# Junction Temperature Optical Sensing Techniques for Power Switching Semiconductors: A Review
## 3 Junction Temperature Optical Sensing Techniques
### IRC Sensing Technique
Some of the established issues with IRC include surface reflectance and uncertainty in local temperature. The probable reason for the significant change in the emissivity was presented as the transparency of the molded lens and reflection from other components of the targeted object [37]. This issue could be mitigated by removing objects around the setup that may likely cause reflection; otherwise, the surface could be painted black. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Year** & **Device** & **F\({}_{\text{S}}\) (Hz)** & **Pixel** & **C\({}_{\text{TH}}\) (\(\upmu\)m)** & \(\varepsilon\) & \(\Delta\)T (\({}^{\circ}\)C)** & **Err** & **Ref.** \\ \hline
2021 & SiC M. & 200 & - & - & 0.73 & 100–160 & \(\pm 1.25\) K & [67] \\
2021 & IGBT & 9 & - & - & - & 39–100 & - & [68] \\
2020 & IGBT & 200 & - & - & -1 & 70–140 & \(\pm 3\)\({}^{\circ}\)C & [69] \\
2017 & IGBT & 100 & - & 10–16 & - & 80–190 & \(\pm 2\)\({}^{\circ}\)C & [70] \\
2016 & IGBT & 160 & - & - & \(\sim\)1 & 30–80 & \(\pm 5\)\({}^{\circ}\)C & [71] \\
2016 & IGBT & 50 & - & - & - & 35–60 & 3.4\% & [72] \\
2016 & IGBT & 106 & \(48\times 4\) & - & - & 25–50 & - & [73] \\
2013 & IGBT & - & 1368 & 100 & 1 & 40–180 & \(\pm 0.12\) K & [65] \\
2012 & IGBT & 100 & - & - & 0.94 & 70–140 & \(<\)3\% & [74] \\
2010 & IGBT & 50 & 320 \(\times\) 256 & - & \(\sim\)1 & 40–70 & \(\pm 2\)\({}^{\circ}\)C & [75] \\
2007 & IGBT & 15 & - & 220 & \(\sim\)1 & 35–60 & \(\pm 1\) K & [66] \\ \hline \end{tabular} \({\text{F}_{\text{S}}}\): frame rate, C\({}_{\text{TH}}\): coating thickness, \(\varepsilon\): emissivity, \(\Delta\)T: temperature range, K: Kelvin, SiC M: Silicon Carbide MOSFET, Err: measurement error.  
\end{table} 
Table 2: Advancements in IRC sensing technique for junction temperature measurement of switching semiconductor devices. Figure 5: Representation of equivalent radiation captured by IRC sensing technique. The uncertainties of local temperature due to variation in surface emissivity are due to changes in radiative recombination properties of semiconductor layers, bonding elements, and coating having different transparency or reflectance properties to infrared radiation, making it difficult to determine the surface emissivity accurately. These issues have been mitigated in the literature with high emissivity coatings [50; 76]. Another alternative to obtain an accurate measurement of emitted radiation from a target is to place micro carbon particles near the targeted surface, eliminating the need for coating [77]. It should be noted that IRC may provide nonuniform temperature estimation if the thickness of the coating varies across the surface. Other factors to consider for the accuracy of the IRC sensing technique, including the spatial resolution and image pixels, which defines the ability to provide details of temperature distribution on the captured surface, which relies on the pixels of the camera's detector and its field of view (FOV) specification with respect to the area the camera sees at any given moment [53]. The spatial resolution of the IR image could be defined with an adapted lens to permit pixels of various sizes and FOV, which was utilized in [75] to improve the accuracy of the IRC camera to about \(\pm 2\)\({}^{\circ}\)C. Also, the influence of system noise and the error during detection could be compensated with auto-calibration and image-processing algorithms embedded in commercially available IR cameras. The summary of benefits and challenges provided by this IRC OBS technique is shown in Table 3. As highlighted before, IRC is attractive to evaluate power electronics circuits' thermal validation, reliability, and temperature performance at a designated distance only when there is a clear path to the targeted object.