Beam scanning chromatic confocal microscopy for high speed three-dimensional measurement

Abstract

The longitudinal depth scanning of confocal microscopy generally has been depending on the mechanical movement of the objective lens or the sample. A chromatic depth scan was proposed to avoid the mechanical translation in confocal microscopy. The chromatic depth scan was effectively realized by focusing the incident broadband light source to different axial positions through an intentionally generated longitudinal chromatic aberration. Since the different spectral components of the light source are focused onto the planes at different depth levels, the polychromatic illumination and the spectrum analysis of the light reflected from the sample can possibly work as an alternative method for mechanical depth scanning. The chromatic depth scanning confocal microscopy is the promising technique for high speed three-dimensional measurement as it does not require the mechanical translation in axial direction. To obtain three-dimensional surface profile, transverse scanning should be accompanied with the longitudinal depth scanning. The implementation of transverse scanning can be categorized as stage scanning, moving pinhole scanning, and moving beam scanning as in confocal microscopy. Stage scanning is a form of using a stationary beam and a moving object. It is conceptually simple and technically accurate as diffraction limited optics can be easily implemented for the on-axis focus. The drawback of stage scanning is that its imaging speed is very slow and it is difficult to implement when the samples are large and heavy. Moving pinhole scanning is conducted by changing the position of the source and detection conjugate points in an image plane. A rotating Nipkow disk using the parallel pinhole array to scan the transverse plane is a good example of moving pinhole scanning. It can greatly improve the imaging speed; however, it has the disadvantages of low light efficiency and high backscatter noise. Moving beam scanning is done by changing the incident angle of...