US2910532A - Arrangement for projecting televised images on to a large screen - Google Patents

Description

Oct. 27, 1959 2,910,532

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United States PatentV O ARRANGEMENT FOR PRQJECTING TELEVISED IMAGES N TO A LARGE SCREEN Michel Auphan, Neuilly-sur-Seine, France, assignor to Societe Generale dElectronique, Monaco-Condamine, a societe of Monaco Application June 14, 1954, Serial No. 436,603 Claims priority, application France June 1S, 1953 6 Claims. (Cl. 17S7.5)

The object of the invention is to provide an arrangement for projecting televised images on to a large screen.

This arrangement is of the kind in which the fluorescent screen of the normal type of cathode-ray tube is replaced by a metallic sheet, the function of which is to modulate locally, in association with a suitable optical system, the light emitted from an external source.

'This metallic sheet or membrane has a refiecting surface and it is in general flat or spherical in the position of rest. It is disposed at a very short distance from an insulating support and, when it is traversed by a pencil of light rays, it charges up the insulating support, especially bysecondary emission. 'Ihe sheet then becomes locally deformed under the effect of electrostatic forces. The optical system usually used is of the kind known under the name of the Toepler optical system which transforms this local deformation into a brilliant spot on the screen. In 'the text which follows, the structure which takes the modulating sheet which has received local deformations proportional Ito the brilliance of each area of the transmitted image, will be known as the mosaic The principle of a modulating sheet of this kind has been described in the addition of No. P.V. 49,65() of July 19, 1951 to French Patent No. 994,390, applied for on June 9, 1949 in the name of the same applicant.

The present invention is more particularly intended to provide improvements both in the mechanical and in the electrical field, to the said modulating sheet and in its conditions of operation.

In the attached drawings:

Fig. l shows a possible form of embodiment of the arrangement for projecting televised images on to a large screen, utilising an optical system provided with grids.

Figs, 2, 3, 3a, 4 and 5 show different improvements of the light-modulating system in the case in which the cathode-ray tube is of the positive-modulation type.

Fig. 6 shows an arrangement in accordance with the invention which is applicable to tubes of the negativemodulation type.

Modulation is known as positive for the purposes of the present description, when the maximum intensity of the cathode beam corresponds to the white portions of the image, and is called negative when this maximum intensity corresponds to the black portion.

Fig. 7 shows a television arrangement on a large screen With an optical system utilising a spherical modulating sheet which forms a concave mirror in its state of rest.

An example of a form of construction of the optical system is given in Fig. l, in which T indicates the cathoderay tube and F the modulating reflecting sheet. In this example, the sheet F is supposed to have a flat surface in the state of rest. A grid, made up of parallel bars G1 is suitably illuminated from a source S t lrough a condenser C. The arrangement of the lens O1 and the sheet F gives an image of this grid G1 in the plane of a second grid G2. The various component members are arranged in such a way that the images of G1 and G2 interlace exactly, that is to say the image of a full bar of the grid G1 is produced e .t 1C@ in the space separating two bars of G2 and vice-versa. The result of this is that no light passes through G2. If, on the other hand, the sheet is locally deformed, the light is deflected and passes between the bars of G2, thus giving on the projection screen E a brilliant trace corresponding to the deformed area. It will be noted that the grids G1 and G2 may be reduced to two bars: there are then two diaphragms having the form of slots. An arrangement of this kind is described in the addition referred to previously.

Emphasis must be placed on the fact that, in order for the clear parts of the televised image to give a lighted area on the screen, its orientation must be locally changed when the modulating sheet F is deformed, this change in orientation being defined by the fact that the normal moves in a plane which should not be parallel to the bars of the grid of the Toepler system. When the said plane of movement of the normal is perpendicular to the bars of the Toepler grid, the modulation is a maximum, since the image of G1 given in the plane of G2 by the small portion of surface considered, has been moved at right v angles to the bars.

If a small portion of the sheet F suffers simply a translation effect, the direction of its normal remains unchanged and the image of the portion considered does not cross the optical system described above. Only the contours of the corresponding area of the screen are illuminated. It follows that the areas of the modulating sheet which correspond to white parts of the screen must assume, under the effect of the electronic bombardment, an undulating form, the direction of these undulations not being prependicular to the direction of the bars of the grids. The system described in the addition referred to above supposed that the texture of the television image itself could introduce this undulation, the density of electronic bombardment being modulated at right angles to the scanning lines. But this modulation is only real if the said lines are clearly separate from each other, which is very difficult to obtain, especially in hiUh-definition Work. It may furthermore be observed that, even if the lines are effectively separated, the light contrast depends on the focussing of the tube, and this is a nuisance.

The methods and arrangements in conformity with the present invention enable the light-modulating sheet to assume an undulating form in all the light portions of the televised image, even if the scanning lines are not effectively separated.

(l) In the usual case in which the tube is positively modulated (Figs. 2 to 5), the electronic bombardment should cause undulations to appear on the modulating sheet. As has been explained in the addition referred to above, the sheet F is generally maintained at a constant distance from the insulating support I by a series of parallel strips or spacers which are called chevrons herein, because of their configuration and which are denoted by H.

In accordance with the invention, the desired undulation may be obtained by the various arrangements which follow hereafter, these arrangements all having a common feature comprising the utilisation of a network which influences the deformation of the sheet:

(a) In the case in which the modulating sheet F is flat, there is arranged, on the side of the electronic bombardment and at a short distance from the sheet, a at grid G composed of parallel Wires, in accordance with the arrangement shown in Fig. 2. The wires cast shadows during the electronic bombardment and the sheet is only deformed in the actual areas bombarded, that is to say it assumes an undulated form following the shadow of the wires of the grid. This grid is parallel to the grids of the optical system considered so as tol obtain maximum modulation, as has been explained above. The insulating support constitutes in this case the wall of the tube, and the light arrives from the side of the insulating support following the arrangement shown in Fig. 1. The chevrons which support the sheet are perpendicular to the bars of the grids as to avoid any troublesome diffraction effects.

(b) In accordance with the arrangement shown in Figs. 3 and 3a, the sheet F has alternate thick and thin portions; the bands having the same thickness are at right angles to the chevrons and parallel to the grids. Under the electronic bombardment, the deformation of the sheet varies in accordance with the thickness of the band in question and this causes the production of the desired undulations.

(c) In accordance with the arrangement shown in Fig. 4, the sheet F is of uniform thickness but the chevrons are crenellated. The sheet is much more free to assume deformation between the tops of each tooth, which are here known as supporting strips, than along the lines of these strips. Under the effect of the electronic bombardment, the sheet also assumes an undulating form. It will, of course, be understood that these teeth should not be visible from the side of the support; the material which constitutes the bottoms of these castellations should thus be opaque.

(d) In a further system (not shown), there is deposited (for example by evaporation under vacuum through a grid), a series of thin bands of a body upon the insulating support. These bands should be sufliciently thin for their optical influence to be negligible, but they should be thick enough for the secondary emission from the support to be modied in the areas at which it is covered by the strips. The chevrons, must of course, be perpendicular to the strips. By employing a suitable speed for the incident electrons, there is obtained a secondary emission greater than lover the strips and less than 1 between the strips, or vice-versa; the sheet will thus be attracted by the bands and repelled by the spaces between the bands or conversely, and it will thus take up'an undulating shape. It will also be clear that, quite apart from the method referred to, in which the strips are obtained by evaporation under vacuum, any means which enables the application to the support, of strips, the secondary emission of which is modied, will fall within the scope of this invention. In accordance with this arrangement, the chevrons are still perpendicular to the grids.

(e) In accordance with an alternative form of the preceding system, the insulating support is again provided with thin parallel strips of a substance, but they are no longer provided with the object of modifying locally its secondary emission.

The nature of the substance is such that it confers on the insulating support, at the areas covered by the deposit, a certain superficial conductivity. This conductivity should be sulicient for it to be able to discharge the insulating support in a time which is much less than the duration of an image.

It will be understood that, at the moment of the electronic impact, the whole sheet is also attracted towards the support, but that after a very short time, only those portions of the sheet which are not faced by strips remain attracted, and this accomplishes the desired undulation of the sheet during almost the whole ofthe duration of an image.

' (f) The arrangement in accordance with Fig. 5 differs from the preceding arrangements by the fact that the chevrons which support the sheet -are parallel and not in this case perpendicular torthe grids G1 and G2 of the Toepler system, but they are built up in a special Way so that they do not produce diffraction of the light in the absence of deformation of the sheet. In fact, these chevrons are composed of two substances 1 and 2, the substance 1 is transparent and the substance 2 is preferably metallic, that is to say opaque withpa reflecting surface. The index and the thickness of the substance 1 are so chosen that the optical thickness of the layer 1 is equal to the distance ofthe moving sheet from the support (the optical length is the product of the real length multiplied by the index). It follows that the light reiiected by the chevrons and by` the sheet has the same phase and that the whole arrangement behaves, fromthe optical point of View, as if the chevrons did not exist. When the sheet is displaced, the phases of the light rays reected by the sheet and by the chevrons are different; the light is dilracted and passes through the grid G2 as in the previous cases.

It is to be noted that, if monochromatic light only is used, the substance 1 is no longer useful, provided that the distance between the sheet S and the support I in the state of rest, is equal to a multiple ofrhalf a wave-length.

(Z) In the case in which it is preferred to give the modulating sheet an undulating form in the state of rest, the electronic bombardment should be such as to restore the sheet to its plane form. The modulation of the tube must therefore obviously be negative. Y

In accordance with the arrangement shown in Fig. 6, which is given by way of example without limitation, the chevrons may then be undulating and the sheet may adhere to the entire length of the chevrons; it is thus undulating in its state of rest (or it may have a castellated shape). The chevrons are perpendicular to the bars of the grids G1 and G2, as in the rst systems described, but the sheet, in its state of rest, corresponds to the maximum of white. When it is subjected to electronic bombardment, it adheres to the insulating support and, in consequence, becomes flat everywhere it is visible, that is to say between the chevrons. The areas which are thus stuck together will therefore correspond to the black part of the image. It is easy to make -up a mosaic of this kind if reference is made to French Patent No. P.V. 648,412 of May 19 1953 made in the name of the present applicant. It is sufficient to proceed as in the case of the alternative form intended to produce supporting strips hidden by opaque bands, but to suppress the second grid during the evaporation of the substance having a powerful chemical activity (substance 7); the sheet will then adhere along the entire length of the grooved chevron.

It will be Well understood that in all the systems which have been described above, except the irst, the mosaic does not need to have a plane surface; it may, in particular, be spherical or it may represent an optically-defined surface. One application, in accordance with the invention, of spherical modulating sheets consists in the elimination of the lens O1 of the optical equipment. The mosaic, which is then a concave mirror, gives directly an image of the grid G1 on the grid G2 (see Fig. 7).

In all the systems described above, no precise information has been given on how the deformations of the sheet were suppressed between each televised image. This result is generally obtained by making the insulating support slightly conductive. This conductivity must be just enough to permitrthe dissipation through the chevrons (which in this case act as conductors) of the charges induced by secondary emission on the insulating support. This dissipation of the charges must be completed in a time equal at most to the duration of a television image (that is to say 25th of a second in the case of the standard used in France). In order to obtain a support which is slightly conductive, either a material can be used whichris conducting inthe mass, or a deposit may be made (in general by evaporation under vacuum) of a conducting substance on to the insulating support.

The alternative form of embodiment described under (1)(e) is especially easy to carry into effect in the case in which the above-mentioned means of suppressing `the deformations is employed, since it is much easier to increase the surface conductivity of a support which'is already a conductor than to make a support which is whollyV Si insulating act as a conductor when, as is the case, the thickness which can be employed for the conducting strips is limited for optical reasons.

In accordance with a further feature of the invention, the presence of an electric held in the vicinity of the sheet facilitates its deformation under the effect of the electronic bombardment. As far as the direction of this electric field is concerned, several cases may be considered:

(1) If it is desired to charge the support negatively, the electric field must prevent all secondary electrons from leaving the support and passing through the moving sheet; in this case, the sheet must be charged to a potential higher than that of the metallisation of the tube.

(2) lf it is desired to charge the insulating support positively, the secondary electrons must, on the other hand, be attracted far from the mosaic. The moving sheet is then charged to a potential less than that of the metallisation of the tube.

With certain insulating supports, the introduction of such an electric field is absolutely necessary in 'obtaining a deformation of the sheet` it has even been found that, the' sheet having been deformed, if its potential is brought back to a value equal to that of the metallisation of the tube, whilst the electronic bombardment is maintained, the sheet resumes its initial state.

In accordance with the present invention, this property may be applied to a new method of wiping out the mosaic, which avoids the use of a slightly conductive support.

in this method, the mosaic is obliterated during the return of each line. During these returns, the potential of the sheet is brought hack to a kvalue equal to that of the metallisation of the tube. O n the other hand, the cathode-ray is not cut off during the return-lines but is brought back to a state of constant current; it is, furthermore, slightly deflected towards the base of the image. lt will thus be understood that'it will obliterate, in the areas which are to be restored, the line or the few lines which follow.

With this wiping-out system, a positive modulation may be used in the case of the mosaics described with reference to Fig. 6. In that case, it is during the course of the line itself that the potential of the metallisation is the same as that of the sheet and during the return-line, the potential of the rnetallisationl is less than that of the sheet, so that during each return-line the sheet re-adheres to the support over the bombarded areas.

On the other hand, in order to increase the electric -elds in the vicinity of the mosaic, there may be an advantage in suppressing the space-charges which could be produced in front of it. In accordance with the invention, a new improvement in the arrangements suggested consist in placing a grid in front of the mosaic, arranged as shown in Fig. 1. In this case, however, the object sought for is different, and there is no need for the grid to be stretched so close to the mosaic. On the other hand, its wires will then be as line as possible so as to intercept as little as possible of the electron ow. This grid may also be replaced by a metallic membrane stretched over a frame, on the condition that it is line enough not to interfere with the passage of the electrons. In both cases, the electric field is applied by establishing a difference of potential between the moving sheet F and the grid or the membrane. The grid (or the membrane) does not require to be very close to the mosaic and can thus be used with a non-plane mosaic, for example a spherical mosaic.

What l claim is:

l. In apparatus for the projection of television images utilizing an optical system and a cathode ray tube, said tube having at least a source of electrons adjacent one end thereof and a modulating element adjacent the other end thereof, said modulating element including an insulating support and a thin reflecting metallic sheet held at a short distance from said support, means for sweeping said sheet with a beam of electrons from said source, said sheet being locally deformable in response to the impingement thereon of said beam of electrons, and means interfaced between said sheet and said source of electrons to limit the impingement of said beam to predetermined areas of said sheet.

2. ln a cathode ray tube for the projection of televised images on a large screen, a modulating element disposed in the position normally occupied by the fluorescent screen in conventional tubes, said modulating element being locally deformable by an electron beam within said tube and an optical system cooperating therewith for locally modulating the light emitted by an auxiliary luminous source, the structure of said modulating element comprising an insulating support, a metallic reflecting sheet held at a short distance from said support, suitable means for supporting said sheet such that normally plane areas become rippled and normally rippled areas become plane upon impingernent thereon of said electronic beam.

3. A modulating element as described in claim 2 including holding bands connecting said reflecting metallic sheet to said insulating support.

4. A modulating element as defined in claim 2 and incorporated in an optical system of the Toepler type comprising a grid disposed on the side of said insulating support opposite to said metallic reccting screen, said grid being parallel to said screen and having its bars parallel to those of the lgrids of said optical system.

5. A structure of a modulating element as dened in claim 2 and incorporated in au optical system of the Toepler type comprising a grid disposed on the opposite side of said metallic reflecting sheet from said insulating support, said grid being parallel to said sheet and having its bars parallel to those of said grids of said optical system, and parallel holding bands connecting said sheet to said insulating support and being perpendicular to said bars of said grids.

6. A structure for a modulating element as defined in claim 2 and incorporated in an optical system of the Toepler type comprising a grid disposed on the opposite side of said metallic reflecting sheet from said insulating support, said grid being parallel to said sheet and having its bars formed on the metallic sheet itself by thickened areas in the form of parallel bands of equal thickness parallel to the bars of said grids of said optical system.

References Cited in the le of this patent UNITED STATES PATENTS 2,510,846 Wikkenhauser June 6, 1950 FOREIGN PATENTS 678,307 Great Britain Sept. 3, 1952 OTHER REFERENCES Ser. No. 354,771, Paehr et al. (A.P.C.), published May 18, 1943.

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