Simulfocus Imaging: Quasi-simultaneous Multi-focus Imaging
A normal camera can only capture an image at a certain focal length at a time. If it is necessary to take several images with different focal lengths, it is necessary to change the focal length for each image. Conventional methods of image synthesis such as holography and light fields have been used to solve this problem, but these methods require image reconstruction process based on the measured information.
In response to this, we have developed Simulfocus Imaging, a technique to capture multiple focal points simultaneously, in collaboration with Professor Shoji Kawahito's group at Shizuoka University. The advantage of this technique is that it can optically capture images at multiple focal lengths and does not require image reconstruction process.
The technology is based on two devices:
1. a TAG lens, which is a resonant liquid lens that oscillates its focal length at about 70 kHz
2. a Multi-Tap Lock-in Pixel Image Sensor, which has multiple image storages called taps and can independently perform multiple exposures with nanosecond accuracy
A single camera system can capture images that are in focus at several different locations at virtually the same time. The following movie demonstrates simulfocus imaging with the microscope and the telephoto system.
The oscillation of the TAG lens is very fast, and its first cycle is very short, about 14.5 us. For this reason, a short exposure time of about 100 ns is required to capture only images with a specific focal length. However, only very dark images can be measured in this short exposure time. On the other hand, since the TAG lens repeats the same oscillation, the focus position passes through the same place many times. Therefore, we succeeded in obtaining a practical brightness by taking an exposure each time it passed through a specific position and integrating the information obtained there. An overview of this operation is shown in Figure 1.
Movie. Simulfocus Imaging of swimming cells, an approaching person and cars.
Figure 1. Schematic figure of the principle of Simulfocus Imaging
Kazuki Yamato, Yusuke Tanaka, Hiromasa Oku, Keita Yasutomi, and Shoji Kawahito : Quasi-simultaneous Multi-focus Imaging Using Lock-in Pixel Image Sensor and TAG Lens, Optics Express, Vol. 28, Issue 13, pp. 19152-19162 (2020) [doi:10.1364/OE.394760]