Phase Mask Increases Depth of Focus

Hank Hogan

Anyone who has taken a picture with a camera only to find some features out of focus will appreciate the recent efforts of a team from Bar-Ilan University in Ramat Gan and from Tel Aviv University, both in Israel. The group has increased the depth of focus of an imaging lens by inserting a nondiffractive binary phase mask at the entrance pupil of the lens. Because the all-optical approach requires no digital postprocessing, it is suitable for use also in ophthalmic applications, said Zeev Zalevsky, a member of the faculty at Bar-Ilan University.

An all-optical approach employing a nondiffractive binary phase mask improves the depth of focus of optical systems. In a setup without the phase mask, close features are out of focus (a). In the same setup with the mask, both near and far features are in focus (b). Courtesy of Zeev Zalevsky.

Key to the approach is the mask, a phase-only element in which some regions generate phase retardation of 90°, or π/2, while others generate none. The phase-retarding features stand about 250 nm high and are created using a photolithographic or stamping technique and etching. This process yields a pattern of features that does not repeat often. Because of this low spatial frequency, the masks are not wavelength-sensitive, and they transmit light well. The effect of the selective phase retardation on incoming incoherent light is an extension of the depth of focus of the lens.

Zalevsky noted that the phase mask must be designed for a given lens but that coming up with the proper layout is not difficult, even on a desktop computer. “The calculations are very, very simple and take less than one second,” he said.

The investigators have constructed phase masks for a variety of lens systems. Only one mask is needed for a given system, even if it consists of three, four or more sublenses. Using the method with a cell phone camera that had an initial minimum focusing distance of 50 cm, they imaged objects as close as 15 cm without sacrificing the ability to focus on objects at infinity.

Zalevsky said that the group is working on improving performance and has developed a new version. Possible uses include camera, endoscopic, medical and ophthalmic applications such as glasses.

The research was funded by Xceed Imaging Ltd. of Petach Tikva, Israel. The company holds the patents and plans to commercialize the technique.

Optics Express, April 3, 2006, pp. 2631-2643.