Can binary phase masks increase the depth of field of panchromatic cameras in practice?

Abstract

A phase mask in the aperture stop of an imaging system can enhance its depth of field (DoF). This DoF extension capacity can be maximized by jointly optimizing the phase mask and the digital processing algorithm used to deblur the acquired image. This method, introduced by Cathey and Dowski with a cubic phase mask, has been generalized to different mask models. Among them, annular binary phase masks are easy to manufacture, and can be co-optimized with a simple unique Wiener deconvolution filter. Their performance and robustness have been characterized theoretically and experimentally in the case of monochromatic illumination. We perform here a theoretical and experimental study of codesigned DoF enhancing binary phase masks in panchromatic imagers. At first glance, this configuration is not optimal for binary phase masks. Indeed, the binary phase masks are most often manufactured by binary etching of a dielectric plate, so dephasing depends on the wavelength. The π radians dephasing is reached for only one wavelength. How do phase masks optimized for a particular wavelength respond to a wide illumination spectrum? Is it possible to take into account the illumination spectrum in the co-optimization of phase masks? What impact does this have on the result? We analyze the behavior of DoF enhancing phase masks in panchromatic imagers in terms of Modulation Transfer Function and of final image quality. The results are experimentally validated with imaging experiments carried out with a commercial lens, a Vis-NIR CMOS sensor and co-optimized phase masks. We study different phase masks co-optimized for different spectrum of illumination. We show that masks specifically optimized for wide spectrum illumination perform better under this type of illumination than monochromatically optimized phase masks under monochromatic illumination, especially when the targeted DoF range is large.