US3087375A - Albada type viewfinder having undulating reflecting mask frame - Google Patents

Description

April 30, 1963 F. PAPKE ALBADA TYPE VIEWFINDER HAVING UNDULATING REFLECTING MASK FRAME 5 Sheets-Sheet 1 Filed June'5, 1958 F/G. la

FIG. 3

FIG?

April 30, 1963 F. PAPKE 3,087,375

ALBADA TYPE VIEWFINDER HAVING UNDULATING REFLECTING MASK FRAME Filed June 5, 1958 3 Sheets-Sheet 2 United This invention relates to finders for photographic cameras and it has particular relation to novel and i1nproved finders of the type in which a reflecting image frame is imaged by a partially permeable mirror at about infinity. The invention also relates to a process for producing such finders.

Among the numerous photographic finders known from the art, in which a reflected image field framing is used, the so-called Albada finders are distinguished by their simple structure. The present invention relates to finders of this type. A reflecting frame, which is located near the insight opening of the finder, is imaged at about infinity by a partially permeable mirror. Such finders can be produced in various modifications, for example also in cooperation with finders according to the Galilci or Newton principle.

Illumination of the frame to be reflected in finders based on the Albada principle is, as a rule, connected with considerable difficulties because it is carried out by a partially permeable mirror, whereby a considerable part of the initially available light is lost. Attempts have been made to avoid the difliculties of illumination by imparting a particular structure to the frame. For example, it has been suggested to make up the frame from small rods, pyramids, reflecting squares, or alternating difluscly reflecting and normally reflecting elements. Furthermore, it has been attempted to improve the frame to be reflected, with regard to its luminosity, by means of luminescent compositions or luminous substances, or to conduct the light to the frame in a roundabout way through socalled light conductors, whereby the partially permeable mirror surface is by-passed. It has been also known to produce a diffusely dispersing frame by depositing the frame to be reflected on :a ground glass surface by vaporization and rendering this surface transparent again by cementing it together with another surface by means of Canada balsam. It is true that this last-mentioned procedure is rather simple. However, a ground glass plate subjected to depositing by vaporization strongly disperses in all directions, so that a considerable part of the light is lost. Therefore, the desired effect is fully obtainable only for certain specific kinds of illumination, but will not be satisfactory particularly when the frame to be reflected is supposed to appear in contrast with large, bright surfaces, eg the sky.

It is a main object of the present invention to improve the brightness conditions in Albada finders in such a manner that the frame to be reflected will appear as brilliant as possible, even under unfavorable illumination conditions, e.g. against a bright sky. The disadvantages of the hitherto known arrangements are thereby avoided.

A considerable improvement in the illumination of the frame can be attained according to the present invention by shaping the frame carrying surface in undulated form. This design or shaping of the carrier body surface, which forms the support or base for the frame, is fundamentally different from the above-mentioned known suggestion of using a diffusely reflecting carrier surface.

By suitable selection of the depth and length of the waves, reflection on the frame can be influenced in such a manner that part of the light irrespective of what Patent G 3,087,375 Patented Apr. 30, 1963 part of the field of vision of the finder it comes fromis reflected from a plurality of the undulatcd surfaces so that it reaches a definite eye or pupil position for imaging the frame by the partially permeable mirror. In carrying out the invention, it is preferred to keep the length of the waves of the frame carrying surface small, as compared to the width of the side strips or border of the open frame. It has been found advantageous to select the wave length in the range of to /2 of the width of the frame side strips or border. Preferably the length of the waves in the undulated surface of the carrier amounts to from 5 to 50 times the wave length of light. Thus, in comparison with the granularity of a ground-glass plate according to the above-mentioned known suggestion, according to the present invention the length of individual waves of the carrier surface, is decidedly large.

For an understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawings. In the drawings:

FIG. la is a diagrammatical sectional view illustrating the scattering or diffusion of light rays stricking a carrier plate having a ground glass surface;

FIG. lb is a view, similar to FIG. 1, illustrating the reflection of light rays stricking a carrier plate formed in accordance with the present invention;

FIG. 2 is a sectional view of a carrier plate embodying the invention and undulated only in the frame carrying portion;

FIG. 3 is a view similar to FIG. 2 illustrating the carrier plate having its entire surface, on which the frame is mounted, formed with undulations;

FIG. 4 is an axial sectional view through an Albada viewfinder embodying the invention;

FIG. 5 is a sectional view through one form of frame carrier assembly embodying the invention;

FIG. 6 is a somewhat diagrammatic perspective view of another form of frame carrier embodying the invention;

FIG. 7 is a sectional view through a carrier plate before the latter is processed in accordance with the invention;

FIG. 8 is a sectional view of the plate shown in FIG. 7 after an initial operation thereon in accordance with the invention; and

FIG. 9 is a sectional view of the plate shown in FIGS. 7 and 8 after completion of processing thereof in accordance with the invention.

According to the present invention, the height of the waves of the carrier surface is preferably considerably smaller than their length and should amount, for example, to only A of the wave length. As a comparison with the prior art, it may be mentioned that the ratio of the height of the waves to their length is considerably smaller than the ratio of height to width in the fissuring of ground-glass plates, where the corresponding ratio is about 1:1. This is explained in more detail by FIG. la, in which 1 denotes a section of a ground-glass plate, the surface of which is diffusely reflecting, while the section of a surface undulated according to the present invention is shown at 11 in FIG. 11). Light ray 2 is scattered at the fine jags 3, 'while the light ray 12 is normally reflected at the waves 13 and merely changes direction and width at the undulated shape.

In contrast to the known structure consisting of a frame to be reflected, which is deposited by vaporization on a ground-glass plate, according to the invention no diffuse scattering or dispersion occurs, but rather a regular or normal reflection. It is true that thereby the direction of reflection is subjected to manifold changes due to the alternation of wave crests, side walls of the waves, and wave troughs, and the irregular arrangement of these waves on the carrier surface. However, the scattering or dispersion of the light by the frame base according to the present invention is considerably narrower than by a ground-glass plate. This fundamentally different effect of reflection, in comparison with diffuse dispersion, renders it possible to considerably improve utilization of the light incident on the frame, with a suitable geometrical optical arrangement of the entire finder system, in carrying out the present invention. This effect is considerably further increased if the undulated carrier surface for the mirror frame is rendered vitreously smooth to have a polished or specular finish and this can be attained by a corresponding heat treatment. A difluse scattering is thus prevented.

In carrying out the present invention, the undulated shape of the surface carrying the mirror frame can be limited to those places to which the frame is applied, e.g. deposited by vaporization. Such an embodiment is illustrated in FIG. 2. However, the entire surface of the carrying member can be of undulated shape, as shown in FIG. 3. It is also contemplated, according to the invention, to alternately arrange undulated and even zones in the carrying surface, in the form of strips, rings or other forms.

It has been found to be of particular advantage if the above described undulated surface of the frame carrier surface-i.e. its cover surface-is not plane, but curved, whereby, for reasons of manufacture, a spherical surface is preferred. Thereby it is of advantage to select the curvature of the carrier surface in such a manner thatat the distance of the eye along the optical axis, contemplated for the regular use of the finderthe light which serves for illumination of the frame is taken from that zone of the image field into which the frame is reflected, whereby the undulated structure is left out of consideration, i.e. an even cover surface of the frame carrier is taken as a basis.

Due to the undulated structure of the surface, which is superimposed on the spherical base surface of the frame carrier, it is possible also to utilize light from other parts of the image field for illuminating the frame, particularly if the respective portion of the frame is supposed to appear opposite to dark zones of the image space. Such an arrangement is illustrated in FIG. 4.

A finder according to the known Albada system consists of three parts, illustrated by Way of example in FIG. 4; a piano-concave front part is shown at 41, the concave surface of which carries the partially permeable mirror and is cemented to the middle part 42. The other end surface of the latter is cemented to the plane-convex back part 43, the convex surface A4 of which is undulated and carries the frame 45, the reflecting surface of which matches the wave-like design of the carrier. A light ray 46 (which is shown in full line) indicates the course of an about parallel incidence. The dotted line indicates the course which the same ray would follow if the frame did not have a wave design. In this latter case, the light would be reflected to the lateral surface of the finder, i.e. would not be utilized.

According to an advantageous embodiment of the finder according to the invention, the undulated carrier surface is connected by cementingafter application of the frame to be reflectedwith the surface of a glass body inserted in front, in the meaning of the direction of the light. The surface to be cemented of such glass body is designed with a suitable curvature, i.e. in the case of a plane carrier surface it is likewise plane, while in the case of a curved carrier surface it has the same curvature as that determined by the undulation of the respective cover surface.

Cementing can be best carried out with a material the index of refraction of which corresponds to that of the glass used. For example, if the carrier lens consists of plate or mirror glass, then the cementing substance should have an index of refraction of about equal value, i.e. n =l.523. The use of ethoxyline resins or epoxy resins has been found to be particularly satisfactory. A mixture of these materials (the index of refraction of which is different) can also be used and, by suitable selection of the mixing proportion, the index of refraction of such mixture can be adjusted to the index of refraction of the glass or synthetic plastic, of which the carrier body consists.

Furthermore, it is also contemplated according to the present invention to provide the undulated surface of the carrier bodyafter application of the frame, cg. by deposition by vaporizingwith a transparent coating which has almost the same index of refraction as the carrier body proper. Commercial plastics, resins and lacquers have been found satisfactory for this purpose. This coating is applied in such a manner that the surface formed by it is smooth or even. Particularly according to this embodiment, cementing with the adjacent surface of a member inserted in front can be dispensed with, so that the surface of the carrier body provided with the coating is uncemented within the finder, i.e. is in contact with air.

In order to render the undulated carrier surface particularly if the entire surface is undulatedclearly transparent again, the above mentioned cementing with a cement which has the same index of refraction as the carrier body, can be substituted by the following procedure.

After application of the frame to be reflected to the carrier glass, the carrier glass is melted together-according to conventional melting procedure-with a suitable second glass in such a manner that the separating surface becomes invisible and only the imbedded metallic frame remains visible in the homogenous appearing glass plate. Deformation of the outer surfaces, which may be caused by this melting process, may be eliminated by treatment according to conventional processes of optical manufacture. Mounting of the frame on an undulated surface can be carried out in analogous manner by means of suitable thermoplastic synthetic materials which can be molded and fused in conventional manner. In a manner customary in glass melting, in the melting of synthetic plastics care should be taken that the melting point of the applied cover material should be lower than the melting point of the base body which carries the frame. The use of a cement is not necessary in this procedure.

FIGURE 5 illustrates a carrier element for the frame produced according to the above described process. In FIG. 5 reference symbol 51 denotes the carrier body consisting of glass or synthetic material. This body has been provided with a wave-like design 52 over its entire surface. The reflecting frame, which has been deposited by vaporization on undulated surface 52, is shown at 53, 53. Reference symbol 54 indicates the transparent body 54 of glass or synthetic plastic, which forms a covering and has been intimately connected by melting with carrier body 51.

The present invention also relates to the production of undulated carrier surfaces for the frame to be reflected. Four preferred procedures are described hereinafter as specific examples. However, it is contemplated according to the present invention to modify these procedures in accordance with particular requirements.

Example I The carrier surface to be prepared should have the above described design and meet the above described requirements in conformity with the present invention. Usually the carrier is made of glass, but it is also contemplated according to the invention to use carriers consisting of synthetic plastics. The surface to be formed with waves may be milled or ground; polished surfaces over the entire surface, or at definite portions intended to carry the frame, or parts thereof, are roughened and this can be done by means of sand jets. Each of the beforernentioned preliminary treatments results in the formation of a milky-white fissured surface, and this surface is smoothened again by heat treatment so that it becomes glassy. Thereby the surface is substantially leveled by the aid of the surface tension, so that in any case the sharp, strongly dispersing edges and jags disappear and a slightly undulated surface having the polished appearance of vitreous glass results. The heat treatment can be controlled without difficulty in such a manner that, in the leveling of the previously distinctly rough surface, the wave design is obtained in the desired form, size and height, whereby the sharp edges, humps, fissures, gaps, and the like, are fused. To the surface thus prepared, a frame, consisting of a reflecting metal or multiple layers of suitable materials, is applied in a manner known by itself, preferably by deposition by vaporizing under vacuum. Although due to the above described heat treatment the originally non-transparent, only translucent surface has become transparent, it does not yield a clear optical image in the meaning of geometric optics, so that, on either side of the frame side strips or border, it is not possible to see directly through this surface, i.e. if not only that zone of the carrier body which carries the frame, but the entire surface, has been subjected to the described pretreatment. However, this surface can be rendered transparent, in a manner known by itself, by cementing it together with a surface of an adjacent optical element of the finder, e.g., a lens or a glass block whose cementing surface has a curvature corresponding to that of the base surface. As already mentioned above, particularly suitable for this purpose is an epoxy resin which consists of two components adapted to polymerize at suitable composition. The cementing body should have an index of refraction which as nearly as possible is equal to that of the carrier body, e.g. plate or mirror glass. The surface to be cemented to the frame carrying surface likewise need not be polished in the optical meaning of this term, and it is sufi"1 cient if it approximately corresponds to the frame carrier surface in its geometric shape. The above mentioned cementing resin has the advantage, in comparison to Canada balsam, that it is considerably more transparent and is, therefore, capable of filling larger gaps too, without perceptible absorption of light. Moreover, it is stronger and more resistant to temperature.

The above mentioned epoxy resin can be used also in mixture with another polymerizable cement, preferably an ethoxyline resin. It is thereby advisable to vary the index of refraction of the mixed product, by varying the amount of the individual components, in the range of 11. 1.514.58. This renders it possible to use glasses other than plate or mirror glass for the manufacture of finders according to the invention, and this may be important particularly in the case of block constructions.

As already mentioned above, it is not necessary to apply the wave design to the entire carrier surface and it may be sufficient to limit this design to a part of said surface, e.g. the part which actually carries the frame.

Such a modification is illustrated in FIG. 6, in which the carrier surface 62 of carrier body 61 is provided with undulated strips 63, to which the frame 64 is applied preferably by deposition by vaporization. In this manner, plane and undulated frame portions are alternately formed.

Example 11 In a second procedure for providing the carrier surface for the frame with the wave design according to the present invention, the following steps are used.

A roughly processed surface, which may be ground, milled or treated with sand jets, is coated with a thin lacquer which-due to its surface tension-levels the sharp inequalities of the ground surface and leaves an undulated surface after drying. By variation of the original degree of roughness, the consistency of the lacquer, and/ or the thickness of the lacquer layer, the wave form, particularly the height of the waves, can be varied within wide limits and adapted to the desired purpose. For example, the use of an epoxy resin, which is preferably diluted with acetone, has been found to be satisfactory, although in its polymerized condition the epoxy resin is not soluble in acetone. However, the mixture is applied as a thin film to the surface to be leveled, and the acetone is vaporized after a short period of time, whereupon the resin residue forms 'a thin coating and undergoes slow polymerization. Thereby an undulated surface is formed which is glossy or polished on the outside and has the same index of refraction as the carrier material. It is, therefore very suitable for subsequent coating with metal. On this undulated surface the frame is deposited by vaporization. This surface can be cemented in the above described manner with a second glass surface, again by means of an epoxy resin, and becomes thereby clearly transparent.

If it is desired to avoid cementing of the frame carrying surface with another surface, the undulated surface can be rendered transparent by the following procedure.

After deposition of the frame on the undulated surface, the latter is coated with a layer of an epoxy resin and then, by means of a member which has a registering surface and consists e.g. of Plexiglas, leveling of the outer surface of the layer is effected. This resin does not adhere to a suitably treated Plexiglas surface and yields an optically unobjectionable surface upon polymerization.

Example 111 A continuous rough surface of the carrier body is first obtained in the manner described in the above Example I or II. This surface is then rendered polished and smooth by subjecting the rough surface to etching with a suitable acid, e.g. hydrofluoric acid or a mixture of hydrofluoric acid with sulfuric acid. If the carrier body consists of an organic plastic, instead of the acid solution a suitable organic solvent should be used in an analogous manner.

Example IV According to a further procedure, the entire carrier surface of the carrier body consisting of glass or a synthetic plastic, or selected portions of said surface, which are designed to carry the frame, are subjected to pressing under heat with a suitably shaped pressing tool whereby the carrier surface is provided with the desired wave design.

In carrying out the above mentioned melting together of the carrier glass, after application thereto of the frame, with a second glass layer, the carrier glass may consist of glass BK4 having a cohesion temperature of about 626 C., to which the frame to be reflected is applied and which is then heated near to its cohesion temperature. A second glass layer consisting, for example, of BKZ having a somewhat lower cohesion temperature of about 528 C. and being fitted in the design of its surface to that of the carrier glass body, is now applied to the carrier glass body. Due to its lower cohesion temperature the second glass layer will reach its final viscosity sooner, e.g. at a lower temperature than the glass of the carrier body, which is softened to a small extent only, so that the frame will be enclosed by the glass layer of lower cohesion temperature, closely, and there will be no separating layer visible between the two glass bodies. These glass bodies should have approximately equal indexes of refraction.

If the carrier body and the layer used for enclosing the applied frame consist of synthetic plastic materials, polymethacryl esters and/ or polyacryl esters, can be used by way of example. Such plastic materials may have a softening point of about C., using for example the materials known in commerce under the name Plexigum M 380 and Plexigum M 320.

The above mentioned deformations of the outer surfaces, which may be caused by the above described process for melting together two glass materials, or two plastic materials, may primarily occur on the outer, optically active surfaces. Such deformations can be eliminated by methods which are conventional in optical techniques, e.g. by grinding and subsequent polishing.

Some examples of synthetic plastic materials which can be used in carrying out the present invention are: polymerized methyl rnethacrylate, polystyrene and phenol-formaldehyde resins.

FIGURE 7 illustrates by way of example a carrier body 70 consisting of a glass plate and having initially a plane surface 71. By treatment with a sand blast device the smooth surface 71 is roughened in such a manner that a surface structure of the design diagrammatically illustrated in FIG. 8 results.

In FIG. 8 the carrier body is denoted 80 and its surface structure produced by the action of the sand blast is denoted 81. The uneven portions of surface 81 have a design corresponding to the grain size of the sand used. In the example shown in FIG. 8 the height 82 of the uneven portions amounts to about 50 and their width 84 is in the range of about to 50 In addition to these coarse surface structure portionswhich are arranged at random, but in a statistically uniform mannerthe portions show smaller uneven parts, which are indicated at 83 and have a size in the range of 0.5 to 3a. The surface 81 shown in FIG. 8 forms the basis for the subsequent thermal treatment in order to produce a wavy surface.

FIGURE 9 also illustrates the carrier body which is denoted by reference numeral 90 here. The wavy surface 91 shown in this figure is obtained by a thermal treatment. In carrying out the latter, first the small uneven portions 83 of FIG. 8 are eliminated by melting so that the second wave form denoted in FIG. 9 by reference symbol '92, results. Upon continued action of the heat treatment the waves 92 are more and more eliminated by melting so that finally the desired flat wave form having a height 93 in the range of 10 to 20 and the width 94- which has been established already in the stage according to FIG. 8in the range of 10 to 50 results. Thereby the waves are arranged at random relative to each other, but within a statistically uniform distance determined by the sand blast treatment.

It will be understood from the above that variation of the Wave length depends on the selection of the sand blast means used. The height of the waves is determined by the duration of the heat treatment, i.e. the height becomes lower if the period of heat treatment is extended.

In producing the frame by deposition from vacuum, preferably gold, aluminum, silver, chromium or copper can be used. In producing the frame of several layers, for example, aluminum fluoride, magnesium fluoride, zinc sulfide, silver fluoride, thorium fluoride and tantalum oxide, can be used.

As an example of the above mentioned epoxy resin which consists of two components adapted to polymerize in mixture with each other, Palatal BK 269 and Palatal Hardener K 269 are mentioned, which are commercial designations of a conventional epoxy resin and a conventional hardener therefor. This resin and hardener are mixed and used in the proportion of 100:38. As a further example of epoxy resins the product sold under the name Araldit is mentioned which is likewise a conventional epoxy resin.

In carrying out the procedure described in the above Example II, the epoxy resin is diluted with acetone, for example in the proportion of 10: 1. The glass block used according to said Example II may be the glass BK7, which has an index of refraction of about n =1.5l68. In carrying out the procedure described in the above Example 11 the same epoxy resin can be used in all steps of the procedure.

In the above Example III, for example, polymerized methyl methacrylate can be used as a plastic and acetone can be used as a solvent.

In carrying out the pressing according to the above Example IV, the surface of a carrier glass body is heated to about 800 C. and the counter surface of the pressing tool, which may for example consist of nickel, chromium or stainless steel, is heated to about 400 C. If the material to be pressed consists of polymerized methyl methacrylate, or a similar plastic, the latter is heated to about 180220 C. and the tool used for pressing, which may for example consist of nickel, is heated to about 70-l00 C.

In the examples described above and diagrammatically illustrated in the drawings, the Waves are arranged at random. However, the waves can be present according to the invention also in a uniform arrangement, for example in the form of concentric circles.

It will be understood from the above that this invention is not limited to the arrangements, designs, materials, steps and other details specifically described above and illustrated in the drawings and can be carried out with various modifications without departing from the scope of the invention as defined in the appended claims.

A closer definition of the above mentioned sorts of glass BK2, B144 and BK7 are mentioned per example in the September 1956 edition No. 350 of the glasscatalogue of Jenaer Glaswerk Schott & Gen., Mainz.

An exact definition of the expression cohesion temperature is given per example in the July 1937 edition No. 5858 of the glass-catalogue of Ienaer Glaswerk Schott & Gen., Jena.

What is claimed is:

1. In an Albada type viewfinder including a front transparent optical element having an eye facing concave surface carrying a partially-light-permeable mirror, and a rear transparent frame carrier optical element spaced axially from the front element and having an object facing surface carrying an image limiting frame comprising a border strip bounding an open image viewing area, the frame being disposed in substantially the focal plane of the mirror for imaging thereby at substantially infinity: the improvement comprising, those portions of the frame carrier surface of said rear optical element underlining the frame being undulated and specular, and the frame having an undulating and reflecting surface conforming to the frame carrier surface; the viewing area bounded by the frame being clear and transparent whereby diffusion of light coming from the field of view of the viewfinder is avoided, and that portion of such light impinging upon the frame is reflected by a plurality of undulated surfaces as a concentrated reflection so that the major part of the light impinging upon the frame is reflected to said mirror for re-reflection to the eye of the observer; the length of the waves of the undulated frame carrier surface being from 0.10 to 0.50 the width of the frame border strip and within the range of from 5 to 50 times the wave length of light; the height of the waves of the undulated frame carrier surface being from 0.10 to 0.25 the length of the waves of the undulated frame carrier surrace.

2. The improvement in Albada type viewfinders, as claimed in claim 1, in which the entire area of the frame carrier surface of said rear optical element is undulated and the rear optical element is laminated with a clear material conforming to the frame carrier surface of the rear optical element.

3. The improvement in Albada type viewfinders, as claimed in claim 1, in which only that portion of the frame carrier surface of the rear optical element having the frame applied thereto is undulated.

4. The improvement in Albada type viewfinders, as claimed in claim 1, including a block of transparent material interconnecting said front and rear optical elements and having end surfaces conforming to the mirror carrying surface of the front optical element and the frame carrier surface of the rear optical element; said block being integrated with said rear optical element and forming a transparent cover for the frame on said frame carrier surface.

5. The improved Albada type viewfinder, as claimed in claim 4, in which the frame carrier surface is convexly curved and has a radius of curvature such that, at the eye position of one using the finder, the light illuminating the frame comes from that zone of the image field into which the frame is reflected.

6. The improved Albada type viewfinder, as claimed in claim 4, in which said frame carrier surface is convex; said block being glass and having a concave end surface. registering with said frame carrying surfaces; and cement joining the concave surface of said block to said frame carrier surface; the index of refraction of said cement being at least equal to the index of refraction of the glass of said body.

7. The improved Albada type viewfinder, as claimed in claim 1, including a transparent coating covering said frame carrier surface and the frame thereon, said coating having an index of refraction substantially equal to that of said frame carrier, and having a plane outer surface.

References Cited in the file of this patent UNITED STATES PATENTS 1,044,135 Buechner Nov. 12, 1912 2,187,057 Sauer Jan. 16, 1940 2,807,122 Upton Sept. 24, 1957 3,041,918 Papke July 3, 1962 FOREIGN PATENTS 996,990 France Sept. 5, 1951 1,004,913 Germany Mar. 31, 1957 OTHER REFERENCES Revue dOptique, vol. 35, No. 1, January 1956, pp. 21-34 cited.

Claims (1)

1. IN AN ALBADA TYPE VIEWFINDER INCLUDING A FRONT TRANSPARENT OPTICAL ELEMENT HAVING AN EYE FACING CONCAVE SURFACE CARRYING A PARTIALLY-LIGHT-PERMEABLE MIRROR, AND A REAR TRANSPARENT FRAME CARRIER OPTICAL ELEMENT SPACED AXIALLY FROM THE FRONT ELEMENT AND HAVING AN OBJECT FACING SURFACE CARRYING AN IMAGE LIMITING FRAME COMPRISING A BORDER STRIP BOUNDING AN OPEN IMAGE VIEWING AREA, THE FRAME BEING DISPOSED IN SUBSTANTIALLY THE FOCAL PLANE OF THE MIRROR FOR IMAGING THEREBY AT SUBSTANTIALLY INFINITY: THE IMPROVEMENT COMPRISING, THOSE PORTIONS OF THE FRAME CARRIER SURFACE OF SAID REAR OPTICAL ELEMENT UNDERLINING THE FRAME BEING UNDULATED AND SPECULAR, AND THE FRAME HAVING AN UNDULATING AND REFLECTING SURFACE CONFORMING TO THE FRAME CARRIER SURFACE; THE VIEWING AREA BOUNDED BY THE FRAME BEING CLEAR AND TRANSPARENT WHEREBY DIFFUSION OF LIGHT COMING FROM THE FIELD OF VIEW OF THE VIEWFINDER IS AVOIDED, AND THAT PORTION OF SUCH LIGHT IMPINGING UPON THE FRAME IS REFLECTED BY A PLURALITY OF UNDULATED SURFACES AS A CONCENTRATED REFLECTION SO THAT THE MAJOR PART OF THE LIGHT IMPINGING UPON THE FRAME IS REFLECTED TO SAID MIRROR FOR RE-REFLECTION TO THE EYE OF THE OBSERVER; THE LENGTH OF THE WAVES OF THE UNDULATED FRAME CARRIER SURFACE BEING FROM 0.10 TO 0.50 THE WIDTH OF THE FRAME BORDER STRIP AND WITHIN THE RANGE OF FROM 5 TO 50 TIMES THE WAVE LENGTH OF LIGHT; THE HEIGHT OF THE WAVES OF THE UNDULATED FRAME CARRIER SURFACE BEING FROM 0.10 TO 0.25 THE LENGTH OF THE WAVES OF THE UNDULATED FRAME CARRIER SURFACE.

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