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

• An amplitude CGH is a diffractive structure that contains alternating features that blocks 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 are 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, and amplitude CGH can be used.  If the optic has low reflectance (<=20%) at the test wavelength, a phase CGH should be used.