## 4 Conclusions  
In this study, a multichannel spectral interference sensor for large-area thickness measurement of transparent films was proposed. In the optical configuration, linear 1-to-4 fan-out fiber bundles were used to connect multiple probes to the imaging spectrometer. Multichannel signals can be detected synchronously, and the integration time is related to the reflectivity of the sample, which is about 2-5 ms. The film thickness measurement range that the proposed system can achieve was calculated at about 250 nm to 90 \(\upmu\)m. Furthermore, a complete method combining the power spectral density estimation algorithm and the quadratic linear fitting algorithm was used to calculate film thickness. Compared with the extremum method and the Fourier frequency transform method, the proposed method was distinguished by a smaller deviation for film thickness. The total measurement and computing time of a single-point film thickness was about 3 ms.  
To verify the thickness measurement performance of the proposed method, two standard film samples of different thicknesses and varieties were measured 20 times. The results demonstrate that the relative deviation of the system is less than 0.38%, and the relative standard deviation is less than 0.044%. In addition, good performance has been achieved in the thickness measurement experiment of electronic-grade polyimide films. Three different thicknesses of multilayer films were accurately distinguished by just one experiment. Furthermore, the large-area thickness measurement speed of the proposed system was four times that of a traditional single-point thickness measurement system. The large-area thickness measurement experiments revealed the periodic thickness variation of the film, which may be related to the production parameters. In addition, the noise points observed in the thickness images are related to wrinkles and dust on the film. Therefore, it is necessary to keep the film flat and the environment clean during online measurement.  
Despite that the scanning speed of film thickness imaging was limited to 6 Hz by the frame rate of the spectrometer and the step time of the scanning stage, the total time for single multichannel spectral acquisition and thickness calculation was successfully reduced to 7.5 ms. The measurement speed can be increased by selecting a spectrometer with a high frame rate camera in future work. We will continue to optimize the algorithm model and the probe design to further improve the accuracy of the measurement system.  
Figure 8: Thickness distribution of two polyimide films with nominal thicknesses of (**a**) 8 \(\upmu\)m and (**b**) 25 \(\upmu\)m.  
**Author Contributions:** Conceptualization, W.H. and H.B.; methodology, W.H.; validation, Z.T., C.W. and Y.S.; formal analysis, W.H.; investigation, Z.D.; resources, H.B.; data curation, W.H.; writing--original draft preparation, W.H.; writing--review and editing, C.W.; visualization, W.H.; supervision, H.B.; project administration, W.H.; funding acquisition, W.H. and H.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Youth Innovation Foundation of Jihua Laboratory (X220171 XP220), the Key Project Foundation of Jihua Laboratory (X210221 TP210), and the National Natural Science Foundation of China (52173282).  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Conflicts of Interest:** The authors declare no conflicts of interest.