Video projector lens system

Claim 2

A lens system for projecting onto a screen a video image formed on a face plate of a cathode ray tube comprising, from the screen side to the tube side: a first positive plastic lens having a screen side convex surface; a second negative plastic lens; a third biconvex lens; and a fourth negative lens having a screen side concave surface, at least the third lens including a glass material, wherein the lens system includes at least one aspheric surface and fulfills the following conditions: 1.0<|f.sub.2 |/f<2.5 ν.sub.2 <40 wherein: f represents a focal length of the whole lens system; f 2 represents a focal length of the second lens; and ν 2 represents an Abbe number of the second lens.

Claim 3

The invention of claim 1, wherein the lens system further fulfills the following condition: 1<d.sub.2 /d.sub.4 <5 wherein: d 2 represents an axial distance between the first lens and the second lens; and d 4 represents an axial distance between the second lens and the third lens.

Claim 4

The invention of claim 2, wherein the lens system further fulfills the following condition: 1.25 <f.sub.1 /f<2.0 wherein: f 1 represents a focal length of the first lens.

Claim 5

The invention of claim 3, wherein the lens system further fulfills the following condition: n.sub.3 >1.55 wherein: n 3 represents a refractive index of the third lens.

Claim 6

The invention of claim 1, wherein the lens system further fulfills the following condition: 1.25<f.sub.1 /f<2.0 wherein: f 1 represents a focal length of the first lens.

Claim 7

The invention of claim 1, wherein the lens system further fulfills the following condition: n.sub.3 >1.55 wherein: n 3 represents a refractive index of the third lens.

Claim 8

The invention of claim 1, wherein both of the first lens and the second lens are made of plastic materials.

Claim 9

A lens system for projecting onto a screen a video image formed on a face plate of a cathode ray tube comprising, from the screen side to the tube side: a first positive lens having a screen side convex surface, said first lens including at least an aspheric surface and being made of a plastic material; a second negative plastic meniscus lens having a screen side convex surface and having a first air space formed between the first lens and the second lens; a third biconvex lens made of a glass material and having a second air space formed between the second lens and the third lens; and a fourth negative lens having a screen side concave surface and having a third air space formed between the third lens and the fourth lens.

Claim 10

A lens system for projecting onto a screen a video image formed on a face plate of a cathode ray tube comprising, from the screen side to the tube side: a first positive lens having a screen side convex surface, said first lens including at least an aspheric surface and being made of a plastic material; a second negative plastic meniscus lens having a screen side convex surface and having a first air space formed between the first lens and the second lens; a third biconvex lens made of a glass material and having a second air space formed between the second lens and the third lens; and a fourth negative lens having a screen side concave surface and having a third air space formed between the third lens and the fourth lens.

Claim 11

The invention of claim 8 comprising the following design parameters, wherein asterisk (*) represents the aspheric surface with the following aspheric surface coefficients Ci: ______________________________________ f = 100 β = -0.061 F.sub.NO = 1.05 ______________________________________ r.sub.1 * 83.39 L.sub.1 d.sub.1 17.01 n.sub.1 1.491 ν.sub.1 57.8 r.sub.2 -442.62 d.sub.2 26.94 r.sub.3 304.12 L.sub.2 d.sub.3 5.47 n.sub.2 1.592 ν.sub.2 30.9 r.sub.4 71.19 d.sub.4 13.39 r.sub.5 93.02 L.sub.3 d.sub.5 19.02 n.sub.3 1.620 ν.sub.3 60.3 r.sub.6 -96.62 d.sub.6 48.74 r.sub.7 * -42.13 L.sub.4 d.sub.7 3.45 n.sub.4 1.491 ν.sub.4 57.8 r.sub.8 ∞ d 4.14 r.sub.A ∞ P d 10.35 n 1.536 ν 50.7 r.sub.B ∞ ______________________________________ aspheric surface coefficients ______________________________________ r.sub.1 r.sub.7 ______________________________________ C.sub.2 -0.47062 × 10.sup.-6 0.27073 × 10.sup.-5 C.sub.3 -0.12549 × 10.sup.-9 -0.73863 × 10.sup.-8 C.sub.4 -0.15711 × 10.sup.-14 0.11481 × 10.sup.-10 C.sub.5 0.50902 × 10.sup.-17 -0.76826 × 10.sup.-14 C.sub.6 -0.26165 × 10.sup.-20 0.20436 × 10.sup.-17 C.sub.7 0.0 0.0 C.sub.8 0.0 0.0 ______________________________________ wherein L represents a lens, r represents a radius of curvature, d represents an axial distance, n represents an index of refraction, ν represents the Abbe number and P represents a face plate.