Projection lenses having large back focal length to focal length ratios

Abstract

A projection lens for use with LCD or DMD panels is provided. The lens has three lens units, the first unit having a negative power and at least one plastic lens element having two aspheric surfaces, a second lens unit having a negative power or a weak positive power and at least one color correcting doublet, and a third lens unit having a positive power and an aspheric surface either on a glass element or on a weak plastic element. The projection lens has a back focal length to focal length ratio of at least 3.0, the ratio being achieved by arranging the first, second, and third lens units so that:D.sub.12 /f.sub.0 >1.0,D.sub.23 /f.sub.0 >0.7,and1.5<(D.sub.12 +D.sub.23 +BFL)/BFL<4.0where: f.sub.0 is the effective focal length of the combination of the first, second, and third lens units; BFL is the back focal length of the combination of the first, second, and third lens units; D.sub.12 is the distance between the first and second lens units; and D.sub.23 is the distance between the second and third lens units.

Description

FIELD OF THE INVENTION

This invention relates to projection lenses and, in particular, to projection lenses which can be used, inter alia, to form an image of an object composed of pixels, such as, a LCD or DMD.

BACKGROUND OF THE INVENTION

Projection lens systems (also referred to herein as "projection systems") are used to form an image of an object on a viewing screen. The basic structure of such a system is shown in FIG. 7, wherein is a light source (e.g., a tungsten-halogen lamp), 12 is illumination optics which forms an image of the light source (hereinafter referred to as the "output" of the illumination system), 14 is the object which is to be projected (e.g., a matrix of on and off pixels), and 13 is a projection lens, composed of multiple lens elements, which forms an enlarged image of object 14 on viewing screen 16. FIG. 7 is drawn for the case of a LCD panel where the output of the illumination system strikes the back of the panel and passes through those pixels which are transparent. DMDs, on the other hand, work by reflection and thus the output of the illumination system is routed to the front of the panel by a prism or similar device.

Projection lens systems in which the object is a pixelized panel are used in a variety of applications, including data display systems. Such projection lens systems preferably employ a single projection lens which forms an image of either a single panel having, for example, red, green, and blue pixels, or three individual panels, one for each color. In some cases, two panels are used, one for two colors, e.g., red and green, and the other for one color, e.g., blue. A spinning filter wheel or similar device is associated with the panel for the two colors and the panel is alternately fed information for the two colors in synchrony with the filter.

There exists a need for a projection lens for use with a pixelized panel which simultaneously has at least the following properties: (1) a very long back focal length, e.g., a back focal length greater than at least three times the lens' focal length; (2) a high level of color correction; (3) low distortion; and (4) low sensitivity to temperature changes.

A long back focal length, i.e., the distance from the last lens surface to the pixelized panel, is needed, especially where multiple panels are used, to accommodate the optical elements, e.g., filters, beam splitters, prisms, and the like, used in combining the light from the different color optical paths which the lens system projects towards the viewing screen. In addition, a long back focal length allows the output of the illumination system to be in the vicinity of the projection lens for output distances which are relatively large. Relatively large output distances are desirable since they provide relatively shallow entrance angles for the light at the pixelized panel which is especially important in the case of LCD panels.

A high level of color correction is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. These problems are typically most severe at the edges of the field. In general terms, the color correction, as measured at the pixelized panel, should be better than about a pixel and, preferably, better than a half a pixel to avoid these problems.

All of the chromatic aberrations of the system need to be addressed, with lateral color, chromatic variation of coma, and chromatic aberration of astigmatism typically being most challenging. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.

In projection systems employing cathode ray tubes (CRTs) a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction is thus needed from the projection lens.

The use of a pixelized panel to display data leads to stringent requirements regarding the correction of distortion. This is so because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, an undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center. Moreover, projection lenses are often used with offset panels, the lens of FIG. 1 being, for example, designed for such use. In such a case, the distortion at the viewing screen does not vary symmetrically about a horizontal line through the center of the screen but can increase monotonically from, for example, the bottom to the top of the screen. This effect makes even a small amount of distortion readily visible to the viewer.

In order to produce an image of sufficient brightness, a substantial amount of light must pass through the projection lens. As a result, a significant temperature difference normally exists between room temperature and the lens' operating temperature. In addition, the lens needs to be able to operate under a variety of environmental conditions. For example, projection lens systems are often mounted to the ceiling of a room, which may comprise the roof of a building where the ambient temperature can be substantially above 40.degree. C. To address these effects, a projection lens whose optical properties are relatively insensitivity to temperature changes is needed.

One way to address the temperature sensitivity problem is to use lens elements composed of glass. Compared to plastic, the radii of curvature and the index of refraction of a glass element generally change less than those of a plastic element. However, glass elements are generally more expensive than plastic elements, especially if aspherical surfaces are needed for aberration control. As described below, plastic elements can be used and temperature insensitivity still achieved provided the powers and locations of the plastic elements are properly chosen.

The projection lenses described below achieve all of the above requirements and can be successfully used in producing low cost projection lens systems capable of forming a high quality color image of a pixelized panel on a viewing screen.

DESCRIPTION OF THE PRIOR ART

Projection lenses for use with pixelized panels are described in various patents including Taylor, U.S. Pat. No. 4,189,211, Tanaka et al., U.S. Pat. No. 5,042,929, Yano et al., U.S. Pat. No. 5,179,473, Moskovich, U.S. Pat. No. 5,200,861, Moskovich, U.S. Pat. No. 5,218,480, Iizuka et al., U.S. Pat. No. 5,278,698, Betensky, U.S. Pat. No. 5,313,330, and Yano, U.S. Pat. No. 5,331,462. Discussions of LCD systems can be found in Gagnon et al., U.S. Pat. No. 4,425,028, Gagnon, U.S. Pat. No. 4,461,542, Ledebuhr, U.S. Pat. No. 4,826,311, and EPO Patent Publication No. 311,116.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to provide improved projection lenses for use with a pixelized panel which simultaneously have each of the four desired properties discussed above. This object is achieved by means of a projection lens which comprises in order from its image side to its object side (i.e., from its long conjugate side to its short conjugate side):

(A) a first lens unit having a negative power and comprising a plastic lens element having at least one aspheric surface; (B) a second lens unit having a negative power or a weak positive power and comprising a color correcting doublet; (C) a third lens unit having a positive power and comprising an aspheric surface either on a glass element or on a weak plastic element; wherein: D.sub.12 /f.sub.0 &gt;1.0 (1) D.sub.23 /f.sub.0 &gt;0.7 (2) 1.5&lt;(D.sub.12 +D.sub.23 +BFL)/BFL&lt;4.0, (3) and BFL/f.sub.0 &gt;3.0 (4) where: (i) f.sub.0 is the effective focal length of the combination of the first, second, and third lens units; (ii) BFL is the back focal length of the combination of the first, second, and third lens units for an object located at infinity along the long conjugate side of the projection lens; (iii) D.sub.12 is the distance between the first and second lens units; and (iv) D.sub.23 is the distance between the second and third lens units.

Limitations (1)-(3) relate to the physical structure of the projection lens. When these are satisfied, limitation (4) can be achieved while still achieving high levels of color correction and low levels of distortion. In certain embodiments, the BFL/f.sub.0 ratio (limitation (4)) is greater than 4.0 and in still further embodiments, it is greater than 5.0.

In addition to limitations (1)-(4), the projection lenses also preferably satisfy the following limitation:

.vertline.EPD.vertline./f.sub.0 &gt;4.0, (5) where EPD is the distance from the pixelized panel to the projection lens' entrance pupil, as seen from the illumination optics. A large .vertline.EPD.vertline./f.sub.0 ratio is desirable because (1) to obtain a bright, uniformly-illuminated image it is important to match the location and size of the exit pupil of the illumination system with the entrance pupil of the lens system and (2) illumination optics generally work best when the exit pupil is located a long distance from the light source. In addition, a large .vertline.EPD.vertline./f.sub.0 ratio is desirable since it causes light from the illumination optics to intersect the pixelized panel at relatively shallow angles where such panels work best. For some embodiments, the .vertline.EPD.vertline./f.sub.0 ratio may be as great as 100 or more.

In certain embodiments, the projection lenses of the invention are designed using the location of the output of the illumination system as a pseudo-aperture stop/entrance pupil of the projection lens (see Betensky, U.S. Pat. No. 5,313,330, the relevant portions of which are incorporated herein by reference). In this way, efficient coupling is achieved between the light output of the illumination system and the projection lens. The projection lenses of FIGS. 1 and 2 were designed in this way.

In accordance with these embodiments, the invention provides a projection lens system which forms an image of an object and comprises:

(a) an illumination system comprising a light source and illumination optics which forms an image of the light source, said image being the output of the illumination system; (b) a pixelized panel which comprises the object; and (c) a projection lens of the type described above, said projection lens having an entrance pupil whose location substantially corresponds to the location of the output of the illumination system.

In some embodiments (e.g., the projection lenses of FIGS. 1 and 2), the magnification of the projection lens system is varied by changing the distance between the pixelized panel and the second and third lens units while holding constant (a) the distance between the second and third lens units and (b) the distance between the first lens unit and the pixelized panel.

The projection lenses of the invention are also designed to be substantially athermal. As discussed below, this is done by: (1) employing plastic lens elements having substantial power only at points in the system where the axial beam height is small, i.e., only in the first unit; and (2) employing only weak plastic elements at other locations in the system, e.g., the third unit. In this way, temperature changes do not substantially affect the overall optical properties of the system and, in particular, do not substantially affect the system's back focal length.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-6 are schematic side views of projection lenses constructed in accordance with the invention.

FIG. 7 is a schematic diagram showing an overall projection lens system in which the projection lens of the present invention can be used.

The foregoing drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The projection lenses of the present invention are of the retrofocus type and comprise three lens units which, as can be seen in FIGS. 1-6, are widely spaced from one another. For example, in the lens systems shown in the figures, the largest spacing between any two lens elements within a unit is less than 30% of the smallest spacing between two units, where the spacing between units (i.e., D.sub.12 and D.sub.23) is measured from the last surface of a unit to the first surface of the next unit. This approximately 30% ratio occurs in the lens system of FIG. 3. For all the other lens systems, the ratio is less than 10%. The lens systems are characterized by a high level of aberration correction and a wide field of view, i.e., a half field of view greater than about 35.degree..

The first lens unit has a negative power and at least one plastic lens element which has at least one aspheric surface. Preferably, the first lens unit comprises a single plastic lens element and even more preferably, that single plastic lens element has two aspheric surfaces. A single element is preferred for the first lens unit since the elements of that unit are generally the largest in the system and thus minimizing the size of those elements reduces the overall cost of the system. In certain embodiments, e.g., to achieve an ultra wide field of view, the first lens unit can include two lens elements, which are preferably both composed of plastic and both have two aspheric surfaces. In general terms, the first lens unit and, in particular, the aspheric surfaces of that unit, contribute to the correction of the system's distortion which, as discussed above, needs to be highly corrected for lens systems used with pixelized panels. The distortion correction is generally better than about one percent at the image.

The second lens unit either has a negative power (FIGS. 1-4 and 6) or a weak positive power (FIG. 5), where a weak positive power means a power (P.sub.2) which is less than about 10% of the power of the combination of the first, second, and third lens units (P.sub.0), and is preferably less than about 5%. The second lens unit comprises at least one color correcting doublet and in some cases may comprise two color correcting doublets (FIGS. 1 and 2). The output of the illumination system is in the vicinity of the second lens unit and thus this unit preferably includes only glass elements which can withstand the heat generated in the vicinity of that output. In general terms, the second lens unit contributes to the correction of off-axis color.

The third lens unit has a positive power and comprises an aspheric surface either on a glass element (FIG. 6) or on a weak plastic element (FIGS. 1-5), where a weak plastic element is one having a power (P.sub.element) whose absolute value is less than about 5% of the power of the combination of the first, second, and third lens units (P.sub.0). The weak plastic element, when used, can be considered as a corrector lens element since its purpose is to correct lens aberrations, as opposed to providing optical power for the system. The third lens unit preferably contains a color correcting doublet, whereby the overall system typically contains two color correction doublets (one in the second lens unit and one in the third lens unit) and may contain three color correcting doublets (two in the second lens unit and one in the third lens unit).

The magnitudes of the powers of the first and third lens units are generally similar and each is substantially greater than the magnitude of the power of the second lens unit.

As discussed above, the projection lenses of the invention are athermalized so that the optical performance of the system, including, in particular, the system's back focal length, does not substantially change as the projection lens is heated from room temperature to its operating temperature. More specifically, the change in the back focal length is preferably less than an amount which will significantly change the modulation transfer function (MTF) of the system, e.g., the change in the MTF should be less than about 5%. For the specific examples presented below, this MTF criterion corresponds to a change in the back focal length of less than about 0.1 millimeters. The desired thermal stabilization of the lens' focus is achieved through the selection and placement in the lens of the plastic lens elements.

Ordinarily, the use of plastic lens elements has the drawback that the refractive index of plastic optical materials changes significantly with temperature. Another effect is the change in shape, i.e., expansion or contraction, of plastic optical materials with temperature. This latter effect is usually less significant than the change in index of refraction.

If only low power plastic lens elements are used in a lens it is possible to achieve a balance between the thermal changes in the plastic optics and the thermal changes in the plastic or aluminum mechanical components of the system, e.g., the lens barrel which is usually the major mechanical source of thermally-caused focus changes. The unrestricted use of optical plastics in a design, i.e., the ability to use at least some plastic lens elements of relatively high power, has advantages in that, since the plastic lens elements can be readily molded, non-spherical optical surfaces (aspherics) can be used to maximize the capability (performance) of a particular lens design. The use of relatively high power plastic elements also leads to a lens having an overall lower cost.

If the net plastic optical power in a design is significant, then athermalization needs to be performed or the focus of the lens will change significantly as the lens' temperature changes from room temperature to its operating temperature. This is especially so with projectors which must transmit significant amounts of light to a viewing screen and thus have an operating temperature significantly above room temperature.

For the projection lenses of the present invention, athermalization is achieved by taking into account the location and power of the plastic lens elements, as well as the marginal ray heights at those elements.

The location of the plastic lens elements is significant in terms of the amount of temperature change the element will undergo and thus the amount of change which will occur in the element's index of refraction. In general, elements close to the light source or the image of the light source will undergo greater temperature changes. In practice, a temperature distribution in the region where the projection lens is to be located is measured with the light source and its associated illumination optics operating and those measured values are used in the design of the projection lens.

The marginal ray height at a particular plastic lens element determines, for a given thermal change, whether changes in the element's index of refraction will be significant with regard to the overall thermal stability of the lens. Elements for which the marginal ray height is small will in general have less effect on the overall thermal stability of the system than elements for which the marginal ray height is large.

Based on the foregoing considerations, athermalization of the projection lenses of the present invention is achieved by (1) using only low powered plastic lens elements at locations where the marginal ray height is large, (2) using high powered plastic lens elements only at locations where the marginal ray height is small, and (3) excluding plastic elements from the second lens unit where the output of the illumination system is located.

The level of athermalization achieved is preferably optimized using a computerized lens design program as follows. First, a ray trace is performed at a first temperature distribution and a back focal distance is calculated. The ray trace can be a paraxial ray trace for the marginal ray. Second, the same ray trace is performed at a second temperature distribution and the back focal distance is again calculated. Neither the first nor the second temperature distribution need be constant over the entire lens but can, and in the typical case does, vary from lens element to lens element. The calculated back focal distances are then constrained to a constant value as the design of the system is optimized using the lens design program.

It should be noted that the foregoing approach assumes that the mechanical mounts for the projection lens and the pixelized panel hold the distance between the last lens surface and the panel substantially constant as the temperature of the system changes. If such an assumption is not warranted, other provisions can be made for performing the athermalization, e.g., a measured value for the relative movement of the mechanical mounts can be included in the process or an alternate distance, e.g., the distance between the front lens surface and the panel, can be assumed to be mechanically fixed.

In addition to a long back focal length, a high level of color correction, low distortion, and low temperature sensitivity, for some applications, it is desirable to have a projection lens with a short focal length so that an adequate level of magnification of the pixelized panel can be achieved. In particular, such short focal length lenses are needed when the pixelized panel is a DMD, which typically has a characteristic dimension, e.g., a diagonal, of less than about 20 millimeters. The projection lenses of FIGS. 3-6 have focal lengths of less than 20 millimeters and thus are suitable for use with DMDs. The lenses of FIGS. 1-2, on the other hand, have focal lengths above 50 millimeters and thus are more suitable for use with LCDs. The color correction of the lenses of FIGS. 1-2 tends to be somewhat better than that of the lenses of FIGS. 3-6, especially in relation to pixel size, since the pixel size for a LCD (FIGS. 1-2) is generally much larger than that of a DMD (FIGS. 3-6).

FIGS. 1 to 6 illustrate various projection lenses constructed in accordance with the invention. Corresponding prescriptions and optical properties appear in Tables 1 to 6, respectively. HOYA or SCHOTT designations are used for the glasses employed in the lens systems. Equivalent glasses made by other manufacturers can be used in the practice of the invention. Industry acceptable materials are used for the plastic elements.

The aspheric coefficients set forth in the tables are for use in the following equation: ##EQU1## where z is the surface sag at a distance y from the optical axis of the system, c is the curvature of the lens at the optical axis, and k is a conic constant.

The abbreviations used in the tables are as follows:

______________________________________EFL effective focal lengthFVD front vertex distancef/ f-numberENP entrance pupil as seen from the long conjugateEXP exit pupil as seen from the long conjugateBRL barrel lengthOBJ HT object heightMAG magnificationSTOP location of aperture stopIMD image distanceOBD object distanceOVL overall length.______________________________________

The designation "a" associated with various surfaces in the tables represents an aspheric surface, i.e., a surface for which at least one of D, E, F, G, H, or J in the above equation is not zero. The designation "c" represents a conic surface for which the k value in the above equation is not zero. All dimensions given in the tables are in millimeters. The tables are constructed on the assumption that light travels from left to right in the figures. In actual practice, the viewing screen will be on the left and the pixelized panel will be on the right, and light will travel from right to left. The pixelized panel is shown by the designation "PP" in the figures.

In Tables 1 and 2, the first lens unit (U.sub.1) comprises surfaces 1-2, the second lens unit (U.sub.2) comprises surfaces 3-10, and the third lens unit (U.sub.3) comprises surfaces 11-17. Surfaces 18-19 represent a glass plate, e.g., a protective plate, at the back end of the lens system. It is not part of the lens system and is not included in the summary of lens properties set forth in Tables 7 and 8, although it is included in the first order property calculations of Table 1 and 2.

In Table 3, the first lens unit (U.sub.1) comprises surfaces 1-4, the second lens unit (U.sub.2) comprises surfaces 5-7, and the third lens unit (U.sub.3) comprises surfaces 9-15. Surfaces 16-18 represent a prism and related DMD components located at the back end of the lens system. These components are not part of the lens system and are not included in the summary of lens properties set forth in Tables 7 and 8, although they are included in the first order property calculations of Table 3.

In Tables 4-5, the first lens unit (U.sub.1) comprises surfaces 1-2, the second lens unit (U.sub.2) comprises surfaces 3-5, and the third lens unit (u3) comprises surfaces 7-13. As in Table 3, surfaces 14-16 represent DMD components at the back end of the lens system, which are not included in the summary of lens properties set forth in Tables 7 and 8, but are included in the first order property calculations of Tables 4-5.

In Table 6, the first lens unit (U.sub.1) comprises surfaces 1-2, the second lens unit (U2) comprises surfaces 3-5, and the third lens unit (U.sub.3) comprises surfaces 7-11. As in Tables 3-5, surfaces 12-14 are DMD components which are not included in Tables 7 and 8, but are included in the first order property calculations of Table 6.

As discussed above, the projection lenses of FIGS. 1 and 2 were designed using the pseudo-aperture stop/entrance pupil technique of Betensky, U.S. Pat. No. 5,313,330. Surface 20 in Tables 1 and 2 constitutes the pseudo-aperture stop. Its location corresponds to the location of the output of the illumination system. As can be seen in the subtables labeled "Variable Spaces," the distance from the pseudo-aperture stop to the pixelized panel, i.e., the "Image Distance," is essentially constant for all focal positions (magnifications) of the projection lens system.

As also discussed above, the projection lenses of FIGS. 1 and 2 can be focused by holding the first unit fixed relative to the pixelized panel and moving the second and third units as a group relative to the panel. Alternatively, the entire projection lens can be moved and then the first unit moved back to its constant position. As shown in Tables 1 and 2, movement of the second and third units through a distance on the order of 5-10 millimeters achieves a change in magnification of approximately six fold. The projection lenses of FIGS. 3-6 are focused by moving the entire lens relative to the pixelized panel.

Tables 7 and 8 summarize various properties of the lens systems of the invention. The values listed in these tables are for the first, second, and third lens units per se and do not include the optical elements after those units shown in Tables 1-6. The BFL values are calculated for an object at infinity along the long conjugate of the system. It should be noted that the EPD values of Table 7 correspond to the EXP values of the prior tables, except for Tables 1 and 2, where the EPD values are obtained from the "Variable Spaces" subtable.

As shown in Table 8, each of lens systems 1-6 satisfy limitations (1) through (5) set forth above. In addition, the table shows that the ratio of D.sub.12 to D.sub.23 lies in the following range for the lens systems of the invention:

1.0&lt;D.sub.12 /D.sub.23 &lt;2.5 (6)

Although specific embodiments of the invention have been described and illustrated, it is to be understood that a variety of modifications which do not depart from the scope and spirit of the invention will be evident to persons of ordinary skill in the art from the foregoing disclosure.

TABLE 1__________________________________________________________________________Lens DataSurf. Clear ApertureNo. Type Radius Thickness Glass Diameter__________________________________________________________________________1 a 368.5410 5.00000 ACRYLIC 106.582 a 45.6769 Space 1 83.333 153.3231 7.50000 FD2 38.394 -72.7876 0.85000 37.245 -78.3469 3.00000 TAF1 36.556 38.4407 9.30000 C10 36.147 -155.9928 0.18000 36.918 -515.5717 3.50000 LAC14 37.139 37.4924 10.30000 F5 38.6510 -382.1087 54.70000 39.7611 -281.0522 9.60000 FC5 58.3112 -67.1076 3.00000 59.7213 a -52.3755 8.00000 ACRYLIC 59.7714 a -54.3902 0.20000 62.8215 -397.9817 6.00000 FD6 63.2316 163.8136 15.00000 FC5 64.2717 -74.5926 Space 2 65.1818 .infin. 7.50000 C5 66.1519 .infin. -89.65000 66.8820 Aperture Image 79.49 stop distanceSymbol Description a - Polynomial asphereEven Polynomial AspheresSurf.No. D E F G H I__________________________________________________________________________1 9.4659E-07 -5.7939E-10 -5.1394E-14 1.8767E-16 -6.7075E-20 7.5390E-242 1.2713E-07 -3.4925E-10 -6.8949E-13 -1.9181E-16 6.0301E-19 -2.2113E-2213 2.1223E-07 4.2506E-10 8.1369E-15 8.1278E-17 6.1788E-22 2.7541E-2314 4.0595E-07 2.6522E-10 1.3626E-13 -8.7488E-17 6.4370E-20 3.6391E-24Variable SpacesFocus Space 1 Space 2 Focal ImagePos. T(2) T(17) Shift Distance__________________________________________________________________________1 106.620 5.370 -0.300 355.0032 101.220 10.770 -0.300 354.9973 111.990 0.000 -0.500 355.007SYSTEM FIRST ORDER PROPERTIES, POS 1OBJ. HT: -1520.0 f/ 4.50 MAG: -0.0380STOP: 0.00 after surface 20. DIA: 79.494EFL: 80.0069 FVD: 520.973 ENP: 67.4373IMD: 355.003 BRL: 165.970OBD: -2056.18 OVL: 2577.15SYSTEM FIRST ORDER PROPERTIES, POS 2OBJ. HT: -900.00 f/ 4.50 MAG: -0.0640STOP: 0.00 after surface 20. DIA: 79.470EFL: 82.4051 FVD: 520.967 ENP: 66.8465IMD: 354.997 BRL: 165.970OBD: -1240.13 OVL: 1761.10SYSTEM FIRST ORDER PROPERTIES, POS 3OBJ. HT: -5300.0 f/ 4.50 MAG: -0.0108STOP: 0.00 after surface 20. DIA: 79.560EFL: 77.7565 FVD: 520.977 ENP: 68.0207IMD: 355.007 BRL: 165.970OBD: -7148.70 OVL: 7669.68First Order Properties of ElementsElement SurfaceNumber Numbers Power f' lpp l'pp__________________________________________________________________________1 1 2 -0.94219E-02 -106.14 3.8404 0.475982 3 4 0.13041E-01 76.678 3.1189 -1.48073 5 6 -0.30437E-01 -32.854 1.1205 -0.549764 6 7 0.16064E-01 62.251 1.2428 -5.04355 8 9 -0.20074E-01 -49.815 1.9145 -0.139226 9 10 0.17619E-01 56.758 0.57800 -5.89077 11 12 0.56303E-02 177.61 8.3458 1.99278 13 14 0.10916E-03 9160.9 445.40 462.549 15 16 -0.70362E-02 -142.12 2.3337 -0.9605910 16 17 0.93462E-02 106.99 7.0674 -3.2181First-Order Properties of DoubletsElement SurfaceNumbers Numbers Power f' lpp l'pp__________________________________________________________________________3 4 5 7 -0.13497E-01 -74.092 -1.0131 -9.08615 6 8 10 -0.22018E-02 -454.17 -3.8246 -12.4309 10 15 17 0.28380E-02 352.36 28.772 16.685__________________________________________________________________________ TABLE 2__________________________________________________________________________Lens DataSurf. Clear ApertureNo. Type Radius Thickness Glass Diameter__________________________________________________________________________1 a 429.0479 5.00000 ACRYLIC 108.022 a 46.0871 Space 1 85.643 308.4044 7.50000 FL5 37.014 -49.2176 0.85000 35.755 -49.6570 3.00000 TAF1 34.146 42.9034 9.30000 C10 31.967 -62.7809 0.50000 32.308 -104.5868 3.20000 TAF1 32.369 37.3326 9.30000 FD8 34.1510 -156.2506 72.27450 35.3611 -1142.8400 9.60000 FC5 61.0012 -68.7114 3.30000 61.7513 a -52.1950 8.00000 ACRYLIC 61.6814 a -56.2840 0.20000 64.6715 -356.0175 5.70000 FD6 64.6516 174.7998 14.30000 FC5 65.4817 -79.1509 Space 2 66.2018 .infin. 7.50000 C5 64.1119 .infin. -89.65000 64.0820 Aperture Image 79.49 stop distanceSymbol Description a - Polynomial asphereEven Polynomial AspheresSurf.No. D E F G H I__________________________________________________________________________1 5.0896E-07 -4.1457E-10 -6.9178E-14 1.8713E-16 -6.7901E-20 7.7140E-242 -6.6893E-07 -2.0408E-10 -6.7153E-13 -1.6876E-16 5.6336E-19 -2.0869E-2213 1.4772E-07 4.0695E-10 -1.4288E-13 2.1258E-16 -1.3073E-19 7.5704E-2314 4.4237E-07 1.4337E-10 2.4751E-13 -2.2448E-16 1.0947E-19 8.6717E-25Variable SpacesFocus Space 1 Space 2 Image FocalPos. T(2) T(17) Distance Shift__________________________________________________________________________1 118.697 5.370 354.996 -0.2702 116.297 7.770 355.021 -0.2703 121.467 2.600 355.015 -0.270SYSTEM FIRST ORDER PROPERTIES, POS 1OBJ. HT: -1160.0 f/ 4.50 MAG: -0.0380STOP: 0.00 after surface 20. DIA: 79.476EFL: 62.9170 FVD: 548.937 ENP: 69.0765IMD: 354.996 BRL: 193.941OBD: -1597.85 OVL: 2146.79SYSTEM FIRST ORDER PROPERTIES. POS 2OBJ. HT: -715.00 f/ 4.50 MAG: -0.0620STOP: 0.00 after surface 20. DIA: 79.472EFL: 63.6221 FVD: 548.962 ENP: 68.8085IMD: 355.021 BRL: 193.941OBD: -968.875 OVL: 1517.84SYSTEM FIRST ORDER PROPERTIES, POS 3OBJ. HT: -4400.0 f/ 4.50 MAG: -0.0100STOP: 0.00 after surface 20. DIA: 79.488EFL: 62.1225 FVD: 548.956 ENP: 69.3830IMD: 355.015 BRL: 193.942OBD: -6153.74 OVL: 6702.70First Order Properties of ElementsElement SurfaceNumber Numbers Power f' lpp l'pp__________________________________________________________________________1 1 2 -0.95218E-02 -105.02 3.7663 0.404562 3 4 0.13672E-01 73.144 4.1130 -0.656383 5 6 -0.34201E-01 -29.239 0.89346 -0.771954 6 7 0.19173E-01 52.156 2.5873 -3.78615 8 9 -0.28491E-01 -35.099 1.3147 -0.469296 9 10 0.22583E-01 44.281 1.0799 -4.51987 11 12 0.67104E-02 149.02 6.8390 0.411188 13 14 -0.24281E-03 -4118.5 -193.50 -208.669 15 16 -0.69649E-02 -143.58 2.0990 -1.030610 16 17 0.88121E-02 113.48 6.7344 -3.0494First-Order Properties of DoubletsElement SurfaceNumbers Numbers Power f' lpp l'pp__________________________________________________________________________3 4 5 7 -0.12825E-01 -77.974 -4.1287 -12.7455 6 8 10 -0.49113E-02 -203.61 -5.8088 -13.5079 10 15 17 0.23238E-02 430.34 31.545 20.224__________________________________________________________________________ TABLE 3__________________________________________________________________________Lens DataSurf. Clear ApertureNo. Type Radius Thickness Glass Diameter__________________________________________________________________________1 a 2023.5141 6.50000 ACRYLIC 79.852 a 34.7639 14.52895 58.683 a 134.3043 6.00000. ACRYLIC 56.354 a 64.3362 78.21745 50.165 -36.0676 2.00000 LAK8 15.966 18.6395 5.50000 SF15 15.617 -65.0770 18.65000 15.538 Aperture stop 31.74455 15.269 582.7881 7.00000 FK5 26.5710 -28.0086 2.50000 SF6 27.5811 -69.3662 0.20000 29.6212 47.6849 8.50000 FK5 31.7113 -47.6849 0.50000 31.8914 a -66.0000 4.50000 ACRYLIC 31.5215 a -51.0000 1.20000 31.4216 .infin. 70.67000 BK7 30.6917 .infin. 3.00000 ZK7 20.3718 .infin. 10.47164 19.93Symbol Description a - Polynomial asphereFocal Shift = 0.03796Even Polynomial AspheresSurf.No. D E F G H I__________________________________________________________________________1 1.6047E-06 2.3044E-10 -2.1672E-15 4.4338E-17 -5.7852E-21 -1.5003E-232 -4.1021E-06 -9.2507E-11 2.0540E-12 1.0280E-15 2.4804E-19 -1.8987E-223 1.0218E-06 -2.3579E-10 -1.1271E-12 3.1803E-16 -7.2539E-19 1.9049E-214 5.1894E-06 6.9149E-10 -6.0898E-12 -2.7710E-15 2.2080E-18 5.3959E-2114 -2.2395E-06 8.9092E-09 5.2995E-12 2.6729E-15 6.1593E-17 -3.4316E-1915 3.7601E-06 7.4597E-09 2.0109E-11 5.3271E-15 -1.2006E-16 3.1859E-20SYSTEM FIRST ORDER PROPERTIESOBJ. HT: -880.00 f/ 3.00 MAG: -0.0100STOP; 0.00 after surface 8. DIA: 14.803EFL: 11.4999 FVD: 271.683 ENP: 44.3891IMD: 10.4716 BRL: 261.211 EXP: -2210.50OBD: -1105.66 DVL: 1377.34First Order Properties of ElementsElement SurfaceNumber Numbers Power f' lpp l'pp__________________________________________________________________________1 1 2 -0.13945E-01 -71.712 4.4322 0.76146E-012 3 4 -0.38850E-02 -257.40 7.9349 3.80113 5 6 -0.59167E-01 -16.901 0.75678 -0.391104 6 7 0.47297E-01 21.143 0.73852 -2.57845 9 10 0.18234E-01 54.841 4.5021 -0.216376 10 11 -0.16830E-01 -59.417 -0.96005 -2.37777 12 13 0.19915E-01 50.213 2.9401 -2.94018 14 15 0.24186E-02 413.46 12.059 9.3184First-Order Properties of DoubletsElement SurfaceNumbers Numbers Power f' lpp l'pp__________________________________________________________________________3 4 5 7 -0.87079E-02 -114.84 -5.3788 -10.2545 6 9 11 0.11761E-02 850.27 15.144 9.1525__________________________________________________________________________ TABLE 4__________________________________________________________________________Lens DataSurf. Clear ApertureNo. Type Radius Thickness Glass Diameter__________________________________________________________________________1 a 832.6757 9.00000 ACRYLIC 94.492 ac 30.0992 120.64190 67.913 -198.0282 3.00000 LAC8 37.204 34.8452 9.50000 FD8 36.315 -180.9231 54.00602 36.026 Aperture stop 42.00000 23.457 97.5036 9.50000 FC5 35.958 -46.4020 3.16000 SF6 36.609 -141.7385 4.60000 38.1810 a -55.3507 6.25478 ACRYLIC 38.5111 a -50.2569 0.70000 42.0512 -297.6356 10.13644 FC5 43.9313 -39.0453 10.00000 45.0714 .infin. 102.50000 BK7 41.0115 .infin. 3.00000 ZK7 26.0016 .infin. 0.99729 26.00Symbol Description a - Polynomial asphere c - Conic sectionFocal Shift = 0.00000Conics Surface Number Constant__________________________________________________________________________ 2 -9.0000E-01Even Polynomial AspheresSurf.No. D E F G H I__________________________________________________________________________ 1 2.0459E-08 -4.3610E-11 6.1077E-14 1.6799E-18 -9.6593E-21 1.5489E-24 2 2.8170E-07 1.0846E-10 -3.4780E-13 5.2951E-16 3.9333E-19 -3.7853E-2210 -7.8808E-06 -3.5347E-09 -2.5932E-12 -1.4259E-15 7.3563E-17 -1.3990E-1911 -2.4815E-06 -2.5456E-09 4.0267E-12 9.9295E-15 -1.9894E-17 5.4499E-21SYSTEM FIRST ORDER PROPERTIESOBJ. HT: -1220.0 f/ 3.00 MAG: -0.0100STOP: 0.00 after surface 6. DIA: 22.201EFL: 16.7012 FVD: 388.996 ENP: 53.0427IMD: 0.997294 BRL: 387.999 EXP: -2799.63OBD: -1617.18 OVL: 2006.17First Order Properties of ElementsElement SurfaceNumber Numbers Power f' lpp l'pp__________________________________________________________________________1 1 2 -0.15753E-01 -63.479 6.2742 0.226802 3 4 -0.24299E-01 -41.154 1.4786 -0.260173 4 5 0.23329E-01 42.864 0.92221 -4.78834 7 8 0.15221E-01 65.700 4.4183 -2.10275 8 9 -0.11605E-01 -86.172 -0.86130 -2.63096 10 11 0.12712E-02 786.67 32.363 29.3857 12 13 0.11024E-01 90.709 7.7351 1.0147First-Order Properties of DoubletsElement SurfaceNumbers Numbers Power f' lpp l'pp__________________________________________________________________________2 3 3 5 -0.29917E-03 -3342.6 -90.725 -100.824 5 7 9 0.38352E-02 260.74 0.66215 -7.5574__________________________________________________________________________ TABLE 5__________________________________________________________________________Lens DataSurf. Clear ApertureNo. Type Radius Thickness Glass Diameter__________________________________________________________________________1 a 12057.0951 9.00000 ACRYLIC 86.182 ac 26.2462 120.77813 61.463 -574.2251 3.00000 LAC8 38.704 41.7117 9.50000 FD8 37.905 -205.2798 54.32126 37.536 Aperture stop 42.00000 24.327 101.0769 9.50000 FC5 35.628 -45.9452 3.16000 SF6 36.239 -144.9350 4.60000 37.7510 a -54.7664 6.23804 ACRYLIC 38.1011 a -50.6562 0.70000 41.5012 -277.7475 9.68102 FC5 43.2113 -39.1902 10.00000 44.2914 .infin. 102.50000 BK7 40.4415 .infin. 3.00000 ZK7 25.0816 .infin. 1.00563 24.63Symbol Description a - Polynomial asphere c - Conic sectionFocal Shift = 0.00000Conics Surface Number Constant__________________________________________________________________________ 2 -9.0000E-01Even Polynomial AspheresSurf.No. D E F G I 1 8.1077E-08 -4.9913E-11 5.9749E-14 1.1278E-18 -1.0091E-20 1.4111E-24 2 -1.1414E-07 4.1498E-10 -1.5660E-13 5.5103E-16 3.2180E-19 -5.1455E-2210 -7.7823E-06 -3.3365E-09 -1.9906E-12 -5.9533E-17 7.6060E-17 -1.3444E-1911 -2.6114E-06 -2.4026E-09 4.3735E-12 1.0618E-14 -1.8774E-17 4.8159E-21SYSTEM FIRST ORDER PROPERTIESOBJ. HT: -1220.0 f/ 3.00 MAG: -0.0100STOP: 0.00 after surface 6. DIA: 23.170EFL: 16.7013 FVD: 388.984 ENP: 47.4679IMD: 1.00563 BRL: 387.978 EXP: -12l4.27OBD: -1622.89 OVL: 2011.88First Order Properties of ElementsElement SurfaceNumber Numbers Power f' lpp l'pp__________________________________________________________________________1 1 2 -0.18768E-01 -53.283 6.0396 0.13147E-012 3 4 -0.18454E-01 -54.190 1.6264 -0.118143 4 5 0.19708E-01 50.741 0.96215 -4.73514 7 8 0.15157E-01 65.977 4.4810 -2.03695 8 9 -0.11907E-01 -83.981 -0.82089 -2.58956 10 11 0.10986E-02 910.28 37.054 34.2737 12 13 0.10863E-01 92.055 7.4695 1.0539First-Order Properties of DoubletsElement SurfaceNumbers Numbers Power f' lpp l'pp__________________________________________________________________________2 3 3 5 0.16469E-02 607.19 14.554 7.36964 5 7 9 0.34689E-02 288.28 0.30695 -7.9026__________________________________________________________________________ TABLE 6__________________________________________________________________________Lens DataSurf. Clear ApertureNo. Type Radius Thickness Glass Diameter__________________________________________________________________________1 a 292.8219 9.00000 ACRYLIC 96.262 ac 28.1642 120.24370 66.853 -266.8324 3.00000 LAC8 35.994 33.2578 9.50000 FD8 34.915 -259.6141 53.16168 34.466 Aperture stop 42.00000 23.387 519.0042 9.50000 FCS 34.738 -47.2408 3.16000 SF6 36.169 -131.0132 5.87579 38.0610 72.7138 10.13644 FC5 42.5111 ac -56.1142 10.00000 42.7412 .infin. 102.50000 BK7 39.7713 .infin. 3.00000 ZK7 25.3114 .infin. 1.02121 24.88Symbol Description a - Polynomial asphere c - Conic sectionFocal Shift = -0.08000Conics Surface Number Constant__________________________________________________________________________ 2 -9.0000E-01 11 -1.4000E+00Even Polynomial AspheresSurf.No. D E F G H I__________________________________________________________________________1 1.4941E-07 -1.6298E-10 9.7911E-14 4.6897E-18 -1.2743E-20 2.1836E-242 9.2188E-07 3.5505E-10 -3.3822E-13 5.5051E-16 3.0208E-19 -1.9330E-2211 1.5012E-06 -4.0662E-09 1.4997E-11 -7.7650E-15 -5.8554E-17 8.2240E-20SYSTEM FIRST ORDER PROPERTIESOBJ. HT: -1220.0 f/ 3.00 MAG: -0.0100STOP: 0.00 after surface 6. DIA: 22.169EFL: 16.6986 FVD: 382.099 ENP: 53.5799IMD: 1.02121 BRL: 381.078 EXP: -879.8830BD: -1616.60 OVL: 1998.70First Order Properties of ElementsElement SurfaceNumber Numbers Power f' lpp l'pp__________________________________________________________________________1 1 2 -0.15668E-01 -63.826 6.7420 0.648462 3 4 -0.24318E-01 -41.121 1.5479 -0.192933 4 5 0.23233E-01 43.042 0.64535 -5.03774 7 8 0.11234E-01 89.012 5.8797 -0.535185 8 9 -0.10813E-01 -92.478 -0.99999 -2.77336 10 11 0.15045E-01 66.469 3.9439 -3.0436First-Order Properties of DoubletsElement SurfaceNumbers Numbers Power f' lpp l'pp__________________________________________________________________________2 3 3 5 -0.61182E-03 -1634.5 -30.284 -38.3574 5 7 9 0.36454E-03 2743.2 19.667 11.551__________________________________________________________________________ TABLE 7______________________________________Lens f.sub.0 D.sub.12 D.sub.23 BFL f.sub.2 EPD______________________________________1 77.03 113.79 54.70 265.98 -418.41 3552 61.86 122.40 72.27 271.94 -213.40 3553 11.50 78.22 50.40 60.08 -114.84 22004 16.70 120.64 96.01 80.31 -3342.6 28005 16.70 120.78 96.32 80.32 607.19 -1214.36 16.70 120.24 95.16 80.33 -1634.5 -879.9______________________________________

Claims

1. A projection lens for forming an image of an object, said lens comprising in order from its image end to its object end:

(A) a first lens unit having a negative power and comprising a plastic lens element having at least one aspheric surface;

(B) a second lens unit having a negative power or a weak positive power and comprising a color correcting doublet;

(C) a third lens unit having a positive power and comprising an aspheric surface either on a glass element or on a weak plastic element;

wherein:

D.sub.12 /f.sub.0 &gt;1.0,

D.sub.23 /f.sub.0 &gt;0.7,

1.5&lt;(D.sub.12 +D.sub.23 +BFL)/BFL&lt;4.0,

and

BFL/f.sub.0 &gt;3.

where:

(i) f.sub.0 is the effective focal length of the combination of the first, second, and third lens units;

(ii) BFL is the back focal length of the combination of the first, second, and third lens units for an object located at infinity along the long conjugate side of the projection lens;

(iii) D.sub.12 is the distance between the first and second lens units; and

(iv) D.sub.23 is the distance between the second and third lens units.

2. The projection lens of claim 1 wherein:

BFL/f.sub.0 &gt;4.0.

3. The projection lens of claim 1 wherein:

BFL/f.sub.0 &gt;5.0.

4. The projection lens of claim 1 wherein:

.vertline.EPD.vertline./f.sub.0 &gt;4.0,

where EPD is the distance from the object to the projection lens' entrance pupil for light traveling from the object to the image.

5. The projection lens of claim 4 wherein:

.vertline.EPD.vertline./f.sub.0 &gt;100.

6. The projection lens of claim 1 wherein the first lens unit comprises at least two aspheric surfaces.

7. The projection lens of claim 1 wherein the first lens unit consists of a single lens element.

8. The projection lens of claim 7 wherein the single lens element has two aspheric surfaces.

9. The projection lens of claim 1 wherein the first lens unit comprises two lens elements.

10. The projection lens of claim 9 wherein each of the two lens elements has two aspheric surfaces.

11. The projection lens of claim 1 wherein the second lens unit comprises two color correcting doublets.

12. The projection lens of claim 1 wherein the second lens unit comprises only glass lens elements.

13. The projection lens of claim 1 wherein the third lens unit comprises a color correcting doublet.

14. The projection lens of claim 1 wherein the BFL does not change substantially as the lens is heated from room temperature to its operating temperature.

15. The projection lens of claim 1 wherein f.sub.0 is less than 20 millimeters.

16. The projection lens of claim 1 wherein:

1.0&lt;D.sub.12 /D.sub.23 &lt;2.5.

17. The projection lens of claim 1 wherein the lens has a distortion which is less than about one percent at the image.

18. The projection lens of claim 1 wherein the lens has a half field of view in the direction of the image greater than about 35.degree..

19. The projection lens of claim 1 wherein the object is a pixelized panel.

20. The projection lens of claim 19 wherein the lens has a lateral color aberration which is less than about a pixel at the object.

21. A projection lens system for forming an image of an object, said system comprising:

(a) an illumination system comprising a light source and illumination optics which forms an image of the light source, said image of the light source being the output of the illumination system;

(b) a pixelized panel which comprises the object; and

(c) the projection lens of claim 1;

wherein said projection lens has an entrance pupil whose location substantially corresponds to the location of the output of the illumination system.

22. A variable magnification projection lens system which forms an image of an object and comprises:

(A) an illumination system comprising a light source and illumination optics which forms an image of the light source, said image of the light source being the output of the illumination system;

(B) a pixelized panel, which comprises the object; and

(C) a projection lens which comprises:

(i) a first lens unit having a negative power and comprising a plastic lens element having at least one aspheric surface;

(ii) a second lens unit having a negative power or a weak positive power and comprising a color correcting doublet;

(iii) a third lens unit having a positive power and comprising an aspheric surface either on a glass element or on a weak plastic element;

wherein the magnification of the projection lens system is varied by changing the distance between the pixelized panel and the second and third lens units while holding constant (a) the distance between the second and third lens units and (b) the distance between the first lens unit and the pixelized panel.