Hybrid refractive/diffractive achromatic camera lens and camera using such

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to hybrid refractive/diffractive lenses and particularly to a refractive/diffractive achromat which is especially suitable for use as a taking (or an objective) lens in inexpensive cameras such as single-use cameras.

2. Description Relative to Prior Art

In order to obtain photographs with good quality images, the lens that focuses the light must be well corrected for aberrations. It is not enough for the objective lens to be corrected for monochromatic aberrations. The lens must also be corrected for chromatic aberrations for a relatively broad range of wavelengths. For each color or wavelength of light incident on a refractive lens, the lens will have a different focal length. It is this property of the lens that give rise to (longitudinal and lateral) chromatic aberrations. Currently, the objective lenses for cameras correct chromatic aberrations by using additional lens elements. However, this creates additional bulk and makes the lens system heavier and more expensive. These considerations are especially important for single use cameras which need to be light weight, compact and inexpensive.

Single-use cameras typically include a one or two element lens utilized at a large F/# so they can be used In a fixed focus mode where everything from two meters to infinity is nearly in focus. Single-use cameras of a single lens element type typically are not corrected for chromatic aberrations, which all singlets tend to suffer from. Lenses used for single-use cameras generally have relatively high levels of monochromatic and chromatic aberrations. Some of the monochromatic aberrations can be corrected in a plastic molded singlet element through the use of aspheric surfaces. However, at some point, the chromatic aberrations will be significantly worse than the monochromatic aberrations therefore limiting the minimum spot size. The resulting unachromatized images can also exhibit color fringing.

Current single-component objective lenses used in single-use cameras are made of low dispersion, low index of refraction materials (usually plastic) to minimize longitudinal chromatic aberration. Thus, in order to reduce the difficulty of correcting for chromatic aberration in a single-element lens system, lens designs have been driven in the direction of reducing dispersion (using low index, high Abbe number glass) in order to obtain the necessary power and reduce the numerical aperture (NA) of the lens. Higher curvature, thicker lenses have therefore been required. Such thicker lenses give rise to manufacturing errors since they are more sensitive to variations in lens thickness, wedge, tilt, and decentering.

Additional lens elements are used to provide chromatic aberration correction in multi-element, more costly, lens systems. When a cemented doublet (comprised of a positive and a negative power lens element) is used to correct for chromatic aberrations, a negative power lens element made of flint glass (i.e. glass having a low Abbe number) is cemented to the positive lens element which is typically made from a crown glass, However, because the negative lens element increases the focal length, the positive lens element is made stronger to compensate for that change in order to keep the original focal length. In order to obtain the necessary power, the positive lens element will thus need to have stronger radii of curvature and to be thicker. Such lenses also sacrifice weight and size in order to accommodate surfaces and elements which compensate for chromatic aberration. Alternatively, two air spaced, roughly symmetrical, lens elements separated by an aperture stop can also be used to get a better system performance. However, an additional element again increases weight and size of the system. Finally, when designing a single element optical system, a designer may use low dispersion glasses that still have a high index of refraction. However, such glasses are expensive.

Although various patents and publications have discussed the use of diffractive elements to compensate for chromatic aberration (see U.S. Pat. Nos. 4,768,183, 5,117,433, 5,044,706, 5,078,513, 5,117,306, 5,161,057, 5,208,701, and 5,229,880), designs for objective or taking lenses in single element cameras have not had any chromatic correction and typically have relatively steep surface curvatures. As previously mentioned, in order to avoid these and other problems, some single-use camera lens systems include two lens elements separated by an aperture stop. Similarly, consumer camera lenses in visible light applications, such as for taking photographs of friends, relatives or nature, use multiple lens elements to correct for chromatic aberrations.

SUMMARY OF THE INVENTION

The present invention deviates from the conventional wisdom in the field of optical design for camera lenses (such as optical objectives) operating in visible spectrum by modifying one of the surfaces of the plastic lens (or a glass lens) in a simple camera objective such as the one used in a single-use camera to achromatize the lens in the visible spectrum with a diffractive surface. This invention is especially useful in simple cameras using lenses with F-numbers of f/5.6 and higher and especially those lenses of f/8-f/12 range and even more preferably f/9 to f/11 range. This is because this lens needs to be produced relatively inexpensively, should not be bulky, needs to have as few elements as possible and yet is able to produce good pictures where the object being photographed may be far away or may be standing as close as two meters away from the photographer. By using a correctly designed diffractive surface the level of aberrations can be reduced by a factor of two or more and the lens can be achromatized over the entire visible spectrum even for a single element. Diffractive surfaces can also incorporate aspheric terms at low cost, thereby, obtaining a thinner, lighter lens. It has been discovered in accordance with the invention that the chromatic aberration (of the light in at least 150 nm range) introduced by a refractive portion of the lens can be corrected by a diffractive portion which is incorporated into the lens thereby providing an improved hybrid refractive/diffractive achromatic lens for an objective camera lens application such as a single-use camera.

The invention also provides an improved taking or objective lens such as one for use in a single-use camera and has one or more of the following aspects: (a) high polychromatic MTF such as MTF value of about 0.4 or higher at a broad range of frequencies; (b) improved chromatic correction in visible spectrum; (c) a singlet rather than a multi-element configuration; and (d) small size and light weight.

Objective camera lenses, including single lens camera lenses, have to focus light in a band-width of almost a 300 nm wavelength range (e.g., from 400 nm and above to 680 nm (with the emphasis on a center portion of this region). A hybrid refractive/diffractive achromat for use in cameras in accordance with the invention comprises a body of optically transmissive material having an index of refraction at a wavelength approximately in the center of said range of at least 1.45, said body having first and second surfaces on opposite sides thereof, said first and second surfaces being intersected successively by an optical axis of said lens which extends in a longitudinal direction, at least one of said surfaces being curved to provide a refractive portion having optical power and introducing longitudinal chromatic aberration, said lens having a diffractive portion having optical power which substantially achromatizes said lens for said chromatic aberration of said refractive portion over at least 490 to 650 nm or at least 440 to 625 nm region.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will become more apparent from a reading of the following description in connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a refractive/diffractive hybrid lens 10 in accordance with the invention shown spaced from photographic film upon which an image is formed, the lens having a curved surface 1 and a diffractive grating surface 3 formed on curved surface 2, the features on the diffractive surface being too small to be seen on the scale of the Figure;

FIG. 2 is a sectional view of the lens shown in FIG. 1 but the diffractive surface features are greatly magnified;

FIG. 3 is a diagrammatic, perspective view of a Fresnel zone pattern which may be formed as by blazing on the diffractive surface of the lens shown in FIGS. 1 and 2, where .lambda..sub.0 is the design wavelength, m is an integer greater than 0, f is the focal length and F designates the focal point;

FIG. 4 is a greatly enlarged side view of the diffractive surface of the lens shown in FIGS. 1 and 2 showing the surface blaze profile of a few zones, the actual thickness or height h of each zone being of the order of 0.8-1.4 .mu.m and the spacing between the zones actually being of the order tens of microns (40 .mu.m-600 .mu.m);

FIG. 4A is an enlarged vie of a single blaze step of the side view of FIG. 4;

FIG. 4B is an enlarged vie of a portion of the blaze surface of the single blaze step of FIG. 4A;

FIG. 5 is a plot of the efficiency of the lens for design wavelength .lambda..sub.0 of 587.6 nm;

FIG. 5A is a schematic diagram of a prior art lens having a binary-type diffractive surface;

FIGS. 6A and 6B are plots of the ray aberration of the refractive diffractive lens of FIG. 1 for on axis, 0.7 field as well as full field. The full field is .+-.32.degree.. FIG. 6A shows the ray aberration corresponding to the vertical axis of 0.40 mm (.+-.0.20 mm) while FIG. 6B shows similar curves where the vertical axis is one wavelength (.+-. 1/2.lambda.);

FIG. 7 is a plot of the on axis, polychromatic MTF of the refractive/diffractive lens of FIG. 1;

FIG. 8 is a plot of an on axis, polychromatic point spread function of the refractive/diffractive hybrid lens of FIG. 1;

FIG. 9 is a plot of the optical path difference in the image plane generated by the refractive/diffractive lens of FIG. 1;

FIG. 10 is a plot of the polychromatic MTF corresponding to the 0.7 field, i.e., a half field angle of 22.4.degree. of the refractive/diffractive hybrid lens of FIG. 1;

FIG. 11 is a plot of a polychromatic point spread function corresponding to the 0.7 field, i.e., a half field angle of 22.4.degree. of the refractive/diffractive hybrid lens of FIG. 1;

FIG. 12 is a plot of the polychromatic MTF corresponding to the full field, i.e., a half angle of 32.degree. of the hybrid lens of FIG. 1;

FIG. 13 is a plot of a polychromatic point spread function corresponding to the half field angle of 32.degree. of the hybrid lens of FIG. 1;

FIG. 14 is a schematic diagram of a second embodiment of a refractive/diffractive hybrid lens made in accordance with the invention and shown forming an image on a photographic film, the lens having a curved surface 1' and a diffractive grating surface 3' formed on curved substrate surface 2', the features on the diffractive surface being too small to be seen on the scale of the Figure;

FIG. 15 is a graph of a diffraction efficiency profile of the lens system of FIG. 1 where the diffractive profile is optimized for .lambda.=0.540 .mu.m; to FIG. 16 is a graph of a diffraction efficiency profile of the lens 20 shown in FIG. 14 where the diffractive profile is optimized for .lambda.=0.510 .mu.m;

FIG. 17 is a schematic drawing of the manufacturing apparatus used to produce diffractive surfaces 3 and 3' of refractive/diffractive hybrid lenses shown in FIGS. 1 and 14;

FIG. 18 shows a sharp diamond tip with a small flat used in an apparatus of FIG. 17;

FIG. 19 shows a prior art rounded diamond tip;

FIG. 20 shows a sharp diamond tip with no flat used in an apparatus of FIG. 17;

FIG. 21 shows that a diamond tip of FIG. 18 is canted at an angle relative to the work piece;

FIG. 22 shows final surface roughness characteristics of the preferred embodiment as seen through the atomic force microscope;

FIGS. 23A-D are plots of polychromatic MTF curves corresponding to 0, 0.5, 0.7 and full field of view of lens 20 of FIG. 14;

FIG. 24 is a schematic drawing showing how lens 10 or 20 may be molded;

FIG. 25 is a schematic diagram of a third embodiment of a refractive/diffractive hybrid lens made in accordance with the invention and shown forming an image on a photographic film, the lens having a curved surface 1" and a diffractive grating surface 3" formed on curved substrate surface 2", the features on the diffractive surface being too small to be seen on the scale of the Figure; and

FIGS. 26A-F are respectively plots of polychromatic MTF curves corresponding to 0, 0.4, 0.6, 0.75, 0.9 and full field of the lens 30 of FIG. 25.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a diffractive/refractive hybrid lens 10 for use in visible light camera applications and more specifically for use in a single-use camera 100 having a camera body 5 and a photoghraphic film 15 supported at a suitable location so that an image is produced by the lens on the film. The lens 10 is a convex-concave single element or singlet having from an object side a convex-curved surface 1 and a Fresnel zone pattern 3 on the concave-curved surface 2 of the lens body which is the surface opposite to the first curved surface 1. Both surfaces 1 and 2 are perpendicular to the optical axis of the lens. The refractive lens is made from optically transmissive material having an index of refraction of at least 1.49. Suitable and preferable material is any moldable material such as optical plastic or glass. If the lens element will be molded, a plastic material is more preferable because it is cheaper and easier to mold. Alternatively, if the lens is not to be molded, a diffractive zone pattern may be diamond turned or cut on the lens surface. If the lens is not to be molded, the lens material does not need to be suitable for molding purposes. FIG. 2 shows the lens 10 and emphasizes its curved surface 2 which defines the refractive portion of the element as well as the Fresnel zone pattern 3 which defines the diffractive portion of the element. An annular ring 16 is part of the lens body and is merely for attachment and location in a camera barrel. The lens F-number is f/11 and the lens accommodates a field angle of .+-.32 degrees. The overall thickness of the lens may be less than 4 mm and it is preferred that it be about 1.0-3.0 mm (millimeters). It is 1.4 mm in this embodiment. The focal length of lens 10 is 35 mm and it accommodates the field angle of .+-.32.degree.. The base radius of the curved surface (or substrate) 2 on which the diffractive surface 3 of the lens 10 is formed (shown in FIG. 3), is located is 11.43 mm to a point along the optical axis on the right of that surface. The diffractive surface 3 of the diffractive portion of the lens element 10 has a radius of curvature that corresponds to an effective focal length f of 364.2 mm. Exemplary dimensions and spacings are set forth in Table 1, the index n.sub..lambda.n n is at the center of range being measured at the nominal wavelength .lambda..sub.n =587 nm (or 0.587 .mu.m) and n.sub..lambda.n is 1.496.

TABLE 1 ______________________________________ Sur- V face Radius Thickness Material Index Number ______________________________________ 1 7.42 1.4 Plexiglass 1.492 57.3 2 11.4343.sup.1 0 Plexiglass 3 NA.sup.2 0 10,000 -3.5 Air 4.309 Air 1. 1. Stop 28.3096 Image -120.000.sup.3 plane ______________________________________ 1 Base radius is 11.4343. This surface is an "internal" surface used for design purposes, i.e. there is no index difference between surfaces 2 and 3 in this embodiment. 2 This surface profile is an asphere; radius of curvature corresponds to focal length of = 364.2. The aspheric profile of the surface as described by equation 12; and where AD = 0.8265207E 8; AE = -0.1041272E 8; AF = 0.612808E 10; and AG = -.1356105E11. 3 Cylindrical shape.

The lens of the first embodiment has a nominal or center wavelength of .lambda..sub.n =.lambda..sub.d =587 nm. The lens is achromatized for 480 to 680 nm bandwidth around this center wavelength. Specifically, the design wavelengths are: .lambda..sub.f =486 nm, .lambda..sub.d =587 nm, and .lambda..sub.c =656 nm.

The depth h of echelons in the zones is shown exaggerated in FIGS. 2 and 4 and may be of the order of a 0.8 to 1.4 micron and it is preferable that they be 0.9 to 1.2 microns. The spacing d between the zones in this embodiment is between 40 and 600 microns (.mu.m). It is preferable that the spacing d be on the order of tens of microns, but it can be 1 to 1000 .mu.m.

It is preferable that the diffractive surface 3 be formed on the curved (base) surface such as surface 2 which acts as a substrate. In this embodiment, the diffractive surface is formed on a concave surface with radius of curvature of 11.43 mm. However, the concave surface 2 in this embodiment is an internal surface of the lens and is not a real, separate surface because the same index material is used to mold the entire lens.

The achieved achromat is a single element or singlet, but in effect works as a cemented doublet because the refractive and diffractive portions of the lens element work together to add to the final total power. It is preferred that the refractive portion of the lens component has 85-97% of total power of the lens component. Table 1, which will be discussed in greater detail below, shows that about 90% of the total power of the achromat is in the refractive portion of the lens element and about 10% is in the diffractive portion of the lens element. Therefore, the achromat behaves much like a regular singlet. Since both surfaces of the refractive portion of the lens element are curved (i.e. the front surface 1 and the internal or substrate surface 2), there are at least two degrees of freedom to modify the lens in order to control aberrations. In addition, one or both of the real or actual surfaces (surfaces 1 and/or 3--i.e. external surfaces) may be aspheric. In this embodiment, aspheric terms on the diffractive surface allow for better aberration correction. The diffractive surface compensates for longitudinal chromatic aberration, but also because of introduction of higher order terms (4th order, 6th order, 8th order and 10th order corresponding to AD, AE, AF and AG coefficients), in the phase function [equation (4)] of the diffractive surface 3, monochromatic aberrations, such as spherical aberration and coma are also substantially corrected.

Consider the design of the diffractive surface 3. The design takes advantages of the wave nature of light. Light travels in waves, which can interfere. If the waves interfere such that the peaks and valleys coincide, the energy in the two waves adds to each other; this is referred to as constructive interference. Note that if one of the waves is delayed exactly one or more wavelengths behind the other, then it is once again in phase, and they will interfere constructively. If the waves line up out of phase, the energy in one wave will cancel the energy in the other; this is referred to as destructive interference.

To design the diffractive surface, a diffractive zone pattern is used, as shown in FIGS. 3 and 4. Such a zone pattern consists of multiple zones Z.sub.i. A focal point, F, is designated at a distance, f, from the center of the pattern. This distance is equal to the focal length. The rings, or zones, are spaced such that the edge of each zone is exactly one wavelength further away from the point F. This way light passing through the pattern at the edges of the zones will be in phase and constructively interfere at the point F.

Using right triangles, an equation can be derived that gives the zone radius or zone spacing r.sub.m as a function of the focal length f (distance from the pattern to F) and the wavelength of light .lambda..sub.0 used to design the zone pattern (i.e. blaze wavelength) and m is a zone number:

r.sup.2 +f.sup.2 =(f+m.lambda..sub.o).sup.2. (1)

Assuming the wavelength of light is much smaller than the focal length, Equation (1) can be reduced to: ##EQU1##

From Equation (3), it can be seen that the diffractive surface has a strong dependence on the wavelength of light used to construct the zones. If the wavelength of light incident on the diffractive surface deviates from the design wavelength, the focal length also changes. This is an important property when the diffractive surface is used to achromatize the refractive element.

Although the light propagating from the edge of zone is in phase when it gets to the focal point F (FIG. 3), light coming through the middle of each of the zones is not yet in phase, and therefore will not interfere constructively. To correct this problem, material is taken off (i.e. it is machined off if the diffractive surface is diamond turned) in a programmed manner in accordance with a profile desired so that the phase is delayed just enough so that at the point F, all the light coming through the surface constructively interferes. This blaze is shown in FIG. 4. Wherein, it can be seen that a step or zone is tapered towards the substrate. The tool is brought in to remove the programmed amount of material and then is brought out as the surface makes a spiral-like cut in the surface.

In the center of the first zone Z.sub.0, where the material is the thickest, the light is delayed exactly one wavelength. Moving away from the center of the pattern, the distance from the focal point increases so that less material is needed at the periphery of zone Z.sub.0. The material is gradually thinned to a minimum at the edge of the first zone, where no additional delay is needed, because the distance at the edge of the first zone is one wavelength further from the focal point than the center of the r