Chromatic aberration correcting element and its application

A chromatic aberration correcting device having a diffractive surface, said diffractive surface comprising:.

• a central surface characterized by rotation symmetry with respect to said optical axis and
• a plurality of ring-shaped surfaces that are concentric with said central surface,
• wherein said central surface, one of said ring-shaped surfaces just outward of said central surface and the remaining ring-shaped surfaces are offset in position by the same step distance t in said optical axis direction, respectively, and
• wherein said diffractive surface is of optically substantial concave or convex shape as a whole when viewed macroscopically, and the contours of said central reflecting surface and said plurality of ring-shaped surfaces are elliptical and said distance t is given by:
• t=A.lambda..sub.0 m/2n
• where .lambda. is an arbitrary wavelength in the operating wavelength range;
• m is an integer;
• n is the refractive index of the medium which is located on the light incident side of said reflecting surfaces;
• and A is the ratio between the major and minor axes of the ellipse (major axis/minor axis);
• and
• wherein said central surface and said plurality of ring-shaped surfaces are reflecting surfaces.

An element which is disposed in an optical system having a light source emitting light of a substantially constant wavelength which shifts over an operating wavelength range due to operating conditions, said element comprising:

• a reflective-diffractive surface;
• wherein said reflective-diffractive surface comprises a plurality of planar ring-shaped reflective surfaces concentrically formed around a central planar reflective surface;
• wherein adjacent ring shaped surfaces are relatively offset in a direction that is parallel to an optical axis by a step distance t or an integral multiple of the step distance t;
• and
• wherein the reflective-diffractive surface is macroscopically convex or concave and wherein the contours of said central reflecting surface and said plurality of ring shaped surfaces are elliptical and said step distance t is defined by the following relationship:
• t=A.lambda..sub.0 m/2n
• where .lambda..sub.0 is an arbitrary wavelength in the operating wavelength range, m is an integer, n is the refractive index of the medium which is located on the light incident side of said reflecting surfaces, and A is the ratio between the major and minor axes of the contours.