Projection lens system

Claim 2

A lens system for projecting an enlarged image of a concave object surface onto a planar display screen, characterised in that the projection lens comprises a first element L1 of low power relative to the power of the whole lens, and separated second and third elements L2 and L3, each of positive power, between the first element and the concave object surface, the powers K1, K2 and K3 of the respective elements being chosen relative to the power K of the whole lens so that -0.20K<K1<+0.15K +0.62K<K2<30 0.80K +0.55K<K3<+0.67K and at least one of the element surfaces being aspheric.

Claim 3

A lens system as claimed in claim 1 characterised in that two of the element surfaces are aspheric.

Claim 4

A lens system as claimed in claim 1 or claim 2, characterised in that each aspheric surface is defined by the following relationship: ##EQU2## where Z is the deviation, in the axial direction, of the surface from a plane normal to the optic axis and tangent to the surface at its pole for a zone of the surface which is at a distance s from the axis, C is the curvature of the surface on the axis, e is a conic constant, and a 4 , a 6 , a 8 and a 10 are constants for the surface.

Claim 5

A lens system as claimed in claim 3, characterised in that the lens is combined with a cathode ray tube face plate FP concave towards the lens system for projecting a faceplate raster onto the screen, the lens system being of focal length 92.1 mm at a wavelength of 525 nm, relative aperture F/1.05, projection angle ±40.0°, a throw of 750 mm providing a magnification of 8×, described substantially as follows: ______________________________________ Surface Axial Axial polar radius thickness separation Refractive mm mm mm index ______________________________________ S1 512.8 L1 5.00 1.5727 S2 493.1 25.63 S3 164.6 L2 21.00 1.5727 S4 -136.9 36.28 S5 482.0 L3 25.00 1.5727 S6 -104.6 52.28 S7 -119.2 FP 12.00 1.5200 S8 -119.2 ______________________________________ Aspheric surfaces: S2, S5 S2 S5 ______________________________________ C .002028 .002075 e 0 0 a.sub.4 .3789 × 10.sup.-6 -.4840 × 10.sup.-6 a.sub.6 .1225 × 10.sup.-9 .1411 × 10.sup.-9 a.sub.8 -.4456 × 10.sup.-13 - .5227 × 10.sup.-13 a.sub.10 .1298 × 10.sup.-16 .5667 × 10.sup.-17 ______________________________________ Element values Focal length, mm Power, mm.sup.-1 Relative power ______________________________________ Lens 92.1 0.01086 1 L1 -24,700 -0.00004 -0.004 L2 133.9 0.0075 0.688 L3 152.4 0.0066 0.604 ______________________________________ where L1, L2 and L3 are successive lens elements from the image end and S1 to S8 inclusive are successive element surfaces, positive surfaces being convex towards the image end and negative surfaces being concave towards the image end.

Claim 6

A lens system as claimed in claim 3, characterised in that the lens is combined with a cathode ray tube faceplate FP concave towards the lens system for projecting a faceplate raster onto the screen, the lens system being of focal length 92.3 mm at a wavelength of 525 nm, relative aperture F/1.2, projection angle ±40.0°, a throw of 750 mm providing a magnification of 8×, described substantially as follows: ______________________________________ Surface Axial Axial polar radius thickness separation Refractive mm mm mm index ______________________________________ S1 plane L1 5.00 1.5727 S2 plane 23.80 S3 215.6 L2 19.00 1.5727 S4 -108.3 42.97 S5 851.3 L3 23.00 1.5727 S6 -97.2 48.97 S7 -119.1 FP 12.00 1.5200 S8 -119.1 ______________________________________ Aspheric surfaces: S2, S5 S2 S5 ______________________________________ C 0 .001175 e 0 0 a.sub.4 .5435 × 10.sup.-6 -.3609 × 10.sup.-6 a.sub.6 .8080 × 10.sup.-10 .2297 × 10.sup.-10 a.sub.8 -.3581 × 10.sup.-14 -.6778 × 10.sup.-14 a.sub.10 0 0 ______________________________________ Element values Focal length, mm Power, mm.sup.-1 Relative power ______________________________________ Lens 92.3 0.01083 1 L1 ∞ 0 0 L2 128.6 0.0078 0.718 L3 153.7 0.0065 0.600 ______________________________________ where L1, L2 and L3 are successive lens elements from the image end and S1 to S8 inclusive are successive element surfaces, positive surfaces being convex towards the image end and negative surfaces being concave towards the image end.