Auto Focusing Under Microscopic Views

Microscopic auto focusing (AF) is an important technique in applications for precision measurement and inspection. This paper presents an AF algorithm composed of three stages (i.e., initial search for direction, a rough search, and a fine search). Sophisticated decision makings and recovery mechanism are built in the algorithm to enhance the overall performance. It is illustrated that a previous measure of image focus value (FV) could be subject to low signal-to-noise ratio. The variance of sub-windowing measures is proposed to enhance the ratio and improve the AF reliability and accuracy. Experimental tests have been widely conducted. Selected results of the tests are included in this paper. Good overshoot responses are achieved via adaptation of the AF step sizes. The steady-state focusing errors are within the tolerance of depth of focus for each lens. Robustness against undesired local peak and abnormal conditions is enhanced by a designated significant level of FV and an integrated recovery mechanism. The AF time spreads over the range of 400-1700 ms and it is less than 1000 ms in average.