# Junction Temperature Optical Sensing Techniques for Power Switching Semiconductors: A Review
## 3 Junction Temperature Optical Sensing Techniques
### IRC Sensing Technique
Moreover, these emissions can further be expressed as:  
\[E_{obj}=\varepsilon_{obj}\ \ \tau_{atm}\ \ \sigma_{atm}\ \ \ (T_{obj})^{4} \tag{2}\]  
\[E_{atm}=\varepsilon_{obj}\ \ \tau_{atm}\ \ \sigma_{atm}\ \ \ (T_{atm})^{4} \tag{3}\]  
\[E_{refl}=(1-\varepsilon_{obj})\ \tau_{atm}\ \ \sigma_{atm}\ \ \ (T_{refl})^{4} \tag{4}\]  
where \(\varepsilon_{obj}\) is the emissivity of the target surface (i.e., the object), \(T_{obj}\), \(T_{atm}\), and \(T_{refl}\), are temperatures of the object, atmosphere, and reflection, respectively, and \(\tau_{atm}\) is the transmittance of the atmosphere. \(\sigma_{atm}\) is Stefan-Boltzmann's constant, which is given as \(5.670\times 10^{-8}\ \mathrm{W/m^{2}/k^{4}}\). Emissivity, which refers to the ability of an object to emit thermal energy [60; 61; 62], dramatically affects the measurement accuracy of the IRC sensing technique [43]. For the IRC-based system to obtain accurate thermal measurement, emissivity of the targeted object must be uniform. For instance, commercial IGBT chips are usually coated with silver solder layers by manufacturers, which significantly decrease its surface emissivity. As such, the surface temperature of the IGBT chip could not be correctly measured by the IRC as reported in [63]. Nevertheless, we can characterize the surface emissivity considering the blackbody and infrared radiation discussed in [64] as:  
\[\varepsilon_{obj}=\frac{(R_{T1}-\ R_{T2})}{(R_{b1}-\ R_{b2})} \tag{5}\]  
where \(R_{T1}\) and \(R_{T2}\) are the infrared emission levels at known temperatures, \(T1\) and \(T2\), and \(R_{b1}\) and \(R_{b2}\) are the equivalent black body emission levels. The advancements in this IRC optical sensing are summarized in Table 2.  
To increase the surface emissivity, [PERSON] et al. [64] employed filtered paint and microspraying equipment to spray the target surface to emphasize further the requirements for improved surface emissivity for reliable junction temperature measurement of an IGBT chip. They ensured that the particle size and paint thickness (<100 \(\upmu\)m) were uniform throughout the surface, which they showed was necessary to achieve homogenous surface emissivity. Another approach for fixing the emissivity is to use signal processors to directly postprocess the output signal of an infrared camera, as shown in Figure 4. Although the approach enhances the complexity of IRC, it has the benefit that it can map the temperature of any semiconductor chip, independent of its shape or composition [50]. As indicated in Table 2, it was observed that the experimental results obtained in [65; 66] have a temperature error of \(\pm 0.12\) K at a uniform emissivity compared to cases where emissivity falls below 1, such as ref. [67]. In this case, a temperature difference of \(\pm 1.25\) K at an emissivity factor of 0.735 has been reported within a temperature range of 100 to 160 \({}^{\circ}\)C. This suggests that the effect of emissivity cannot be overlooked. Furthermore, frame rate and spatial resolution are also essential factors in evaluating the performance of IRC. The frame rate is the speed at which the camera updates the temperature readings to be displayed on the screen. An IRC with a high frame rate is desirable to capture rapid temperature changes. Due to the limited refresh rate of 9 Hz exhibited by the IRC used by [PERSON] et al. [68], the junction temperature variation could not be captured when the applied pulse width modulation (PWM) was raised above 10 Hz at the gate of IGBT. An IRC of sampling rate above 100 Hz, employed in [69; 71], reported capturing transient temperature across the junction with an accuracy of \(\pm 3\)\({}^{\circ}\)C in the temperature range of 70 to 140 \({}^{\circ}\)C. The existing costly IRCs available in the market exhibit frame rates up to 200 Hz with better scanning speeds. Nevertheless, the accuracy and transient temperature measurement capability could be improved. In this case, the IRC images are sampled once a steady thermal state is reached.