Diffractive multifocal optical device

DETAILED DESCRIPTION The invention relates to a diffractive multifocal optical element comprising a phase zone plate containing annular concentric zones in which the zones are spaced substantially proportional to .
sqroot.
n and the zones possess stepped facets that introduce a discontinuity in optical path length of less than .
lambda.
/2 where .
lambda.
is the design wavelength.
The invention encompasses a phase zone plate containing annular concentric zones possessing facets which provide an alternating stepped repetitive pattern in accordance with .
sqroot.
n spacing in the optical element and wherein the depth of the steps of the facets are less than .
lambda.
/2(.
eta.
'-.
eta.
), where .
eta.
' and .
eta.
are the indices of refraction of the lens and the medium in which the lens is interacting and .
lambda.
is the design wavelength.
In a particular embodiment of the invention, the optical element comprises facets within the annular concentric zones providing an alternating stepped repetitive pattern wherein: 1.
the facet of one of the alternating zones has an inclined curved profile that is interrupted at the zone boundary by another curved profile providing the differently inclined curved facet of the other alternating zone, 2.
the zones are spaced substantially proportional to .
sqroot.
n, 3.
the depth of the facets are less than .
lambda.
/2, 4.
the zones are cut so as to direct yellow light to at least two primary focal points in at least adequate intensities for visual usage at each such primary focal point, and, 5.
but for the alternating pattern, the element would not have such intensity.
The invention encompasses a bifocal optical element of the Cohen lens design wherein the odd and even zones of the phase zone plate a.
conform to r.
sub.
n .
perspectiveto.
.
sqroot.
nd.
lambda.
spacing, b.
are contiguous and free of a non-refractive step interface at at least every other zone boundary, and leave a sloped profile at such contiguous interface, c.
the cross-section of each odd zone has the same general profile and the cross-section of each even zone has the same general profile, d