Radially Symmetric-Tangent Phase Mask to Obtain Invariant Imaging System to Defocus

In this paper, we propose a radially symmetric tangent phase mask to obtain the modulation transfer functions nearly symmetric through the in-focus plane and more invariant over a wide range of defocus. Imaging performance of the proposed phase mask is compared with the quartic phase mask and conventional imaging system based on the use of the evaluation functions, such as the modulation transfer function, the point spread function and the simulation images. The results demonstrated that the proposed phase mask has superior imaging performance in extending the depth of field.