A dual-channel single-spectrometer chromatic confocal coherence sensor is proposed for simultaneous measurement of film thickness and refractive index. Compared with the conventional single-channel single-spectrometer approach, the proposed method converts the multiplicative coupling between interferometric and confocal signals into an additive form by introducing an additional detection channel and eliminating the need for an external reference arm. As a result, the optical thickness resolution is improved by one order of magnitude. For validation, a sensor system is established, and film specimens with different materials and thicknesses are measured. The results demonstrate that the dual-channel configuration introduces no additional structural complexity while improving the optical thickness resolution from 50 µm to 5 µm, with film thickness and refractive-index measurement errors less than 11 µm and 0.02, respectively.
Zhou et al. (Mon,) studied this question.