PROJECTION-LENS SYSTEM

Abstract: An achromatic temperature-independent projection lens system is described for projecting a magnified image of a scene reproduced by means of a reproduction-tube onto a projection screen, which lens system comprises two correction groups and one main group which provides substantially the entire power of the lens system. The main group comprises a cemented doublet of a convexo concave lens and a biconvex lens and has two spheric outer surfaces. The correction groups each comprise one lens whose surface facing the image side is aspheric.

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 having one aspheric surface, a positive second or main group having two spherical convex outer surfaces, and a third group comprising a planoconvex lens whose concave surface which faces the image side is aspheric, the lenses of the first group and of the third group being made of a transparent plastics material. The invention also relates to a colour-television projection system provided with at least one projection-lens system of this kind. Such a projection-lens system is disclosed in the United States Patent Specification no. 4,438,081, which system is used for projecting a scene in one colour onto a projection screen, where superposition of three monochrome images results in a colour picture, for example a colour-television picture. The third lens group from the image side is a field-curvature correction lens, also referred to as "Field flattener", which provides compensation for the Petzval curvature of the two other lens groups. The system shown in Fig. 1 of Unites States Patent Specification no. 4,348,081 has a main group comprising one biconvex lens. If this lens is made of glass, which is a stable material, the focal length of this lens, which largely determines the focal length of the entire system, is substantially temperature-independent. The two other groups are made of a transparent plastics which is lighter in weight than glass, so that the weight of the entire projection-lens system remains within acceptable limits despite the fact that the lens elements have comparatively large diameters, for example of the order of 100 mm. Since a part of the power of the entire lens system is provided by the first group and the third group, which are made of a plastics whose refractive index is temperature dependent, the focal length of the entire system is still temperature dependent. Moreover, the projection-lens system in accordance with United States Patent Specification no. 4,348,081 provides the desired imaging only for a small range of wavelengths around the central wavelength of the radiation which passes through the system; for radiation in a wider range of wavelengths the picture will not be in focus owing to the variation in focusing as a function of the wavelength. In view of the comparatively large bandwidth of the radiation emitted by the reproduction tubes, in particular the reproduction tube with the green phosphor, it is desirable that in practice an achromatic main lens is used. The present invention provides a projection lens system which meets this requirement and which, in addition, has a focal length which is substantially temperature-independent. Further, the projection-lens system in accordance with the invention can have a smaller focal length and a larger angle of field than comparable known projection-lens systems. In accordance with a first aspect of the invention the projection-lens system is characterized in that the main group provides substantially the entire power of the lens system and comprises a cemented doublet of a convexo-concave and a biconvex lens, which lenses have substantially equal refractive indices and different dispersions. An important practical aspect of the projection-lens system in accordance with the invention is that the system can be designed in such a way that the diameters of consecutive lens elements decrease viewed from the object side. Since in a colour-television projection system the monochrome images are superimposed on the projection screen, the reproduction tubes, which are arranged in-line, should be inclined relative to one another, i.e. the normals of the two outer reproduction tubes should make a specific angle with the normal to the inner reproduction tube. If the diameters of the lens elements decrease towards the image side, i.e. if the projection-lens system has a tapered shaped, this angle can be smaller than when conventional projection lens systems are employed, which is desirable in view of the optical properties of the projection screen. In accordance with a second aspect of the invention a projection-lens system comprising two single correction lenses each having one aspheric surface and a main group comprising a cemented doublet has a large angle of field and a small focal length and this projection-lens system also has a satisfactory modulation-transfer function (MTF) and an acceptable luminance in the corners of the field if the parameters of this system have the values specified in Claim 3. The angle of field is to be understood to mean the angle between the optical axis of the system and the chief ray of a beam which issues from the edge of an object or scene and which is still accepted by the lens system. 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 drawing, in which: Fig. 1 is a schematic perspective view of a colour-television projection system, and Fig. 2 shows a projection-lens system in accordance with the invention which may be employed 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 split 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, a green and a blue image appear. The schematically shown projection-lens systems, 6, 7 and 8 associated with the reproduction tubes project these images 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 to reflect the obliquely upward 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 images must be superimposed on the projection screen. For this purpose, the reproduction tubes, which are arranged in line, are slightly inclined towards each other, i.e. the normals to the screens of the tube 3 and 5 make a small angle with the normal to the screen of the tube 4. In the projection screen 15 the radiation of the three beams is scattered over a comparatively large angle in the Y-direction, i.e. in 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 magnified images from the reproduction tubes. Each of the projection-lens systems 3, 4 and 5 should image the scene on the face plate of the associated reproduction tube onto the projection screen with a high imaging 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 manufacture these lenses from light-weight materials. Further, since the number of lens elements should be as small as possible, at least some of these elements should have aspheric refractive surfaces in order to ensure that the projection lens system is corrected adequately and has a satisfactory optical transfer function. Moreover, the focal length of the entire projection-lens system should remain as constant as possible in the presence of temperature variations. For this purpose, substantially the entire power of the lens system is provided by the main group and this main group comprises a glass lens, whilst the two other groups comprise lenses made of a transparent plastics with a lower weight than glass. In accordance with the invention the main group comprises a cemented doublet of a concave convex lens L₂ having surfaces S₃ and S₄ and a biconvex lens L₃ having surfaces S₄ and S₅, as is shown in Fig. 2. The refractive indices of the lenses L₂ and L₃ are equal as far as possible, whilst the dispersions of these lenses differ substantially. The lens L₂ may be made, for example, of glass of the type "Schott F₂" having a refractive index n = 1.625 and the lens L₃ may be made of glass of the type "Schott SK5" having a refractive index n = 1.592. In Fig. 2 the image side is situated to the left of the drawing. Although in practice the rays propagate from right to left, the radiation path in Fig. 2 is shown from left to right, as is permissible in optical systems of this type. In addition to the main group g₂, the projection lens system comprises a first correction group g₁ comprising a single meniscus lens L₁ of, for example, polymethyl methacrylate (PMMA) having refractive surfaces S₁ and S₂ and a second field-curvature correction group g₃. The last-mentioned group comprises a single lens L₄ which has a concave refractive surface S₆ and which may also be made of PMMA. Instead of PMMA the lenses L₁ and L₃ may be made of, for example, polycarbonate. The second surfaces S₇ of the lens L₄ is plane and is positioned against a disc-shaped holder CP. A coolant, comprising, for example, water and glycol, flows in this holder. Such a coolant is necessary because the reproduction tube should have a high brightness, so that 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 diminish. Surprisingly, it has been found that a projection-lens system having a sufficiently small focal length and a sufficiently large angle of field and providing adequate correction for aberrations can be obtained, which comprises only two aspheric refractive surfaces, instead of more than two as is customary in this type of system. The surfaces S₃ and S₅ of the main group then need not be made aspheric, so that a time-consuming and expensive process may be dispensed with. The aspheric surfaces are the surfaces S₁ and S₆, i.e. surfaces of plastics elements. The lenses having the aspheric surfaces can be manufactured by means of a moulding or pressing method. The aspheric surfaces S₁ and S₆ 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. A projection lens system in accordance with the invention, in which the lenses L₁ and L₄ are made of PMMA, the lens L₂ is made of glass type "F2" and the lens L₃ is made of glass "SK5", which projection-lens system has a numerical aperture N.A. of approximately 0.35 and a focal length F of approximately 85 nn, has axial curvatures C of the lens surfaces, axial distances d between these surfaces, and refractive indices n with the following values, viewed from the image side: whilst the aspheric coefficients a 2i of the two aspheric surfaces S₁ and S₆ are as follows: The projection-lens system described here is particularly suitable for use in the green channel, which has the widest band, in a colour-television projection system. However, such projection lens systems may also be employed 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 viewed from the image side, a first group having one aspheric surface, a positive second or main group having two spherical convex outer surfaces, and a third group comprising a planoconvex lens whose concave surface which faces the image side is aspheric, the lenses of the first group and of the third group being made of a transparent plastics material, characterized in that the main group provides substantially the entire power of the lens system and comprises a cemented doublet of a convexo-concave and a biconvex lens, which lenses have substantially equal refractive indices and different dispersions. 2. A projection-lens system as claimed in Claim 1, characterized in that the diameters of consecutive lens elements decrease, viewed from the object side. 3. A projection-lens system as claimed in Claim 1 or 2, characterized in the focal length F is approximately 85 mm and the numerical aperture N.A. is approximately 0.35, and the axial curvatures C of the lens surfaces, 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 a2i of the two aspheric surfaces S₁ and S₆, which 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, are given by: 4. 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