Powerless field-corrective lens

DETAILED DESCRIPTION Curvature of an optically imaged field, denoted .
rho.
, arises in an optical imaging system when off-axis illumination of the imaging optics is utilized.
As the illumination moves further from the optical axis, field curvature increases.
The field curvature is a function which depends upon the profile of the lens within the imaging system as can be defined as follows: ##EQU1## where y is defined as the distance from the optical axis to a given annular zone and R is the lens curvature.
The term K is a constant of proportionality required to form the equality.
In association with the teachings of the present invention, a corrective field curvature -.
rho.
thus may be introduced to essentially cancel an inherent field system curvature .
rho.
.
FIG.
1 illustrates a side view of an exemplary powerless non-planar field curvature corrective lens 10 which is capable of introducing the field curvature required to essentially cancel the inherent system field curvature.
As shown, lens 10 includes a flat (i.
e.
, plano) surface 12 and a series of stepped zones 14, building upon plano surface 12 so as to form a stepped profile plano-convex structure.
Each zone 14 is configured to comprise a step run 16 parallel to surface 12 and a step rise, or height, 18 perpendicular to surface 12, where this parallel/perpendicular configuration ensures that lens 10 imparts no additional power (i.
e.
, magnification) to the system.
The step height S is chosen to provide a 2.
pi.
m (m=1,2,3, .
.
.
) phase shift to the optical wavefront passing therethrough, where field curvature corrective lens 10 is designed so that each step comprises an essentially identical height S.
As will be discussed in detail below, there exist applications where the step height S varies across the profile of the device, for example, when correcting for spherical aberration, as discussed above and described in more detail below.
However, for arrangements such as that illustrated in FIG.
1 where the only required correction is for field curvature, the step height S will remain essentially constant