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Focusable front projection lens systems for use with large screen formats

  • US 5,936,776 A
  • Filed: 09/12/1997
  • Issued: 08/10/1999
  • Est. Priority Date: 09/27/1996
  • Status: Expired due to Fees
First Claim
Patent Images

1. A variable focus, front projection, lens system for forming an image on a screen which has a diagonal greater than 1.5 meters, said system being optically coupled to a cathode ray tube by a coupling fluid during use, said system and said coupling fluid having an optical power Φ

  • , said system comprising in order from the system'"'"'s image side;

    (a) a first lens unit which primarily corrects aperture dependent aberrations, said first lens unit having an optical power Φ

    1 and comprising at least one aspherical surface;

    (b) a second lens unit having an optical power Φ

    2 and comprising in order from the system'"'"'s image side;

    a negative, biconcave, first lens element composed of glass;

    a positive, biconvex, second lens element composed of glass;

    a positive, biconvex, third lens element composed of glass; and

    a fourth lens element which comprises at least one aspherical surface and is composed of plastic;

    (c) a corrector lens unit having an optical power Φ

    CR and comprising at least one aspherical surface; and

    (d) a third lens unit which provides correction for the field curvature of the lens system, said third lens unit comprising a lens element which is in contact with said coupling fluid during use of the lens system and which has two surfaces both of which are concave to the image, said third lens unit and said coupling fluid having an optical power Φ

    3 ;

    wherein(i) Φ

    1 >

    0;

    (ii) Φ

    2

    >

    0.7;

    (iii) Φ

    CR <

    0;

    (iv) Φ

    3 <

    0;

    (v) the corrector lens unit and the third lens unit are separated by a distance DCR/3 which is varied during focusing of the image on the screen, said focusing being able to accommodate a range of screen diagonals of at least 1.5;

    1; and

    (vi) said lens system and coupling fluid have a modulation transfer function at the cathode ray tube which varies over said range of screen diagonals such that the ratio RMin/Max of the minimum value of the modulation transfer function at 5 cycles/millimeter to the maximum value of the modulation transfer function at 5 cycles/millimeter over said range of screen diagonals for any field position up to 0.85 of the maximum field is greater than 0.5.

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