US2889735A

US2889735A - Beam splitter comprising colour separating planoparallel plates

Info

Publication number: US2889735A
Application number: US483384A
Authority: US
Inventors: Wieberdink Ate
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1954-02-06
Filing date: 1955-01-21
Publication date: 1959-06-09
Anticipated expiration: 1976-06-09

Description

SEARCH ROOM June 9, 1959 A. WIEBERDINK BEAM SPLITTER COMPRISING COLOUR SEPARATING PLANOPARALLEL PLATES Filed Jan. 21, 1955 INVENTOR ATE wqssanomx fl? AGENT United States Patent BEAM SPLITTER COMPRISING COLOUR SEP- ARATIN G PLAN OPARALLEL PLATES Ate Wieberdink, Emmasingel, Eindhoven, Netherlands, asslgnor, by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Application January 21, 1955, Serial No. 483,384

Claims priority, application Netherlands February 6, 1954 3 Claims. (Cl. 88-1) Beam splitters comprising colour separating planoparallel plates to break the light emanating from a source of light into a plurality of colour components are used, among other things, in colour television. In this case the light emanating from an object is broken up into three fundamental colours. With the aid of a camera the light of a particular fundamental colour is converted into an electric signal; the three signals thus obtained or the linear combinations thereof can be converted again in a suitable receiver into a picture of the object in its natural colours.

Such beam splitters are, in general, composed of two intersecting, colour separating planoparallel plates, each of which transmits substantially completely particular parts of the visible spectrum and each of which reflects substantially completely other parts thereof.

These beam splitters have a limitation in that at the intersection of the two planoparallel plates the effect of the colour separation is disturbed, which, in the finally reproduced picture, produces a troublesome ribbon, whose breadth is directly proportional to the ratio between the thickness of the planoparallel plates and their length, measured in a direction at right angles to their line of intersection. In order to obviate this unwanted phenomenon, it has been suggested to compose the beam splitter of four prisms joined together in a manner such that the separation planes constitute two intersecting surfaces, each of which is provided with a material producing the colour separation.

In the situation involving separation of the light emanat ing from an object into three colour components, two aspects may be distinguished, i.e.

(1) It may be required that the pictures produced by the beam splitter inthe three fundamental colours should not be deformed,

(2) It is not required that the pictures should be undeformed at least as far as their geometrical association is concerned.

The first-mentioned case may for example apply to the production of a picture in a particular fundamental colour on the photo-electric cathode of a television camera operating by electron beam scanning. It will be obvious that, if such a picture is deformed, also the image finally reproduced by a receiver will be deformed.

The second case applies for example to the scanning of a colour film by means of a white light beam, the passing light being separated into three fundamental colours. The light of a particular fundamental colour is captured by a photo-electric cell. In this case, however, the geometrical coherence of the pictures produced on the photo-electric layer of the cell is not essential.

In both cases the beam splitter comprising two intersecting, colour separating planoparallel plates exhibit the disadvantage described above; in both cases the beam splitter comprising four prisms obviates it.

It will, however, be obvious that this solution is comparatively costly, since the four prisms must be ground accurately to fit exactly to one another; they must, more- Hce over, be satisfactorily achromatic in order to avoid colour dispersion in the prisms themselves.

The invention has for its object to provide a materially simpler solution to avoid the occurrence of a dark ribbon in the image to be finally reproduced in the second case, i.e. if the images in the fundamental colours, as far as their geometrical coherence is concerned, are not required to be undeformed, and has the feature that the beam splitter is composed of two parts separated from one another in space, each part comprising a V-shaped body of transparent material, which may be subdivided, the limbs of each V-shaped body being constituted by planoparallel plates, the peaks or apices of the bodies facing one another and the plane in which lie both the line of intersection of the outer surfaces of one V-shaped body and the line of intersection of the inner surfaces of this body and the corresponding plane of the other V-shaped body lie at least substantially in the same plane, the colour separating material, at least with the part located on the front side of the beam splitter, being applied to those sides of the planoparallel plates which face the front side of the beam splitter.

The invention will be described more fully with reference to the accompanying drawing, in which Figs. 1 and 2 show embodiments of known beam splitters,

Fig. 3 shows one embodiment of a beam splitter according to the invention, and

Fig. 4 serves to explain the invention.

Fig. 1 shows a sectional view of a known beam splitter, composed of two intersecting planoparallel plates 1 and 2, having layers of colour separating material 3 and 4. Such a layer may for example be composed of one or more layers of dielectric material, the thickness of the layer being required to be in a given proportion to the wavelengths of the part to be transmitted and the part to be reflected of the light spectrum. For example the layer 3 can reflect red light, and pass blue and green light; the layer 4 can reflect blue light and pass red and green light. The plates 1 and 2 serve as supports for the colour separating material; they must of course be transparent and are mostly made of glass.

The fact that the crossing of the two supports, one of which comprises two parts (the support 2 of the figure) has a disturbing efiect, is illustrated by two light beams 5 and 6. The light beam 5 is broken up by the layer 3 into a reflected part 5,, containing mainly red light, and a passed part 5 containing mainly green and blue. The first part passes unhindered the layer 4 and the layer 2, apart from the refraction in the planeparallel plate 2, and the second part, subsequent to its passage through the plate 1, strikes the layer 4. This layer breaks the light from the plate 1 into a reflected part 5 containing mainly blue light, and a passed part 5 containing mainly green light. Upon considering the light beam 6, it is evident from the figure that the passed part of the light striking the layer 3 is no longer broken up into a green part and a blue part and that this part leaves the beam splitter, moreover, in an unwanted direction.

These phenomena do not occur with another known beam splitter shown in Fig. 2. This beam splitter comprises four

prisms

1, 2, 3 and 4, the colour separating material being applied to the surfaces at which two prisms engage one another. The layer 5 may, for example reflect red light and pass blue and green light;

avoid unwanted colour dispersion in the prisms themselves. However, this solution is quite costly.

Fig. 3 shows one embodiment of a beam splitter according to the invention. It comprises two elongated V-shaped parts 1 and 2, separated from one another in space and each constituted by two planoparallel plates of transparent material, i.e. part 1 by plates 3 and 4 and part 2 by plates and 6. The plates 3 and 4 intersect one another in a plane 7 of intersection, and the plates 5 and 6 in a plane of intersection 8. These intersection planes 7 and 8 are each also defined by the two lines formed by the inner intersecting and the outer intersecting surfaces of the V-shaped bodies 1 and 2. These planes of intersection need not correspond to the material boundaries of the

planoparallel plates

3 and 5 or 5 and 6 respectively, although this is to be preferred, since all

plates

3, 4, 5 and 6 may have equal dimensions and the said surfaces may be ground at the same angles; moreover, the application of the colour separating layers is then most simple. However, one part may be made of one piece or of two pieces as indicated in Fig. 3 for part 2. The two planes of intersection 7 and 8 form at least substantially a common plane; the peaks or apices 9 and 10 of the V-shaped parts 1 and 2 face one another; d designates the distance between the corners 9 and 10 of the parts 1 and 2. This distance may be zero without any objection. The colour separating or color selective material is applied in

layers

11, 12, 13, and 14 to those sides of the planoparallel plates which face the front side of the beam splitter. The incident light on the front side is designated by the horizontal arrow to the left of the beam splitter.

This is required for the part 2 lying on the front side i.e. the part lying on the side where the light from the light source comes in, if at least the occurrence of components emanating at an unwanted angle should be avoided. This is not required for the other part 1, but the application of the colour separating

layers

12 and 13 to the sides of the planoparallel plates 3 and 4 facing the front side has the advantage that the light reflected by the

layers

12 and 13 need not traverse twice the plates 3 and 4. The layers 11 and 12 may for example reflect red light and allow green and blue light to pass, and the layers 13 and 14 reflect blue light and allow green and red light to pass.

The colour splitter according to the invention does not exhibit the disadvantages inherent in the colour separator shown in Fig. 1, and it is of materially simpler construction than the beam splitter shown in Fig. 2. The angles at which the planoparallel plates of the separate parts intersect one another, i.e. the angles a and a which are preferably 90 each, need not be equal to one another whereas it will be obvious that the tolerances for the angles b of the prism (see Fig. 2) are within very narrow limits.

The fact that the beam splitter according to the invention does not exhibit the disadvantages inherent in the beam splitter shown in Fig. 1 is illustrated in Fig. 4. Each of the

incident light rays

1, 2 and 3 is broken up, independent of the impact area on the beam splitter, into a green component 1 2 and 3, respectively, a blue component 1 2 and 3;, respectively and a red component 1,, 2,. and 3 respectively, while no components emerges at an unwanted angle, with the result that no light-sensitive element is struck as in the case with the light ray 6 of Fig. 1.

From Fig. 4 it is evident that the emerging red and blue light beams do not contain rays in the hatched parts. This means that the geometrical coherence of the images produced by these beams of the object is interrupted. In many cases this is not an objection. Reference has been made to the case in which a colour film is scanned by means of a white beam, the components produced by the splitting action of the beam splitter being captured each by a photocell. A further example is given by the case in which images are produced by a beam splitter on the photo-electric cathode of a television camera tube for light scanning, the light spots required for scanning being images of the same light source describing a scanning frame under the action of suitable deflection means, these images being also formed via the same beam splitter. Since the geometrical coherence both of the images of the object and of the images of the light spot describing the scanning frame is interrupted, however in completely the same manner for the two, so that the local relationship between the scanning frame and the pictures remains the same, this does not affect adversely the picture to be finally reproduced. As used herein' with reference to the plates of the beam splitter, the term planoparallel connotes that the oppositely-facing surfaces of the plates are planar and parallel to one another.

What is claimed is:

1. An optical device adapted for separating incident light into a plurality of emergent light beams containing different color components or vice versa, comprising a pair of elongated, substantially V-shaped, in cross-section, bodies each comprising a pair of substantially transparent, planoparallel plates joined at their edges to form the apex of the V, said pair of bodies being mounted adjacent one another with their apices aligned with and facing one another and with their plates facing outwardly in opposite directions to form, as a whole, a generally X-shaped configuration, the lines defined by the inner intersecting and the outer intersecting surfaces of the planoparallel plates of each of said bodies each defining a plane, the thus defined planes of both bodies being substantially coplanar, first and second layers of light reflective and transmissive, color-selective material on the two inner surfaces of one of said bodies, and third and fourth layers of light reflective and transmissive, color-selective material on two V-shaped surfaces of the other of said bodies, said first and second layers possessing different reflective and transmissive properties to different color components but each of them possessing the same reflective and transmissive properties as that of the third and fourth layers extending in the same general direction and thus all the layers cooperating to separate incident light into the differently-colored light beams.

2. An optical device as set forth in claim 1 wherein one of the V-shaped bodies is constituted of a pair of members joined along the said plane defined by the lines formed by the inner intersecting and the outer intersecting surfaces.

3. An optical device adapted for separating incident light into three emergent light beams containing different color components or vice versa, comprising a pair of elongated, substantially V-shaped, in cross-section, bodies each comprising a pair of substantially transparent, planoparallel plates joined at their edges to form the apex of the V, the angles included between the plates of each of the bodies being about the same, said pair of bodies being mounted adjacent one another with their apices aligned with and facing one another and with their plates facing outwardly in opposite directions to form, as a whole, a generally X-shaped configuration, the lines defined by the inner intersecting and the outer intersecting surfaces of the planoparallel plates of each of said bodies each defining a plane, the thus defined planes of both bodies being substantially coplanar, first and second layers of light reflective and transmissive, color-selective material on the two inner surfaces of one of said bodies, and third and fourth layers of light reflective and transmissive, color-selective material on the two outer V-shaped surfaces of the other of said bodies, said first and second layers possessing different reflective and transmissive properties to different color components but each of them possessing the same reflective and transmissive properties as that of the third and fourth layers extending in the same general direction and thus all the layers cooperating to separate incident light into the diflerenfly-colored light beams.

References Cited in the file of this patent 6 Schroeder Iune16, 1953 Rehorn Feb. 23, 1954 Sachtleben et a1. Mar. 16, 1954 FOREIGN PATENTS France June 20, 1905 France Mai. 20, 1926