Transient state liquid crystal image bar with contrast enhancement

• US 4,767,190 A
• Filed: 12/04/1986
• Issued: 08/30/1988
• Est. Priority Date: 12/04/1986
• Status: Expired due to Fees

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Perregaux, Alain E., Dir, Gary A.
• Primary Examiner(s): Miller, Stanley D.
• Assistant Examiner(s): Gallivan, Richard F.
• Application Number:
  US06/937,806
• Time in Patent Office: 635 Days
• Field of Search: 350/339 R, 350/346, 350/347 E, 350/331 R, 350/334, 350/347 R
• US Class Current: 349/3
• CPC Class Codes: G02F 1/133531 characterised by the arrangement of polariser or analyser axes
• CPC Class Codes: G02F 1/13363 Birefringent elements, e.g. for optical compensation
• CPC Class Codes: G02F 1/133638 Waveplates, i.e. plates with a retardation value of lambda/n
• CPC Class Codes: G02F 1/1393 the birefringence of the liquid crystal being electrically controlled, e.g. ECB-, DAP-, HAN-, PI-LC cells G02F1/1396, G02F1/141 take precedence
• CPC Class Codes: G02F 2413/01 Number of plates being 1

Abstract

An improved transient state liquid crystal image bar having enhanced contrast with lower required driving voltage. The image bar is of the type having a nematic liquid crystal cell configured as a single array of dot shutters energizable by a single freqency voltage with crossed polarizer and analyzer. The polarizing axis of the polarizer is at 45 degrees with the liquid crystal molecules in their rest state. Thus, when all the image bar shutters are energized, it is in the non-transmission condition. For bursts of light to pass, the voltage to selected dot shutters are interrupted and reapplied. In one embodiment, the improvement is achieved by an optical compensator or thin prism placed between the polarizer and the liquid crystal cell. In another embodiment, a quarterwave plate is placed between the liquid crystal cell and the analyzer, with the analyzer being rotated about the image bar'"'"'s optical axis a predetermined angle from its initial crossed position with respect to the polarizer, so that there is increased extinction of the light exiting from the liquid crystal cell, when the liquid crystal material is in its normally stable energized state.

Claims

1. An improved transient state liquid crystal image bar for producing latent electrostatic images on a moving, precharged photoconductive member of an electrophotographic printer one line of pixels at a time, the image bar having a nematic liquid crystal cell configured as a single array of dot shutters individually energizable by the selective application of a single frequency driving voltage and having the polarizing axes of the polarizer and analyzer approximately crossed, the polarizing axis of the polarizer being at 45 degress with the liquid crystal molecules in their homogeneous relaxed state, and substantially perpendicular to the optical axis of the image bar, so that the image bar is non-transmissive when all of the dot shutters are energized, bursts of light for erasing pixels of precharged background area of the photoconductive member being passed by selected dot shutters through the interruption and reapplication of the voltage applied thereto, wherein the improvement comprises:

an optical compensating member being placed between the liquid crystal cell and either the polarizer or analyzer, the compensating member being oriented in a predetermined position relative to both the polarizing axis of the polarizer or analyzer and the molecules of the nematic liquid crystal material in their homogeneous non-energized state, so that the elliptically polarized light, generated by the incomplete homeotropic state or tilt of the energized liquid crystal molecules as the linearly polarized light enters and passes therethrough, is prevented from entering the analyzer, whereby the contrast between the transmissive and non-transmissive state of the image bar is increased and the driving voltage is concurrently reduced.

2. The improved image bar of claim 1, wherein the optical compensating member is a relatively thin transparent prism between the polarizer and the liquid crystal cell, the deflection angle of light passing through the prism being matched to the deflection angle of light passing through the liquid crystal molecules in their energized state.

3. The improved image bar of claim 1, wherein the optical compensating member is a quarter-wave plate between the liquid crystal cell and the analyzer;

and wherein the polarizing axis of the analyzer is rotated relative to the polarizing axis of the polarizer and the quarter-wave plate for a predetermined number of degrees (Φ

) so that the polarizing axis of the analyzer is coincident with the linearly polarized light exiting from the quarter-wave plate.

4. The improved image bar of claim 3, wherein the predetermined number of degrees (Φ

) is 25 degrees or less.

5. A transient state liquid crystal image bar for producing latent electrostatic images on a moving, precharged photoconductive member of an electrophotographic printer one line of pixels at a time through the selective passage of bursts of polarized light along an optical axis that is normal thereto, comprising:

a nematic liquid crystal cell configured as a single array of dot shutters individually energizable by the selective application of a single frequency driving voltage, the nematic liquid crystal material contained therein being of the type having its molecules in the homogeneous orientation when in the non-energized or relaxed state;

a polarizer and an analyzer being parallel to each other and having their polarizing axes approximately crossed, the polarizing axis of the polarizer being at 45 degrees with the liquid crystal molecules in their homogeneous relaxed state, and substantially perpendicular to the optical axis of the image bar, so that the image bar is non-transmissive when all of the dot shutters are energized, bursts of light for erasing pixels of precharged background area of the photoconductive member being passed by selected dot shutters through the interruption and reapplication of the voltage applied thereto; and

a relatively thin prism having a predetermined light deflection angle for light passing therethrough, the prism being positioned between the polarizer and the liquid crystal cell, the deflection angle for light passing through the prism being matched to the deflection angle of the light passing through the liquid crystal molecules in their homeotropic energized state, so that the elliptically polarized light, which would normally be generated by the incomplete homeotropic state or tilt of the energized liquid crystal molecules without the prism, does not occur, whereby the contrast between the transmissive and non-transmissive state of the image bar is increased and the driving voltage is concurrently reduced.

6. A transient state liquid crystal image bar for producing latent electrostatic images on a moving, precharged photoconductive member of an electrophotographic printer one line of pixels at a time through the selective passage of bursts of polarized light along an optical axis that is normal thereto, comprising:

a nematic liquid crystal cell configured as a single array of dot shutters individually energizable by the selective application of a single frequency driving voltage, the nematic liquid crystal material contained therein being of the type having its molecules in the homogeneous orientation when in the non-energized or relaxed state;

a polarizer and an analyzer being parallel to each other and having their polarizing axes approximately crossed, the polarizing axis of the polarizer being at 45 degrees with the liquid crystal molecules in their homogeneous relaxed state, and substantially perpendicular to the optical axis of the image bar, so that the image bar is non-transmissive when all of the dot shutters are energized, bursts of light for erasing pixels of precharged background area of the photoconductive member being passed by selected dot shutters through the interruption and reapplication of the voltage applied thereto;

a quarter-wave plate being positioned between the liquid crystal cell and the analyzer, one of the main axes of the quarter-wave plate being aligned with the polarizing axis of the polarizer, so that the linearly polarized light entering the liquid crystal cell exits therefrom as effectively linearly polarized light having been rotated about the optical axis of the image bar by a predetermined angle Φ

; and

said analyzer being rotatably adjusted about said optical axis and relative to the polarizer and quarter-wave plate by an amount equal to said predetermined angle Φ

, so that the effectively linearly polarized light exiting from the quarter-wave plate is coincident with the polarizing axis of the analyzer, whereby the contrast between the transmissive and non-transmissive state of the image bar is increased and the driving voltage is concurrently reduced.