1. A method for quantitative optical phase imaging of a sample characterized by a surface, the method comprising: a. generating a first and a second replica field of an image field, each of the image field and the first and second replica fields characterized by a respective optical phase, the second replica field cross-polarized and shifted in a shift direction transverse to a normal to the surface of the sample by a transverse offset; b. spatially Fourier transforming the first and second replica fields; c. retarding the second replica field by four successive phase shifts; d. inverse Fourier transforming the first and second replica fields; e. passing the first and second replica field through an analyzer polarizer and superposing the first and second replica fields on a detector array to create four successive detector signals; and f. solving the four successive detector signals to derive a gradient of the optical phase of the image field.