Rapid Autofocus Method for Optical Microscopes Using Dual Photodetectors

摘要

Aiming at solving the problems of low-efficiency and high-cost of existing auto-focusing methods for microscopic imaging, an on-line auto-focusing method is proposed and developed using dual photodetectors. Firstly, two identical photodetectors are placed in symmetrical positions with equal distance away from the focal plane in the image space. One is in front, the other is behind the focal plane. The photodetectors convert the light intensity signals originating from the sample into two electric signals, and their difference is proportional to the amount of defocus. Using a pre-calibrated curve between the differential voltage signal and the amount of defocus, the actual distance of the current sample from the focal plane can be measured in real time. The sign of the differential signal determines the direction of defocus, i.e., before or behind the focal plane. The sign and magnitude of the differential signal is used to control the direction and number of steps of a motor, which drives the carriage to move toward the focal plane and bring the sample into sharp focus, realizing the on-line automatic focusing of the microscope. The results of experiments show that the system can achieve efficient and stable automatic focusing. Under 40X objective lens, the effective working range of the proposed method is ±40um. Within this working range, the time required to achieve auto-focusing is 0.90702s, with the majority time, i.e., 0.907s spent on mechanical movement while 0.02ms for sensing the defocus. The major advantage of the proposed method is its high efficiency defocusing sensing at an updating frequency of 50k Hz. Further, it is easy to implement and with a low cost. Therefore, it is reasonable to expect that the proposed autofocusing method would have wide applications include classroom teaching, laboratory microscopic imaging, and inline product quality inspection.