US2415211A

US2415211A - Image projection system

Info

Publication number: US2415211A
Application number: US476411A
Authority: US
Inventors: Russell R Law
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1943-02-19
Filing date: 1943-02-19
Publication date: 1947-02-04
Anticipated expiration: 1964-02-04

Description

lies. t, ist? 6 Claims.

The present invention relates to an image form- 4ing lor projection system, and more particularly t e. novel optical train giving improved refrac- 'tive and light transmission effects.'

The invention is disclosed herein as being embodied in a system for projecting television pictures or the like, in which the optical system is of the type comprising a spherical mirror and a spherical-aberration correcting plate. As explained in Patent No. 2,273,801, granted to Daniel O. Landis, February 17, 1942, it has been found that a large television picture can be projected with sufficient illumination and with good denition by employing a specially designed optical system of the above mentioned type. Whe-n the present invention is employed in conjunction with the projection .system disclosed :1nd claimed in the Landis patent, improved results will be obtained as will hereinafter appear.

It is anrobject of Vthe present invention to prov vide a novel and improved optical system of the type employing a spherical mirror in which the optical efficiency of the system is greatly enhanced.

. The present invention is concerned with changing the refraction of light at the boundary be tween two4 media by selecting the contacting media in view of optical properties such as their respective indices of refraction. One of the media must maintain its configuration at thecontactin'g boundary, and therefore a transparent solid is employed; The remaining medium is, in accordance with the present invention, a iuid such as a transparent liquid or a transparent semi-solid, such as a jell. Moreover, in accordance with the invention, immersion of the effective ren-acting surface oi the solid medium may be extended to embrace other elements of the optical train or system, 'and may for certain applications be carried to the point of total immersion of all of the effective optical elements o; the system. In the instance last described the medium selected will preferably 'ne a liquid, and in its selection other properties may be considered, such as its heat conductivity, so as it may serve es a coolant, its dielectric strength. so that it may serve as an electrical insulator, and its density, so that it may provide X-ray shielding.

If the correcting plate is immersed in a transf parent medium of suitable index of refraction, the curvatures and variations in thickness of the correcting plate must be greatly exaggerated to Conversely, bymaking a suitable choice of dif- 55 non SYSTEM olii-p, 1i-leraar Corporation of 'mrc lon i9, No. $76,411

(Cl. Sii-24) ference of index of refraction between the correcting plate and the liquid. the ngure, curvatures or variations in thickness might oe emggez-ated to any desired degree.

Accordingly, another important object of the present invention is to exaggerate the curvature of a spherical-aberration correcting lens, whereby the requirements on precision of figuring the correcting lens are less stringent.

Still another object ci the invention isto make hanced light ciiiciency.

A still further object of the invention is substantially to eliminate the eiect of halation, mal:- ing possible improved contrast in an image projected by the optical system.;

2g A son further object of the invention n to er lect a redaction in the throw distance of o. projection system, and more particularly a televisionv image projection system.

A still further object of the invention is to provide protection for the principal reilecting surface of an optical system employed in projecting images, and more particularly in a television image projection system.

A still further object of the present invention is to provide for more edective electrical insulation and cooling 'of a.- television picture reproducing device such as a cathode ray tube and to provide shielding from radiations emanating from such a tuoe when operatedat high voltages, these results being attainable in accordance with the invention separately or together.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which:

40 Figs. 1 to 3 are diagrams which are referred to" in describing principles of the invention existing in an optical system embodying the invention;

Fig. 4 is a side view of a television projection system embodying the inventio-nin modified form,

and

Figs. 5 and 6 are views similar to Fig. 4, illustrating other .inodications of the invention.

Fig. 7 is adiagram illustrating the gain in light emclency in accordance with the invention.

5o Referring to Fig. 1, there is illustrated en optical system of general application embodying the invention. This illustrative system comprises a spherical mirror ID and n correcting plate Il. The face i 2 of the correcting plate is formed or figured to bring light rays from all points of the t mirror in protection screen it', which is ahov 'e loer" 1 r than at n, 1li the c t A t there is a such as e. cathode ray tube, for i point indicated. This point may ce e. 1; cipal focus of the curved mirror, but between the center of eurvaturahi. and the principal locus rrlien'an e embodying the invention is to be i. icction Work. The principal :focal i way between the mirror and its c ture 2l. Light rays from the sou i5 rciiected from the mirror' l0 neer its edge have their focus at a point marked normal edge locus" in the absence of the correcting plate il. The path of these rays after leaving the correcting plate is indicated by the dashed line i3. The correcting plate il in conjunction with the optical medium i9 is figured within comparatively libera-l tolerance limits provided by the invention to refract these raysjust enough to maire them focus on the projecting screen M. Ille correcting plate, it will be understood, is figured so as to make al1 of Athe rays originating from the sourcel and reected from the mirror come to a focus on the screen I4.' The solid line 23 indicates the corrected ray path corresponding to the unccrrected ray path I8.

The way in which the tolerance limits are made less stringent by immersing the correcting plate as taught by this invention is clearly illustrated by the following examples.

Referring now to Fig. 2, there is shown, in a conventional manner, a. pencil or ray 25 traveling in a transparent medium, for example, glass 2li-having an index of refraction of 1.5, The boundary 2l of the glass 26 which is in contact with air makes an ang-le indicated by 2B with respect to a line 29 drawn so as to be normal to vthe'ray'. The ray 25 is bent so that it follows the path 3l after emerging from the glass 26.

The angle through which the ray is refracted is `indicated-by reference character 32.

Fig. 3 shows a ray 34 through transparent glass 26 having the same index' of refraction as the glass 26 of Fig. 2. The surface of the glass 23 is in contact with a transparent medium 35 such as glycerin which has a lower index of refraction than glass, namely 1.4729. The ray 34, as it emerges from the first medium into thesecond medium, is to be bent or retracted through the angle 32, which is the same as the angle 32 of Fig. 2, and is so indicated m Fig. 3. The emerging ray 35 of Fig. 3 is to pass through a point 31 corresponding to an identically located point 3'! Vin Fig. 2. In order to fulfill this condition it; will be found that the boundary 3S between the media. 25 and 35 must maize an angle tl with respect to a line l2 drawn so as to be-normal to the ray 34. It will be seen from this brief study of Figs. 1 and 2 that if the angle 22 of Fig, 2 is unchanged, the angle 32 will be decreased with the'surface 21 of the glass 26 immersed in .a liquid such as the liquid 35.

Referring again to Fig. l oi the drawings, it will be seen, in View of Figs; 2 and 3, that if the ray indicated by the solid line 23 is to intersect the axis in the plane of the screen I4, the curvature of the correcting plate I l is exaggerated when the medium I9 is in interfacial contact with the surface` l2 of the correcting plate Il. By making a suitable choice Vof 'difference in index of refraction between the correcting plate H and the immersion medium, the curvature of this correctingr e ted to any desired degree. .el etcct is the carne en medium he nrcuring th resting plate may be For er1-111131113, whereas the con ent may require the the cor- 1red to cn accuracy el 0.0001 inch, if the curi/at e and variations in. tl'liclmess are 'exe' :crateri ten-fold by the immersion principle, wh l1 is employ-cd in a nove] manner in the present invention,` the correcting plate need only be Egured to accuracy or 0.001 inch to give the same performance.

The optical medium lfl may be a liquid or a. viscous colloidal suspension and is maintained in facial contact with the surface i2 by a transparent plate @it which is secured in any suitable manner to the plate H along its edge td, for example, by cement which is not soluble in the liquid transparent medium I9 or the liquid com ponent of the jell.

The lens Il may be formed from e. plastic by molding or casting, since thel precision o1 figuring this lens or correcting plate is greatly reduced by employing the present invention as pointed out above. As specific examples of the ray correcting combination H and l0, Lucitc (methyl methacrylate) having an index of refraction approximately 1.52 tc 1.53 was used in conjunction with glycerin or Russian mineral oil as the uid medium. Another suitable liquid medium is a product obtainable on the market under the name Texaco Capella AA. These liquids which are mentioned by way of example may also be employed when the correcting plate H is formed of polystyrene. The molding of correcting plates from any of the known, transparent plastics to an accuracy of 0.0001 inch such as would be required for a conventional spherical mirror and 40 correcting plate arrangement is exceedingly difiicult, whereas with an accuracy 0.001 inch such as would be required in a system in which the face l2 of 4the correcting plate is immersed as described above permits the employment of conventional molding technique.

The point i0 may be on the end t0 of a cathode ray tube (not shown) upon which a television picture appears. When the tube is active numerous light points, in addition to the point I8-,

will be effective to illuminate the screen I4, and it will be understood that configuration of the tube end l0 and the din'ensions,A aperture, and the figured surface I2 of the correcting plate will beselected in view of considerations set forth in detail in the lundis patent above referred to.'

and in a patent to E. G. Bamberg, No. 2,298,808, granted October 13, 1942.

It will be understood that the principles disclosed above in connection with Fig. 1 of the drawings may be applied in terrestial and astronomical telescopes, cameras and image projectors generally. The location of the elements in the optical system for each case will be governed by Well known comiderations. For example, in an astronomical telescope, a. curved photographic plate will be placed at the focus of the system in Fig. 1 as the local siuface will be curved.l

Fig. 4 of the drawings shows a television projection system embodyingthe invention in which the entire optical system, with the exception o!A the viewing screen, is immersed in an optical 1118-.. dium 52, such as a liquid or Jell selected in view. of .considerations pointed out above. In addition?, to having an index of refraction which 4isf less'.

than-the index .of refraction of the correcting ravtube ill.

.aa-laan' pirate di., the fluid constituting the critical rnc-- diam la n cercbly in view of other proscrit at out aber-e, namely its heat con elasticity', .s dielectric strength, and' its density. A spherical mirror iid is ,positioned so that it re ceivcs fern the active end or the cathode i duid tight casing te retains the duid-optical medium 52. Suitable supports, such as rings 5l'. are positioned within the casingii with the rings spaced apart sufficiently to provide a groove in which theedse of the mirror is seated. A flange di." provides a recess in which the edge of the correcting plate 53 is seated andan opening in the center of the correcting plate receives the elongated portion of the cathode raytube di. This may be accomplishedby using a cement insoluble in or impervious to medium 52.

The casing 56 vis provided with brackets or flanges' t2 so that it may be secured as a compact optical unit to a 'support of any desired type such as the interior of a cabinet. The gured surface E3 of the 'correcting plate 53 is immersed as well as the active end of the cathode ray tube 5i and the mirror 54. A screen G6 is spaced from the mirror 54 at a. distance depending upon the optical design of the optical system, including the mirror 54 and the correcting plate 53.

The placement distance of the screen 66 with respect to the mirror 54 is an important feature of the invention in the form disclosed in Fig. 4 of the drawings. traverses the 'medium 52 before emerging into the air after passing through the correcting plate 53 and travels in'air from the correcting' plate 53 to the screen 65. The screen 65 may be placed closer to the mirror 54 since the throw distance is reduced. This result is attained because the principal rays from the mirror 54 form a. cone, the' axis ofwhich is substantially normal to the boundary between the correcting plate 53 and the air;A This effect corresponds Vto the effect observed when an image is viewed normally to the `boundary between two media such as water and air. When the object is .in water, for example, and is viewed from the air in this manner, there sno lateral displacement of the object but an lapparent', change in distance of the object from the observer, the object appearing to be closer. Approximatelv a 33% reductionin throw distance is attained.

There is also realized substantially twofold gain in light eiiiciency when the reilective sur- :face 5d of the mirror and the tube end 55 are immersed. This gain in light eiciency comes about because the oblique rays leaving the tube face are not' refracted away 'from the normal when the index of refraction of the immersion iluid matches that of the tube face, and as a consequence, more of them are collected by a mirrorfof a given size.- This is illustrated in Fig.

7 of the drawingsin which a segment of the tube end 55 is shown. P indicates a luminous point.

' Only the smaller cone 561 of light rays leaving the point P actually gets picked u'p at the mirror 54 when it is not immersed, while the light from the large cone 511 is picked up when the tube face and the mirror are immersed.

This change in the critical angle of reflection also eliminates the `effect of halation, making `possible improved' contrast 1in the image appearing on theA screen 68.

The surface of the mirror 54 is protected from dust. dirt, corrosive .effects or aqueous moisture and other deleterious substances which cause Light from the tube end 55 corrosion :or disintegration of the silveri. :duelli boliche-ri surface ci the silver or the ir .glee-.s surface. 'llzc liquid medium may serve as a ccolant for so much ci the tube lll as is immersed therein. This cooling eilect may he enhanced by providing a radiator having se able conduit corsie-cliche to the casing so that heat absorbed by the optica inediuru may be radiated to the air'. A circulating means may be providedfor circulating mediinu through tlier-ad-iator, and e. ian or other arrangement may bc used for caurinn motion of air in the neighborhood of the radiator in a well knownrnanner. l

As pointed out above. .themedium of lil may be selected for its Iii-ray absorbent properties s as to provide a measure of .Protection against hard rays emitted from the cathode ray tube lil. The cathode ray tube will usually by of the type employing higher than normal operating volt ages, so as to attain a brilliant image at the end of the tube 55, and these high voltages whichirequently are in excess of fifty to seventy lrilovolts tend to give rise to X-ray radiation from the rimpacted target area.

Fig. 5 of the drawings illustrates a modification of the arrangement shown in Fig. 4 i'n which all of the elements of the optical system, including one face of the viewing screen lll, are immersed ln a liquid optical medium 7! selected in rie-w of the considerations mentioned abovegoverning the selection of the medium 52 indicated in 4 of the drawings. Reference character I2 indicates a cathode ray tube which may be similar to the...wr

tube 5I illustrated in Fig. i `of the drawings. A correcting plate I3 surrounds the neck of the tube in the manner shown in Fig. 4 of the drawings and illustrated more in detail in the two patents referred to above. Any known or desired means may be employed to support the 'tube 12 and the correcting plate 13 in a housing or container 16, which is filled with the liquid optical medium 1I, having an index of refraction lower than the index of refraction of the'jcorrecrlng plate 13. The spherical mirror 'Il may be supported at or adjacent to one end of the container 16 by suitable meanssuch as that shown in Fig. 4 of the drawings.

The viewing screen is preferably of translucent materialso thatthe image formed thereon may be viewed externally of the container 75. The arrangement just described possesses the advantage of substantially eliminating chromatic aberration which is present in some degree in the op tical system illustrated by Fig. 4 of the drawings. The arrangement disclosed by Fig. l of the drawings is in general the preferred modification of the invention. However, where 800 line television pictures, for example, are to be projected the arrangement disclosed in Fig. 5 ol.' the drawings is to be preferred, since with such a high definition, it is desirable to compensate for chromatic aberration even though it involves thesacrlilce of the 33% reduction in throw distance attained with the modification of Fig. 4 of the drawings. Tests show that chromatic laberration resultin! from the arrangement of Fig. d of the drawings are not detrimental to the quality of television pictures reproduced in accordance with present day standards. i

The 'all-immersion arrangement of Fig. 5 o! the drawings provides increased insulation for the tube 'I2 and its electrical connections. The liquid optical medium will then beselected with this` property in mind as well` as other desirable proporties mentioned shove. it the ined-lum is ingroscepic to en appreciatie degree, the liquid e be l apparatus. l-

Flg.' 6 of the drawings discicsesan arrange ment wherein chromatic aberration ci the entire projection cell or unit, indicated generally hy reference character 1Q, is compensated for in a novel manner. In selecting the liquid optical nieduni il! which is contained Within the cell i9, the property of dispersion must be considered in conjunction with the dispersive power o a liquid optical medium 82 contained in Va screen cell indicated generally by the reference character 133.

This screen cell comprises a transparant Weil 24 and a translucent Wall t8, the latterserving as the viewing screen. These walls are joined toesther in any suitable manner so as to provide the liquid tight cavity between them, which contains the liquid medium 82. For achromatism the product of the dispersion of the medium Bl in the'projection cell 'mand the distance 8B from the projection cell ln to the screen cell 8f3 should equal the product lof the dispersion of the medium 32 in the screen cell 83 and the thickness 89 of the screen cell. By way of example, the transparent liquid in an embodiment of the invention operated for test purposes' was mineral oil and the liquid in the screen cell was carbon disulphide (CS2).

VFrom the foregoing it will be seen that the invention provides' a method of and means for giv ing a prescribed exaggeration of the figure of a correcting lens or plate useful for general optical purposes.. -Also. the invention provides a television picture 'projection system having a tele'- visionreproduction tube and reective mirror imrnersed with a corrective lens in a common rnediurn to form a-'projection cell. It will be understood in this connection by those skilled in the v tart that4 an bf the mechanical arid optical refine-V ments disclosed in thetwo patents referred to above may he applied to television .projection ap paratus embodying the present invention. For example, the arrangement disclosed by Fig. 3 of the Landisv patent, including the mirror I9, may be followed in designing a television projector em bodying this inventionA The invention further provides for use 0f such a projection cell with a screen cell containing a suitable medium and so proportioned as to provide achromatism.

The nature of -the invention will be determined A from the foregoing, and the scope .thereofis dened in the appended claims. i

Having now described the invention, what is claimed and desired to be secured by Letters Patent 'is the following: Y

4l. An optical system comprising a concave recting plate functions in the same manner as a.

correcting plate having unexaggerated curvatures in Contact with air.

2. An optical syst-em ior image projection and enlarging comprising a light source adapted to emit light to produce a bidimensional optical pictiue image oi liu-ite erea'suhstoutiaih' centered on tvc optical oi' the erstem. a iight rciiccting t ricca of revolution having its concave eurfece positioned to relient the iight milling from scid source. on euphoric-al zone plate positioned to it sive the rcected light projected from acid light source, said noue plate haring a surface and lacing arranged eirtcrnal to the light ,cth from the licht source to the reiiector and axially aligned with each and adapted to correct the spherical aberrations introduced into the reflected iight rays ha' the relecting surface, and means for exaggerating the zones of said .e1-incrical tone plate comprising a liquid optical medium immer-sing the gurcd surface of said zone plate. means for maintaining the liquid optical medium in contact with the zones of said aspherical zone plate, said zone plate and said liquid optical medium having selected differences of their refrective indices giving the saine optical eectas would'he obtained with an unimrnersed and unexaggerated correcting plate.

3. A projection system comprising a concave spherical mirror, a correcting plate having a figured surface and positioned to receive light re il'ec-ted 'from said spherical mirror, a projection tube having an object surface therein which is Y positioned at a conjugate focus of said system1 and a iuid tight enclosure for maintaining a. fluid optical medium of selected' difference of refrac tive index in physical contact with said mirror, the figured surface of said correcting plate and the object surface of said tube. to immerse the Iigured surface of said correcting 'plate whereby the selected differences of the refractive indices of the plate and immersing medium permit the iigured surface vof the correcting plate to be greatly exaggerated andwhich in connection with the said immersing vmedium gives the same optical eii'ect that would be obtained with an unimmcrsed correcting plate.

4. A projection system comprising a concave .spherical mirror,`a correcting plate having an optically active surface positioned to receive and transmit light rellected from said spherical vmirror, a projection tube having an object Vsurface therein which is positioned at a conjugate focus of the system, a viewing screen receiving light in the 4form of a readily perceptible image transmitted by said correcting plate, and a. iluid optical medium in wihch said projection tube. the optically active surface of said correcting plate, said mirror and the light receiving faccio! said viewing screen are immersed, said uid optical medium having an index of refraction differing from that of said correcting plate.

5. An optical system comprising a concave spherical mirror, a correcting plate positioned to transmit light received fromsaid sphellcal l mirror. a projection tube having an object surface therein which is positioned at a coniugate focus of the system, a viewing screen'spaced from said spherical mirror, a liquid optical medium ln the light path from said projection tube object surface to said correcting plate, and a second -liquid optical medium in contact with said viewing screen so as to be traversed by light transmitted by said correcting plate for a portion of its path, said iirst named liquid optical medium having index of refraction differing from that 0f said correcting plate, said ilrst land second liquid optical media having values of dispersion such that the product of. the portion of the light path in linear units along the axis lof said system from said correcting plate to said second liquid optical medium multiped by the dispersion velue of seid rst named liquid optical medium s-egual to the product of the path in linear units treversedby light. passing 'from seid correcting niet@ to seid viewing so en through Vseid second uid optical medium multiplied by the dispersion velue of said second named cguid opte el medium.

"whereby said eptice system is aehromatic.

6. An optical eombnet'ou comprising a, spherical mirror with aberrations end e. correcting plete for the aberrations for transmitting directed light in which the refractive effect of the plate is decreased so that' tolerance requirements in4 forming the element may be relaxed, said plete having e. figured surface, the curvature of the figured surface being exaggerated, a fuid optical medium in Contact with and mmel'sng seid gured surface, theud optical medium having a, derent index of refraction so reete'd to the ndex of refraeton of the material of the guxed

Number Name Date

2,2%,802 Ncoll Sept.` 15, 1942 2,093,228 Ogoblnsw.. Sept. 14, 1937 2,273,801 Landis Feb. 1'?, 1942 2,1?0,97 9 Straubel Aug. 29, 1939, 695,606 Gmn Mar. 18, 1902r 959,785 Kellner Sept. 13, 1910 2,229,302 Martin, et al Jan. 21, 1941 2,200,166 Cristiana. et al May 14. 1940