Abstract:
A projection objective having a fixed focal length for digital projection includes the following arrangement of the lenses in the sequence from the image (enlargement side) to the object (reduction and illumination side): a first, negative lens, a second, negative lens, a third lens, a fourth, positive lens, a fifth lens, a sixth, positive lens, a seventh, negative lens, an eighth, positive lens, and a ninth, positive lens. By means of such an objective, the quality requirements in the field of digital projection can be met with a minimal number of lenses.

Description:
CLAIM OF PRIORITY  
       [0001]     Applicant hereby claims the priority benefits under the provisions of 35 U.S.C. § 119, basing said claim of priority on European Patent Application Serial No. 06 012 841.0, filed Jun. 22, 2006. In accordance with the provisions of 35 U.S.C. § 119 and Rule 55(b), a certified copy of the above-listed European patent application will be filed before grant of a patent.  
       BACKGROUND OF THE INVENTION  
       [0002]     The field of application of the invention is digital projection, in particular for digital cinema. The projection objective finds application, for example, as a base objective with an anamorphic attachment.  
         [0003]     Projectors used in digital cinema are distinguished through a beam-splitting prism between the chips and the objective. The glass path is up to 119.5 mm, which causes a longitudinal chromatic aberration, as well as a spherical aberration. These aberrations must be corrected by the projector objective. Thus, objectives calculated for other circumstances are fundamentally unsuitable.  
         [0004]     Mainly, zoom objectives are offered on the market. However, for fixed installations, a zoom objective is not necessary. Moreover, the conversion of the chip format to the image format 2.35:1 advantageously takes place in digital cinema with the aid of an anamorphic attachment. In order to reduce the dimensions of the attachment and thus the costs, it is necessary that the screen side aperture diaphragm not be too far away from the front lens. This goal is difficult to achieve with zoom objectives.  
         [0005]     Described in DE 103 53 563 B4 is an objective with fixed focal length that meets the requirements. However, in all of the specified embodiment forms, this objective contains an aspherical surface, which increases the production costs.  
       SUMMARY OF THE INVENTION  
       [0006]     The goal is an objective with fixed focal length that meets the quality requirements in the field of digital projection, in particular digital cinema, with a minimum number of lenses. The distance from the screen side aperture diaphragm to the front lens is to be dimensioned in such a way that a clear reduction of the mass of the required anamorphic attachment results.  
         [0007]     Although the requirement for imaging quality makes the use of special glasses with an abbe number value of ν d &gt;75 appear indispensable, nevertheless the use of these glasses is to be limited to as few lenses as possible, since they involve high costs both in their acquisition and their processing. Particularly high costs are involved in the use of lenses consisting of calcium fluoride or the use of glasses with a value ν d &gt;85, which should be avoided. Likewise to be avoided are aspheric surfaces, since the number of lenses is to be kept as small as possible.  
         [0008]     The above-stated goal of the invention is achieved through a projection objective according to the features set forth herein in claim  1 , thus through a projection objective of fixed focal length for digital projection with the following arrangement of the lenses in the sequence from the image (enlargement side) to the object (reduction and illumination side): 
        a first, negative lens,     a second, negative lens,     a third lens,     a fourth, positive lens,     a fifth lens,     a sixth, positive lens,     a seventh, negative lens,     an eighth, positive lens, and     a ninth, positive lens.        
 
         [0018]     According to a particular embodiment of the invention according to claim  2  herein, the projection objective is formed with the following arrangement of the lenses in the sequence from the image (enlargement side) to the object (reduction and illumination side): 
        a first, negative lens with a convex surface on the image side and a concave surface on the object side,     a second, biconcave lens,     a third lens with a concave surface on the image side and a convex surface on the object side,     a fourth, biconvex lens,     a fifth lens with a concave surface toward the object side,     a sixth, biconvex lens,     a seventh lens with a concave surface toward the object side,     an eighth, biconvex lens, and     a ninth, biconvex lens.        
 
         [0028]     In particular, it is planned that the fifth lens will have a convex or concave surface toward the image side and/or the seventh lens will have a concave or convex surface toward the image side.  
         [0029]     In addition, between the fourth and fifth lenses a stop is advantageously arranged.  
         [0030]     With regard to the relationships of the values of the index of refraction n d  and the abbe number ν d , the following value ranges are preferably fulfilled:  
                                       Lens No.   n d     ν d                     1   Greater than 1.6   Less than 62       2   Between 1.58 and 1.8   Less than 62       3   Greater than 1.65   Less than 41       4   Between 1.6 and 1.8   Between 45 and 62       5   Greater than 1.64   Between 30 and 45       6   Less than 1.74   Greater than 44       7   Less than 1.75   Between 40 and 66       8   Less than 1.50   Greater than 70       9   Less than 1.50   Greater than 70                  
 
         [0031]     In particular, the following value ranges for these parameters result:  
                                       Lens No.   n d     ν d                     1   Greater than 1.7   Less than 50       2   Between 1.56 and 1.8   Less than 62       3   Greater than 1.65   Less than 35       4   Between 1.6 and 1.8   Between 45 and 60       5   Greater than 1.65   Between 30 and 45       6   Less than 1.7   Greater than 45       7   Less than 1.75   Between 40 and 61       8   Less than 1.50   Greater than 70       9   Less than 1.50   Greater than 70                  
 
         [0032]     It is considered to be especially advantageous when the abbe number ν d  is greater than 85 for all of the lenses.  
         [0033]     The seventh and the eighth lenses are preferably cemented together.  
         [0034]     In the objective according to the invention, the correction, as well as the back focal length of the objective, preferably allow a beam splitter between the objective and the object to be projected. This beam splitter possesses a glass path of 80 mm to 130 mm, preferably 110 mm to 130 mm, in particular 115 mm to 125 mm.  
         [0035]     The invention makes possible an object side aperture diaphragm that is very far away from the object. The objective side aperture diaphragm is at least 800 mm, preferably at least 1200 mm, in particular at least 2000 mm away from the object.  
         [0036]     It is considered to be especially advantageous, especially for cost reasons, when all of the lens surfaces of the lenses of the projection objective are spherical or planar.  
         [0037]     Further features of the invention are represented in the dependent claims and the explanation of the embodiment examples, in connection with which it is remarked that all individual features and combinations of individual features are essential to the invention.  
         [0038]     These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]     Illustration 1 is a partially schematic side view of a lens section embodying the present invention.  
         [0040]     Illustration 2 is a graph of the modulation transfer function for the lens illustrated in  FIG. 1 .  
         [0041]     Illustration 3 is a graph of the vignetting for the lens shown in  FIG. 1 .  
         [0042]     Illustration 4 is a graph of the distortion of the objective for the lens shown in  FIG. 1 .  
         [0043]     Illustration 5 is a partially schematic side elevational view of another embodiment of the present invention.  
         [0044]     Illustration 6 is a graph of the modular transfer function for the lens embodiment shown in  FIG. 5 .  
         [0045]     Illustration 7 is a graph of the vignetting for the lens embodiment shown in  FIG. 5 .  
         [0046]     Illustration 8 is a graph of the distortion of the objective for the lens embodiment shown in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]     For purposes of description herein, the terms “upper”, “lower”, “right”, “left”, “rear”, “front”, “vertical”, “horizontal” and derivatives thereof shall relate to the invention as oriented in Illustrations 1 and 5. However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise.  
         [0048]     The invention is represented through two embodiment forms, without being limited to these examples.  
         [0049]     Illustration 1 shows a lens section of an exemplary embodiment of the present invention. The precise optical data of this projection objective are given in claim  12  herein, the projection objective according to this embodiment form being constituted as follows:  
         [0050]     Data for lenses 1 to 9:  
                                                                                     Radius   Thickness               Notation   Surface   [mm]   [mm]   n d     ν d                                  Lens 1   1   215.573   7.400   1.8052   25.4           2   60.054   19.873       Lens 2   3   −86.116   6.198   1.7292   54.5           4   214.588   5.812       Lens 3   5   −159.767   25.000   1.7174   29.5           6   −97.046   1.422       Lens 4   7   233.759   19.000   1.7292   54.5           8   −110.662   133.385       Stop   9   planar   34.694       Lens 5   10   119.584   11.300   1.8061   40.6           11   52.245   3.364       Lens 6   12   192.641   7.500   1.5168   64.2           13   −65.831   12.288       Lens 7   14   −37.013   5.000   1.5827   46.6       Lens 8   15   293.564   15.600   1.4970   81.6           16   −48.182   0.200       Lens 9   17   181.597   15.000   1.4970   81.6           18   −65.785   17.000       Beam Splitter   19   planar   116.500   1.5168   64.2       Protective       Glass   20   planar   3.000   1.5085   61.2           21   planar   0.507       Object       planar                  
 
         [0051]     For practical reasons, the objective is described in reversed position (illustrated from the enlargement side to the reduction side). A negative sign before the radius means that the center point of the lens surface lies on the enlargement side relative to the apex. The surfaces 1 to 18 describe the objective as claimed; the surfaces 19 to 22 represent the beam splitter, protective glass, and chip of the projector.  
         [0052]     In the illustrated embodiment, the glass block with a total thickness of 119.5 mm represents a simplified optical model of the beam splitter in the projector. For practical reasons, the objective is described in reversed position (illustrated from the enlargement side to the reduction side). The focal length of the objective is 57 mm, and the relative aperture is 1:2.5. On the reduction side, a circle with a radius of 18 mm is usable, which on the enlargement side leads to a maximum projection angle of 2w=35.1°. The distance of the aperture diaphragm from the first lens apex on the enlargement side amounts to 77.4 mm. On the reduction side, the distance of the aperture diaphragm is over 6 meters. This means that the objective is nearly telecentric on this side and thus optimally matched to the illumination system of the projector.  
         [0053]     The objective requires only nine spherical lenses, with only the last two lenses consisting of glasses with an abbe number ν d &gt;75.  
         [0054]     In the conventional digital projector, an individual picture element has a size of 13.68 μm. This corresponds to approximately 73 pixels per millimeter or approximately 36 line pairs per millimeter that the projector can at most represent. According to experience, a projection objective must be able to represent twice as many line pairs per millimeter as the associated digital projector in order for the objective to be perceived as high quality by the viewer. Thus, in the judgement of the objective, the modulation transfer function (MTF) of up to 72 line pairs per millimeter is to be considered. Illustration  2  shows this MTF. The MTF was calculated at the wavelengths of 460 nm, 545 nm, and 620 nm, wherein the middle wavelength was evaluated with the weight 2 and the other wavelengths with the weight 1. The MTF exhibits good contrast values at 36 line pairs per millimeter and at 72 line pairs per millimeter. The slight drop-off of the contrast from the center to the edge indicates a very good correction of the lateral chromatic aberration. The correction of this aberration is especially important in digital projection, since it leads to troublesome color edges.  
         [0055]     Illustration  3  shows the vignetting, and Illustration  4  shows the distortion of this objective.  
         [0056]     Illustration  5  shows a further embodiment example. The focal length of the objective was increased to 69 mm. The precise optical data of this projection objective are also given in claim  13  herein. The projection objective according to this embodiment form is constituted as follows:  
         [0057]     Data for lenses 1 to 9:  
                                                                                     Radius                   Notation   Surface   [mm]   Thickness [mm]   n d     ν d                                  Lens 1   1   489.067   7.500   1.7440   44.9           2   76.083   17.565       Lens 2   3   −65.740   7.500   1.7552   27.6           4   166.123   4.232       Lens 3   5   −1167.668   24.000   1.7552   27.6           6   −95.446   0.992       Lens 4   7   191.825   23.000   1.6584   50.9           8   −101.162   132.991       Stop   9   planar   19.359       Lens 5   10   −79.551   24.881   1.6727   32.3           11   92.129   7.145       Lens 6   12   107.711   13.349   1.6584   50.9           13   −108.508   25.694       Lens 7   14   448.856   6.000   1.6204   60.3       Lens 8   15   56.510   17.000   1.4970   81.6           16   −133.601   0.200       Lens 9   17   107.738   9.199   1.4970   81.6           18   −409.370   17.000       Beam Splitter   19   planar   116.500   1.5168   64.2       Protective       Glass   20   planar   3.000   1.5085   61.2           21   planar   0.522       Object       planar                  
 
         [0058]     The practical reasons, the objective is described in reversed position (illustrated from the enlargement side to the reduction side). A negative sign before the radius means that the center point of the lens surface lies on the enlargement side relative to the apex. The surfaces 1 to 18 describe the objective as claimed herein; the surfaces 19 to 22 represent the beam splitter, protective glass, and chip of the projector.  
         [0059]     In the objective according to this illustrated embodiment, the relative aperture amounts to 1:2.5. On the reduction side, a circle with a radius of 18 mm is usable, which on the enlargement side leads to a maximum projection angle of 2w=29.2°. The distance of the aperture diaphragm from the first lens apex on the enlargement side amounts to 90 mm. Through the reduced projection angle, the increase of the aperture diaphragm distance relative to that of the first example is unproblematic with regard to the use of anamorphic attachments. On the reduction side, the distance of the aperture diaphragm is over 5 meters. This means that the objective is nearly telecentric on this side, and this optimally is matched to the illumination system of the projector.  
         [0060]     Illustration  6  shows the MTF, Illustration  7  shows the vignetting, and Illustration  8  shows the distortion of this objective.  
         [0061]     In the foregoing description, it will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims by their language expressly state otherwise.