WO1991007681A1

WO1991007681A1 - Optical image processing

Info

Publication number: WO1991007681A1
Application number: PCT/GB1990/001740
Authority: WO
Inventors: Geoffrey Arthur Coleridge Boyd
Original assignee: Memotech Computers Limited
Priority date: 1989-11-10
Filing date: 1990-11-12
Publication date: 1991-05-30
Also published as: AU7041191A

Abstract

An array of hollow tubes with internal reflecting surfaces is positioned in front of a CRT to provide magnification and reduce screen reflections. The magnification enables images from a stack of CRT's to be rendered contiguous. The same array can be used as a rear projection screen providing high contrast. The hollow reflection elements can be formed from a stack of corrugated laminations.

Description

OPTICAL IMAGE PROCESSING

This invention relates to optical image processing.

More particularly, the invention relates to apparatus and methods of optical image processing for use with image bearing surfaces such as CRT screens and for use with image projectors.

In a variety of both interior and exterior applications, there is a requirement to provide a large screen display of typically 2 to 10 metres diagonal. Mechanical considerations rule out the use of a single CRT of this size and essentially two different approaches to the problem have emerged.

By the use of a wide variety of projection techniques, a screen display can be produced with considerable magnification. The drawback of projection techniques is, however, that they normally require viewing in dark room conditions and are in general unsuited to use in daylight. Attempts have been made to improve projection systems by using a separate, high powered projection device for each colour or by stacking projection devices in an array. There is, though, a considerable cost penalty in achieving increased performance with either of these techniques. Also, the difficulty remains of ambient light being reflected from the projection screen.

It has been proposed to use fibre optic face plates with CRT's or projection screens, in an attempt to enhance contrast. Reference is directed for example to EP-A-0 357 070. It is however an inherent defect with such usage of optical fibres that at least one additional media/ambient boundary is introduced leading to further superious reflections.

An alternative to projection, in the production of large screen displays, is the use of stacked array of CRT's with electronic signal processing of an incoming video signal such that each CRT displays a different, magnified portion of the original image. So called "video walls" can range in size from typically 3x3 to 16x16 CRT's and sophisticated electronic signal processing techniques have been developed to produce a variety of visual effects. Whilst video walls have achieved significant success, they do have two drawbacks. The first is that the inevitable gap between the screen edges of abutting CRT's produces the appearance of a grid overlay which can be distracting. Secondly, screen reflections of ambient light will usually prevent outdoor daylight viewing.

It is an object of one aspect of the present invention to provide improved optical image processing apparatus for cooperation with a CRT or other image bearing surface in which either or both of these disadvantages are overcome.

Accordingly, the present invention consists in one aspect in optical image processing apparatus comprising a generally planar array of hollow optical elements having reflecting interior walls, the elements being disposed to collect light from an image-bearing source at one side of the array and arranged through internal reflection of light in each element to generate a processed image for viewing at the opposite side of the array.

Advantageously, the elements increase in at least one transverse dimension in a direction normal to the image bearing surface so as to provide magnification in the processed image.

In will be understood that, in this way, the image from each of a stacked array of CRT's can be magnified sufficiently to produce an array of contiguous images, thus avoiding the problem of gaps between CRT's.

Thus, in one form, the present invention consists in an array of CRT's having respective screen surfaces in a common screen plane, in combination with optical image processing apparatus, comprising for each CRT an array of internal reflection elements disposed to collect light from respective contiguous regions of the associated screen and arranged through internal reflection of light in each element to generate a processed image for viewing, the processed images corresponding with the respective CRT's being disposed contiguously in a common viewing plane. The internal reflection elements can be arranged so as to limit the angle at which light is collected or transmitted by the element. In this way, spurious off-angle reflections of ambient light are avoided; normal reflections of ambient light are less serious because conventional anti-reflection coatings can be optimised to deal with these.

One method of restricting the angle of light collection or transmission is to provide an end region of each internal reflection element in which internal reflection is inhibited.

The apparatus according to this invention will find application with single CRT's or the like, where - for example - it is desired to reduce screen reflection or provide magnification.

It is an object of a different aspect of the present invention to provide improved optical image processing apparatus for cooperation with image projection means to reduce the problem of screen reflections of ambient light.

Thus, in yet a further aspect, the present invention consists in optical image processing apparatus serving as a projection screen for a projected image, comprising an array of hollow optical elements having reflecting walls and disposed to receive light over respective contiguous regions of a projected image, arranged through internal reflection of light in said elements to generate a processed image for viewing.

This invention will now be described by way of example with reference o the accompanying drawings in which:-

Figures 1A and 1 B are schematic representations in side and front elevation of a conventional video wall illustrating one problem which is addressed by the present invention;

Figures 2A and 2B are similar representations illustrating the use of the present invention; Figures 3A and 3B are diagrams illustrating the dimensions of a single internal reflection element for use with the present invention;

Figure 4 is a diagram illustrating the orientation of one internal reflection element with respect to a CRT;

Figure 5 is a section to an enlarged scale of one internal reflection element;

Figures 6A, B and C are sketches illustrating possible orientations of internal reflection elements; and

Figures 7A and B are front and plan views of apparatus according to the invention illustrating a preferred constructional technique for the internal reflection elements.

The problem that arises from the gap between the screens 10 of abutting CRT's 12 is shown clearly in Figures 1A and 1B, and needs no further explanation. Referring to Figures 2A and 2B, the solution offered by the present invention is to provide on each CRT screen 10 an array 14 of internal reflection elements 16. These elements take the form of hollow plastics tubes metallised on the interior surface and having a rectangular cross section which increases in dimension away from the CRT screen. Put briefly, each element 16 collects light from a defined region of the screen 10 and, through internal reflection, generates a magnified image at the front edge 18 of the array. The magnification is such that the images from abutting CRT's are contiguous and the impression is created of a continuous picture over the CRT area. At any sensible viewing distance, the front edges of the tubes are not seen.

The design and manner of operation of the internal reflection elements will now be described in greater detail. The example is taken of a 10x10 video wall constructed with monitors of dimensions 591.0mm horizontal by 465.5mm vertical (W_hxW_v) using a CRT with an active phosphor area of 543.5mm by 406.0mm (V_hxV_v). The display should be designed to handle standard video signals with bandwidths specified by the PAL and NTSC standards. The viewing angle should be 100° horizontal (Θ_h) and 60° vertical (Θ_v).

The bandwidth of the video signal determines the pixel size. This is of course the rectangular cross sectional dimensions (D_h, D_v) of the tube and determines, in turn, the number of tubes needed for each monitor. The ratio of the monitor dimensions to those of the CRT screen determines the magnification (M_h, M_v) required and hence the ratio of maximum (D_h, D_v) to minimum (T_h, T_h) dimensions of the tapering tube element. The difference between the monitor dimensions and the CRT screen dimensions also determines the minimum length L of the tube elements and thus the conic angles σ_h, σ_v. The ratio of the viewing angles in the horizontal and vertical directions (Θ_h, Θ_v) determines the ratio of the horizontal to vertical sides of the tube element (D_h:D_v). Finally, as will be described in more detail later, the viewing angles (Θ_h, Θ_v) determine the required masking depth at the outer end of each tube element.

The video signal bandwidth and the number of video lines determines the pixel dimensions as follows. Sampling theory states that a signal with bandwidth limit of frequency F_c is completely determined if it is sampled at a frequency greater than the Nyquist limit F_N = 2xF_c. Because of multiple internal reflections, the light emerging from an individual tube element can be regarded as having lost spatial coherence. This then sets the resolution limit. Provided that the cross section of the tube element is smaller than the pixel size as determined by sampling theory, the full resolution of the image will be preserved. In other words, using the Nyquist criterion, we can ensure that any incoherency introduced by multiple internal reflections within each tube element does not compromise the final resolution of the image. Consider a PAL-1 (UK) signal with a bandwidth limit specified as 5.5MHz and 288 lines visible per field and 576 lines per frame. (Other PAL systems are specified lower at 5.0MHz and NTSC at 4.2MHz and 240 lines). The signal should thus be sampled at least 11MHz on the active line of 52 μs. This gives a total of 572 pixels horizontal and 576 pixels vertical for a full video frame. With a large video wall of size 10x10, each monitor would therefore require a minimum of 58 tube elements horizontal and 58 tube elements vertical to be within the Nyquist limit. In the preferred example, the array comprises 59 tubes at 10mm horizontal and 77 tubes and 6mm vertical for each monitor. This gives a total display of 590 pixels horizontal by 770 pixels vertical for the 10x10 video wall. It is useful at this point to contrast that resolution with a typical size of 300x200 pixels for certain forms of known outdoor display.

With a chosen dimension D_h of 10mm and a desired ratio Θ_h/Θ_v of 100°:60°, a value for D_v of 6mm as obtained. The minimum magnification is set by the ratio of the size of the monitor to the size of the CRT screen in the horizontal and vertical directions. Therefore

minimum M_h = W_h/V_h = 59.0/543.5 = 1.0874

minimum M_v = W_v/V_v = 465.5/406.0 = 1.1466

In the preferred example a magnification of 1.15 is selected in both the horizontal and vertical directions although there is no necessity for the magnifications to be equal.

Referring to Figure 3A, it can be seen that:-

Tan a/2 = (D-T)/(2xL)

The minimum dimensions of the tube element can be calculated from the known magnification factor as follows:- T_h = D_h/1.15 = 8.6957mm

T_v = D_v/1.15 = 5.2174mm

Effectively an active region of the CRT screen is thus defined having dimensions:-

V'_h = 59xT_h = 513.0mm

V'_v = 77xT_v = 401.7mm

The length L of the tubular element must be selected so that, as seen in Figure 4, the tube can at least extend to the edge of the monitor and typically, the angle made with the axis of the CRT should be small, say less than about 20°. Thus:-

L > (W-V)/2xSin(a_χ)

and with a maximum value selected for a_χ == 2200°°,, minimum values of L for the horizontal and vertical directions can be calculated as:

L_h > 1 14.03

L_v > 93.3

In the preferred example, a. value of L = 1 12.5mm is chosen and the conic angles can be calculated as follows:-

σ_h = 2xTan^'1((D_h-T_h)/(2xL)) = 0.667°

σ_v = 2xTan-¹((D_v-T_v)/(2xL)) = 0.399° The design parameters can accordingly be summarised as follows:

A region L_N at the inner end of each tube element and a region L_] at the outer end is left matt black to define the maximum angles at which light can enter and leave the tubular element. Thus, there are shown in Figure 5, the light rays of maximum angle which can enter the tube element from either end. It will be clearly understood that light incident at a greater angle would strike the matt portion and thus not be reflected onward.

It is of course imperative that, with selected values of a, L, D and T, it is ensured that light emerging from the viewing angle must be collected from the CRT surface. By Fermat's principle, (the reversibility of the light rays constructed for any optical system) any light entering within the viewing angle cannot be reflected back out from the mirrored walls of the cone section. Any light which emerges from a mirrored tube for viewing must have been either emitted from the phosphor or, is ambient light collected within the viewing angle range and reflected from the glass CRT surface. By using conventional anti-reflective optical coatings, the latter can be reduced to negligible proportion. The masking at the inward end of the tube again ensures that the coating is effective. This is because only angles of incidence within the emergent angle range (122° horizontal and 72° vertical in the preferred example) and can reach the screen surface from ambient light collected and then only within the viewing angle range. This maximises the wavelength range over which a given coating is effective. These features help maximise the overall contrast of the display.

The masking at the inward end of each tube element further serves to limit the angle over which light is collected from the phosphor and prevents cross talk from adjacent pixels. This is important because the active phosphor is spaced from the tube element at least by the thickness of the screen glass.

A calculation can be made of the number of multiple reflections in the worst case for particular design parameters and it is of course necessary for this number to be within a sensible range. It can be shown that with the dimensions of the preferred embodiment the maximum number of horizontal reflections is 17 and of vertical reflections 12. This is satisfactory with the coefficients of reflectivity than can be achieved with low-cost metallisation techniques.

It will be possible to avoid all direct reflection from the glass CRT surface, even at orthoganol viewing angles, by suitable design of the tube elements. An example of such a design technique will now be described with reference to Figures 6A, 6B and 6C.

Figure 6A shows for the purposes of comparison a representation of internal reflection elements arrayed as previously discussed. The tubular elements 50, defined between walls shown diagrammatically at 52, are straight although as previously described, they increase in width away from the CRT screen. Depending upon the exact design parameters, there is the possibility of light in certain regions of the screen, reflecting directly off the CRT with no internal reflection in the tubular elements. It has been explained that the use of anti-reflective layers is helpful in this regard but the modification shown in Figure 6B removes the difficulty without recourse to additional screen layers. In this case, the element walls 52 are cranked in at least one plane, preferably the horizontal plane in the case of a vertical CRT screen. Whilst retaining the previously described magnification effect, this cranking of the element walls eliminates the direct reflection path in all screen regions. It will be understood of course that the internal reflection elements can be orientated in very many different ways, beyond the simple form illustrated in Figure 6B, to achieve the desired elimination of a direct light path.

The use of non-straight internal reflection elements can be taken further as illustrated in Figure 6C, to provide an additional advantage. By arranging for a pair of adjacent wall elements 54 and 56 to be cranked in opposite directions and to converge to a point at both surfaces, a "dead zone" 58 can be created. No light passes through such a dead zone 58 which can accordingly be used to accommodate structural supports or other items. In the case of larger screens, this ability to incorporate structural elements which can extend across the screen without impairing the viewed image, is extremely useful. The internal reflection elements themselves need not be self-supporting and proper registration can be assured without the need for precise interlocking between adjacent elements.

The skilled man will recognize that internal reflection elements can be produced in a great number of different ways.

Tube elements can in one embodiment be produced by lightweight plastics mouldings designed to interlock to form a rigid array. The internal surfaces (except of course for the masked regions) are aluminised to provide the desired reflectivity. It should be understood that it is not essential for tubes of rectangular cross section to be employed; a honeycomb array of hexagonal section tube would be an alternative. Indeed, it is not essential for the abutting tube sections to fit together precisely and it would be possible, for example, to employ tube elements of elliptical section. In a further modification, it would be possible to replace an array of interlocked tubular elements with a single plastics moulding having similar geometry. For the specific application of an outdoor display, it is proposed to employ a waterproof plastics gasket carrying the array of internal reflection elements and optically bonded to the CRT screen.

A particularly preferred construction technique will now be described with reference to Figures 7A and 7B.

In this embodiment, the internal reflection elements are not discrete units, neither are they aggregated in an integral structure. Rather, the elements are defined by a stack 100 of 100 laminations 102 and 104 which rest freely, one upon the next. The laminations 102, 104 are castellated in transverse section as shown in Figure 7A with the laminations 102 being mirror images of the laminations 104.

The laminations may be formed in a rolling or pressing operation from high purity rolled aluminium sheet of typical gauge 200 to 250 micrometers. The elements can then be chemically polished, before or after assembly, to a highly reflecting surface.

In longitudinal section, as shown in Figure 7B, the laminations take the general form illustrated in Figure 6C, with the creation of dead zones 106 vertically extending support rods 108 are accommodated in the dead zones 106 and are bonded to a thin frame support (not shown). The support rods 104 pass with narrow clearance through apertures (not shown) in the laminations 102 so that accurate alignment between the laminations is assured. In this way a stable array is created.

It will be seen that the internal reflection elements are arranged in staggered layers with the spacing of the layers being half the depth of the elements. This variation in depth between neighbouring elements in a horizontal plane is indicated in Figure 7A, with elements of an upper layer being designated "1 " and those of the lower layer "0".

In a modification, the laminations are produced through vacuum forming in plastics material and are metallised on both surfaces to provide the necessary degree of reflectivity. The use of aluminium laminations will, however, offer the advantages of mechanical strength, anti-static and ease of repolishing. Whilst the described examples utilise relatively small magnification sufficient only to close the gap between adjacent CRT's, it would be possible to increase the magnification sigπficantly and to increase the area of the display for a fixed monitor array. Alternatively, the number of monitors required to produce a display of fixed size could be reduced. This would lead to the possibility of substantial cost savings.

It is the case that the brightness levels achievable with colour CRT's are considerably lower than with monochrome. One embodiment of the present invention seeks to provide a colour display using only monochrome CRT's. In this arrangement, the tubular elements are provided with red, blue or green colour filters. The arrangement of filters could be on the basis of individual tubular elements or groups of adjacent elements could be assigned the same filter colour. The electronic signal processing used to divide an input video signal over the monitors of the video wall would be further adapted to generate for the RBG signals displays in separate pixels, these pixels corresponding of course with the arrangement of filters over the tubular elements. Each tubular element (or group of tubular elements in the case where a group of adjacent elements shares the same filter colour) would always be used to display a pixel of the same colour.

Whilst the present invention has been described principally in relation to video walls, it is not to be regarded as restricted to this particular application. The advantages of magnification and avoidance of screen reflections will apply to applications utilising single CRT's. In these cases, it may be necessary to reduce the dimension of the internal reflection elements to maintain the desired resolution. Moreover, the invention is not restricted to the use of CRT's and will find applications in image processing of a wide range of image bearing screens.

Indeed, the invention will find still further application in projection arrangements where the invention provides the screen itself. Thus, it has previously been described that an array of internal reflection elements can be arranged to transmit light from an image bearing screen such as a CRT, whilst avoiding reflection of ambient light from the elements themselves. It has further been explained that direct light paths (not involving internal reflection) can be eliminated by the use of cranked or other non-straight elements. If, then, an image is projected on the rear surface of an array (with appropriate selection of element size in relation to pixel dimension) a viewer from the front will see a high contrast image unimpaired by screen reflections, even in daylight or other high intensity ambient lighting. The invention can be regarded as providing a "black" screen which nonetheless transmits the rear projected image. Since there is no change of refractive index from the projection source to the eye, there are no spurious reflections from phase boundaries.

It is believed that this application of the present invention will be of considerable use in rear projection systems and especially in projection video systems currently under development. These employ video driven LCD projection plates which can provide good resolution even in relatively large projected images. They have, however, been handicapped hitherto by image intensities, due to poor contrast, much lower than those achieveable with CRT or other "active" screens. Use of the "black" screen according to this invention, as a passive screen in .place of conventional translucent screens, should enable high contrast to be achieved even at these lower intensities.

This invention has been described by way of examples only and still further variations are possible without departing from the scope of the invention.

Claims

CLAI S

1. Optical image processing apparatus comprising a generally planar array of hollow optical elements having reflecting interior walls, the elements being disposed to collect light from an image-bearing source at one side of the array and arranged through internal reflection of light in each element to generate a processed image for viewing at the opposite side of the array.

2. Apparatus according to Claim 1 , wherein the elements are tapered so as to provide magnification in the processed image.

3. Apparatus according to Claim 1 or Claim 2, wherein the array of hollow optical elements is formed from a stack of laminations.

4. Apparatus according to Claim 3, wherein each lamination is castellated.

5. Apparatus according to Claim 4, wherein each lamination has at least one metallic surface.

6. Apparatus according to any one of the preceding claims, wherein said elements are cranked or curved such that there exists no straight light paths through the array.

7. An array of CRT's or the like having respective screen surfaces in a common screen plane, in combination with optical image processing apparatus, comprising for each CRT or the like an array of internal reflection elements disposed to collect light from respective contiguous regions of the associated screen and arranged through internal reflection of light in each element to generate a magnified image for viewing, the magnified images corresponding with the respective CRT's or the like being disposed contiguously in a common viewing plane.

8. An array according to Claim 7, wherein each internal reflection element is defined between opposing reflective sheets.

9. An array according to Claim 7 or Claim 8, wherein each internal reflection element is cranked or curved.

10. An array according to Claim 9, wherein there are formed dead zones between said internal reflection elements to accommodate supports.

11. Optical image processing apparatus serving as a projection screen for a projected image, comprising an array of hollow optical elements having reflecting walls and disposed to receive light over respective contiguous regions of a projected image, arranged through internal reflection of light in said elements to generate a processed image for viewing.

12. Apparatus according to Claim 11 , wherein the hollow elements are defined between opposing reflective sheets.

13. Apparatus according to Claim 12, wherein at least a proportion of said optical elements are cranked or curved to provide dead zones for the accommodation of supports extending in the plane of the array.

14. A rear projection screen comprising a stack of metallic or metallised laminations contoured as to define a two dimensional array of hollow internal reflecting elements.