What is the difference between an amplitude and a phase CGH?

An amplitude CGH is a diffractive structure that contains alternating features that block and transmit light.

A phase CGH is a diffractive structure that contains alternating features that add a phase shift to light.

The difference between the two types of CGHs is the amount of light diffracted in a particular direction. This is called Diffraction Efficiency.

The choice of using an amplitude or phase CGH is based on the reflectance of the optic being tested. As a rule of thumb, if the optic has high reflectance (>20%) at the test wavelength, an amplitude CGH can be used. If the optic has low reflectance (<=20%) at the test wavelength, a phase CGH should be used.