GB2237443A - Light modulating device - Google Patents

Abstract

A light modulating device for use in a projection optical system comprises a visco-elastic light modulating layer (66) disposed over a unidirectionally conductive faceplate (28) of a cathode ray tube for applying an electrical pattern by way of one set of electrodes to the layer in order to form elemental phase diffraction gratings, and a further set of electrodes is provided which are capacitively coupled with the first set of electrodes for supplying electrical signals externally of the tube for controlling the electrical pattern applied by the tube.

Description

LIGHT MODULATING DEVICES This invention relates to light modulating devices of the type which employ an electrically actuable light modulating layer.

Examples of such devices are described in US 3882271, US 4529620, US 4626920, US 4639788 and US 4641193. In these devices, the light modulating layer is of a visco-elastic material. One surface of the layer is reflective, and the other surface has a semiconductor array applied to it which is operable to apply a pattern of electrical charge to the layer in accordance with an input video signal. The layer responds by forming a pattern of depressions or ripples in the reflective surface in accordance with the applied charge pattern, which can then be projected using a Schlieren optical system, for example, with the ripples acting like a phase diffraction grating so as to form a projected image.

Disadvantages of this arrangement are that the semiconductor array is large and is therefore complicated and expensive to manufacture, and, furthermore, in view of the very large number of transistors which are employed, yield can be expected to be low and therefore unsatisfactory.

Another example of such a device is described in H. R.

Luxenberg et al, "Display Systems Engineering" McGraw-Hill, pages 329 to 332. In this device, the light modulating layer is of oil, and the charge pattern is formed by a cathode ray tube having a faceplate provided with an array of through-conductors. Electrons are supplied to the through-conductors by a modulated scanning electron beam from a conventional electron gun and also by flood guns, and electrons are emitted from the through conductors by secondary emission. It is suggested in this publication that the through-conductors can be charged to one or two stable potentials, and that switching between these two potentials can be achieved by changing the energy of the flood gun or the writing gun.Disadvantages of this device are that flood guns need to be employed, and that a binary image is produced, that is to say an image in which pixels have either of two levels, without any grey scale between the two levels. This form of image is therefore not satisfactory for, for example, television type images.

One form of light modulating device provided by the invention uses a cathode ray tube and a light modulating layer, and means are additionally provided for supplying electrical signals to the layer externally of the tube in order to control the electrical pattern applied by the tube. This therefore obviates the need for a flood gun as used in the known arrangement described above.

In one specific arrangement of the invention, the control signal has a first portion to enable pixels of a group to be cleared to a first state and a second portion to enable selected pixels to be changed to a state between the first state and the second state by the cathode ray tube. Thus gradation of the pattern can be provided. In one version of the invention, all of the pixels are changed to the second state and then pixels are selectively changed back towards the first state by the tube.

In a preferred form of the invention, each pixel is defined by first and second elements which have complementary electrical signals applied thereto, and preferably the polarities of the signals are alternated for successive cycles of writing to the light modulating layer. This has the advantage of preventing a permanent set being formed in the light modulating layer.

In a preferred form of the invention, a set of electrodes is provided for applying electrical signals to the light modulating layer, with each electrode being directly coupled to the faceplate and being capacitively coupled to a source of the external signals. The capacitive coupling may be provided by a dielectric layer and a further set of electrodes between the first-mentioned set of electrodes and the faceplate. This provides a simple construction.

Specific embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a projection video system which utilises the present invention; Figure 2 is a partial cross sectional view of the faceplate of the cathode ray tube shown in figure 1 taken along the section lines 2-2 shown in figures 3A to Figures 3A to 3H are partial sectional views taken along the planes marked A to H in figure 2; Figure 4 is an electrical circuit diagram used to explain the electrical operation of each pixel on the faceplate; Figure 5 illustrates a secondary emission characteristic; Figure 6 is a view showing the arrangement of electrical pads on the faceplate; Figure 7 is a series of diagrams illustrating the waveforms arising in the circuit shown in figure 4;; Figure 8 is partial sectional view through the light modulating layer to illustrate how the elemental phase diffraction grating is produced; and Figures 9A and 9B are similar to figures 3E and 3G illustrating a modified arrangement of electrodes.

Referring to figure 1, a cathode ray tube 10 has an electron gun 12 and a power supply 14 which supplies drive voltage to the electron gun 12 and a grounded collector terminal 16. A signal processing unit 18 receives a video signal on line 20 and supplies a modulating signal to the electron gun 12 on line 22 and deflection signals to vertical and horizontal deflection coils, one of which is shown at 24. The signal processing unit also supplies signals on a bus 26 to the faceplate 28 of the cathode ray tube 10, for a purpose to be described below.

Referring to figures 2 and 3C, the faceplate 28 is formed by a matrix of mutually insulated, electrically conductive pins 30 which are bonded together with a glass filler 32. The pins 30 extend through the thickness of the face plate 28. For further description of the manufacture of such a face plate, reference is directed to J James Stone "The Videograph Electrostatic Printing Process", IEEE Transactions on Electron Devices, Vol. Ed-19, No 4 April 1972, and to A R Hildebrand et al "Microconductors in Glass", Electrical Design News July 1963.

Referring to figures 2 and 3B, on the inner surface 34 of the faceplate 28, there is disposed a layer 40 of electrically insulating material having an array of apertures 41 therein. Referring to figures 2, 3A and 3B, on the insulating layer, there is formed a matrix of pairs of generally rectangular, electrically conductive pads 36, 38 which are in register with respective apertures 41 and each of which has a portion 36', 38' which extends through the respective aperture into contact with at least one of the pins 30 of the faceplate 28. Referring to figures 2 and 3A, the pads 36, 38 are separated by a rectangular network 42 of electrically conductive material disposed on the insulating layer 40. The network 42 is grounded by terminal 16 and acts as a collector for electrons which are secondarily emitted by the pads 36, 38. The patterns of conductors are formed by well-known methods, such as vapour deposition and photo-etching.

It will be appreciated from the above that each of the pads 36, 38 is in electrical contact with some of the pins 30, and therefore the voltage on each of the pads 36, 38 is carried across to the outer surface 35 of the faceplate 28.

Referring to figures 2 and 3D, the outer surface 40 of the face plate 28 is formed with an insulating layer 42 and an array of conductors 44, 46, each of which extends through the insulating layer 42 and through a further layer 48. Each of the conductors 44, 46 makes electrical contact with at least one of the pins 30 (which at its other end contacts a respective pad 36, 38).

Referring now to figures 2 and 3E, sets of electrodes 50, 52 are formed on the insulating layer 42. A set of electrodes 50 and a set of electrodes 52 are provided for each row of pads 36, 38 extending across the width of the face plate 28. The electrode set 50 comprises a common electrode portion 54 extending across the width of the faceplate, and for each pair of pads 36, 38 an L-shaped portion 56 connected to the common portion 54. Similarly, the electrode set 52 includes a common portion 58 extending across the width of the faceplate 28, and an L-shaped portion 60 for each pair of pads 36, 38. As shown in figure 3E, the L-shaped portions 56, 60 and the common portions 54, 58 are interleaved.

Referring to figures 2 and 3F, the layer 48 is of a dielectric material, and the conductors 44, 46 extend through the thickness of the dielectric material.

Referring now to figures 2 and 3G, there are disposed on the outer surface of the dielectric material 48 pairs of electrodes 62, 64 which are electrically connected to the conductors 44, 46, respectively. The electrodes 62, 64 are U-shaped and such that each overlies the L-shaped portion 56, 60 and part of the common portion 54, 58 of respective parts of the electrode sets shown in figure 3E. Thus, these electrodes form capacitors.

Referring to figures 2 and 3H, the dielectric layer 48 and electrodes 62, 64 are covered by a layer of visco-elastic material 66 of the type described in the above mentioned US patents, and the visco-elastic material 66 is covered by a layer 68 of silver which is optically reflective, and which is electrically connected to ground or to a bias voltage.

Referring to figure 4, there is shown an electrical equivalent of the arrangement described above. The common portion 54 of the electrode set 50 can be supplied externally with a voltage of Vla, which is supplied to one side of a capacitor Ca formed on the one hand by the electrode portions 54, 56, on the other hand by the electrode 62, and by the dielectric layer 48. The electrode 62 is electrically connected to the pad 36 to which electrons Ei can be added by the electron beam from the electron gun and from which electrpns Es can be emitted by secondary emission, thus affecting the voltage V2a. The voltage V2a is applied to the visco-elastic material 66 to cause it to deform.Similarly, a voltage Vlb can be applied externally to the common electrode portion 58, and a voltage V2b arises on the pad 38 and electrode 64 and is applied also to the visco-elastic material 66.

Referring to figure 5, there is shown a graph illustrating the ratio of the rate of emission Es of secondary electrons to the rate of reception Ei of incident electrons from the electron beam as a function of the energy E of the electron beam. In operation of the apparatus, the electron beam is set to have an energy in the range 70 between upper and lower values 72, 74, so that the secondary emission ratio Es/Ei is greater than unity, and the beam current is modulated by the input video signal between zero and a maximum value.

Figure 6 shows the arrangement of the pairs of pads 36, 38 and the system which will be used in this specification to designate pairs of the pads. The pairs of pads 36, 38 are arranged as n columns and m rows, with the pads 36, 38 of each pair being arranged in the row direction. As an example, the number n of columns may be about 1500, and the number m of rows may be about 1100. The pairs of pads 36, 38 of the top row are numbered from left to right (1, 1) to (1, n) and the pairs of pads 36, 38 of the left hand column are numbered from top to bottom (1, 1) to (m, 1). The rows are arranged in the line scanning direction of the electron beam, and the columns are arranged in the field scanning direction of the electron beam. The column electrode sets 50, 52 for each row are designated 50(1) 52(1), 50(2), 52(2), ..., 50(m), 52(m).These electrodes are connected to the bus 26 from the signal processing unit 18 (figure 1).

Referring to figure 7, there is shown an example of voltages which arise for the first row. The voltages change during four distinct time periods: period T1 during which the elements on the line are sequentially erased; period T2 when the elements on the line are simultaneously set; period T3 when the elements on the line are sequentially written to; and period T4 when the voltages on the elements are held.

At the beginning of the sequential erasure period T1, the voltages Vla(l) and Vlb(l) applied to the common electrode sets 50(1) and 52(1), respectively, are set to zero volts, having previously been at opposite voltages of, for example, +25V and -25V. Due to the capacitive coupling of the capacitors Ca and Cb, the voltages V2a and V2b of all of the pads 36, 38 on the line are either decreased or increased by an amount approaching 25V from initial values which may have been between +25V as shown by waveform V2a(l,i), and -25V as shown by V2b(l,i). The electron beam is then used to scan the row with maximum beam current and firstly it strikes pad 36 at location (1,1).As shown by V2a(l,1) this voltage is initially -20V, and therefore the secondarily emitted electrons transfer to the collector 42 at higher voltage (OV) and thus there is a net loss of electrons from the pad 36 (1,1) and its voltage accordingly rises to a stable value of OV. The electron beam next strikes pad 38 at position (1,1) where the voltage is initially +20V, as shown by waveform V2b(1,1). Accordingly, the secondarily emitted electrons do not transfer to the collector 42, because it is at lower potential (OV), and thus the secondarily emitted electrons are attracted back to pad 38(1,1) and there is a net gain of electrons, resulting in the voltage of pad 38 (1,1) reducing to the stable zero value. The electron beam then strikes pads 36, and then 38 at position (1, 2), and so on.

As shown at 76 and 78 in waveforms V2a(1,i) and V2b(l,i) if the initial value of the voltage on the pad 36 or 38 is zero, then it remains zero. Once the electron beam has finished scanning all of the positions in the first row, the voltages V2a and V2b of all of those positions are stabilised to zero.

Once the voltages have been stabilised they are set in the period T2. In order to do this, the voltage Vla(l) on the common electrode 50(1) is decreased to -25V, and the voltage of Vlb(l) on the common electrode 52(1) is increased to +25V. Due to the capacitive effect of capacitors Ca and Cb, the voltages of all of the pads 36 of the first row are decreased to approximately -25V, as shown by waveforms V2a(1,1), V2a(1,i) and V2a(1,n). Conversely, the voltages of all of the pads 38 of the first row are increased to +25V, as shown by waveforms V2b(1,l), V2b(1,i) and V2b(l,n). Thus, the pads 36, 38 of the first row are set to opposite voltages.

Then, the sequential writing period T3 begins. During this period, the electron beam is scanned along the first row from position (1,1) to (1,n) and the beam current is modulated between zero and the maximum current, in accordance with the input video signal, in order to change the voltages on the pads. For example, as illustrated in figure 7, the beam is at the maximum beam current for location (1,1). Accordingly, when the electron beam strikes pad 36 at location (1,1), because the initial voltage is -25V, the secondarily emitted electrons are collected by the collector 42 and so there is a net loss of electrons, which is sufficient to result in the voltage of pad 36 at location (1,1) increasing substantially to OV.When the electron beam next strikes pad 38 at location (1,1), due to the initial voltage of +25V, the secondarily emitted electrons return to the pad 38 resulting in a net gain of electrons which is sufficient to decrease the voltage substantially to OV. The electron beam then scans the remaining positions in the line with the beam current being modulated between the maximum and zero levels. As shown by waveforms V2a(1,i) and V2b(1,i), the beam current here is at zero, and therefore the voltages are not changed but remain at -25V and +25V respectively. By way of further example, as shown in wavefrms V2a(1,n) and V2b(1, n), the beam current here is at an intermediate level which results in the voltages changing from -25V to -12V and from +25V to +12V, respectively.It will therefore be appreciated that by virtue of the sequential erasure steps, the simultaneous setting step and the sequential writing steps, the voltages of alternate pads can be set to values between OV and +25V selectively, and the voltages of the remaining pads can be set to values between -25V and OV, selectively.

After the sequential writing period T3, the hold period T4 takes place. In this period, the voltages on the pads 36, 38 are not changed. It should be noted that the duration of the hold period T4 is many times greater, for example 500 times greater, than the erasure, setting and writing periods.

Once the erasure, setting and writing steps have been carried .out for the first row, they are then carried out sequentially for the second to m-th rows, whilst the first row is being held. The entire process is then repeated. From the above, it will be appreciated that a voltage pattern can be applied to the electrodes 62, 64 over the entire surface of the face plate.

From the following description, it will be appreciated that, if a voltage V2a or V2b which is applied to the light modulating layer is always of the same polarity, then there is a risk that a permanent set will be introduced into the light modulating layer which will decrease the efficiency of the device. In order to avoid this problem, the polarity of the signal Vla for each successive field of writing is alternated, as shown in figure 7, and similarly the polarity of the signal Vlb is also alternated and is of the opposite polarity at only one time to the signal Vla. Thus, the signals V2a and V2b also alternate in polarity for successive fields of writing to the light modulating layer.

In the arrangement described above, it is necessary for the electron beam to scan each line once with a constant beam current to perform the erasure step and then to scan the same line with beam modulation to perform the writing step. This therefore requires the electron beam to be indexed down the face plate from one row to the next after each pair of scans. As an alternative to this arrangement, the beam may be scanned in a different manner so that the writing step for one row does not immediately follow the erasure step. For example, the beam could be indexed and controlled so as to perform the following sequence: erase row 6, write row 1, erase row 7, write row 2, erase row 8, write row 3, ..., erase row m, write row (m-5), with the additional necessary preliminary steps for erasing the first five rows and final steps for writing to the last five rows.As a further alternative, rather than setting rows one at a time, the common row electrodes 50, 52 may be arranged in groups, so that, for example thirty rows are set simultaneously.

With the above arrangements, it will be appreciated that the electron beam must scan each row twice, that is once for erasing and once for writing. As an alternative, a dual-beam cathode ray tube may be employed so that for each scan in the row direction one beam erases a row, whilst the other beam writes to a previously erased row. As a further alternative it is possible to store a previous field of image information in a frame store, and then, instead of writing to the faceplate the current field of image information, the difference between each pixel setting for the current field and for the stored previous field is written to the faceplate. This obviates the need for erasure step described above.As a further alternative, rather than suppressing the electron beam during inter-line flyback, the beam may be used during flyback with increased beam current to erase the pixels on the next line.

In the above arrangement, it will be appreciated that it is necessary to ensure that the electron beam or beams correctly scan the rows and do not deviate during a row scan from one row to the next or scan between two rows. In order to do this, beam-indexing techniques which have been used in known beam-index cathode ray tubes may be employed.

Referring now to figure 8, the effect of the voltages written to the electrodes 62, 64 is illustrated. In the case where the voltages are substantially zero, as shown in the left-hand part of figure 8, there is no substantial effect upon the visco-elastic layer 66, and therefore incident light upon the silver layer 68 is specularly reflected. However, in the case where the voltages on the electrodes 62, 64 are +25V and -25V respectively, as shown in the right-hand portion of figure 8, the visco-elastic layer 66 is caused to deform, so that the silver layer 68 provides a localised, generally sinusoidal, phase grating, and accordingly incident light is diffracted.

Furthermore, in the case where the voltages on the electrodes are at intermediate levels, for example +12V and -12V as shown in the middle portion of figure 8, an amount of light is diffracted, and thus gradation of the amount of diffracted light is provided.

Referring back to figure 1, a Schlieren optical system includes a light source 80, which projects light onto a series of Schlieren bars 82 which act to reflect the incident light through a Schlieren lens 84 to the faceplate 28. Light which is specularly reflected by the faceplate 28 returns to the Schlieren bars 82 and is then reflected back in the direction towards the light source 80. However, any +lust and -1st order diffracted light upon return through the Schlieren lens 84 passes through the series of Schlieren mirrors 82 and is projected by a projection lens 86 onto a screen 88. Accordingly, the pattern of the portions of diffraction grating produced by the face plate 28 result in a corresponding image pattern on the projection screen 88.

The arrangement described above produces a gradational monochrome image on the projection screen 88.

However, it will be appreciated that three arrangements as described above can be used with red, green. and blue light sources to produce a combined colour image on a single projection screen 88.

In the arrangement described above, the capacitors have U-shaped electrodes, as illustrated in figures 3E and 3G. It will be apparent that many other shapes of electrodes are possible. For example, second L-shaped electrode portions may adjoin the L-shaped portions 56, 60 in figure 3E so as to form W-shaped capacitor electrodes, and a corresponding modification may be made to the electrode.

In the above described arrangements, the diffraction caused by the visco-elastic layer is in the direction D as shown in figures 1 and 3G. Thus, it is necessary to arrange the Schlieren bars in the manner as shown in figure 1 and extending in the direction S as shown in figure 3G. In an alternative arrangement, the direction of diffraction can be arranged to be at right angles to that described above, and accordingly the Schlieren bars 82 would also be arranged at right angles to the direction described above. One form of electrodes to provide this feature is shown in figures 9A and 9B, which show modifications to the arrangement shown in figures 3E and 3G respectively.The common electrode 54 has a pair of portions 98 extending at right angles thereto, and similarly, the common electrode 58 has a pair of electrode portions 100 at right i angles thereto and interleaved with the electrodes 98. As shown in figure 9B, the modified electrodes 102, 104 are U-shaped and interleaved, and are connected to the conductors 46, 44, respectively. This arrangement causes diffraction in the direction D shown in figure 10B, and therefore the Schlieren bars 82 are arranged in the direction S as shown in figure 9B.

In the above arrangement a Schlieren lens 84 is used as part of the optical system. However, the need for this lens may be obviated by forming the faceplate of the cathode ray tube and the reflective surface of the light modulating layer so as to be concave and part-spherical.

Although the device described above utilises a visco-elastic light modulating layer as described in the above-mentioned US patents, other light modulating means may be employed, such as the oil film described above which is also used to form elemental diffraction gratings, or a liquid crystal layer which is used to modulate the polarisation of the incident light.

Claims (19)

1. A light modulating device comprising an electrically actuable light modulating layer disposed over a unidirectionally conductive faceplate of a cathode ray tube for applying an electrical pattern to the layer, and means for supplying an electrical signal to the layer externally of the tube for controlling the application of the electrical pattern applied to the layer by the tube.

2. A device as claimed in claim 1, wherein the pattern is arranged as an array of pixels, the external electrical signals being supplied to a group of the pixels at a time.

3. A device as claimed in claim 2, wherein an electron beam formed by the cathode ray tube is operable selectively to change the voltages of elements associated with the pixels towards a datum voltage, and wherein the external electrical signal is operable to change the voltage levels of all the pixels in a group.

4. A device as claimed in claim 2 or 3, wherein each group of pixels comprises at least one line of the pixels...

5. A device as claimed in any of claims 2 to 4, wherein the external electrical signals are supplied sequentially and cyclically to the groups of pixels.

6. A device as claimed in any of claims 2 to 5, wherein the external electrical signal has a first portion to enable the group of pixels all to be cleared to a first state and a second portion to enable selected pixels of the group to be changed to a state between the first state and a second state.

7. A device as claimed in claim 6, wherein the second portion of the external electrical signal comprises one portion to cause the group of pixels all to be set to the second state and another portion to enable pixels selectively to be changed towards the first state by the tube.

8. A device as claimed in any of claims 2 to 7, wherein each pixel is defined by first and second elements to which complementary electrical signals are applied.

9. A device as claimed in claim 8 when dependent on claim 5, wherein the polarities of the external electrical signals for successive cycles alternate.

10. A device as claimed in any preceding claim, including a set of electrodes for applying electrical signals to the layer, each electrode being directly coupled to the faceplate and being capacitively coupled to a source of the electrical signals.

11. A device as claimed in claim 10, wherein a dielectric layer extends over the faceplate between the faceplate and the light modulating layer, the electrodes being formed on the surface of the dielectric layer which is nearer to the light modulating layer, and a second set of electrodes being formed on the surface of the dielectric layer which is nearer the faceplate to form the capacitive coupling.

12. A device as claimed in claim 11, wherein the first-mentioned set of electrodes are coupled to the faceplate by conductors which extend through the dielectric layer to the faceplate.

13. A device as claimed in any preceding claim, wherein the electrical pattern is applied to the light modulating layer so as to form elemental phase diffraction gratings in an external surface of the light modulating layer.

14. A device as claimed in any preceding claim, in combination with a Schlieren projection optical system.

15. A light modulating device substantially as described with reference to the drawings.

16. A method of writing an electrical pattern to an array of electrodes on an inner surface of an electrically conductive faceplate of a cathode ray tube, the faceplate having a collector electrode adjacent the electrodes of the array and having an electrically actuable light modulating layer on an outer surface of the faceplate, the method including the steps of using an external signal to cause a potential difference between a group of the electrodes of the array and the collector electrode, and then scanning said group of electrodes with a modulated electron beam so that the secondary emission ratio at those electrodes to which the beam is applied is greater than unity and so that the potential difference between those electrodes to which the beam is applied and the collector electrode reduces.

17. A method as claimed in claim 16, further comprising the preliminary step of scanning said group of electrodes with an unmodulated electron beam so that the potential difference between all of the electrodes of the group and the collector electrode reduces substantially to zero.

18. A method as claimed in claim 16 or 17, wherein said steps are repeated for other groups of the electrodes.

19. A method substantially as described with reference to the drawings.