A 'Tuned' fresnel lens.

In this memo we describe a technique to manufacture Fresnel lenses where the grooves are "phase-synchronized" at a given wavelength. This means that the light from different grooves will superpose coherently at the focal point, at the specified wavelength. Such a "tuned" Fresnel lens is akin to a phased- array antenna, its focusing performance will be better than that of ordinary Fresnel lenses by orders of magnitude and can, in principle, equal the performance of a conventional lens of the same diameter (diffraction limit).