A high resolution and long working distance bifocal interference confocal imaging technique

Abstract A bifocal interference confocal microscopy system (BICMS) is proposed in which bifocal lenses are used to overcome the tradeoff between high resolution and long working distance, which cannot be achieved simultaneously in devices with small numerical apertures (NAs). In this work, a bifocal lens is introduced in the proposed system to ensure signals (generating the interference) originate from the same point on the surface of the object to be measured. The annular interference region generated by the bifocal lenses can be used to sharpen the width of the central bright spot. This reduces the full width at half maximum for the normalized lateral intensity point spread function, thereby improving the stability, anti-disturbing capabilities, and lateral resolution. A theoretical analysis (using a wavelength of 632.8 nm and an objective NA of 0.1) demonstrated BICMS improved the lateral resolution of the confocal microscope from 2.34 μ m to 1.33 μ m, which is 1.7 times that of a CMS; in the case of NA = 0.2, BICMS improved the lateral resolution of the confocal microscope from 1.24 μ m to 0.92 μ m, which is 1.3 times that of the CMS. As such, the proposed system is bringing the possibility of a wide range of applications for high-resolution confocal microimaging systems.