Comparative study of defocus techniques in wavefront sensing systems utilizing dual out-of-focus intensity images

In this study, we have analysed and compared different techniques for generating defocus in wavefront sensing systems that rely on dual out-of-focus intensity images for wavefront measurement. A mechanical displacement (MD) approach uses a stage to physically move a sensor to obtain the two out-of-focus images and process them. This approach enables the full resolution of the sensor, avoiding the use of additional optics, and has high repeatability (dependent on the stage). The challenge with MD is that the use of a mechanical stage makes it unsuitable for video recording or for applications with in vivo samples that require fast image acquisition. The second is a multiplexing approach using a prism and a beam splitter to capture two out-of-focus images simultaneously, in the same sensor. This approach allows video recording and real-time image acquisition, necessary for certain test objects. However, the drawbacks may be a reduction in the intensity of the light source, due to the beam splitter, possible lower resolution, the need for a larger sensor, and its fixed propagation distance along the optical paths. An electrically tunable lens (ETL) approach obtains the two out-of-focus images by changing the focal plane of the lens, thereby varying the object propagation distance at a high acquisition rate, allowing video recording and the use of the full sensor resolution as no additional optics are required. However, this approach might be limited in terms of spatial resolution and exposure time due to the diameter of the ETL. The goal of this study is to systematically evaluate and compare the three approaches, enabling the identification of the most optimal method for specific applications by critically assessing their respective advantages and limitations.