Phase shift multifocal zone plate

DETAILED DESCRIPTION The purpose of a multifocal lens, is to increase the number of foci to which incident light will converge.
Unfortunately, whenever this has been done, it has been at the expense of the image brightness at each of the foci.
The present invention makes use of the fact that multifocal zone plates have many subsidiary zone plate foci in addition to the multiple Fresnel lens foci.
By properly adjusting the annular zone spacings in a multifocal zone plate, we can shift some of the zone plate foci until they coincide with some of the Fresnel lens foci.
In this case there will be a significant increase in intensity of light at these double foci.
The present invention is summarized in a multiple power optical device having a plurality of alternating odd and even, annular, concentric zones, bounded on the outside by radii r.
sub.
n, with n=1,3,5, .
.
.
, for the odd zones and n=2,4,6, .
.
.
, for the even zones; first focal power means within at least some of the odd zones for directing incident parallel light to a first focal point; second focal power means within at least some of the even zones for directing incident parallel light to a second focal point different from said first focal point; wherein the radii r.
sub.
n of said odd and even zones are substantially proportional to the square root of n; and wherein the absolute value of r.
sub.
1 is set equal to .
sqroot.
.
lambda.
d, with .
lambda.
equal to the wavelength under consideration, and d is substantially equal to the reciprocal of the absolute value of the difference between the first and second focal powers.
An important advantage of this invention, is that its performance as a multifocal optical element is not limited by dim focal images inherrent in the usual multifocal lens designs.
Other objects and advantages of the present invention will be more fully apparent from the following description of the preferred embodiments when taken in conjunction with the accompanying drawings.