1988-01-22
1989-01-10
Corbin, John K.
G02B 1700
Patent
active
047969849
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical imaging systems and, in particular, to optical imaging systems for forming an image of an object at unit magnification.
U.S. Pat. No. 3,748,015 describes an optical imaging system for forming an image of an object at unit magnification and high resolution by means of convex and concave spherical mirrors arranged with their centres of curvature coincident. The mirrors are arranged to produce at least three reflections within the system and they are used in the system. Two off-axis conjugate areas at unit magnification are in a plane which contains the centre of curvature, the axis of the system being an axis normal to this plane and through the mirror centre.
Any unit magnification system that consists of two equal halves disposed symmetrically about a central aperture stop is intrinsically free from various types of aberration, in particular all orders of distortion and coma. The prior art system shares this characteristic, but it suffers from other aberrations and limitations which restrict its performance. In particular:
(a) It is inherent in the design that it only gives sharp imagery over a quite narrow annular area in the focal plane, so that it is necessary to use the system with a narrow arcuate slit aperture exposing only this area, and to copy the object (mask) to an image surface by scanning them both, in synchronism, across this aperture. This requires extreme mechanical precision if one micron or less is to be resolved; and the scanning process is obviously slower than direct exposure over the whole area.
(b) For any aberration free optical system, there is an inherent limit of resolving power, which is proportional to the numerical aperture (N.A.) and in the prior art system there is a practical limit to the attainable N.A. In the specific embodiment of the prior art (and in practical realisations thereof) this maximum N.A. is about 0.18 (equivalent at f/2.78) which gives a theoretical limit of resolution (at which the optical transfer function falls to zero), at a wavelength of 400 nm, at a spacing of 1.1 .mu.m. This is certainly inadequate for future requirements. A larger N.A. could only be realised by moving object and image further from the centre, and as this is done the aberrations increase, and the masking slit width decreases rather rapidly.
In order to overcome this drawback, we have devised a means of improving the correction of aberrations.
In Sov.J.Opt.Technol. 50(3), March 1983 153, there are described unit magnification imaging systems with compensation meniscus lenses. However, the design parameters of the systems described therein are constrained so that the monocentric meniscus is wholly on the object side of the secondary mirror so that it is not traversed a second time by light between the first and second mirror reflections. We have found that, contrary to this teaching, far greater improvement in imagery arise when the meniscus lies between the mirrors.
According to the present invention, there is provided a substantially unity magnification image forming system comprising at least one convex mirror and at least one concave mirror, said mirrors being supported with their centres of curvature substantially coincident and means to define a location for an object the image of which after at least three reflections including at least one reflection at each of said mirrors is a real image at a second location and further comprising a monocentric meniscus lens between said concave and said convex mirrors substantially to give overall correction for Petzval sum, to produce with said meniscus lens at said second location a stigmatic image of an object at said first location.
The invention will now be described, by way of example, with reference to the accompanying drawings, in which;
FIG. 1 is a diagrammatic representation of a prior art catoptric imaging system
FIGS. 2, 3, 4a and 4b are diagrams used in explaining the operation of the imaging system of FIG. 1
FIGS. 5a-b are diagrammatic representations of a second prior art imaging system; and
F
REFERENCES:
patent: 3748015 (1973-07-01), Offner
patent: 4171871 (1979-10-01), Dill
patent: 4293186 (1981-10-01), Offner
patent: 4344676 (1982-08-01), Shafer
Corbin John K.
Gass Rebecca D.
National Research Development Corporation
LandOfFree
Optical imaging systems does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical imaging systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical imaging systems will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2105276