GB2278222A - Spatial light modulator - Google Patents

Abstract

A spatial light modulator comprises a liquid crystal layer formed of pixels 32 and a black mask 33 sandwiched between substrates 31 and 34. The substrate 31 has formed therein graded refractive index micro lenses 40, each of which is aligned with and adjacent a respective pixel 32. Such an arrangement may be used in high resolution imaging devices, for instance for three dimensional displays. The lenses may be replaced by a parallel barrier formed of a black mask having an array of slits or pin holes. <IMAGE>

Description

SPATIAL LIGHT MODULATORThe present invention relates to a spatial light modulator (SLM). Such a modulator may be used in apparatuses for converting spatial and temporal information into a high resolution array of image picture elements (pixels), for instance so as to provide a lenticular three dimensional imaging apparatus or a high resolution display or printer.

Figure 1 of the accompanying drawings shows a three dimensional display of the type disclosed in BritishPatent Application No. 9210399.3. The display comprises an array of light sources 1 to 8 connected to a control circuit 9 for sequentially illuminating the light sources one at a time. Light from the light sources is directed via an illumination lens 10 to a hybrid sandwich 11. The hybrid sandwich 11 comprises a first lenticular screen 12, a SLM 13, a diffuser 14, and a second lenticular screen 16. The control circuit 9 controls the SLM 13 by supplying in sequence spatially multiplexed image data.

This apparatus thus combines spatial and temporal multiplexing in order to produce a three dimensional display with a large number of views. In the hybrid sandwich 11, a high resolution image is produced at the diffuser 14 and is re-imaged by the second lenticular screen 16 so as to provide the three dimensional image.

Figure 2 shows an apparatus for effectively enhancing the resolution of a SLM 24. The apparatus comprises an array of illuminators 21 and a control circuit 26 which illuminates the illuminators 21 one at a time in sequence. Light from each of the illuminators is directed via an on axis correction lens 22 to a micro lens array 23 and the SLM 24. Each of the micro lenses of the array 24 focuses the incident light through a corresponding pixel of the SLM 24 onto a common image plane 25 at which may be located a film for printing applications or a diffuser for display applications. The control circuit 26 supplies image data to the SLM 24 such that, for each of the illuminators 21, an image sub-pixel is provided at the image plane 25.This is repeated for each of the illuminators 21 so that each pixel of the SLM 24 provides a plurality of sub-pixels at the image plane 25 which combine to produce an image of resolution greater than that of the SLM 24.

In the apparatuses shown in Figures 1 and 2, each lenticule or micro lens of the screen 12 or the array 23 focuses the incident light through a corresponding aligned pixel of the SLM 13 or 24. Thus, the screen or array and the SLM are required to be aligned accurately and to direct incident light with minimum loss through the corresponding pixel while avoiding cross-talk from adjacent pixels.

Figure 3 of the accompanying drawings shows an arrangement of the lenticular screen or micro lens array 30 and a known type of SLM. The SLM comprises a liquid crystal device having a substrate 31 which is approximately 1 mm thick and to which the micro lens array 30 is bonded. The liquid crystal device (LCD) comprises a liquid crystal layer a few Rm thick providing a plurality of pixels 32 spaced apart by a black mask 33.

For instance, the black mask may carry control electronics in the form of thin film transistor circuitry for controlling the addressing and transmissivity of the individual pixels 32. The pixels 32 and the black mask 33 are disposed between the substrate 31 and a further substrate 34, also about 1 mm thick. The rear surface of the substrate 34 coincides with an image plane 38.

The typical pixel size is 100 to 200 zm. Imagine takes place in the image plane 38, which requires microlenses with low numerical apertures, for instance of focal length 2mm and diameter 100 to 200 Rm.

Figures 4a to 4c of the accompanying drawings illustrates several possible paths for light which is incident on the micro lens array 30. When used, for instance, in apparatuses of the types shown in Figure 2, light from the light sources is imaged at the image plane 38. For light which is incident normally with respect to the plane of the pixels 32, as shown in Figure 4a, each micro lens of the array 30 focuses the light substantially through the corresponding pixel 32a so that substantially no light is lost through blocking by the black mask 33 and the intensity of all of the incident light is controlled by the transmissivity of the pixel 32a.

As the angle at which the light is incident increases, part of the light which is incident on the LCD is obscured by the black mask 33 until, as shown by the ray paths 35 in Figure 4b, passage of light is substantially completely prevented by the portion 33a of the black mask.

For increasing angles of incidence of the light as illustrated by the ray paths 36 in Figure 4c, "second order imaging" occurs. In other words, light which is incident on the microlens passes not through the corresponding pixel 32a but through an adjacent pixel 32b. This represents "crosstalk" and is most undesirable.

According to the invention, there is provided a spatial light modulated as defined in the appended Claim 1.

Preferred embodiments of the invention are defined in the other appended claims.

It is thus possible to provide a spatial light modulator in which masking of off-axis light and second order imaging of extreme off-axis light rays are eliminated or substantially reduced. Such an arrangement is therefore suitable for use in a wide variety of applications, including apparatuses of the types shown in Figures 1 and 2.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which:Figure 1 shows a schematic cross-sectional view of a three dimensional display apparatus;Figure 2 shows a cross-sectional view of an enhanced resolution SLM;Figure 3 is a diagrammatic cross-sectional view of an arrangement which may be used in the apparatuses ofFigures 1 and 2;Figures 4a to 4c are views corresponding to Figure 3 illustrating passage of light through the arrangement;Figure 5 is a diagrammatic cross-sectional view of a SLM constituting a first embodiment of the invention;Figures 6a to 6c are views similar to Figures 4a to 4c, respectively, illustrating passage of light through theSLM of Figure 5;Figure 7 is a diagrammatic cross-sectional view of a SLM constituting a second embodiment of the invention;;Figure 8 is a diagrammatic cross-sectional view of a SLM constituting a third embodiment of the invention;- and Figure 9 is a diagrammatic cross-sectional view of a SLM constituting a fourth embodiment of the invention.

Like reference numerals refer to like parts throughout the drawings.

The SLM shown in Figure 5 comprises a first substrate 31,LCD pixels 32, a black mask 33, and a substrate 34 similar to those shown in Figure 3. However, the SLM ofFigure 5 differs from the arrangement of Figure 3 in that the micro lens array 30 is replaced by an array of micro lenses formed in the substrate 31. The micro lenses 40 are shown as graded refractive index (GRIN) micro lenses formed in a surface of the substrate 31 which is adjacent the LCD, with each micro lens lying adjacent and being aligned with a corresponding pixel 32.

As shown in Figure 6a, light which is incident normally on the front surface of the substrate 31 passes through each micro lens and pixel substantially without masking by the black mask 33. Also, off-axis light 35 which is masked by the black mask 33 as shown in Figure 4b passes through the SLM as shown in Figure 6b substantially without attenuation. Light 36 which is incident at more extreme angles passes through each micro lens 40 and through the corresponding pixel as shown in Figure 6c.

Thus, the correct "first order imaging" takes place whereas light incident at the same angle with the arrangement as shown in Figure 4c is subjected to second order imaging by the adjacent pixel 32b. Accordingly, masking and consequent fluctuations in intensity are eliminated or substantially reduced by the SLM shown inFigure 5 so that substantially uniform light throughput is obtained as the direction of incident light changes.

Further, crosstalk produced by extreme off-axis light is eliminated or minimised. The arrangement of Figure 5 provides increased freedom of illumninator positioning compared with the arrangement of Figure 3. The SLM is therefore suitable for use in a wide range of applications, such as in the apparatuses shown in Figures 1 and 2.

The SLM shown in Figure 7 differs from that shown inFigure 5 in that the substrate 34 also has formed therein graded index microlenses 50. Such an arrangement may be advantageous for some applications so as to avoid crosstalk, for instance in the lenticular screen 16 shown in Figure 1. Further, alignment of the graded index microlenses 40 and 50 with the corresponding pixels 32 can be facilitated, thus easing manufacture of SLMs.

The SLM shown in Figure 8 differs from that shown inFigure 7 in that the microlenses 60 have the same pitch as and are aligned with the microlenses 40. Each pair of aligned microlenses 40 and 60 thus forms a compound lens of increased performance. For collimated incident light, the compound lenses form an image at an image plane 51 which substantially coincides with an outer surface of the LCD substrate 34.

The SLM shown in Figure 9 differs from the previously described SLMs in that both the substrates 31 and 34 are plane or conventional and do not include lenses.

Instead, the black mask 33 is extended so as to form an array of slits or pin holes, each of which is aligned with a respective LCD pixel 32. The black mask 33 thus forms a parallax barrier which, compared with the use of microlenses, provides reduced light through-put but does not suffer from optical aberrations. Although the SLM shown in Figure 9 may be used with a diffuser at the image plane 38 to provide a display for applications in which relatively low intensity is permissible, it may be of more use in printing applications where the reduced light through-put may not be a disadvantage.

Claims (26)

1. A spatial light modulator comprising a first substrate incorporating a first array of lenses and a light modulating layer having a first surface adjacent the first substrate.

2. A modulator as claimed in Claim 1, in which the first substrate has an imaging surface disposed adjacent the first surface of the light modulating layer.

3. A modulator as claimed in Claim 1 or 2, in which the first array of lenses is a microlens array.

4. A modulator as claimed in any one of the preceding claims, in which the lenses of the first array are converging lenses.

5. A modulator as claimed in Claim 4, in which the lenses of the first array are spherically converging lenses.

6. A modulator as claimed in Claim 4, in which the lenses of the first array are cylindrically converging lenses.

7. A modulator as claimed in any one of the preceding claims, in which the first array of lenses comprises a graded refractive index lens array.

8. A modulator as claimed in any one of the preceding claims, further comprising a second substrate adjacent a second surface of the light modulating layer.

9. A modulator as claimed in Claim 8, in which the second substrate incorporates a second array ot lenses.

10. A modulator as claimed in Claim 9, in which the second substrate has an imaging surface disposed adjacent the second surface of the light modulating layer.

11. A modulator as claimed in Claim 8 or 9, in which the second array of lenses is a microlens array.

12. A modulator as claimed in any one of Claims 8 to 10, in which the lenses of the second array are converging lenses.

13. A modulator as claimed in Claim 12, in which the lenses of the second array are spherically converging lenses.

14. A modulator as claimed in Claim 12, in which the lenses of the second array are cylindrically converging lenses.

15. A modulator as claimed in any one of Claims 8 to .14, in which the second array of lenses comprises a graded refractive index lens array.

16. A modulator as claimed in any one of the preceding claims, in which the light modulating layer comprises a liquid crystal layer.

17. A modulator as claimed in any one of the preceding claims, in which the light modulating layer comprises a plurality of picture elements.

18. A modulator as claimed in Claim 17, in which each of the picture elements is substantially aligned with a respective lens of the first array of lenses.

19. A modulator as claimed in Claim 17 or 18 when dependent on Claim 9, in which each lens of the second array of lenses is substantially aligned with at least one of the picture elements of the light modulating layer.

20. A spatial light modulator comprising a light modulating layer and an opaque mask disposed at or adjacent the light modulating layer and having formed therein an array of apertures to provide a parallax barrier.

21. A modulator as claimed in Claim 20, in which the array of apertures comprises a plurality of elongate parallel slits.

22. A modulator as claimed in Claim 20, in which the array of apertures comprises a two-dimensional array of pinholes.

23. A modulator as claimed in any one of Claims 20 to 22, in which the light modulating layer comprises a liquid crystal layer.

24. A modulator as claimed in any one of Claims 20 to 23, in which the light modulator comprises a plurality of picture elements.

25. A modulator as claimed in Claim 24, in which each of the picture elements is substantially aligned with a respective aperture of the array of apertures.

26. A spatial light modulator substantially as hereinbefore described with reference to and as illustrated in Figures 5 and 6 or Figure 7 or Figure 8 orFigure 9 of the accompanying drawings.