Fast image-free autofocus method for passive FSPI microscopy.

Autofocus is crucial for capturing sharp images with imaging devices for information acquisition. Traditional autofocus strategies based on post-processing become less efficient for passive FSPI microscopy of yet low temporal resolution. In this Letter, a fast and image-free autofocus system is proposed for passive FSPI microscopy. Based on the complementary design of an optical path, the system can measure the focus degree at 5000 fps while maintaining a high light efficiency for imaging. The proposed system can be easily combined with existing trinocular microscopes, which provides a welcomed boost to the practicability of passive FSPI microscopy.