations with two IRTs, with or without an external temperature reference source (ETRS) for temperature compensation. The combination of an IRT and an ETRS forms a screening thermograph (ST). We investigated the effects of viewing angle (θ, 0-75°), ETRS set temperature (TETRS, 30-40 °C), ambient temperature (Tatm, 18-32 °C), relative humidity (RH, 15-80%), and working distance (d, 0.4-2.8 m). We discovered that STs exhibited higher accuracy compared to IRTs alone. Across the tested ranges of Tatm and RH, both IRTs exhibited absolute measurement errors of less than 0.97 °C, while both STs maintained absolute measurement errors of less than 0.12 °C. The optimal TETRS for EBT detection was 36-37 °C. When θ was below 30°, the two STs underestimated calibration source (CS) temperature (TCS) of less than 0.05 °C. The computer simulations showed absolute temperature differences of up to 0.28 °C and 0.04 °C between estimated and theoretical temperatures for IRTs and STs, respectively, considering d of 0.2-3.0 m, Tatm of 15-35 °C, and RH of 5-95%. The results highlight the importance of precise calibration and environmental control for reliable temperature readings and suggest proper ranges for these factors, aiming to enhance current standard documents and best practice guidelines. These insights enhance our understanding of IRT performance and their sensitivity to various factors, thereby facilitating the development of best practices for accurate EBT measurement.

Keywords: ISO/TR 13154; accuracy; ambient temperature; atmosphere transmittance; elevated body temperature; environmental effects; external temperature reference source; infrared thermograph; relative humidity; thermography; viewing angle.

Figure 1

Principle of the total radiation received by an IRT. [$\sigma$: Stefan–Boltzmann constant, $\epsilon$: emissivity, $\tau$: atmospheric transmittance, $T_{refl}$: reflected temperature, T: object temperature, $T_{atm}$: atmosphere temperature].

Figure 2

Schematic of the experimental setup. The red dash-dotted box shows the ST systems.

Figure 3

Accuracy of (a) IRT-1 and (b) IRT-2 without ETRS compensation. Horizontal dashed lines represent the recommended laboratory accuracy and the rectangular gray area denotes the required evaluation range of 34 °C to 39 °C.

Figure 4

Effects of temperature compensation and $T_{ETRS}$ on the accuracy of (a) ST-1 and (b) ST-2. Horizontal dashed and solid lines represent the recommended laboratory accuracy and offset errors, respectively, and the rectangular gray area denotes the required evaluation range of 34 °C to 39 °C.

Figure 5

Effect of viewing angle on temperature accuracy for the two STs. The error bars represent the standard deviation.

Figure 6

Effects of ambient RH and temperature: $T_{IRT}-T_{CS}$ and $T_{ST}-T_{CS}$ versus (a) ambient RH with ambient temperature at 24 °C and (b) ambient temperature with ambient RH at 35%. The working distance was kept at 0.8 m.

Figure 7

Effect of the working distance: $T_{IRT}-T_{CS}$ and $T_{ST}-T_{CS}$ versus working distance with (a) $T_{ETRS}$ = 37 °C and (b) $T_{ETRS}$ = 35 °C.

Figure 8

Computer simulation results with $T_{CS}$ = 37 °C. ((a): estimated τ based on environmental factors. (b): $E_{e,total}$ received by IRT based on the estimated τ. (c): calculated $T_{CS}$ measured by an IRT assuming τ = 1. (d): calculated $T_{CS}$ measured by an ST assuming τ = 1 and $T_{ETRS}$ = 35 °C).

Figure 9

Computer simulation of $T_{ETRS}$ effect: Offset error $\left(T_{ST}-T_{CS}\right)$ of an ST to measure a CS at 37 °C with different $T_{ETRS}$ values assuming τ = 1. ((a): $T_{ETRS}$ = 20 °C; (b): $T_{ETRS}$ = 30 °C; (c): $T_{ETRS}$ = 36 °C; (d): $T_{ETRS}$ = 38 °C; (e): $T_{ETRS}$ = 40 °C; (f): $T_{ETRS}$ = 50 °C).

Figure 10

Computer simulations depicting the effects of (a) ambient RH, (b) ambient temperature, and (c) working distance, with both $T_{CS}$ and $T_{ETRS}$ set at 37 °C. If not specified, the default values for ambient temperature and distance were set to 24 °C and 0.8 m, respectively. For the sake of comparison, the ambient RH values were set at 35% and 50% in cases (b,c), mirroring the experimental conditions depicted in Figure 6b and Figure 7a.