US2297443A - Television image projection system - Google Patents

Description

Sept. 29, 1942. M. VON ARDENNE 2,297,443

TELEVISION IMAGE PROJECTION SYSTEM Filed March 18, 1940 2 Sheets-Sheet 1 MTTOBNEK Sept. 29, 1942. M. VON ARDENNE 2,297,443

TELEVISION IMAGE PROJECTION SYSTEM Filed March 18, 1940 2 Sheets-Sheet 2 Patented Sept. 29, 1942 UNITED STATES PATENT OFFICE TELEVISION DWAGE PROJECTION SYSTEM Manfred von Ardenne, Berlin-Lichterfelde, Germany; vested in the Alien Property Custodian Application March 18, 1940, Serial No. 324,539

In Germany November 29, 1938 2 Claims.

recting the light as modulated by the target upon a viewing screen.

The arrangements hitherto disclosed are designed, however, in the first place in accordancewith electrical and electro-optical view points, while the view points concerning the light eifects have been taken into consideration to the extent only in which they were reconcilable with the other requirements laid down. Now, within the scope of the present invention a number of proposals for the construction of the projection-optical system shall be made which have the common characteristic that the useful light will be an especially high percentage of the initial light radiated into space by the light source.

The purposes of the present invention may best be understood from the following specification and claims, particularly when considered with the drawings wherein like reference characters represent like parts and wherein:

Figures 1, 2 and 3 show various forms of the present invention;

Figure 3a shows in detail a portion of the target electrode shown in Figure 3;

Figures 4, 5 and 6 show further forms of the present invention; and,

Figures 6a and 6b show in detail alternate forms of an element shown in Figure 6.

In all of the forms shown and described herein the desirable degree of optical efficiency is brought about by the introduction of an optical concentrating means or lens at the place of the relay or crystal screen, or in direct proximity thereof. Thus the construction of storage receiver tubes is rendered similar to the mode of construction of the customary diapositive projectors, or movie projectors. Although such mode of construction appears obvious, considerable inventive ability and special optical auxiliary means were necessaryas will later be appreciated--in order to bring about a favorable phototechnical degree of efliciency in view of the available space and the various forms of construction of the storage tubes.

The simplest way of achieving the purpose of this invention exists in the arrangement shown in Figure 1 wherein a transparent crystal screen is used. In thi arrangement, by positioning the relay or crystal screen 26 near the rear side of the tube 22 it was possible to recreate efi'ectively the optical path of a projection apparatus equipped with a condenser lens. BY means of the concave reflector I2 shown in back of the source of light It], a large part of the light from the source which would be radiated towards the rear may be utilized. The concentration of the light upon the main projection lens 20 is obtained by the condenser lens l4 separated only by the polarization filter Hi from the rear of the tube 22. The second (crossed) polarization filter i8 is situated between the projection lens 20 and the frontal wall of the tube 22. In view of the desirability of obtaining a high quality of optical reproduction, an optically plane window 24 is cemented or otherwise attached in a vacuum tight fashion to the front wall of the tube 22. This window 24 must be in an entirely tension-free state as is the case of the rear wall of the tube so that a complete darkening can be attained when desired.

In the form shown in Figure 1, which requires a transparent and conducting layer at the crystal screen, the said layer instead of being made of zinc oxide as suggested in one or more of the above mentioned applications, may be produced from a special type glass having high conduction property which recently appeared on the market.

In projection tubes with doubled utilization of the electro-optical efiect through refiexion of the light in the relay or crystal screenunit, the introduction of light concentrating means in the proximity of the relay screen involves much greater difiiculties than in the case of the arrangement operating with a transparent crystal area. The optical systems designed for operation with reflection type screens are shown in Figures 2 through 6 and utilize throughout special methods and auxiliary means which will be taken up respectively in conjunction with the arrangement shown. It is particularly the solutions of the problems involved with respect to reflection type tubes which are of greatest importance since the means required at the electrical end are much less complicated than the means required for the operation with the transparent type screen.

In the form shown in Figure 2 the concentration of the light from source I0 upon the projection lens 26 must be accomplished through the use of a lens having a long focal distance, and is ultimately achieved by using a crystal platetd having associated therewith a reflecting layer curved in the manner of a hollow or concave mirror 27. In view of the fact that in the case of quartz crystals this form has revealed a normal piezo-electric behavior, there can be no doubt regarding the normal electro-optical effect in the case of the above form of the crystal. This solution does not, however, appear to be particularly suitable since the making of the curved crystal screen is somewhat diflicult. Furthermore, as in the form shown in Figure 1, the scanning electron ray 28 is highly inclined with respect to the relay screen so that keystone correction measures are necessary in order to reduce distortion of the image. Much more favorable results may be obtained in this connection in the forms wherein reflection and electron scanning are at right angles to the relay or crystal screen and on opposite sides thereof as already proposed in the above mentioned applications. In order to realize from this arrangement a high degree of light efficiency it is proposed in accordance with Figure 3 to arrange in the direct proximity of the relay screen an intermediate condenser lens I!) which can be adapted for a comparatively wide focal distance since it is twice permeated by the light rays. Only through the use of the intermediate condenser lens IQ is it entirely possible to get completely satisfactory results with small, and therefore inexpensive, projection lenses without parts and especially border parts of the relay screen remaining in the dark state. If the intermediate condenser l9 were not used it would be necessary in any event to choose in the interest of a uniform alignment of the image field a diameter of the projection lens 20 which is larger than the diagonal of the relay screen. In similarly arranged optical devices it was hitherto not possible to introduce an intermediate condenser at this point along the optical path because the source of light always caused a reflection on the outer face of the condenser lens which brought about a considerable disturbance of the projection image. An appreciable advancement is seen in the'fact that only in this arrangement does the detrimental surface reflection no longer cause any disturbance since it is extinguished or rather reduced to about /L of its original intensity in view of the provided crossed polarizers I6 and I8. In order to avoid reflection losses between the relay or crystal screen and the intermediate condenser it may be advisable to establish in accordance with known methods optical contacts between the screen 32, the tube wall and intermediate condenser IS. The reflection at the front side of the crystal suitably takes place through the total reflection grating of the screen as disclosed in the above referred to application and especially in applications Serial #306,611, filed Nov. 29, 1939.

A variation of the arrangement just described is shown in Figure 4. In this arrangement the illuminating light source instead of being brought out of the optical axis of the path of the projection rays by means of a prism 2| as in Figures 2 and 3, is brought in from the outside by means of an annular light source Ilia and annular refiector l2a. An annular polarizing screen lGa is, of course, used. Again the intermediate condenser I9 has the effect that a rather small projection lens 20 is suflicient. Now, the fact is that in providing the arrangement with an annular light source no reflection surface which reflects in a plane can be employed since the central part of the relay screen would then remain in the dark. For this reason care is to be taken that the reflection takes place with a certain indistinct directional characteristic (diffusion) and without de-polarization of the light. This condition can befulfllled for instance when the reflection takes place on small metal spheres which, as in the case of the iconoscope, are deposited in the vacuum by volatilization and which at the same time present the necessary lateral insulation with respect to each other.

A higher degree of efliciency of the luminous effects can be attained in the arrangement of Figure 5 in which the reflection grating has impressed thereon a'directional characteristic by which the light may be concentrated. In a grating having total reflection and screen elements of pyramidal shape at the outside of the auxiliary layer, the desired directional characteristic canbe brought about in that the points of the pyramidal screen elements are displaced relative to the base of said elements. The direction of this displacement of these points extends outwards in order to provide a concentrating effect. The production of such a complicated reflection screen is comparatively simple if the screen plate, instead of being worked on its individual elements, is produced by way of casting, or rolling by means of a tool to be made but once althoughits production is cumbersome. In both cases care must be taken in view of the unity of the screen structure that the tool is kept free from any foreign detrimental deposits and that the mass of the glass completely fills the smallest spaces of the auxiliary tool before hardening. The latter condition can be achieved if the production is carried out under high pressure and in the vacuum.

In some of the hitherto proposed arrangements there is placed in the optical axis in front of the main lens an auxiliary mirror, or an auxiliary prism 2| for changing the path of the illumination ray. This prism reflection means is dispensed with in the arrangement shown in Figure 6 which is a rather advantageous solution. The path of the projection beam in this case has a great similarity with the light path of a normal projection arrangement. The main condenser lens I effects the concentration of the useful light in the cross section of the projection lens 20. The axis of the path of the illumination beam is slightly inclined with respect to the common axis of the relay or crystal screen and projection lens 20. Though the electro-optical effect is thereby theoretically slightly reduced, the fact is that this reduction amounts to but 3.5% in the case of the inclination of the axis of 30 (20 inclination of the axis in the crystal) as shown. This extremely slight weakening of the electro-optical effect by the inclined arrival can thus be taken into account. The illustrated guiding of the projected light is brought about in that there is employed for instance a total reflection prism screen in which the surfaces of the prisms are oriented in accordance with Figure 5a. Unfortunately, in the arrangement of Figures 6 and 6a a part of the light will be lost in the total reflection screen 44 depending on the inclination of the axis. Since in this case it is the inclination of the axis in the optical medium and not the inclination outside said medium which matters, more favorable conditions can be obtained if the auxiliary layer with total reflection screen II is formed of a material which has a particularly high index of diflraction. For the inclination of the axis of 30 as shown and at a diffraction index of 1.75, which can be easily attained with optical glass, the angle of inclination in the medium is only 17. At this angle of inclination the loss in the reflection screen is about 53%. Slight losses of light are attainable principally wher the reflection screen, instead of operatingin accordance with the principle of the total reflection, acts as shown in Figure 6b through reflection on reflecting screen elements 46 arranged in a star-like fashion. In the last mentioned construction it is necessary however that the screen elements (for instance metal spheres deposited after volatilization) are insulated with respect to each other with the result that a certain part of the light will be lost at the region between the individual elements.

More favorable results as regards light effects are obtained in an arrangement not shown in the drawing in. which the operation is carried out in the manner of Figur 2 except that the relay screen is not curved and the projected light is directed upon the relay screen in an oblique direction as shown in Figure 6 and that there is employed a stair-like mirror screen as shown in Figure 622 but in which the reflecting layer is a coherent layer. In this arrangement the light may pass into the projection lens 20 without substantial losses at the relay screen. Care must be taken to insure that between the stair like mirror and the rear side of the crystal or relay screen a certain electrical conduction exists in order to avoid polarization charges. For accomplishing this the new special glass having a high conduction property can be suitably employed, for instance.

Various other obvious modifications may be made in the present invention without departing from the spirit and scope thereof and it is desired that any and all such modifications be considered within the purview of the present invention except as limited by the hereinafter appended claims.

I claim:

1. An optical system for a television receiver including a cathode ray tube, a target electrode in said tube, said target electrode including a crystal plate having a reflecting surface on one side thereof, said reflecting surface being in the form 01' a plurality of prismatic elements, means for scanning the said one side of the target electrode with a modulated beam of electrons to produce a charge image on the crystal plate, a source of polarized light, means for directing polarized light upon the unscanned side of the target electrode, said crystal plate operating to rotate the plane of polarization of the light in accordance with the intensity of the electrical charge image, and a projection lens and polarizing screen for directing the light reflected from the reflecting surface upon an observation screen, the prismatic elements of the reflecting surface being so constructed as to produce complete internal reflection and to return the light rays substantially normally to the plane of the target electrode.

2. An. optical'system for a television receiver including a cathode ray tube, a substantially transparent target electrode in said tube, said target electrode including on one side a prismatic reflecting surface, means for scanning said one side of the target electrode with a, modulated beam of electrons to produce an electrical charge image on the target electrode, a source of light, means including a polarizing screen for directing light from said source obliquely upon the unscanned side of the target electrode, said target electrode including a crystal plate operating to rotate the plane of polarization oi. the light projected thereon in accordance with the intensity of the electrical charge image, said target electrode having a high index of refraction whereby the light rays will be directed at a less oblique angle through the transparent target electrode on to the prismatic reflecting surface thereof, said prismatic reflecting surface being so constructed as to produce complete internal reflection of the light rays and to return the light rays through the crystal plate, and a projection lens in axial alignment with the target electrode for directing the reflected light upon a viewing screen.

MANFRED VON ARDENNE.

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