Novel Chromatic Differential Confocal Matrix Sensor Using Multiband Spectral Images

In this letter, a novel chromatic differential confocal matrix (CDCM) sensor is proposed and demonstrated for accurate and fast three-dimensional (3D) profiling without mechanical scanning. In contrast to conventional chromatic confocal microscopy (CCM) methods that require spectral expansion and a peak-intensity-wavelength detection algorithm for axial positioning, our CDCM employs the intensity difference between a pair of spectral confocal images to accurately discriminate surface height. Due to the elimination of spectral expansion required by a conventional CCM, the CDCM has an at least one order of magnitude higher axial sensitivity and surface profiling efficiency than a conventional CCM. In the CDCM, axial scanning is achieved by residual chromatic aberration of an achromatic objective and by constructing a difference intensity signal between two spectral confocal images. Spectral confocal images are acquired in a parallel manner by using a digital micromirror device as an array of illumination pinholes and digitally applying the virtual detection pinhole concept. Quantitative evaluation experiments with the proof-of-principle sensor system indicate that with a <inline-formula> <tex-math notation="LaTeX">$10\times $ </tex-math></inline-formula>NA 0.25 objective, the CDCM can achieve better than <inline-formula> <tex-math notation="LaTeX">$0.1~\mu \text{m}$ </tex-math></inline-formula> axial accuracy with an axial measurement range of approximately <inline-formula> <tex-math notation="LaTeX">$28~\mu \text{m}$ </tex-math></inline-formula>, a lateral resolution of <inline-formula> <tex-math notation="LaTeX">$0.87~\mu \text{m}$ </tex-math></inline-formula>.