Self-reference dispersion correction for chromatic confocal displacement measurement

Dispersion is a very common phenomenon in the broadband optical systems. Chromatic confocal technology uses the chromatic aberration of lens to decode the axial displacement of the sample surface by the focus-wavelength of the reflected spectrum. Dispersion characters of the light source and sample surface greatly influence the reflected spectrum except the useful chromatic aberration, leading to focus-wavelength shift in many displacement measurements. Thus a novel and convenient strategy is proposed to correct the dispersion of the unstable light source or the alterable sample surface. The strategy provides a self-reference spectrum from a prior scanning process to normalize the reflected spectrum. Theoretical analysis demonstrates that the normalized reflected spectrum meets well with the point spread functions of the passive unit, which includes the fiber coupler, probe and spectrometer. Hence, the focus-wavelength of the normalized reflected spectrum is insusceptible of the light source and sample surface. Robustness tests with different light sources and sample surfaces showed that, the biggest wavelength shift of the self-reference strategy was reduced to less than 1/4 of that from the non-reference strategy. The results demonstrated that the proposed self-reference dispersion correction significantly improves the robustness and accuracy against different light sources and sample surfaces in chromatic confocal displacement measurement.

Automated summary

Dispersion is a common phenomenon in broadband optical systems that affects displacement measurements on sample surfaces. A novel strategy using self-reference spectrum corrects the dispersion from unstable light sources or alterable sample surfaces, making the focus-wavelength unaffected. Robustness tests demonstrate that the self-reference dispersion correction significantly improves measurement accuracy against different light sources and sample surfaces.