GB2156537A - Temperature stability arrangement in a combined laser recording and color projection system - Google Patents

Abstract

A combined laser 20 recording and projection system utilizing locally erasable recording medium e.g. smectic liquid crystal display cells 54,56 is provided with a compartment 82 filled with a thermally conductive liquid. The display cells 54,56 and the combining beam splitter 48 are immersed in the liquid and the temperature of the liquid is controlled to provide a uniform surface temperature distribution for the display cells. Additionally, the refractive index of the liquid may match that the beam splitter, thereby eliminating the requirement of an anti-reflection coating. <IMAGE>

Description

SPECIFICATION Temperature stability arrangement in a combined laser recording and color projection system BACKGROUND OF THE INVENTION This invention relates to a laser recording and projection system and, more particularly, to an arrangement for providing temperature stability and uniformity for smectic liquid crystal display cells utilized in such a system.

U.S. Patent No. 4,345,258, which issued to Robert C. Tsai and William J. Kidwell, on August 17, 1982, discloses a laser recording and projection system in which two channels of data are recorded by laser action on smectic liquid crystal display cells and are simultaneously projected, by back lighting the display cells, upon a display screen. In such a system, the uniformity of temperature distribution across the display cells is a critical parameter for high resolution operation. One approach to controlling the temperature of the display cells is to utilize an indium tin oxide thin film heater in addition to external peripherally placed nichrome wire heaters. It is also desirable to be able to measure the temperature of the display cells. One approach to temperature measurement is to use a remote or peripherally located thermocouple embedded at the edge of the liquid crystal layer.However, the thermocouple measures accurately only one area (an edge) of the liquid crystal layer, not the center, or active region, of the display cell. Performance measurements of such display cells have shown that liquid crystal temperature variations do exist and pose a serious problem in terms of system performance.

It is therefore a primary object of this invention to provide an improved arrangement for maintaining temperature stability and uniformity of the display cells in a laser recording and projection system.

SUMMARY OF THE INVENTION The foregoing and additional objects are attained in accordance with the principles of this invention in a recording and projection system of the type utilizing a focused laser beam and a locally erasable recording medium positioned at the laser beam focal point, by providing an arrangement for maintaining a uniform temperature distribution on the surface of the recording medium, which arrangement comprises a housing defining an enclosed compartment, means for mounting the recording medium within the compartment, a thermally conductive liquid filling the compartment sufficiently to cover the recording medium, and means for controlling the temperature of the liquid.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing will be more readily apparent upon reading the following description in conjunction with the drawings wherein: Figure 1 is a simplified drawing schematically illustrating a combined laser recording and color projection system utilizing an arrangement according to this invention; Figure 2 is a top plan view of an arrangement according to this invention for use in a two channel laser recording and projecting system; and Figure 3 is a top plan view schematically illustrating an arrangement in accordance with this invention for use in a four channel laser recording and projection system.

DETAILED DESCRIPTION Fig. 1 schematically illustrates a laser recording and projection system in which the present invention finds utility. The system shown in Fig. 1 includes a laser 20 providing an output beam 22 which is reflected by a mirror 24 and enters an acousto-optic moduiator 26. As is well known in the art, the acousto-optic modulator 26, upon command of control circuitry 28, selectively prevents or allows the beam 22 from exiting therefrom.

After the beam 22 leaves the acousto-optic modulator 26, it passes through a polarization rotator 30. The polarization rotator 30 is illustratively a one-half wavelength retardation plate rotatable by means of a stepper motor 32 under the control of the control circuitry 28, to rotate the polarization of the beam 22 within a range of from 0" to 90 with respect to the polarization characteristics of the subsequent optical elements in the system of Fig.

1. Thus, the control circuitry 28 may selectively energize the stepper motor 32 to rotate the half wave length retardation plate 30 so as to produce a laser beam having a desired polarization between a first plane of polarization and a second plane of polarization.

The appropriateiy polarized laser beam then passes through a beam expander 34 and impinges upon the mirror 36 of the X-deflection galvanometer 38 which is under the control of the control circuitry 28 to deflect the laser beam along the horizontal direction as it enters the relay lens 40. The relay lens 40 operates to focus the X-deflected laser beam from the mirror 36 upon the mirror 42 of the Y-deflection galvanometer 44. The laser beam is then focused by a focus lens 46 and passes through the dichroic polarizer plate 48.

The dichroic polarizer plate 48 has the characteristic that it either reflects or transmits the laser beam passing therethrough, depending upon the plane of polarization of the laser beam. Accordingly, the beam from the laser 20, having been suitably deflected by the Xdeflection galvanometer 38 and the Y-deflection galvanometer 44, is focused by the focusing lens 46 to the focal point 50 or 52, depending upon the polarization impressed on the laser beam by the half wavelength retarda tion plate 30. Positioned at the focal points 50 and 52 are locally erasable recording mediums, preferably smectic liquid crystal display cells 54 and 56. As is known, the laser beam can be utilized to "write" information selectively into the display cells 54 and 56.

Once information is "written into" the liquid crystal display cells 54 and 56, it can be stored or, if desired, the cell can be erased, in whole or in part, by applying a suitable AC voltage to the cells from the AC sources 58 and 60. Light may then be projected through the cells 54 and 56 and through a projection system for displaying the information "written into" the cells.

The focused laser beam, having an energy distribution determined by the angular position of the half wavelength retardation plate 30, is scanned across the display cells 54 and 56 and selectively blanked and unblanked by acousto-optic modulator 26 under the control of the control circuitry 28, to write the information onto the display cells 54 and 56. At the same time that the focused laser beam is recording data on the display cells 54 and 56, these cells are being backlighted so that the images thereon are projected to a suitable display screen. The projection system includes a Xenon lamp 62. The Xenon lamp 62 is at the focal point of a condensing lens 64 which collimates and directs the output light of the lamp 62 to a dichroic plate 66.The dichroic plate 66 contains dichroic thin film coatings that transmit light energy in the range of visible wavelengths at the red end of the spectrum and reflects light and energy at the green end of the spectrum.

Thus, the light from the lamp 62 is divided into a short wavelength visible beam (the green beam) which is reflected by the plate 66 and a long wavelength visible light beam (the red beam) which is transmitted by the plate 66. The reflected short wavelength visible light beam is directed by a mirror 68 to backlight the display cell 54 and the long wavelength visible light beam that is transmitted by the plate 66 is directed by a mirror 70 to backlight the display cell 56. A green filter 72 is interposed on the short wavelength visible light beam and a red filter 74 is interposed in the long wavelength visible light beam. These dichroic color trim filters operate at normal incidence and help to eliminate unwanted wavelengths thereby producing saturated primary colors.Accordingly, the image which was written into the display cell 54 is backlit with light of a first color and the image which was written into the display cell 56 is backlit with light of a second color.

The plate 48 also has dichroic properties indentical to those of the plate 66 in the visible spectrum so that the light band that was reflected by the plate 66 will also be reflected by the plate 48 and the light band transmitted by the plate 66 will also be transmitted by the plate 48. Therefore, the reflected light band from the plate 66 will be projected through the display cell 54 and this band will again be reflected by the dichroic in the plate 48 to pass through the projection lens 76 to a suitable display screen. In like manner, the band transmitted by the plate 66 will pass through the display cell 56 and will also be transmitted by the plate 48 to the projection lens 76 and onto the suitable display screen. However, plate 48 has other optical properties in the near infrared spectrum.Additional dichroic thin film coatings are present which act to analyze the polarization state of the infrared laser beam. Orientation of the laser polarization vector is manipulated between vertical (0 ), horizontal (90 ) and midway (45 ) to allow the light valves to be addressed individually or simultaneously.

Plate 48 transmits horizontally polarized energy and reflects vertically polarized.

In the aforedescribed system, it is highly desirable to be able to control the temperature of the display cells 54 and 56. In particular, the temperature should preferably be uniform across the surface of the cells 54 and 56. To achieve the desired effect, the cells 54 and 56 are immersed in an isotropic temperature controlled liquid bath. As is shown in Fig. 2, a housing 80 is provided, illustratively defining two compartments 82 and 84. The display cells 54 and 56 as well as the mirrors 68 and 70 and the dichroic polarizer plate 48 are mounted in the compartment 82. The dichroic plate 66 is mounted in the compartment 84.

The filters 72 and 74 are supported in the wall 86 which separates the compartments 82 and 84. In accordance with the principles of this invention, the compartment 82 is filled with a thermally conductive liquid of an amount sufficient to cover the cells 54 and 56. Temperature control of the liquid filling the compartment 82 is through the use of flexible silicon rubber/fiberglass insulated wire element heaters 88 mounted on the external walls of the compartment 82. The heaters 88 are illustratively of the type manufactured by Electro-Flex Heat, Inc., of Bloomfield, Connecticut. A thermocouple (not shown) may be mounted within the compartment 82 to monitor the temperature of the liquid. Since the cells 54 and 56 are at the same temperature as the liquid, the thermocouple in the liquid also measures the temperature of the cells, thereby eliminating the use of thermocouples mounted directly on the cells.

The heat conductive liquid in the compartment 82 serves not only as the heating medium for the cells 54 and 56, but also as the index of refraction medium for the imersed thin film color beam splitting coatings on the plate 48. Accordingly, the heat conductive liquid is chosen to have an index of refraction which substantially matches the refractive in dex of the plate 48, thereby eliminating any induced optical aberrations caused by inserting a parallel plate into converging and diverging light beams. Typically, the plate 48 is made of fused silica glass having a refractive index of 1.46 and the heat conductive liquid is chosen accordingly. Preferably, the heat conductive liquid is a blend of silicone liquids such as a code 5610 laser liquid produced by R. P. Cargille Laboratories, Inc., of Cedar Grove, New Jersey.This oil has the characteristics of a refractive index substantially matching that of the fused silica of which the plate 48 is made, high optical transmission, appropriate thermal conductivity, and low electrical conductivity so that connections to the display cells 54 and 56 may be left exposed in the liquid without shorting out.

The walls of the housing 80 are formed with a number of windows. Thus, there is a window 90 which is an entrance window for the projection light. There is a window 92 which is an exit window for the projection light. There is a window 94 which is an entrance window for the laser beam. Additionally, the filters 72 and 74 are mounted in appropriate windows in the wall 86. The compartment 84 is not filled with oil but merely with ambient air.

As shown in Fig. 2, the housing 80 comprises a self contained module holding the optical elements enclosed by the broken lines in Fig. 1. This modular construction provides the advantage that the optical elements may be pre-aligned and ruggedly mounted.

Although the system described hereinabove is a two channel display system, this invention may also be utilized in a four channel system, as schematically depicted in Fig. 3. Thus, for such a system, there is provided a housing 100 with an internal wall 102 dividing the enclosure into a first compartment 104 and a second compartment 106. The first compartment 104 is filled with air and the second compartment 106 is filled with the thermally conductive liquid. Mounted in the first compartment 104 are beam splitters 108 and 110, along with an entrance window for the projection light.The compartment 106 includes beam splitters 11 4, 116, 11 8 and 120, as well as mirrors 122 and 1 24. Additionally, the compartment 106 includes the display cells 126, 128, 1 30 and 132, with appropriate windows in the wall 102 holding the color trim filters 134, 1 36 and 1 38. The laser light enters the system through the laser focus lenses 140 and 142 and the projection light leaves the housing 100 through the exit window 144.

Accordingly, there has been disclosed an improved temperature control arrangement for use in a laser recording and projection system. This arrangement possesses a number of advantages. The liquid crystal light valves are maintained at an isotropic temperature. The thermocouple for measuring the temperature of the display cells need not be mounted on the cells, but rather may be in the thermally conductive liquid at a point remote from the cells. The coating designed for the beam splitter plates is optimized, with no anti-reflection coating being required. Due to the modular construction, the optical elements are prealigned and remain in alignment without requiring external adjustments. The housing may be constructed as a sealed unit so that no dust particles may accumulate on the "writing surface" of the display cells. Using a heated liquid eliminates display flicker which was previously caused by air convection currents due to the external display cell heating units. Additionally, increased light efficiency will allow a commensurate decrease in projection lamp power which will help in solving the remaining display flicker problem by using a lower powered lamp having the electrodes closer together.

It is understood that the above-described embodiments are merely illustrative of the application of the principles of this invention.

Numerous other embodiments may be devised by those skilled in the art without departing from the spirit and scope of this invention, as defined by the appended claims.

Claims (2)

1. In a recording and projection system of the type utilizing a focused laser beam and a locally erasable recording medium positioned at the laser beam focal point, an arrangement for maintaining a uniform temperature distribution on the surface of the recording medium comprising: a housing defining an enclosed compartment; means for mounting said recording medium within said compartment; a thermally conductive liquid filling said compartment sufficiently to cover said recording medium; and means for controlling the temperature of said liquid.

2. The arrangement according to Claim 1 wherein said recording and projection system further includes a dichroic beam splitter mounted within said compartment and covered by said liquid, and wherein the refractive index of the liquid substantially matches the refractive index of the beam splitter.

2. The arrangement according to Claim 1 wherein said recording and projection system further includes a dichroic beam splitter mounted within said compartment and covered by said oil, and wherein the refractive index of the liquid substantially matches the refractive index of the beam splitter.

3. The arrangement according to Claim 2 wherein said beam splitter is made of fused silica glass and said liquid has an index of refraction of 1.46.

4. The arrangement according to Claim 1 wherein said liquid has the characteristic of low electrical conductivity.

5. The arrangement according to Claim 1 wherein said liquid is a blend of silicone liquids.

6. A recording and projection system substantially as herein described and as illustrated by the accompanying drawings.

CLAIMS Amendments to the claims have been filed, and have the following effect: New or textually amended claims have been filed as follows:-