Miniature Hybrid Optical Imaging Lens

The present invention relates generally to multiple lens imaging systems, and more particularly to an extremely small and light weight lens system that corrects for imaging and chromatic aberrations whereby near-television quality picture transmission is achieved.

A large number of applications in today' technology require the use of small and light weight "television" cameras. Typical of such uses, but not limited to, are camcorders, surveillance cameras, etc. The physical laws of optics govern the formation of images, with refraction, reflection, and diffraction each affecting the transmission of light through a series of lenses. In addition to the curvature of each lens surface, image quality is affected by the wavelength of the light forming the image, since the index of refraction of glass is a function of wavelength. Traditionally, imaging aberrations are reduced by adding compensating lenses. Lens systems consisting of six lens elements are common. Lens systems of the Cooke triplet type (having three lens elements) represent the minimal practical configuration. Typical of miniature commercial lens systems are:

1. Securis Model TVX;

2. Sony Model XC-999;

3. Toshiba Model IK-M40A;

4. Fulnix Models TM-540 and TM-545W;

5. Watec Models WAT-906 and WAT-307;

6. Marshall Model V1206.

The larger number of lens units increases the difficulty of lens alignment (with fixtures). Also, the increase of number of lenses increases both the length and weight of the system. The typical "small" commercial lens system is typically 15 to 50 mm long, and weighs 3 to 8 oz.

Accordingly, it is an object of the present invention to provide a lens system having a shorter distance from the first lens element to its detector plane and good aberration reduction.

Another object of the invention is to provide a lens system having reduced weight compared to those of the prior art.

It is still another object of the present invention to provide a lens system wherein a very thin diffractive optical element is added to correct for aberrations without adding substantially to the weight of the system.

These and other objects of the present invention will become apparent upon a consideration of the drawings referred to hereinafter, and a complete description thereof.

In accordance with the present invention, there is provided a small and light weight lens system that utilizes six principal components arranged along an axis of the system. There is an initial optical stop to define the aperture of the system. In one embodiment a convex-plano refraction lens is positioned behind the stop, the curvature of the convex surface being spherical. A thin diffractive optical element immediately follows the refraction lens, either being attached thereto or formed thereon. In this embodiment, it is a sixteen level binary optical element designed at a center wavelength of 0.5876 micrometers. Element No. 4 is a field stop (baffle), and in this embodiment this is followed by a concave-plano lens, with the concave surface having a spherical curvature. Immediately following the last lens is the detector plane for receiving a semiconductor detector array or other form of imaging detector.

FIG. 1 is a schematic drawing illustrating a lens system developed to accomplish the above-stated objects.

FIG. 2 is an enlarged drawing illustrating the diffractive optical element utilized in the present invention, with a binary step surface approximation to the desired phase profile. Both illustrated contours are only simulations.

Referring first to FIG. 1, shown schematically therein at 10 is a lens system developed to accomplish the objects of the invention. There are six essential elements encased in a housing 12 having an optical axis 14. All of the enumerated elements are mounted to have an optical axis corresponding to the axis 14 of the system.

Light from a distant object enters the system 10 through a fixed aperture stop (it could be variable) 16. Spaced closely to this stop 16 is a refractive lens in the form of a convex-plano lens 18. The convex surface 20 of lens 18 is typically spherical. Although a refractive lens with other radii of curvature could be utilized, thereby giving more degrees of freedom for optimization, this particular lens is readily available and functions sufficiently for this application. Immediately adjacent to the planar surface 22 of lens 18, or formed as a part thereof, is a diffractive optical element 24. Details of the surface thereof are sketched in FIG. 2. This surface is formed to approximate the phase profile given by the following equation. ##EQU1## where .PHI.(r) is the phase (related to thickness), .lambda. is the design wavelength, r is the radius from the optical axis 14, and a.sub.2, a.sub.4, a.sub.6 are coeffecients for r.sup.2, r.sup.4 and r.sup.6, respectively. The phase distribution could be implemented by a continuously varying surface relief; however, such a surface is difficult to fabricate. It can be approximated by steps as a binary surface. Such a stepped surface can be formed by photolithographic methods, ion milling or other processes for providing very shallow and narrow steps. As stated, this can be formed directly upon the planar surface of the refractive lens 18, or can be formed separately and applied to that planar surface.

Immediately following the diffractive optical element 24 is a baffle 26 which serves as a "field stop", generally having an area greater than the stop 16. Another refractive lens 28, in the form of a concave-plano lens, performs as a field flattener. The concave surface 32 of lens 28 is typically spherical. In the embodiment illustrated, the radius of curvature of the concave surface 32 matches the curvature of the convex surface 20. However, as with the first lens configuration, other radii of curvature could be chosen to provide the needed correction for aberrations.

FIG. 2 illustrates how a continuously varying thickness for the diffractive element 24 can be approximated with binary steps. This is shown greatly enlarged for purpose of illustration. Further, the continuously varying curvature 34 is not that computed from the above-listed equation: it is just an illustration of the type of diffractive lens. It can be seen that this curvature 34 can be approximated by a series of steps 36, these being more easily fabricated. As set forth above, such a stepped surface can be fabricated using conventional photolithographic processes, by ion milling, etc., all of which will be known by persons skilled in the art. FIG. 2 only shows a one-dimensional radial contour of the diffractive element; the full surface being a surface of revolution generated by the one-dimensional contour around the optical axis 14. Further, this drawing illustrates a sixteen level lens as utilized in the present invention; however, any number of steps could be used to approximate the results of the computation of the equation.

An embodiment of the lens system described herein was constructed having the following specifications. The lens system had an F/# of 5, a focal length of 6.1 mm and a full field of view of 30.degree.. The aperture stop 16 had a clear aperture diameter of 1.22 mm. This was spaced 1 mm from the vertex of the convex-plano refractive lens 18. The radius of curvature of the convex surface 20 was 3.101 mm, and the thickness along the optical axis 14 was 3 mm. This lens was made from BK7 glass and had a diameter of 6 mm. The flat rear surface was etched by photolithographic techniques to form a sixteen level diffractive surface to approximate the above-cited equation. This rear surface was positioned in the system so as to be 0.5 mm from the field stop 26, this stop or baffle having a clear aperture diameter of 2.14 mm. The stop/baffle 26 was positioned 2.814 mm from the vertex of the final refractive lens element 28. This element 28 was fabricated from BK7 glass, with the concave surface 32 having a radius of curvature of -3.101 mm, a diameter of 6 mm, and a center thickness of 0.5 mm. The planar rear surface was positioned 0.1775 mm from the detector plane 30. These dimensions produced a total lens system only 8 mm long from the first lens element to the detector plane.

For this particular lens the diffractive element 24 was designed for an object two (2) meters from the lens and a center wavelength of 0.5876 micrometers and optimized for chromatic aberrations over the visible wavelength range of 0.4871 to 0.6563 micrometers. The values for a.sub.2, a.sub.4 and a.sub.6 in the previous equation were 8.475.times.10.sup.-3, -8.792.times.10.sup.-3 and 1.591.times.10.sup.-3, respectively, and where r and .lambda. are expressed in millimeters.

From the foregoing, it will be recognized that a very small lens system has been provided that has many uses. These uses include, but are not limited to, space projects, visual systems for robotic machinery, surveillance, and other applications where the camera must be as unobtrusive as possible.