GB2252172A - Colour optical output system - Google Patents

Abstract

Colour optical output apparatus comprising: a white light source 16; an output screen 10 onto which an output colour picture is imaged; a plurality of spacial light modulator reflector arrays 20a, 20b, 20c; means for controlling each array in dependence upon a different colour component signal; a dichroic mirror for splitting the white light beam into optical colour component beams, each beam being directed on one array to be modulated; and a combiner for recombining the modulated optical colour beams to provide a combined beam to the imaging plane; wherein the dichroic mirror and the combiner share a common surface. <IMAGE>

Description

COLOUR OPTICAL OUTPUT SYSTEM This invention relates to a colour optical output system of the kind employing spatial light modulators responsive to different colour signals to produce separate modulated colour component signals which are optically combined to form a colour output signal.

One example of an application of such a system is in large scale displays; another is in colour printing.

A spatial modulator is an optical component which is controllable to modulate an incident light beam. One example of such a modulator is the deformable mirror device (DMD) developed by Texas Instruments and described in, for example, US 4856863, US 4615595, US 4596992 and other patents to the same asignee.

Such devices comprise miniature mirrored cantilever beam elements carrying electrodes so as to be electro statically deflectable between two positions. The extent of the deflection can be controlled by the applied electrostatic potential to provide variable degrees of reflection, or the device can be operated in a binary manner by applying predetermined electro static potentials to switch between discrete deflection states. Using an array of such devices, static potentials to switch between discrete deflection states. Using an array of such devices, each individually addressable, a two dimensional image can be reproduced by exposing the array to an incident light beam, modulating the incident beam by controlling the individual mirror devices from a picture signal, and collating the beam reflected in a particular direction.The small size and fast switching times of devices of the kind described in the above mentioned patents makes them usable at video picture data rates, enabling the display of television or video moving images on a display screen onto which the collated beam is projected.

The incident beam is not scanned, as is an electron beam, but illuminates the entire device. In order to display a colour image, therefore, it is necessary to provide three separately illuminated deflector arrays, one controlled responsively to each primary colour or primary colour combinations, and to optically combine the modulated beams reflected from each device onto a single optical display. US 4680579 suggests (Figures 5 and 7) a system of this kind in which separate colour filters are placed in front of each mirror device, to produce different primary coloured modulated output signals.

Inevitably, however, such filters result in considerable loss of optical power since only certain wave lengths are passed; for each of the primary colour filters, light corresponding to the other two primary colours is effectively unused so that either three separate white light sources, or one white light source of considerable power, is required. In applications such as large displays, for use for example in cinemas, the electrical power required is in Kilowatts and thus as well as being wasteful of electrical power, the system of US 4680579 would lead to considerable heat dissipation which makes realization of compact equipment difficult.

We have realized that this is a problem, and our solution is to provide, instead of colour filters of the type which reject light outside their pass band, colour splitters which split incident light into separate colour components which are spatially separated. It is thus possible to use a single optical source, split into separate primary component colours, each component colour beam being modulated by a separate spacial modulator array and then recombined by an equivalent colour combiner device.

However, an essential constraint on such a display system is that the optical path length of the colour components signals must be equal (assuming that the arrays are of identical shape) so that identical modulated signals can be effectively recombined.

This requirement, together with the geometry of deformable mirror device illumination, leads to an arrangement of splitter and combiner devices occupying a substantial spacial volume. In some applications this is undesirable.

We solve this problem by using a single surface to perform both a splitting and a combining function.

This permits a considerable reduction in the volume of the optical unit. Also, it enables the path lengths for each colour component signal to be greatly reduced, thus simplifying the lense system required for display projection. Further, since the number of optical components is reduced the alignment task is simplified. Moreover, since a single component splits and combines the same colour signals, the possibility of spectral differences in the recombined signal is reduced.

Other aspects of the invention, together with preferred features and advantages, will be apparent from the following description.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows schematically the elements of a colour optical output system; Figure 2 shows schematically the structure of a spacial light modulator array device in one embodiment of the invention; Figure 3 shows schematically the optical illumination of a device of Figure 2; Figure 4 shows an alternative to Figure 3; Figure 5 shows schemetrically a splitter system of the invention.

Figure 6A to 6C shows schematically the optical behaviour of splitter elements employed in Figure 5; Figure 7 shows schemetrically the passage of beams through a splitter element of Figure 5; Figure 8A shows a perspective view of a preferred embodiment; Figure 8B and 8C show corresponding plan and elevation views; Figure 9 shows a detail of Figure 8; and Figure 10 shows a further preferred embodiment.

Referring to Figure 1, an output system comprises a projection plane 10 receiving the combined colour output signal. In a display system, as described hereafter, the projection plane comprises an illuminatable display screen 10. Three deformable mirror device spacial light modulator arrays 12a, 12b, 12c are controlled by separate colour control signals. Each colour control signal relates to a primary colour (red, green and blue). The signals are supplied from a signal generator 14, which is supplied with a composite colour video signal.

Illumination sources 16a, 16b, 16c provide an illumination signal for each corresponding modulator array 12a, 12b, 12c. The modulated signals from the modulators 12a, 12b, 12c are different colours; in the prior art this is achieved by either using different coloured light sources 16a, 16b, 16c, or by positioning a colour filter in the path from each source 16 to the respective modulator array 12 or after the modulator array 12.

The coloured signals from each modulator array 12a, 12b, 12c are then re-combined by a pair of combining elements 18a (combining the signals from 12a and 12b) and 18b (adding to this combination the signal from the modulator 12c). The combined beam is imaged onto the display screen 10 by a display lens arrangement (not shown).

Referring to Figure 2, a deformable mirror device array for use in the invention comprises an array of typically m x n deflectable mirror devices; typically, on the order of 500 x 500 devices for a low resolution display or 2000 x 2000 devices for a high resolution display. The array 20 is connected to an addressing circuit 22 which receives the colour signal from the circuit 14, and addresses each of the respective reflectors M11 - Man, as described in our earlier application number 9100188.3 dated 4 January 1991 (Agents ref. 3203201). Each reflector is thus operated between one of two reflection states corresponding to different reflector positions; an "on" state in which reflected light is directed in a first path 24a and an "off" state in which light is directed in a second path 24b.The second path 24b is disposed to lie away from all optical oponents of the system, and will not be described in greater detail.

Thus, when viewed along the "on" path 24a, at an instant the array 20 displays a two dimensional image, those modulators which are set to a first deflection state appearing bright and those which are set to a second deflection state appearing dark.

Referring to Figure 3, the angle through which each reflector is deflected between the two states is relatively small and thus, in order to achieve good discrimination between the two states the incident light beam from the source 16 is directed towards the array 20 at an angle a (from the normal to the display) of around 20 degrees. When an individual reflector device M is lying parallel to the plane of the array 20, the incident beam is reflected at a corresponding angle of 20 degrees to the normal along path 24b, but when the control signal from the addressing circuit 22 sets the deflector N into a second deflection state at an angle to the plane of the array 20, the incident beam is reflected out along the normal angle to the array on the path 24a.

Referring to Figure 4, the beam from the source 16 is essentially conical and illuminates the entire array 20. Likewise, the reflected beam 24a is also connical and wide and thus the incident and reflected beams overlap each other to a substantial distance 1 from the array 20.

Referring to Figure 5, in systems according to the invention, the three light sources 16a, 16b, 16c shown in Figure 1 are realised as virtual light sources derived from a single white light lamp 16. A pair of splitters 30a, 30b are provided to separate the colour components of the white beam from the lamp 16; each splitter 30a, 30b comprises a dichroic mirror element disposed, at an inclined angle, across the beam from the lamp 16.

As is known in the art, a dichroic element employs optical birefringence so that an incident beam is split into two beams, which are refracted through different angles. By employing multiple thin layers (on the order of optical wave lengths) a dichroic device can be arranged so that the two beams have complimentary spectral contents; in one type of dichroic mirror, illustrated in Figure 6a, optical frequencies below a predetermined frequency are transmitted whilst those above are reflected; in a second type, frequencies below a predetermined frequency are reflected whilst those above are transmitted.

The device 30a therefore has the characteristics shown in Figure 6a and accordingly reflects blue wave lengths to form a deflected beam appearing to originate from a virtual blue source 16c. Red and green wave lengths are transmitted. The second splitter device 30b has the characteristic illustrated in Figure 6b and consequently reflects red optical wave lengths to a reflected beam appearing to originate from a virtual source 16a. The beam transmitted through the splitter 30b consequently comprises green wave lengths and appears to originate from the position of the lamp 16.

The use of such dichroic spacial wave length splitters has several advantages; firstly, there is little waste of optical power since the splitters have an efficiency approaching 90% and all portions of the optical spectrum are used, and secondly, the transmitted and reflected beams from each splitter are essentially complementary in frequency content so that when recombined the splitting will not have introduced spurious colourations and the combined beam will have approximately the spectral distribution of the white beam from the light source 16.

When this illumination arrangement is employed in the system shown in Figure 1, the combiners 18a, 18b are conveniently also dichroic elements; the combiner 18a corresponds to the splitter 30b and consequently efficiently reflects the modulated red beam from the array 12a into the same path as the efficiently transmitted green beam from the array 12b, and the combiner 18b corresponds to the splitter 30a and consequently efficiently reflects the modulated blue beam from the array 12c into the transmitted path of the green and red beam from the combiner 18a to produce a combined white beam for imaging onto the display screen 10.

However, referring once more to Figures 3 and 4, because of the narrow angle of incidents required for deformable mirror devices the length 1 from the array 20 over which the incident and reflected beams overlap is substantial. In general, combiner and splitter components must be placed outside this distance L so that the splitter does not effect the reflected beam and the combiner does not effect the incident beam.

This embodiment of the invention therefore requires long optical paths in the system of Figure 1.

Referring to Figure 7, in a preferred aspect of the invention, the splitter 30a and combiner 18b form a single surface, as do the splitter 30b and combiner 18a. The single surface can thus be placed within the overlapping incident and reflected beams without causing difficulty. Although Figure 6 shows only the passage of a single ray, it will be appreciated that the widths of the incident and reflected beams are such that they very substantially overlap at the position of the splitter/combiner elements, as shown in Figure 8.

Referring to Figures 8a to 8c, a white light source 16 comprising a high power lamp generates light along an incident light path which is in a plane normal to that of a display screen 10. For example, the light source 16 may be positioned above the display screen 10. A planare digital mirror display device 20b is positioned spaced apart from and in a plane parallel to the screen 10, and the light source 16 is arranged to illuminate the array 20b at an angle of 20 degrees to its normal axis. The array 20b is arranged to deflect the incident beam to illuminate the screen 10 via a projection lens 40.

Positioned within the path of the incident and deflected rays are a pair of upright splitter/combiner mirrors 30a/18b, 30b/18a which are at an inclination, rotated about the vertical axis relative to the plane of the screen by some angle (typically between 20 and 70 degrees, and preferably 45 degrees) such as to reflect the incident beam to further digital mirror deflector arrays 20a, 20c.

The arrays 20a, 20c are positioned at a distance such that the optical path traversed from each array 20a 20c to the screen 10 is the same. The first splitter/combiner mirror has the characteristic shown in Figure 6a and consequently reflects a blue light component beam to a digital mirror display array 20a which is modulated in response to the blue colour component of the picture to be displayed.

Consequently, the reflected beam is deflected virtically by 20 degrees but is substantially horizontally unmodified. The splitter 30a transmits red and green wave length components substantially unattenuated.

The second splitter 30b reflects red wave lengths to a second digital mirror device array 20c which is modulated in response to the red colour component signal of the picture to be reproduced and consequently deflected 20 degrees vertically. The second splitter 30b allows the green optical wave lengths to pass substantially unattenuated, to be deflected by a third digital mirror device array 20b responsive to the green colour component signal of the picture to be reproduced.

The modulated green beam passes unattenuated back through both splitter/combiners through the projection lens 40 and onto the screen 10. At the first splitter/combiner reached 18a, the modulated beam from the red digital mirror device array 20c is reflected into the same path as the modulated green beam and at the second splitter/combiner 30a/18b the modulated signal from the blue digital mirror device array is reflected back into the same path so that the signal at the projection lens 40 comprises the recombined colour signals.

The spectral responses illustrated in Figures 6a and 6b are in fact dependent upon the angle of incidence of light upon the dichroic mirror in question. The spectral position of the transition (or transitions) between reflection and transmittence and/or the magnitude of either can therefore be different for two beams incident upon the mirror from different angles.

Thus, even if the splitter and combiner elements are identical (as is the case if the two are combined as a single dichroic mirror as above), the recombined beam can differ spectrally from the spectral sum of the modulated colour component signals from each array 20a, 20b, 20c, and consequently an unwanted colour cast can be introduced into the reproduced picture.

Further, stray light caused by unwanted transmission rather than reflection, or vice versa, through a combiner or splitter tends to reduce the contrast ratio available from the apparatus since it will usually eventually find its way to the display screen 10.

In the apparatus illustrated in Figures 8a - 8c, if, as shown, the splitter/combiners 30a/18b, 30b/18a comprise vertical planes then precisely this problem arrises since whilst functioning as splitters incendent light is transmitted and reflected at a vertical angle of 20 degrees whereas modulated light is transmitted and reflected at a vertical angle normal to each splitter/combiner. Thus, the proportion of each colour signal in the combined signal display differs from the original proportions split by the splitters 30a, 30b.Referring to Figure 9, this problem is particularly noticable with a divergent beam of the kind which may be employed in the embodiment of Figure 8a due to the short path lengths enabled by that embodiment, because not only does the angle of the incident beam differ from 50a, 50b, 50c differ from the angle of the modulated beam 51a, 51b, 51c in each case but also the ratio of the angles of each incident beam 50a to its corresponding reflected beam 51a will differ from the top to the bottom of the picture. A variable degree of colour cast within the picture may thus be generated, which is subjectively undesirable.

Referring to Figure 10, we solved this problem by inclining the splitter/combiner in the vertical plane at an angle equal to half the angle, in the vertical plane, between the incident beam from the light source 16 and the reflected beam from the array 20. In this case, with a modulation angle of 20 degrees, the mirror is consequently inclined at 10 degrees. It will be apparent from Figure 10 that, although the angles at which the incident and reflected beams 50a 50c, 51a - 51c impinge upon the splitter/combiner vary, in each case the angle between each incident and reflected beam is the same so that the colour balance of the recombined signal is correct.Because the angles of impingence increase towards the lower part of the combiner, and consequently of the picture reproduced on the display screen 10, some slight variation of intensity across the picture will occur, but this is generally acceptable.

Referring once more to Figure 8a, because in this embodiment the planes of the splitter/combiners are now no longer vertical the two arrays 20a, 20c must also be rotated somewhat.

The use of colour selective mirrors such as dichroic mirrors makes efficient use of the white light beam (up to 90% of which may usefully be employed) compared to the use of separate white light beams for each array (in which case, using typical filters, the blue light uses only 11% of the available energy).

Whilst the invention has been described with reference to a display device it will be equally apparent that it could be applied to a colour printer, where the display screen 10 is replaced by means for fixing the image produced onto a medium.

Equally, other types of spacial frequency splitter components than dichroic mirrors could be employed.

Claims (8)

CLAIMS:

1. Colour optical output apparatus comprising: a beam source; an output plane onto which an output colour picture is imaged; a plurality of spacial light modulator reflector arrays; control means for controlling each said array in dependence upon a different colour component signal; splitter means for splitting said source beam into optical colour component beams, each such beam being directed on one such array to be modulated thereby; and combiner means for recombining the modulated optical colour component beams to provide a combined beam to said imaging plane; wherein said splitter means comprise dichroic optical elements.

2. Colour optical output apparatus comprising: a beam source; an output plane onto which an output colour picture is imaged; a plurality of spacial light modulator reflector arrays; control means for controlling each said array in dependence upon a different colour component signal; splitter means for splitting said source beam into optical colour component beams, each such beam being directed on one such array to be modulated thereby; and combiner means for recombining the modulated optical colour component beams to provide a combined beam to said imaging plane; wherein said splitter means and said combiner means share a common surface.

3. Apparatus according to claim 1 or claim 2 further comprising a display screen positioned at said imaging plane.

4. Apparatus according to any preceding claim comprising three arrays each controlled to correspond to a primary colour.

5. Apparatus according to any preceding claim in which said splitters and/or said combiners comprise dichroic mirrors.

6. Apparatus according to any preceding claim in which said dichroic mirrors are disposed in the path of said source beam at an angle such as to deflect a colour component of said source beam in a plane different to that in which said arrays deflect said source beam.

7. Apparatus according to claim 2 in which, for each said array, the deflection angle is such that the incident and deflected beams substantially overlap and a splitter/combiner is positioned in the region in which said beams overlap.

8. Apparatus substantially as described herein with reference to the accompanying Figure 8a to 8c.