GB2142203A - Television projection apparatus - Google Patents

Abstract

An incoherent light source such as an arc-lamp 2 illuminates a linear light modulator 1 e.g. a row of light valves, controlled by a video line signal. The video line image is projected onto a screen 5 via an oscillating 4 or multi-faced mirror, the scanning being synchronised to the modulator 1 video information such that a complete image is projected. <IMAGE>

Description

SPECIFICATION Television project system The invention relates to atelevision projection system.

Known TV projectors may be classified broadly into three groups. The first group is based on the use of one or more cathode ray tubes (CRTs) in conjunction with an optical projection system comprising one or more mirrors or lenses. The optical projection system forms an image of the CRTfaceplate on a remote viewing screen to provide a large TV picture. In practice, limitations on achievable phosphor brightness limittheavailable Iightoutputfrom such TV projectorstoavaluetypicallyinthe range 100-1000 lumens, the upperfigure demanding large, high brightness CRTs and large aperture projection optics.

Furthermore, high contrast and resolution are difficult to maintain in the picture when the phosphor layer generating the primary image is being driven hard to maximise light output, while colour registration is often poor, at least over part of the picture area, because the most efficient way of projecting a coloured image is to use three separate coloured CRTs, necessitating complex geometrical coverage adjustments to obtain correct superimposition ofthe red, green and blue images on a remote screen.

In the second group of projectors, a static optical projection system is retained, but the CRT phospher is replaced by a layer whose optical transmittance or reflectance can be altered locally by suitable electronic addressing means, to achieve a transmissive or reflective passive image which may be projected using suitable incident illumination. Such TV projectors are often referred to as lightvalves; their extremely high potential light output stems from the replacement of a limited brightness phospher layer with a source image (eg Xenon arc) having uptoten thousand times greater intrinsic brightness.Known devices include the Gretag Eidophor and General Electric projectors (oil control layer deformed locally by scanned electron beam to yield TV image in conjunction with schlieren projection optics), the LEP Titus projector (electro-optic crystal layer in which locally induced birefringence creates a reflective image in conjunction with crossed polarisers) and the Hughes liquid crystal projector (another polarisationbased device using a liquid crystal layer). These systems are capable of light outputs in the range 300-7000 lumens, but only the high intensity Eidophor can approach the upperfigure using massive, expensive hardware.

In theory, much greater light output should be obtainable from light valve projectors: ignoring transmission losses and heat dissipation problems, an f/1.4 lens collecting light from a 50 mm x 50 mm arc source image could project over 350,000 lumens. However, in practice light is lostthrough unavoidable transmission losses and aperture limitations in the optical systems employed, and there are generally power handling limitations (thermal control problems) associated with the lightvalve layer itself.

The third group of TV projectors comprises optical beam scanners in which one or more beams of light of specified wavelength are scanned in rasterfashion over a remote screen by a combination of optical lineand frame-scanning systems. The line- and framescanning systems are synchronised with incoming TV signals which are used to modulate the beam intensities as required to produce a TV picture. With appropriate system design, this type of scanner can achieve very high resolution, high contrast and excellent colour registration. However, in order to achieve efficient line- and frame-scanning using known devices, the optical beam must be of small diameter and low divergence, necessitating the use of laser sources to achieve a reasonable light output.

LasercolourTV projectors currently under development are limited to around 1000 lumens output by the characteristics of available lasers.

ATV projector according to one aspect of the invention incorporates no light-emitting CRT devices, but includes one or more linear array light modulators to generate one or more complete TV lines, together with an optical frame-scanning mechanism.

A preferred arrangement of TV projector according to the invention will now be described byway of example only and with reference to the accompanying drawings in which: Figure 1 is a side viewof the TV projector according to the invention, Figure 2 isa rearviewoftheTVprojectorofFigure 1, Figure 3 shows in graphical form oscillations of the mirrorand parts of the oscillations which are used for frame-scanning, Figure 4 shows a sideview corresponding to part of Figure 1 of an alternative arrangement ofthe invention, Figure 5 shows an arrangement in which three modulators may be used to produce three colours for colourtelevision projection, Figure 6 shows the way in which the TV line generators may be curved according to the position in the lens focal plane, and, Figure 7 shows an alternative arrangement incorporating fibre optics.

Figures 1 and 2 showthe basic configuration of a projector using a single linear array modulator. In Figures 1 and 2, an array modulator 1, brightly illuminated by a suitable source and condenser system 2, and controlled to display one full line of a completeTV picture, is imaged by a lens 3 onto a viewing screen 5 via a moving mirror4, oscillating at the normal TVframe rate. As the mirror oscillates, the image ofthe linear array modulator (the TV "line") is swept down the screen generating the illusion of a continuous display image. The instantaneous position of the frame-scanning mirror 4 is accurately encoded and used to triggerthe output of appropriate TV line information from theTVframe store 6tothe array modulator 1.

The drawing(s) originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.

The claims were filed later than the filing date within the period prescribed by Rule 25(1) of the Patents Rules 1982.

In orderto provide a light output competitive with establishedTV projectors, the optical components shown in Figure 1 are relatively large. Using an efficient array modulatorl approximately200 mm long containing 800 discrete elements, an f/1.4 lens 3 approximately 300 mm in diameter and an oscillating plane mirror 4 approximately 450 mm square, a peak output in excess of 2000 lumens is achievable. It is howeverdifficultto drive such a large oscillating mirrorthrough an angle of perhaps 1 00to achieve a linear angular scan with fastflyback at a repetition rate of 50-60 Hz because ofthe enormous torque required to accelerate the inertial load associated with the mirror.We have, however, adopted a resonant oscillating system for the mirror mount to achieve an adequate deflection angle, using as much as possible ofthe resulting sinusoidal scan to minimise dead time in the frame-scanning mechanism using appropriate timing pulses and a sufficientlyversatileTV line readout mechanism, 80% or more ofthe sinusoidal frame scan can be employed on both the upward and downward sweeps ofthe mirror, as indicated in Figure 3.

It is preferred that several TV lines from either several linear array modulators 1 orfrom a matrix array modulator are projected simultaneously as shown in Figure 4. Benefits ofsuch an arrangement include a proportional increase in light output, with simultaneous reduction in the vibrational amplitude required from the frame scanning mirror. In addition, the response time associated with individual array modulators may be proportionally increased without detriment to picture quality. As stated above, appropriate encoding pulses in conjunction with a sufficiently versatile frame store readout mechanism would enable the correct TV line information to be fed simultaneously to several array modulators.

To provide a TV display in full colour, the lens 3 in Figures 1 and 2 is, in one embodiment, and achromatic projection lens and the linear array modulators comprise interleaved red,green and blue transmitting elements. A preferred configuration employs separate linear modulators 1 comprising entirely red, green or blue transmitting elements, thereby allowing full TV line resolution to be maintained in all colours.

Although individual coloured TV line generators may be separated in the focal plane of the projection lens, appropriate distribution ofTV line information from the TVframe store ensures correct superimposition of the red, green and blue coloured images on the viewing screen 5.

Some ofthe design problems and expense associated with providing a large lens 3 of adequate optical performance may be eased by altering the geometry ofthe linear array modulatorsforming individual TV lines. This facility is particularly useful when several red-, green- and blue-transmitting linear modulators are employed to provide a colour TV projector with a very large output. For example, in one arrangement the red, green and blue linear modulators 1 R, 1 G, 1 B lie respectively in adjacent parallel planes displaced along the axis ofthe projection lens 3, as shown in Figure to reduce the degree of colour correction required in the lens. Furthermore, individual linear modulators 1 may be curved according to their position in thefocal plane ofthe lens 3 as shown in Figure 6, to correct residual geometrical distortion of the lens 3.In some cases it is beneficial to include convexorconcave curvature ofthe arrays 1 towards the projection lens 3 to compensate for residual field curvature.

In order to provide a compact optical system with efficient illumination of indivual TV lines, which may incorporate distortion correction as outlined above, a preferred arrangement employs optical fibres 11 to guide lightfrom a suitable source 10 tothefocal plane of the projection lens 3 via an array of individual modulator elements 12 as indicated in Figure 7. Each element of the modulator array 1 controls the intensity of lighttransmitted by a single fibre. The fibres 11 would beformed into one or more linear arrays in the focal plane of the lens 3 to constitute one or more completeTVlines for projection bythe lens 3.

As an alternative to the large frame-scanning mirror external to the projection lens 3, a smallerframescanning mirrorcould be interposed between the projection lens 3 and the lineararray modulators 1, or the array modulatorsthemselves (or the projection lens) could be vibrated to achieve frame scanning.

We have thus described a TV projector using conventional lightsources in which one or more completeTVlinesaregenerated by one or more brightly-illuminated, parallel-driven, linear array modulators, the TV line generators being imaged onto a remote viewing screen by a suitable projection lens and frame scanning being provided by a vibrating plane mirror or by relative motion between the TV line generators and the projection lens.

One or moreTVframe stores may be used to distributelV line information to one or moreTVline generators at appropriate times during the framescanning cycle.

The frame scanning timebase may be distorted to suit the characteristics of a resonant vibrating mechanism.

Afull colour image may be provided bythe use of one or more linear array modulators transmitting predominantly blue, green or red light respectively.

The shape and location of individual linear array modulators in the focal plane of the projection lens may be adjusted to compensate for deficiencies in the performance of the lens.

The TV line generators may be constructed using optical fibres.

Claims (17)

1. A projector apparatus comprising an incoherent light source, a line modulator as defined, means for passing light from the light source to the line modulator, means for passing a video signal to the line modulator, and means for passing the light influenced bythe line modulatorto a screen, the light passing means including a scanning apparatus to scan the part image formed by the line modulator across the screen to build up at least part of an image.

2. Apparatus as claimed in claim 1 in which there is provided more than one line modulator.

3. Apparatus as claimed in claim 2 in which each linemodulatorisarrangedto receive a video signal in respect of different parts of the complete image.

4. Apparatus as claimed in claim 2, in which there are atleastthree line modulators, there being provided at least one line modulatorforeachvideo signal relating to the green, the red, and the blue component ofthe image.

5. Apparatus as claimed in any of claims 1 to 4 in which the or each line modulator comprises a plurality of discrete components.

6. Apparatus as claimed in any of claims 1 to4 in which the or each line modulatorcomprisesa single component arranged to transmit or reflect incident light to a different extent along its length.

7. Apparatus as claimed in any of claims 1 to 6in which the lightfrom the light source passes to the line modulatorand is thereby modulated in respect of its intensity, and is transmitted through the line modulator to the screen.

8. Apparatus as claimed in any of claims 1 to 6 in which the light from the light source is passed to the line modulator, is reflected therefrom with an intensity which is modulated by the line modulator, and the reflected light is passed to the screen.

9. Apparatus as claimed in any of claims 1 to 8 in which the line modulator is curved.

10. Apparatus as claimed in anyofclaims 1 to 9 in which the scanning apparatus comprises a movable mirror.

11. Apparatus as claimed in claim 1 Oin which the movable mirrorcomprisesan oscillating mirror.

12. Apparatus as claimed in claim lOin which the movable mirrorcomprises a plurality of mirrors in the form of a mirror drum which is rotated about its axis.

13. Apparatus as claimed in any of claims 1 to 9 in which the scanning apparatus comprises one or more movable lens.

14. Apparatus as claimed in any of claims 1 to 13 in which the light is passed to and from the or each line modulator by optical fibres.

15. Apparatus as claimed in claim 5 and claim 14 in which light is passed to each discrete component by an associated optic fibre and from each discreet component by a respective optic fibre.

16. Apparatus as claimed in claim 15 in which the scanning means comprises means to move the optical fibres.

17. A projector apparatus as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawings.