# Infinite conjugate imaging with a Radial Polynomial Grating

The Radial Polynomial Phase Departure option on a Two-point holographic optical element grating definition allows for radial phase (OPL) to be added to a grating definition using the following form:

The Radial Phase polynomial can be used to implement the parabolic approximation of a spherical wavefront by setting the A_2 coefficient of the polynomial to 0.5*n'/d' and using a diffraction order of -1. In this construction, n' is the refractive index of the transmitted wavefront and d' is the distance of the image point from the grating surface.

For example, consider a plane wave incident on a grating with a desired focus position at d=25mm. If the transmitted refractive index is 1.0, then A_2 = 0.5*(1/25) = 0.02. This configuration is shown in the example file below.

Example File: RadialPolynomialInfinateConjugate.frd