PROJECTION-LENS SYSTEM

Abstract: A projection-lens system is described for projecting a magnified image of a scene formed by means of a reproduction element onto a projection screen, which system comprises two correction groups and one main group which provides substantially the entire power of the system. The main group comprises a glass preform of which at least one of the outer surfaces is provided with a layer of a transparent plastics having an aspheric outer profile. This projection-lens system is substantially temperature-independent, provides a satisfactory correction for aberrations, and has a short focal length and a comparatively large numerical aperture.

Description: The invention relates to a projection-lens system for projecting a magnified image of a scene reproduced by means of a reproduction element onto a projection screen, which lens system comprises, in this order and viewed from the image side, a first group of lenses, of which at least one outer surface is aspheric, a positive second or main group, of which at least the outer surface which faces the object side is aspheric, and a third group comprising a negative lens whose surface which faces the image side is concave and aspheric, the elements of the first group and the third group being made of a transparent plastics. The invention also relates to a colour-television projection system comprising at least one projection-lens system of this kind. Such a projection-lens system is disclosed in United States Patent Specification No. 4,348,081 and is intended for projecting a scene in one colour on a projection screen, where three monochrome images are superimposed to form a colour picture, for example a colour-television picture. In the known projection-lens system the third group comprises a planoconcave lens, also referred to as a field-curvature correction lens or "field flattener", which compensates for the Petzval curvature of the two other lens groups. The main group may be a single biconvex lens or may comprise two spaced single positive lenses. The lens system in accordance with United States Patent Specification No. 4,348,081 is not achromatic. Moreover, in particular in the versions comprising three lenses, the modulation transfer function in the field corners is minimal and the luminance is not satisfactory. In those versions of the projection-lens system in accordance with United States Patent Specification No. 4,348,081 in which the main group comprises at least one aspheric surface all the lens elements are made of a transparent plastics. The entire projection-lens system is then comparatively light in weight but on account of the substantial temperature dependence of the refractive indices of the relevant plastics the focal length of such a system will vary strongly with temperature. United States Patent Specification No. 4,348,081 also describes a system with a main group comprising one glass lens. This lens does not have any refractive surface with an aspheric profile, so that the projection-lens system does not provide a satisfactory correction for aberrations. Forming aspheric surfaces on glass lenses is time-consuming and expensive. In the system comprising a glass main lens described in United States Patent Specification No. 4,348,081 a part of the total power of the projection-lens system is provided by the first group and the third group, which are made of a transparent plastics whose refractive index varies with the temperature, so that the focal length of the entire system is still temperature dependent. It is the object of the present invention to provide a projection-lens system whose focal length is substantially temperature-independent, which has a satisfactory modulation-transfer function over the entire field, which also provides an acceptable luminance in the corners, and which can be manufactured cheaply. The projection-lens system in accordance with the invention is characterized in that the main group provides substantially the entire power of the system and comprises at least one glass lens substrate, at least that outer surface of said substrate which faces the object side being provided with a layer of a transparent plastics having an aspheric outer profile. The invention is based inter alia on the recognition of the fact that the temperature-independent high-quality main group with aspheric refractive surfaces required in a projection-lens system can be manufactured comparatively cheaply by the use of a glass preform or lens substrate which is provided with one layer or two layers of a transparent plastics having aspheric outer profiles by means of a replica or copying method using dies having aspheric profiles. By the use of the inventive concept it is possible to design a projection-lens system having a small focal length and a large angle of field. The term angle of field is to be understood to mean the angle between the optical axis and the chief ray of a beam which issues from the edge of the object or scene and which is still accepted by the system. When a projection-lens system with a large angle of field is used in a projection system, for example a colour-television projection system, this system can be compact, which is a substantial practical advantage. A preferred embodiment of the projection-lens system in accordance with the invention is characterized further in that the main group comprises a cemented doublet of a biconvex lens and a concavo-convex lens, whose lens substrates have substantially equal refractive indices and different dispersions. This system has the advantage that the axial chromatic aberration is substantially smaller than in the projection-lens systems known until now. The axial chromatic aberration is to be understood to mean the variation in the axial position of the image as a function of the wavelength of the radiation used. An important practical aspect of the projection-lens system in accordance with the invention is that it is possible to design the system in such a way that the diameter of the lens elements decreases, viewed from the object side. As in a colour-television projection system the monochrome images should be superimposed on the projection screen, the reproduction tubes, which are arranged in-line, should be inclined towards one another, i.e. the normals to the two outer reproduction tubes should make a specific angle with the normal to the centre reproduction tube. If the diameters of the lens elements decrease towards the image side, i.e. in the case that the projection lens system is trough-shaped, this angle can be smaller than in conventional projection-lens systems. Reducing this angle is favourable in view of the optical properties of the projection screen. An embodiment of the projection lens system in accordance with the invention in which the two outer surfaces of the main group are provided with an aspheric layer of a transparent plastics exhibits very good properties. Surprisingly, it has been found that it is possible to obtain practically the same good properties if only one of the outer surfaces of the main group is provided with an aspheric layer. The projection-lens system in accordance with the invention may be characterized further in that the third group comprises two lenses, of which the lens at the image side is a meniscus lens whose concave surface faces the image side and of which the second lens is a planoconcave lens whose concave surface faces the image side, and the concave surfaces of both lenses are aspheric. However, preferably the projection-lens system in accordance with the invention is characterized further in that the third group comprises one meniscus lens whose power is substantially zero and whose convex surface faces the image side. The invention will now be described in more detail, by way of example, by a description of its use in a colour-television projection system. For this, reference is made to the accompanying drawings, in which Fig. 1 is a schematic perspective view of a colour-television projection system, and Figs. 2, 3 and 4 show embodiments of the projection-lens system in accordance with the invention intended for use in said colour-television projection system. The colour-television projection system shown in Fig. 1 comprises a colour television receiver 1. An input of this receiver, which is coupled to, for example, an aerial 2, receives a colour television signal, which is divided into a red signal, a green signal and a blue signal. These signals are applied to three separate reproduction tubes, in the present example three cathode-ray tubes 3, 4 and 5, on whose fluorescent screens a red picture, a green picture and a blue picture appear. The schematically shown projection-lens systems 6, 7 and 8 associated with the reproduction tubes project these pictures onto a projection screen 10. For the sake of clarity only the chief rays of the beams emitted by the reproduction tubes are shown. A mirror 9 is arranged between the projection-lens systems and the projection screen and reflects the upwardly inclined beams from the cathode-ray tubes to the projection screen. This mirror folds the radiation path, so that the projection system can be accommodated in a cabinet of comparatively small depth without thereby reducing the length of the radiation path. The three monochrome pictures must be superimposed on the projection screen. For this purpose the reproduction tubes, which are arranged in-line, are slightly inclined towards one another, i.e. the normals to the screens of the tubes 3 and 5 make a small angle with the normal to the screen of the tube 4. In the projection screen 10 the radiation of the three beams is scattered over a comparatively large angle in the Y-direction, which is the horizontal direction for the viewer W, and over a smaller angle in the Z-direction, which is the vertical direction for the viewer. The viewer W sees a picture which is a superposition of the enlarged images formed by means of the reproduction tubes. Each of the projection lenses 3, 4 and 5 should image the scene on the screen of the associated reproduction tube on the projection screen with a high image quality out to the edges of the picture. Such a projection-lens system should have a large numerical aperture and the image distance corresponding to a specific magnification should be as small as possible. The lenses of the projection-lens system are comparatively large, so that it is desirable to make these lenses of light-weight materials. Further, since the number of lens elements should be minimal, at least some of these elements must have aspheric refractive surfaces in order to ensure that the projection-lens system is adequately corrected and has a satisfactory optical transfer function. Moreover, the focal length of the entire projection-lens system should be temperature-independent and should remain constant over a wide range of wavelengths. The present invention provides a class of projection-lens systems by means of which the above-mentioned partly conflicting requirements are met in an optimum manner. Fig. 2 shows a projection-lens system in accordance with a first embodiment of the invention. Viewed from the image side, or the projection screen which is taken to be situated to the left of Fig. 2, this system comprises a first or correction group g₁, a main group g₂ and a third or field-curvature-correction group g₃. The last-mentioned group comprises a single lens L₅ which has a concave refractive surface S₁₀ with an aspheric profile and which is made of a transparent plastics, for example polymethyl methacrylate (PMMA). The second surface S₁₁ of the lens L₅ is plane and is positioned against a flat holder CP. A cooling liquid, for example comprising water and glycol, flows through this holder. Such cooling is required because the reproduction tube must have a high brightness. Without cooling the temperature of the fluorescent material on the face plate FP of the tube would rise substantially, as a result of which the brightness of the tube could decrease. The first lens group g₁ comprises two correction lenses L₁ and L₂ with surfaces S₁, S₂ and S₃, S₄, respectively. The surfaces S₁ and S₃ are concave and have aspheric profiles. The surface S₂ of the lens L₁ is convex and has a spheric profile and the surface S₄ of the lens L₂ is plane. The lenses L₁ and L₂ are made of a transparent plastics, for example the aforementioned PMMA. Instead of PMMA the groups g₁ and g₃ may also be made of, for example, polycarbonate. The main group g₂, which provides substantially the entire optical power of the lens system and which in principle may comprise a single lens, should comply with more stringent requirements than the correction lenses L₁, L₂ and L₅. The focal length of the main lens should be substantially independent of temperature variations in order to ensure that the focal length of the entire projection system ramains constant in the case of temperature variations and the image formed on the projection screen always remains in focus. This lens comprises two aspheric refractive surfaces S₅ and S₉. The main lens comprises a lens substrate, or preform, LS having two convex surfaces S₆ and S₈, represented by broken lines in Fig. 2, which are each provided with a transparent plastics layer LA₁ and LA₂, respectively, with an aspheric outer profile S₅ and S₉, respectively. Almost the entire power of the main lens is provided by the glass body of high optical quality and good thermal stability. The aspheric plastics layers are thin and a variation in the shape or refractive index of these layers as a result of temperature variations has only a minimal effect on the behaviour of the entire lens. The aspheric layers can be applied to the lens substrate LS by means of a replica process. For this process use is made of dies with inner profiles which are negatives of the desired outer profiles of the layers to be formed. A transparent plastics in a sufficiently soft condition, for example an ultraviolet-polymerisable plastics, is deposited on the lens substrate after which the dies are pressed into this material. Subsequently, the plastics is cured, for example by exposure to ultraviolet light, and the dies are removed, so that the lens is formed without any further operations being necessary. Suitably, the main group g₂ comprises two lens elements L₃ and L₄ which are cemented to each other and which have surfaces S₅, S₇ and S₇, S₉, respectively, as is shown in Fig. 2. The refractive indices of the materials of which these lens elements are made are equal as far as possible and the dispersions are unequal. Such a doublet provides a satisfactory correction for axial chromatic aberrations. Each of the lenses L₃ and L₄ may comprise a glass lens substrate having one side provided with an aspheric layer LA₁ and LA₂, respectively. In an embodiment as shown in Fig. 2 the projection lens system may have a numerical aperture of, for example, 0.375 and a focal length of, for example, 94 mm. In Figs. 2, 3 and 4 the image side is situated to the left of the drawing. Although in practice the rays propagate from right to left, the radiation path is shown from right to left in Figs. 2, 3 and 4, which is permissible in this type of optical system. Fig. 3 shows a second embodiment of the projection lens system in accordance with the invention, which is improved in comparison with that shown in Fig. 2. The improvement concerns a larger numerical aperture, for example N.A = 0.385, and a smaller focal length, for example F = 85 mm, and consequently a shorter length of the system, whilst the correction group g₁ comprises only one lens L₁. In practice, the theoretically anticipated reduction in the quality of the projection-lens system shown in Fig. 3 in comparison with that shown in Fig. 2 is hardly perceptible, inter alia owing to inevitable manufacturing tolerances. The lens L₁ has a convex surface S₁ with an aspheric profile and a spherical concave surface S₂ and may be made of PMMA. The main group comprises a doublet, the lenses L₂ and L₃, with convex aspheric outer surfaces S₃ and S₇ and a common spherical inner surface S₅. Each of the lenses L₂ and L₃ may comprise a glass lens substrate with an aspheric plastics layer. The field-correction lens L₄ has an aspheric concave surface S₈ and a plane surface S₉ and may be made of PMMA. The aspheric surfaces S₁, S₃, S₇ and S₈ may be characterized by: where Y is the distance from a point on the aspheric surface to the optical axis of the lens and Z is the distance between the projection of this point on the optical axis and the intersection of the optical axis with the aspheric surface. For an embodiment of the projection-lens system as shown in Fig. 3, in which the lenses L₁ and L₄ are made of PMMA, the lens L₂ is made of glass of the type "Schott SK5" and the lens L₃ is made of glass of the type "Schott F2", the axial surface curvatures C, the axial distances d between these surfaces and the refractive indices n have the following values, viewed from the image side: whilst the aspheric coefficients a 2i of the surfaces S₁, S₃, S₇ and S₈ are as follows: Fig. 4 shows another embodiment of the projection-lens system. For the same focal length and numerical aperture this system has one aspheric surface less than the system shown in Fig. 3, which is advantageous for reasons of production engineering. In the main group g₂ the biconvex lens and the concavo-convex lens have been interchanged in comparison with the main group shown in Fig. 3. In the projection lens system shown in Fig. 4 only the lens with the smallest refractive index, namely the lens L₃, which is for example made of "SK5" glass, has a layer LA₁ with an aspheric outer surface S₅. The lens L₂ of, for example, "F2" glass has a spherical outer surface S₃. An embodiment of the projection-lens system as shown in Fig. 4 has the following values for the surface curvatures C, the axial distances d , and the refractive indices n : whilst the coefficients a 2i of the aspheric surfaces S₁, S₆ and S₇ are as follows: The projection-lens system described here is very suitable for use in the green channel, having the widest band, in a colour television-projection system. However, such projection-lens systems may also be used in the blue channel and the red channel of such a colour-television projection system.

Claims: 1. A projection lens system for projecting a magnified image of a scene reproduced by means of a reproduction element onto a projection screen, which lens system comprises, in this order and from the image side, a first group, of which at least one outer surface is aspheric, a positive second or main group, of which at least the outer surface which faces the object side is aspheric, and a third group comprising a negative lens whose surface which faces the image side is concave and aspheric, the elements of the first group and the third group being made of transparent plastics, characterized in that the main group provides substantially the entire power of the system and comprises at least a glass lens substrate, at least that outer surface of said substrate which faces the object side being provided with a layer of a transparent plastics having an aspheric outer profile. 2. A projection-lens system as claimed in Claim 1, characterized in that the main group comprises a cemented doublet of a biconvex lens and a concavo-convex lens, whose lens substrates have substantially equal refractive indices and different dispersions. 3. A projection-lens system as claimed in Claim 1 or 2, characterized in that the diameters of the successive lens elements decrease viewed from the object side. 4. A projection-lens system as claimed in Claim 1, 2 or 3, characterized in that each of the outer surfaces of the main group is provided with a layer of a transparent plastics having an aspheric outer profile. 5. A projection-lens system as claimed in Claim 4, characterized in that the third group comprises two lenses, of which the lens at the image side is a meniscus lens whose concave surface faces the image side and of which the second lens is a planoconcave lens whose concave surface faces the image side, and the concave surfaces of both lenses are aspheric. 6. A projection-lens system as claimed in Claim 4, characterized in that the third group comprises one meniscus lens whose power is substantially zero and whose convex aspheric surface faces the image side. 7. A projection-lens system as claimed in Claim 6, characterized in that, viewed from the image side, the axial curvatures C of the lens surfaces, the axial distances d between the surfaces and the refractive indices n of the lens materials have the following values: the focal length F = 85 mm, the numerical aperture N.A. = 0.385, and the coefficients a2i of the aspheric surfaces S₁, S₃, S₇ and S₈ which are characterised by: where Y is the distance from a point of the aspheric surface to the optical axis of the lens and Z is the distance between the projection of this point on the optical axis and the intersection of the optical axis with the aspheric surface, comply with: 8. A projection-lens system as claimed in Claim 2, characterized in that the third group comprises one meniscus lens whose power is substantially zero and whose concave surface faces the image side. 9. A projection-lens system as claimed in Claim 8, characterized in that, viewed from the image side, the axial curvatures C of the lens surfaces, the axial distances d between the surfaces, and the refractive indices n of the lens materials have the following values: the focal length F = 85 mm, the numerical aperture N.A. = 0.385, and the coefficients a2i of the aspheric surfaces S₁, S₅ and S₆ which are characterized by: where Y is the distance from a point on the aspheric surface to the optical axis of the lens and Z is the distance between the projection of this point on the optical axis and the intersection of the optical axis with the aspheric surface, comply with: 10. A colour-television projection system comprising three cathode-ray tubes, one for each of the primary colours red, green and blue, a projection screen, and three projection-lens systems, which are each arranged between one of the cathode-ray tubes and the projection screen, characterized in that at least the projection-lens system in the radiation path from the green cathode-ray tube is a projection-lens system as claimed in any one of the preceding Claims.

Inventors: BRAAT, JOSEPHUS JOHANNES MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: G02B13
IPC codes: G02B13, G02B3, G02B7