Patent Publication Number: US-2002001044-A1

Title: Projection apparatus and method of image projection

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to projection apparatus which are arranged to generate images by projecting light representative of the images onto a display screen. The present invention also relates to methods of projecting images on to a screen.  
       [0003] 2. Description of the Prior Art  
       [0004] Known apparatus for generating visual images include the Cathode Ray Tube (CRT) in which a signal representing an image is arranged to modulate beams of electrons within a vacuum tube. The electrons are arranged to strike a phosphor lined wall of the tube which is divided into individual pixels. The pixels contain different phosphor elements which emit light of different colours when hit by the electrons. The modulation of the electron beams thereby creates a coloured image from the different coloured pixels elements. Other apparatus include Liquid Crystal Displays in which the optical properties of pixels which make up the displays are changed in accordance with signals representative of pixels of the image to be displayed.  
       [0005] Although it is possible to manufacturer CRT displays to a relatively large size, if a display is required to produce a picture to be viewed by a large audience, the manufacture of CRT displays and LCD displays becomes difficult. For displays which are required for large audiences it know to use a projection apparatus, such as, for example a projection television.  
       [0006] Projection televisions are typically arranged to generate an image from a signal representing the image using, for example, a smaller CRT. Light generated by the CRT is projected on to the screen. Projection televisions are known to include front and rear projection arrangements. Generally, but not exclusively, the front projection televisions are arranged to project the image on to a separate screen, whereas for rear projection televisions, the image is projected from behind a viewing side of the screen (referred to herein as a projection side) which forms an integral part of the television.  
       [0007] As with CRT displays, projection television displays are arranged to form colour images by projecting three different components of the image representative of red, green and blue components of the image on to a screen. However, in order to provide an acceptable representation of the colour image, the three components must be projected onto to the screen with the effect that the three components are superimposed whereby the components converge with each other. This superposition is achieved by providing some arrangement whereby the image components are aligned at a plane in which the display screen is disposed. If the three components are not aligned then the coloured image suffers from reduced definition, which is disturbing for the viewer. Arranging for the three components of the colour image to convergence is exacerbated in projection television, because typically each component of the image is generated with a separate CRT. Furthermore, an optical arrangement for projecting the image components onto the screen, particular for rear projection televisions, can require that at least one and usually two of the red, green and blue projectors are offset at an angle.  
       [0008] Generally in order to provide an arrangement in which the colour components of the image are arranged to converge, projectors of each of the three components are provided with an adjustment means. For the example of projectors which utilise a CRT to generate the colour component of the image, the CRT is provided with a deflection coil or choke, for each of the horizontal and vertical directions, which are arranged to change a position of the projected image on the screen in dependence upon horizontal and vertical biasing adjustment voltages applied to the deflection coil. However, although the adjustment voltages can be pre-set by the manufacturer in the factory so that the three colour components of the image are aligned, influences on the magnetic field of the CRT, temperature and ageing effects generally cause the colour components to again diverge. To this end, it is known to provide projection televisions with a convergence arrangement whereby the three colour components are again arranged to converge.  
       [0009] One such convergence arrangement provides a plurality of sensors which are disposed on the display screen. This arrangement is disclosed in European Patent serial number EP 0 852 447 A. Each of the sensors is exposed to a test projection from each of the projectors. The test projections are projected at a plurality of predetermined positions either side of the sensors and measurement signals detected by the sensors for each of the predetermined positions are integrated to provide an average measurement signal. The displacement of the test projections is controlled to the effect of locating a relative displacement of the test projections which provides a maximum value of the average measurement signal. In this known arrangement, the sensors are photodiodes. In other previously proposed arrangements, the sensors are photo-voltaic (sollar) cells. The solar-cells are used because the latency in the measurement signal in response to the test projection produced from the photo-voltaic cells is conveniently matched to a typical rate of processing of a microprocessor. The alignment process is therefore conveniently performed by the microprocessor.  
       [0010] The convergence arrangement in known systems requires the user to manually trigger the adjustment process during which the test projections are visible on the screen, and the projected image is not displayed. This is a cause of some inconvenience and disturbance to viewers.  
       SUMMARY OF THE INVENTION  
       [0011] According to the present invention there is provided a projection apparatus for generating an image by projecting light representative of the image on to a display screen, the apparatus comprising at least one projector operable to receive a component signal representative of a component of the image and to project light representative of the component on to the display screen, the projector having an adjustment means for adjusting the relative position of the projected image component on the display screen in accordance with an adjustment signal, a convergence processor coupled to the adjustment means and operable to adjust a relative position of the image component on the display screen in response to a measurement signal generated by a sensing device in response to a test projection received by the sensing device, wherein the sensing device is operable to produce a measurement signal having a predetermined output value when the relative position of the test projection is substantially optimum, and the convergence processor is operable to displace successively the test projection from a first position, until the value of the measurement signal corresponds to the predetermined output value, the adjustment signal being adjusted in correspondence with the relative displacement of the test projection from the first position to the position at which the measurement signal corresponds to the predetermined output value.  
       [0012] Embodiments of the present invention utilize a sensing device which generates a measurement signal which produces a predetermined output value only when the test projection is at an optimum position for determining the alignment of the colour component. This provides an advantage because the convergence processor, which controls the alignment processes is only required to displace the test projection in one direction only.  
       [0013] As explained in the above referenced known convergence arrangement, disclosed in EP 0 852 447 A, the sensing device produces only an output measurement signal representative of the relative amount of light received by the sensor. As a result, when the sensing device is illuminated by the test projection, the control processor is unable to determine whether the test projection is illuminating one side of the sensing device or the other. As a result, for each controlled sample position of the test projection, the test projection must be positioned first on one side of the test sensor and then positioned on the other side of the sensor. The measurement signal produced by the sensor at each position is integrated, and the controlled sample positions are then arranged to be progressively moved to the effect of maximizing the integrated measurement signal.  
       [0014] The present invention is therefore provided with an advantage in that the control processor which controls the convergence arrangement has reduced complexity in comparison to known arrangements. This is because the control processor is only required to adjust the position of the test projection until the measurement signal from the sensing device reaches the predetermined value, at which point the adjustment signal is considered to be optimal. In contrast the convergence processor of the known arrangement must be arranged to search either side of the sensing device for the optimum adjustment signal. As such the convergence processor according to an embodiment of the present invention, can be implemented in hardware rather than a software controlled processor as is required in known systems. As a result the speed of operation of the convergence processor and hence the alignment process is substantially increased. The increased speed of operation further facilitates implementation of a convergence arrangement in which the alignment process is performed autonomously and contemporaneously with the generation of the image by the projectors.  
       [0015] The measurement signal from the sensing device may be signed, the sign of the measurement signal being indicative of whether the test projection is one side of the optimum alignment position or the other side. As such, the complexity of the convergence processor can be further reduced, simplified and the alignment facilitated by providing the convergence processor with a measurement signal which indicates which side of the optimum alignment position the test projection is positioned. The convergence processor can therefore take correction action to adjust the adjustment signal to move the test projection in a direction opposite to the side on which it is positioned.  
       [0016] The predetermined value of the measurement signal corresponding to the optimum position of the test projection, may be a maximum output value of the measurement signal, produced by the sensing device as the test projection passes over the sensing device. However in preferred embodiments, the predetermined output value is a null output value, being zero, or substantially close to zero. As such the convergence processor may be arranged to adjust the position of the test projection until the measurement signal is equal or substantially equal to zero. As will be understood the predetermined output value may be detected by comparing the measurement signal with a threshold, and a logical output generated from the comparison. The threshold value may be set at zero or slightly above zero in dependence upon a relative detection accuracy required. It will be appreciated however that this is but one example of the predetermined output value of the measurement signal corresponding to the optimal position.  
       [0017] The measurement signal may include a second output signal, the signed output signal being a first output signal, the second output signal providing a peak output value when the test projection is at the optimum alignment position. Detection of the optimum alignment position is further facilitated by providing a second output signal which reaches a maximum value when the test projection is at the optimum position with respect to the sensing device.  
       [0018] In a preferred embodiment, the sensing device comprises first and second sensors coupled to a comparator and arranged to produce the null output when each of said first and second sensors receives substantially the same amount of the test projection, and a positive or negative output when the first or the second sensors receives more of the test projection than the other. The sensing device may also include an adder coupled to the first and second sensors and arranged to add the output signals from each sensor, the output from the adder providing the second output signal. In one embodiment, the first and the second sensors are arranged on a diagonal line formed on a notional quadrangle, and the test projection is shaped and arranged to illuminate the first and second sensors when in said substantially optimum alignment position on the diagonal line. By arranging the sensors in diagonal line with respect to the horizontal and vertical axes of the projected image, the substantially optimal alignment position may be determined for the horizontal and the vertical adjustment signal components contemporaneously.  
       [0019] According to an aspect of the present invention there is provided a television apparatus having a receiver for detecting a television signal and for recovering from the television signal an image signal representative of an image, and the projection apparatus for generating the image from the image signal.  
       [0020] According to an aspect of the present invention there is provided a convergence processor for use in a projection apparatus, the convergence processor being operable to generate an adjustment signal for an adjustment means of a projector, for changing the relative position of an image component projected by the projector, in accordance with a measurement signal received by the convergence processor from a sensing device in response to a test projection produced by the projector, the sensing device producing a measurement signal having a predetermined output value when the relative position of the test projection is at a substantially optimum alignment position, to displace successively the test projection from a first position, by a predetermined amount, and to detect the value of the measurement signal which corresponds to the predetermined output value, the adjustment signal being set in correspondence with the relative displacement of the test projection from the first position to the position at which the measurement signal is corresponds to the predetermined output value. In preferred embodiments. The convergence processor may be implemented as an integrated circuit.  
       [0021] According to an aspect of the present invention there is provided a method of projecting an image having at least one component onto a display screen, the image component being represented as an image component signal, the method comprising the steps of projecting a test projection on to the screen, sensing a relative position of the test projection, using a sensing device which is operable to produce a measurement signal having a predetermined output value when the relative position of the test projection is aligned at a substantially optimum position, displacing successively the test projection from a first position, by a predetermined amount, detecting when the value of the measurement signal corresponds to the predetermined output value, and setting the adjustment signal in correspondence with the relative displacement of the test projection from the first position to the position at which the measurement signal is equal to the predetermined output value. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022] The present invention will be described further, by way of example only, with reference to a preferred embodiment thereof as illustrated in the accompanying drawings, in which:  
     [0023]FIG. 1A provides an illustrative block diagram of a projection television apparatus,  
     [0024]FIG. 1B provides an illustrative block diagram of the projection television of FIG. 1A, configured as a rear projection arrangement,  
     [0025]FIG. 2A is a schematic block diagram of a previously proposed projection processor,  
     [0026]FIG. 2B is a schematic block diagram showing four sensors disposed on a display screen which forms part of a projection television configured for use with the projection processor of FIG. 2A,  
     [0027]FIGS. 3A, 3B,  3 C and  3 D, provide an illustration of a test projection displayed with respect to the sensors of the display screen shown in FIG. 2B,  
     [0028]FIG. 3E is a graphical representation showing a relationship between the magnitude of a measurement signal from the sensors with respect to a position of the test projection,  
     [0029]FIG. 4 is a schematic block diagram of a projection processor according to embodiments of the present invention,  
     [0030]FIG. 5 is a schematic block diagram of a display screen according to embodiments of the present invention,  
     [0031]FIGS. 6A, 6B and  6 C provide a representation of a first test projection displayed with respect to the sensor of the display screen shown in FIG. 5, illustrating a first phase of an alignment process according to a first embodiment of the present invention,  
     [0032]FIGS. 6D, 6E and  6 F provide a representation of a second test projection displayed with respect to the sensor of the display screen shown in FIG. 5, illustrating a second phase of the alignment process of the first embodiment,  
     [0033]FIGS. 7A and 7B provide a representation of a first test projection displayed with respect to two sensors of a sensing device, illustrating a first phase of an alignment process according to a second embodiment of the present invention,  
     [0034]FIG. 7C is a graphical representation showing a relationship between the magnitude of a measurement signal from the sensing device with respect to a position of the test projection,  
     [0035]FIGS. 7D and 7E provide a representation of a second test projection displayed with respect to the two sensors, illustrating a second phase of the alignment process of the second embodiment,  
     [0036]FIGS. 8A to  8 D provide a schematic representation of an alternative arrangement of the first and second test projections, according to the first and second phases of the second embodiment,  
     [0037]FIGS. 9A, 9B and  9 E show a representation of two sensors forming a sensing device and a test projection according to a third embodiment of the present invention,  
     [0038]FIGS. 9C and 9D provide a graphical representation showing a relationship between the magnitude of a first and second output signals forming the measurement signal from the sensors with respect to a position of the test projection, according to the third embodiment, and  
     [0039]FIG. 10 provides a schematic block diagram of a display screen and the two sensors according to an alternative arrangement of the third embodiment. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0040] Embodiments of the present invention find application with any form of projection apparatus, including front and rear projection arrangements. However an illustrative example embodiment of the present invention will be described with reference to a rear projection arrangement, and more particularly to projection televisions. A rear protection television is illustrated schematically in FIGS. 1A and 1B.  
     [0041] In FIG. 1A a projection apparatus, generally  5 , is arranged to project an image  1  onto a display screen  10  which extends in a horizontal X and vertical Y direction. The projection apparatus  5  has three projectors  20 ,  22 ,  24  and a projection processor  30 . The projection processor  30  is connected to the projectors  20 ,  22 ,  24  by parallel connectors  12 . An image signal I, which represents the image  1  which is to be projected onto the display screen  10 , is received by the projection processor  30  and separated into three component signals I R , I B , I G , which are representative of red, green and blue components of the image. Each projector  20 ,  22 ,  20  receives a respective component signal I R , I B , I G , from the conductors  12  and generates an image component corresponding to the component signal. The three projectors  20 ,  22 ,  24  are thereby arranged such that the red, green and blue components of the colour image  1  are superimposed on the display screen  10  to form the colour image  1 .  
     [0042] Although the example embodiment has three projectors generating red, green and blue components, it will be appreciated that in other embodiments, a projection apparatus according to an embodiment of the invention may have two or only one projector, which is arranged to project an image of any wavelength both visible and invisible to the human eye. However, the present invention finds particular utility with projections apparatus having two or more projectors which are arranged to generate image components having different colours in which the light from each of the projectors has at least one different wavelength. In other embodiments the projectors may project components having the same colour.  
     [0043]FIG. 1B shows a particular embodiment of a projection apparatus  25  which is configured as a rear-projection apparatus. In the following description, the region A on which image components from the projectors  20 ,  22 ,  24  are projected on to the display screen  10  will be referred to as the projection side of the display screen  10 , and the region B on the side of the display screen  10  from which the image is viewed will be referred to as the viewing side. Each projector  20 ,  30 ,  40  is arranged to project its respective image component via an optical arrangement, which includes a mirror  15 , to form the image on the projection side. The image is reflected by the mirror  15  onto the display screen  10  so that the image may be viewed by a viewer from the viewing side.  
     [0044] In the example embodiments shown in FIGS. 1A and 1B, the projectors  20 ,  22 ,  24  are formed from smaller CRTs which generate the light forming the red, green and blue projected image components. The image component generated by each of the projectors  20 ,  22 ,  24  must be aligned in some way so that the superposition of the image components provides a colour image with good definition. Arranging for the three components of the colour image to converge is made more difficult for projection televisions, because typically each component of the image is generated with a separated CRT. Furthermore, the optical arrangement of the projectors  20 ,  22 ,  24  can require that two of the red, green and blue colour image components (typically red and blue) are offset at an angle, particularly where the three projectors are arranged in line. It is generally therefore necessary to adjust the relative position on the screen of each of the three components, in the factory during a final production phase, to the effect that the image components are aligned. To this end, the projectors  20 ,  22 ,  24  are usually provided with an adjustment means whereby the relative position of the projected image on the screen can be adjusted. For an example implementation in which the projectors include CRTs, the adjustment means is formed from a deflection coil or choke (DCh, DCv) for each of the horizontal (X) and vertical (Y) directions to which an adjustment signal is applied. The adjustment signal may be any predetermined signal, however in the present illustrative embodiment, the adjustment signal is a voltage for the vertical (Vy) and the horizontal (Vx) deflection coils (DCh, DCv).  
     [0045] During the factory setting, the red, green and blue image components are aligned to achieve a desired picture definition. An alignment is performed typically whilst the projector television is within a constant magnetic field. An alignment process is performed in which an operator visually changes the adjustment signal components (Vx, Vy) applied to the horizontal and vertical deflection coils, for each of the three image components until the three components are aligned. The adjustment values for the horizontal and vertical directions may be applied using potentiometers, but more typically are stored in a memory as a digital value and applied via D/A converters to the deflection coils of the respective projectors  20 ,  22 ,  24 . The value of the adjustment signals which are stored in memory are known as factory settings or correction values.  
     [0046] Although the image components are arranged to converge using the correction values set during the alignment process performed in the factory, the effects of stray magnetic fields, temperature changes and ageing effects can cause the three image components to once again diverge. As such it is known to provide projection televisions with an arrangement for automatically performing the alignment process, when manually triggered by the user. However, as will be explained shortly, known arrangements for automatically performing the alignment suffer several disadvantages, one of which is that the user must manually trigger the alignment process. A further disadvantage is that whilst the alignment process is being performed the projected image cannot be displayed.  
     [0047] In order to better appreciate the many advantages provided by embodiments of the present invention, a previously proposed alignment process and arrangement will be briefly described in the following paragraphs with reference to FIGS. 2 and 3, where parts also appearing in FIGS. 1A and 1B have the same numerical references. For comparison an example of a known arrangement is provided in the above-mentioned European patent serial number EP 0 852 447 A.  
     [0048] In FIG. 2A, the projection processor  30  receives the image signal I, from a television receiver  32 . The receiver  32  is arranged to recover the image signal I, from a television signal received from an antenna  34 . The image signal I is fed to a video device  44  which separates the image signal into the component signals I R , I B , I G  which are applied to the three respective projectors  20 ,  22 ,  24 . The projection processor has a system controller  38  formed from a microprocessor, which generally controls the projection apparatus  25 , with the system controller  38  operating as a master and all other units configured as slave units. The system controller  38  has an associated memory  36  for storing program instructions and data.  
     [0049] The projection processor  30  also comprises convergence driver  42  for controlling the spatial alignment of image components, in combination with a convergence processor  52 . The convergence driver  42  has an associated memory  40  for storing the correction values (factory settings) as described above.  
     [0050] However, to allow for automatic adjustment of the components, the previously proposed projection apparatus  25  utilises generally four sensors  47 ,  48 ,  49 ,  50 . The sensors  47 ,  48 ,  49 ,  50  are typically photocells and are arranged at the periphery of the screen  10  as illustrated in FIG. 2B. The height and width of the photocells typically spans several pixels, so that for example, the height of the photocell has a dimension in the X and Y direction equivalent to 10 lines of the projected image. The alignment process according to the previously proposed arrangement will now be explained with reference to FIG. 3, where parts also appearing in FIGS. 1A, 1B,  2 A and  2 B have the same numerical references.  
     [0051] Following a manually press of a reset switch (not shown) by the user, the system controller  38  instructs the convergence processor  52  implemented as a second microprocessor  52  to enter an automatic alignment process. As shown in FIGS. 3A to  3 E, during the automatic alignment process the image signal is isolated by the video device  44 , such that only a test signal representative of a test projection is applied to the projectors  20 ,  22 ,  24 . Hence whilst the test projection is being projected the projected television image cannot be seen. As shown in FIGS. 3A, 3B,  3 C and  3 D, the test projection comprises a substantially rectangular image  46  which is projected at a number of predetermined locations. Typically, the area of the image is large with respect to the sensors  47 ,  48 ,  49 ,  50 . As such, although the sensors  47 ,  48 ,  49 ,  50  are mounted in a blanking region (not shown) formed in an over scan area on the projection side of the screen  10 , the test projection  46  can be seen during the automatic alignment process from the viewing side. The test projection is typically arranged, in known manner, to traverse towards the sensors  47 ,  48 ,  49 ,  50 , providing at each of the predetermined positions a measurement signal from the sensors.  
     [0052] The measurement signal from the sensors  47 ,  48 ,  49 ,  50  is received by the convergence processor  52  via an equaliser  51  and an A/D converter  53 . The equaliser  51  applies a filter to the output from the sensors  47 ,  48 ,  49 ,  50  in dependence on the wavelength of the light produced by the projector to compensate for the non-linear frequency response of the sensors  47 ,  48 ,  49 ,  50 .  
     [0053] After a first pass over the sensors  47 ,  48 ,  49 ,  50 , the position at which the test projection is projected onto the sensor is adjusted by the convergence processor and the measurements repeated, as illustrated in FIGS. 3A, 3B and  3 C.  
     [0054] The convergence driver  42 , under the control of the convergence processor  52  continues this process for sensors  47  and  48 , traversing from both directions as indicated in FIGS. 3A to  3 D until the location of all the peak outputs have been stored. Thereafter, the convergence processor  52  calculates an arithmetic mean of these stored values to derive a horizontal offset value.  
     [0055] The sensors  49 ,  50  are used to generate vertical offset values in a similar manner. This process continues until horizontal and vertical offset values have been calculated for the plurality of image components which are stored in memory. The convergence processor  52  is arranged to receive the correction values determined during the factory alignment and the offset values generated during alignment process. The convergence processor  52  then generates an adjustment signal for each image component which provides for improved alignment of the image components. The alignment signals are applied to the projectors  20 ,  22 ,  24  via the amplifier  46  to the deflection coils DCh, DCv, (adjustment means) of the respective projectors  20 ,  22 ,  24 . The alignment process is then terminated and the projected image according to the television signal again displayed.  
     [0056] In addition to the disadvantage that the known arrangement must be manually triggered by the user, and the disadvantage that the projected image cannot be displayed during the alignment process, the projection processor shown in FIG. 2A is expensive to manufacture because it requires two microprocessors for implementing the system processor  38  and the convergence processor  52 . This is because the sensors  47 ,  48 ,  4 ,  50  are generally large, being photo cells, which produce a measurement signal having a considerable lag with respect to the time at which the test projection is received by the sensor. Furthermore, because of the limited accuracy with which the test projection can be controlled, the test projection is relatively large, so as to ensure that the test projection is received by the sensors. Also, the output of the sensor is ambiguous, in that the same output value will be produced by the sensor whether the test projection is to the left or to the right (in the horizontal direction) of the sensor. For these reasons several passes of the sensor must be made by the test projection, and the resulting value of the measurement signal integrated in order to obtain a satisfactory indication of an optimum alignment position corresponding to the peak value of the measurement signal. This in turn requires the use of a microprocessor to implement the convergence processor  52 . The previously proposed alignment process and convergence processor is therefore expensive and furthermore requires at least four sensors in order to correctly align the image components.  
     [0057] A first embodiment of the present invention is shown in FIG. 4, where parts also appearing in FIGS. 1, 2 and  3  bear the same designated references. In FIG. 4 a projection processor according to an embodiment of the present invention corresponds substantially to the previously proposed projection processor shown in FIG. 2A, and so only the differences will be described. In FIG. 4, the microprocessor  58  which forms the convergence processor  52  has been replaced by a hardware implemented convergence processor  120 . Furthermore the convergence driver  42  has been replaced with an enhanced convergence driver  142 , for which there is no connection to the video device  44 . Instead, a test signal is provided from the convergence driver  142 . The projection processor  130  is also provided with a pre-processor  140  connected to the sensor  100 ,  200 ,  210 , and a timer  122 .  
     [0058] An alignment process performed by the convergence processor  120  shown in FIG. 4 according to a first embodiment of the present invention will now be explained with reference to FIGS. 5 and 6 where parts also appearing in FIGS.  1  to  4  have the same designated references.  
     [0059] The first embodiment shown in FIGS. 5 and 6 illustrates an alignment process performed by a convergence processor  120  using only a single sensor, whilst contemporaneously projecting the image I representing the image signal onto the display screen  10 . As such, two of the disadvantages associated with the previously proposed alignment process are overcome or at least improved, because the projected image according to the television signal can be displayed contemporaneously with the test projection and hence alignment of the image components is performed whilst the television image is being projected. In addition, there is no longer a requirement for the user to manually trigger the alignment process, because this can be performed periodically whilst the picture is being displayed. The alignment process can be automatically triggered after a predetermined alignment period has passed since the last alignment, which in the example embodiment of FIG. 4 is measured using the timer  122 . Yet further, the alignment process is simplified and the sensor arrangement made less expensive, because only a single sensor is required. For this reason the convergence processor can be implemented in hardware.  
     [0060] The embodiment illustrated in FIG. 5 utilises one sensor  100 . The sensor  100  is disposed on the projection side in a blanking region formed around a periphery of the screen  10 .  
     [0061] As already explained, typically television images are displayed on a screen of some kind on a side obverse to the viewing side. The images are generally projected to fill the display screen  10 . However, the display screen typically includes an over-scan area or a so-called beznet  12  which is opaque and therefore obscures a part of the image projected in this area from the viewing side. The remainder of the image may be viewed in a visible picture area  14 . Typically, the area of the beznet  12  represents around 7% of the area of the display screen  10 . It is well know to provide the beznet  12  in order to prevent the user from viewing any blanking regions formed in the scanned image which may become visible as a result of image drift.  
     [0062] In this embodiment, the sensor  100  is disposed on the beznet  12 . Preferably, but not exclusively, the sensor  100  is disposed centrally within the upper horizontal region of the beznet  12 . However, it will be appreciated that the sensor may be positioned at any suitable point within the beznet  12 .  
     [0063] According to the present embodiment the sensor  100  is a photodiode or phototransistor which generates a photovoltaic response at each of the wavelengths of the components I R , I G , I B . Photodiodes are one example of a group of sensors having a narrow field of view, such that only light which is in close proximity to the sensor  100  will result in an output measurement signal being generated. Furthermore, preferably the sensor  100  has a sufficient response time to ensure that the rise and decay of the output signal has a minimal lag with respect to the incident light and that the output signal is proportional to the flux levels of the incident light.  
     [0064]FIGS. 6A to  6 F illustrate an alignment process according to the first embodiment. The alignment process can generally be considered as comprising two phases. In a first phase, a vertical offset to the vertical component of the adjustment signal Vy is determined, and in the second phase the horizontal offset to the horizontal component of the adjustment signal Vx is determined. The vertical and horizontal offsets for each image component have an effect of once again aligning the image components.  
     [0065] A test signal is generated, by the convergence driver  142  in a systematic way for each of the three image components. The alignment for each component is effected separately in the same way, and so the alignment of one image component only will be explained. The test signal is received by the video device  144  and combined with the image signal I. The test signal represents a test projection  170 . The test projection  170  is displayed on the screen contemporaneously with the projected image  1 .  
     [0066] For determining the vertical offset to the vertical component Vy adjustment signal, the test projection  170  preferably has a small dimension in the vertical Y direction and a large dimension in the horizontal X direction. The test projection  170  has a small dimension in the vertical Y direction so that light will only be incident on the sensor  100  when the test projection  170  is in close proximity to the sensor  100 . As a result the vertical adjustment signal Vy can be determined by simply detecting a peak output from the measurement signal produced by the sensor  100 .  
     [0067] The test projection  170  has a large dimension in the horizontal X direction so that the test projection  170  will be more likely to intersect the sensor  100  even though there may be alignment errors in the horizontal X direction.  
     [0068] In the first phase of the alignment process, the test projection  170  is arranged to be projected at a first predetermined position in close proximity, but vertically to one side of the sensor  100 . In preferred embodiments, the first predetermined position is derived from the vertical correction value of the factory setting of the vertical adjustment signal, which is store in memory  40 . The convergence processor  120  in combination with the convergence driver  142  adds an offset to the vertical correction value of the adjustment signal Vy applied to the deflection coil, to position the test projection  170  at the first predetermined position. The vertical offset has a value such that although the image has become misaligned vertically, based on worst-case conditions, the vertical offset value ensures that the test projection  170  is projected to the required side of the sensor  100  as illustrated in FIG. 6A.  
     [0069] Thereafter, given that the vertical location of the test projection  170  in relation to the sensor  100 , the vertical offset value is adjusted such that the test projection  170  is projected closer to the sensor  100 , here in the direction Y as illustrated in FIG. 6B and the output of the sensor  100  may then be measured again. This adjustment process continues until the test projection  170  has passed over the sensor  100  and the measurement signal from the sensor begins to reduce as illustrated in FIG. 6C. The pre-processor  140  may include filters which have an effect of equalising the output of the sensors in response to the red, green and blue versions of the test projection.  
     [0070] Accordingly, it is possible to determine the location of the maximum output from the sensor  100  with respect to an vertical offset value, the maximum output being indicative of an image aligned in the vertical direction Y. This offset value is stored and applied to its respective component.  
     [0071] A consequence of changing the adjustment signal whilst the projected image is being displayed contemporaneously with the test projection, is that the projected image will also move. However an aspect of embodiments of the present invention is that, as a consequence of the fact that the alignment process can be performed continuously, only small adjustments are required to move the test projection until it reaches the optimum position over the sensor. To this end the first predetermined position of the test projection may be changed for each performance of the alignment process. This is also because of the narrow field of view of the sensor, which may be photodiode and the narrow width of the test projection in the vertical plane.  
     [0072]FIGS. 6D to  6 F illustrate the second phase of the alignment process to the effect of determining the horizontal adjustment signal (Vx) to align the image component in the horizontal direction. As before, a test signal is generated and combined with the image signal I. The resulting test projection  160  is projected on to the sensors. However, the vertical displacement of the test projection and projected image is set in accordance with the corrected vertical alignment signal value Vy determined in the first phase of the alignment process. This is because in the vertical direction the projected image has already been correctly aligned. Therefore the test projection will lie in a horizontal plane which intersects the sensor. As a result the test projection as shown in FIGS. 6D, 6E and  6 F, can be arranged to be much smaller, with dimensions in the order of the dimensions of the sensors area. The test projection  160  preferably has a small dimension in the horizontal X direction. To accommodate any tolerances in the vertical adjustment signal, the test projection may have a larger dimension in the Y direction in order to increase the probability that the test projection intersects the sensor when moved in the horizontal plane. In preferred embodiments the test projection  180  is substantially ovoid, although in other embodiments the test projection may be dot shaped, corresponding to the shape and dimensions of a detection area of the sensor  100 .  
     [0073] The test projection  160  is arranged to be projected at a first predetermined position which is preferably to one side of the sensor  100  in the horizontal direction X. Again the first predetermined position may be derived from the vertical correction value corresponding to the factory set vertical adjustment signal, which is stored in memory  40 . However, the projected image may have become misaligned horizontally. As such, the first predetermined position for the horizontal alignment is determined from the corrected vertical value (factory setting) and a worst-case condition horizontal offset value which may be applied to the vertical adjustment signal, to ensure that the test projection  160  is projected to the required side of the sensor  100  as illustrated in FIG. 6D.  
     [0074] Thereafter, the horizontal offset value is adjusted in accordance with the measurement signal to determine the alignment position of the projected image from a maximum value of the measurement signal. To this end, the test projection is projected closer to the sensor  100 , as illustrated in FIG. 6E and the output of the sensor  100  is measured again. This adjustment process continues until the test projection  160  has passed over the sensor  100  and the output of the sensor begins to reduce as illustrated in FIG. 6F.  
     [0075] Accordingly, the location of the maximum output from the sensor  100  is determined with respect to a horizontal offset value, the maximum output being indicative of an image aligned in the horizontal direction X. This horizontal offset value is stored and applied to the respective deflection coil, by the convergence driver  142 , via the amplifier  46 .  
     [0076] The first and second phases of the alignment process are then applied to determine the offset values for the remaining image components.  
     [0077] However, whilst it is clear that the technique described in general terms above may be used to determine the maximum output of the sensor, the accuracy to which the maximum may be determined can be improved. One such technique is to perform smaller adjustments to the predetermined position of the test projections  170 ,  180 . Another is to average the derived offset values over a predetermined period. Alternatively, a first set of measurements is taken using large adjustments and thereafter further measurements are taken using smaller steps in the region of the initially estimated maximum. Clearly, however, any combination of techniques may be adopted for a situation and the optimum technique determined based on factors such as what length of time is available to perform the adjustment and the availability processing resources.  
     [0078] Second Embodiment  
     [0079] A second embodiment of the present invention will now be explained with reference to FIGS. 7 and 8. For the second embodiment, the single sensor is replaced by a sensing device SD which comprises two sensors which are arranged to detect the same test projection. An example arrangement of the second embodiment of the present invention is shown in FIGS. 7A to  7 D, where parts also appearing in FIGS.  1  to  6  bear the same designated references.  
     [0080] As shown in FIG. 7A, two sensors  200 ,  210  are disposed on the display screen  10 . The sensors  200 ,  210  are arranged to be aligned in the vertical direction Y with the vertical distance between the two sensors  200 ,  210  being a predetermined amount. Preferably, but not exclusively, the sensors  200 ,  210  are located in proximity to one of the edges of the display screen  10 , and in proximity to each other, so that each sensor can receive light from the same test projection. Although in this embodiment the sensors  200 ,  210  are illustrated as being located at a substantially central point along the edge of the display screen  10 , it will be appreciated that the sensors  200 ,  210  may be positioned at any suitable location. Again the sensors  200 ,  210  generate a photovoltaic response at each of the wavelengths of the image components and have a narrow field of view. The sensors  200 ,  210  may therefore be implemented as photodiodes, photo-transistors or the like.  
     [0081] Generally the second embodiment operates in accordance with the alignment process already explained for the first embodiment. Therefore, only those parts of the alignment process which differ from the first embodiment will be explained. However, generally, the alignment process is arranged to detect an optimum alignment position from a null output of the sensing device SD corresponding to a situation in which the two sensors  200 ,  210  receive the same amount of light from the test projection  220 .  
     [0082] As shown in FIG. 7A, the test projection of the first phase of the alignment process is arranged to have a shape which corresponds with the shape of the test projection provided as an example for the first embodiment. The test signal applied to the video device  144 , from the convergence driver  142  therefore represents a test projection  220  which preferably has a smaller dimension in the vertical Y direction than in the horizontal X direction, to improve the likelihood that the test projection  220  will pass over the sensors  200 ,  210 , when detecting the optimum alignment position. The test projection  220  has a large dimension in the horizontal X direction such that the test projection  220  will intersect the sensors  200 ,  210  even if there are alignment errors present in the horizontal X direction. However, to ensure that each sensor receives some light from the test projection, when in an optimum alignment position, the test projection  220  is preferably arranged to have a size in the vertical Y direction which is larger than the predetermined vertical distance between the two sensors  200 ,  210 .  
     [0083] As with the first embodiment the test projection is projected at a first predetermined position and then at a plurality of other predetermined positions corresponding to offset values applied to the vertical adjustment signal. As before, the first predetermined position may be determined from the corrected vertical adjustment determined by the manual alignment process applied in the factory. However unlike the first embodiment the optimum position is determined not from the peak output from the sensor, but from a null output corresponding to a position of the test projection at which the sensors receive the same amount of light. Preferably however, the first predetermined position is arranged such that at least one of the sensors receives light from the test projection. To this end, the convergence processor determines the predetermined dimension in the X direction of the test projection  230  such that the test projection  230  will illuminate at least one of the sensors  200 ,  210 , based on a worst-case error from the factory correction of the vertical adjustment.  
     [0084] If the test projection is misaligned, as illustrated in FIG. 7A, one or other sensors will receive more light and the respective magnitude of the output from the two sensors  200 ,  210  will differ. Should the respective outputs of the two sensors  200 ,  210  be equal then this indicates that the test projection is vertically aligned, centred between the two sensors  200 ,  210 , as illustrated in FIG. 7B  
     [0085] In this embodiment, the output of the two sensors  200 ,  210  are received by a comparator  222 , which forms part of the pre-processor  140 . The comparator  222  subtracts the output of the two sensors  200 ,  210  to form a measurement signal which is illustrated by a response line  224  plotted graphically in FIG. 7C. In the situation illustrated in FIG. 7A, the output of the comparator  222  has a particular signed value. This is shown as a negative value in FIG. 7C. The measurement signal from the comparator therefore provides an indication of whether the test projection is one side of the optimum alignment position or the other side. Thus, in the present example, the sign of the value indicates, for the situation illustrated in FIG. 7A, that the test projection  220  should be repositioned closer to the sensor  200  by adjusting the offset value. Had the output of the comparator had the opposite sign then this would indicate that the test projection  220  should be repositioned closer to the sensor  210 . The sign of the measurement signal therefore provides an indication of the relative position of the test projection with respect to the sensors. The magnitude of the offset value applied can be determined in proportion to the magnitude of the measurement signal.  
     [0086] As before, once the vertical offset of the adjustment value (Vy) has been established in the first phase, the horizontal offset adjustment (Vx) is determined following a corresponding displacement of the test projection form a predetermined starting position. FIGS. 7D and 7E illustrate corresponding steps of the second phase of the alignment process for the horizontal adjustment. However the test projection used to find the horizontal adjustment is differently shaped. The second test projection is shaped and configured to provide a null measurement signal, at the output of the comparator, when light from the test projection is received equally by the two sensors. As shown in FIGS. 7D and 7E, the test projection in preferred embodiments is substantially ovoid, and dimensioned such that the dimension in the Y direction is larger than the distance between the two sensors  200 ,  210 , to ensure that the sensors receive light from the test projection. As before the offset determined for the horizontal alignment is applied to increase the likelihood that the test projection will be projected onto the two sensors.  
     [0087] As for the vertical offset adjustment in the first phase, if the test projection  230  is misaligned, one or other sensors receives more light and the respective magnitude of the output from the two sensors  200 ,  210  will differ, for example, as illustrated in FIG. 7D. Should the respective outputs of the two sensors  200 ,  210  be equal then this indicates that the test projection is horizontally aligned, centred between the two sensors  200 ,  210 , as illustrated in FIG. 7D, which corresponds to the zero point  226  of FIG. 7C.  
     [0088] In this embodiment, the output of the two sensors  200 ,  210  are received by the comparator  222 , in the pre-processor  140 . The comparator  222  subtracts the output of the two sensors  200 ,  210 . In the situation illustrated in FIG. 7D, the measurement signal produced from the comparator will have a particular signed value. The sign of the value will indicate, in this illustration, that the test projection  230  should be repositioned by applying an offset value to cause the test projection  230  to move in the direction opposite to the direction X. Had the output of the comparator had the opposite sign then this would indicate that the test projection  220  should be repositioned in the X direction. The magnitude of the output of the comparator is also used to determine the magnitude of offset value to be applied. As soon as the null value  226  from the comparator  222  is detected the offset adjustment value is considered as the value to apply to align the image component.  
     [0089] Accordingly, it is possible to determine the location of the null value with respect to a vertical offset value and a horizontal offset value. The vertical offset value and horizontal offset value are stored and applied to the vertical and horizontal deflection coils to align the image component. The process steps of the alignment process are repeated to determine the offset adjustment values for the remaining image components.  
     [0090] As will be understood, other arrangements can be used to detect the alignment of the test projection using two sensors. In another embodiment, the comparator may be an adder and the output from each of the sensors may be added to produce a composite measurement signal. The composite output signal will not however provide an indication of the relative position of the test projection with respect to the sensor. The output from the adder may be received by a further comparator, which compares the composite measurement signal with a predetermined threshold. This threshold may be derived given a desired degree of accuracy for alignment. Alternatively, the null value of the comparator may be set to zero. This allows for more accurate alignment of the offset values. In another embodiment, the pre-processor  140  may include filters to filter the outputs of the two sensors  200 ,  210 . The filters may be calibrated such that the two sensors  200 ,  210  output a substantially equal value when the test projections  220 ,  230  are aligned in the horizontal and vertical direction respectively.  
     [0091] In other embodiments of the invention the two sensors of the sensing device SD described above for the second embodiment of the invention are not aligned vertically, but instead aligned horizontally as shown in FIGS. 8A to  8 D, although otherwise the alignment process corresponds and so will not be repeated. However, the test projections  240 ,  250  whilst having the same overall shape, are arranged to be substantially rotated by 90 degrees with respect to the version of the test projections appearing in FIGS. 7A, 7B,  7 D and  7 E.  
     [0092] Third Embodiment  
     [0093] A third embodiment of the invention is illustrated in FIGS. 9A to  9 E. This embodiment has a sensing device SD′ having two sensors  200 ,  210  disposed on the display screen  10 . The sensors  200 ,  210  are arranged to be centred on diametrically opposite corners of a notional quadrangle having a predetermined vertical and horizontal dimension. The sides of the notional quadrangle are arranged to be substantially parallel to the respective edges of the display screen  10 . Preferably, but not exclusively, the sensors  200 ,  210  are located in proximity to one of the edges of the display screen  10 . However, each sensor  200 ,  210  may be located in proximity to a different edge of the display screen  10 . Although in this embodiment the sensors  200 ,  210  are illustrated as being located at a substantially central point along the edge of the display screen  10 , it will be appreciated that the sensors  200 ,  210  may be positioned at any suitable location.  
     [0094] The alignment process according to the third embodiment of the invention corresponds substantially to the alignment process described for the first and second embodiments, and so only the differences from the first and second embodiments will be explained. As with the second embodiment, the third embodiment of the invention is arranged to detect an optimum alignment position when the two sensors of the sensing device SD′ receive the same amount of light from the test projection. The alignment process according to the third embodiment is shown in FIGS. 9A, 9B,  9 C,  9 D and  9 E.  
     [0095] For the third embodiment, the test signal represents a test projection  260 , which preferably has a vertical dimension Y which is larger than the predetermined vertical distance between the two sensors  200 ,  210  such that the test projection  260  will intersect at least one of the sensors  200 ,  210  even though there may be alignment errors in the vertical Y direction. The test projection  260  preferably has a dimension in the horizontal direction X which is larger than the predetermined horizontal distance between the two sensors  200 ,  210  such that the test projection  260  will intersect at least one of the sensors  200 ,  210  even though there may be alignment errors in the horizontal X direction.  
     [0096] In contrast to the second embodiment, the offset values of both the vertical and horizontal adjustment values are adjusted to detect for the optimum position of the test projection, rather than determining the offset values separately. As before the test projection starts at a first predetermined position, in which at least one of the sensors receives light from the test projection. When the test projection  260  is misaligned, one or other sensors receive more light and the respective magnitude of the output from the two sensors  200 ,  210  will differ, as illustrated in FIG. 9A. Should the respective outputs of the two sensors  200 ,  210  be equal then this indicates that the test projection is aligned, centred on the notional line  270  which bisects a line which joins the centres of the two sensors  200 ,  210 , as illustrated in FIG. 9B. Thereafter, both the horizontal and vertical offset values may be adjusted by an amount and magnitude determined from the output signals from the sensors to determine offset values where the maximum magnitude occurs.  
     [0097] In this embodiment, the output of the two sensors  200 ,  210  are received by the comparator  222 ′ within the pre-processors  140 ′. The comparator firstly subtracts the output of one sensor from the other to determine whether the image is horizontally and vertically aligned, which will occur when the output of the comparator is zero. In the situation illustrated in FIG. 9A, the output of the comparator will have a particular signed value. The sign of the value will indicate, in this illustration, that the test projection  260  should be repositioned closer to the sensor  210  by applying a horizontal offset value. Had the output of the comparator had the opposite sign then this would indicate that the test projection  260  should be repositioned closer to the sensor  200 . The magnitude of the output of the comparator is used to determine proportionately the magnitude of the offset value to apply. The pre-processor  140 ′ also includes an adder which adds the output of the two sensors  200 ,  210  to form a composite output signal. This is illustrate in FIG. 9C, with the output from the comparator  222 ′ illustrated in FIG. 9D. The measurement signal is formed from the output of the comparator  222 ′ and the adder  262 . The optimum position is determined from the position of the test projection at which the comparator is substantially at zero  264  and the output from the adder is at a peak  266 .  
     [0098] The horizontal and vertical offset values are both adjusted such that the test projection  260  is moved along the line  260  and the output of the sensors  200 ,  210  measured again. This adjustment process continues until the test projection  260  has passed over the aligned position, as shown in FIG. 9C, and the output of the sensors begins to reduce.  
     [0099] Accordingly, the location of the maximum output from the sensors  200 ,  210 , is determined with respect to a vertical and horizontal offset value, the maximum output being indicative of an image aligned in the vertical direction Y and the horizontal direction X. These offset values are stored to be later applied to its respective component.  
     [0100] An alternative arrangement according to the third embodiment of the invention is shown in FIG. 10, where parts also appearing in FIGS. 5 and 9 have the same designated references. In FIG. 10, the display screen  10 , is shown with the two sensors forming the sensing device SD′ separated at opposite edges of the screen  10 . The test projection in the alternative arrangement is separated into first and second test projections  260 . 1 ,  260 . 2 . In the alternative arrangement of the third embodiment, the alignment process operates in the same way, however the two sensors are separated and illuminated by the two separate test projections  260 . 1 ,  260 . 2 . The two test projections  260 . 1 ,  260 . 2  are displaced with respect to each other in the vertical and horizontal directions by known amounts Ax, Ay. Hence functionally the alignment process is performed in the same way.  
     [0101] Although particular embodiments of the invention has been described herewith, it will be apparent that the invention is not limited thereto, and that many modifications and additions may be made within the scope of the invention. For example, various combinations of the features of the following dependent claims could be made with the features of the independent claims without departing from the scope of the present invention.