Louvered film for unidirectional light from a point source

Abstract

A process for producing film including radiation-opaque louver-like elements of divergent, or convergent, orientation. Pairs of such films for example, using radio-opaque louvers, are valuable as Bucky grids for X-rays.

Parent Case Text

This application is a continuation-in-part of my copending application, Ser. No. 284,403, filed Aug. 28, 1972, which was a continuation-in-part of my application, Ser. No. 128,280, filed Mar. 26, 1971, now abandoned.

Description

This invention relates to a process for the production of film or sheet materials having divergent or convergent radiation-opaque louver-like elements. It further relates to the sheet materials produced by the process and to laminates of these sheet materials with one another with axes at different angles with or without cover sheets and to laminates of single sheets with one or two cover sheets.

It is known from U.S. Pat. No. 3,524,789 of Olsen that film materials or sheet materials may be produced including louver-like elements transverse to the plane of the sheet and substantially normal thereto. Such films with parallel louver elements are known to collimate incident light and to be valuable for the control of optical aperture. In this case and elsewhere, parallel indicates parallelism of the planes of the louver elements within .+-. 3.degree. in extremes although usual deviation is much less, of the order of .+-. 0.5.degree. to .+-. 2.degree.. It is also known to produce film in which the louvers are uniformly canted at an angle such that transparency is only apparent when viewed in a particular direction. Fortuitously, at times, sheet film has been produced in which there was a gradual change in angle of cant of successive louverlike elements; but no control has hitherto been possible such that desired ranges of cant or sequences were possible.

For certain purposes, it is desirable that light rays be made divergent or convergent without the use of lenses. A particular problem in this respect is with regard to X-rays where it is desired to "focus" for the purpose of giving sharper pictures by absorption of secondary rays. The focusing of X-rays by the usual optical lense system is, of course, not feasible. Handmade devices known as Bucky grids have been produced for such purposes. Because these are made with considerable difficulty by manual operations, they are exceedingly expensive, on the order of $100 or more per square foot. Such grids are usually mounted between the film and patient and oscillated slightly during exposure to avoid producing a shadow of the grid in the negative.

It is an object and aim of this invention to provide film material capable of divergent or convergent orientation of light and other radiant energy. Other aims and objects will become apparent hereinafter.

It had been found, in accordance with these objects and aims, that differential distortion of film or sheets having louverlike elements transverse and normal to the surfaces is possible under conditions such that substantially any desired pattern of convergent and/or divergent louvers is feasible. The procedure is illustrated herein with particular reference to simple converging or diverging sheet material, but it will be recognized that variations on this procedure can be introduced quite readily using louvers opaque to various wavelengths, variously colored or combinations of convergence and divergence.

The essential procedure is first, to bond the radiation transparent thermoplastic sheet containing transverse substantially parallel radiation-opaque louver-like elements between relatively rigid or dimensionally stable, but flexible, cover sheets, most suitably of metal, using enough heat to promote adhesion to give a hot laminate, second, to form the still hot laminate produced in the first step around a curved surface so that the louver-like elements of the original thermoplastic sheet are parallel to the axis of the curved surface and remain substantially parallel to one another and third, after cooling, separate the deformed thermoplastic sheet from the cover sheets. Flattening means is then applied as a fourth step. In many cases the step required is to press the deformed sheet into planarity using heat. In other cases, where the sheet is relatively very thin, of the order of about 1 mm. or less, it may be usable directly as produced because it then flattens spontaneously to a sufficient extent to give divergence of the elements or it may be mounted so as to maintain an essentially flat configuration.

Without wishing to be bound by the theory, it would appear that the flattening step may introduce strain which must be relieved by thermal flattening in the case of thicker materials, but is not so great as to require relief in the case of thinner materials.

In one variation of the invention, the cover sheets are fastened to each other along one edge so that angle of cant of the louver elements is progressive from one edge to the other of the light diverging sheet. It will be recognized that it is possible to start with a thermoplastic sheet including regularly canted louver elements, that is all canted at the same angle, and by this procedure superpose further progressive cant. For example, starting with a sheet having 15.degree. cant, a progressive cant of 0.degree. to 15.degree. progressive cant to give one having 0.degree.-30.degree. or two such sheets can be butted to a sheet having +15.degree. to -15.degree. cant to give a combination of +30.degree. to -30.degree.. Such techniques are useful for preparing very large sheets which have a close point for convergence of elements as will be evident.

The final flattened sheet may, in some instances, be used directly or it may be covered with clear transparent cover sheets on one or both surfaces of two or more may be joined together (back to face) with their louver elements non-parallel and particularly at right angles, either with or without the use of clear transparent cover sheets. It will, of course, be also possible to employ louver-like elements which are colored or which have particular properties. A particularly useful embodiment of the invention is one in which the louver-like elements are composed of a radio-opaque substance, e.g., red lead, or powdered lead in a suitable compatible thermoplastic binder and two such diverging sheets are cemented together at right angles to give a screen which can be employed as a Bucky grid to absorb secondary radiation, i.e., scattered or stray rays. The range of angles of cant is readily controlled so that convergence can be at any desired distance from the screen. The point of convergence is the point at which the radiant energy source, i.e. X-ray source, is placed.

It will be recognized that refractive indices of thermoplastic materials used will affect various radiation differently and visible light will focus differently from X-rays.

I am aware of the procedures proposed for producing Bucky grids described in U.S. Pat. Nos. 2,122,135 and 2,133,385 which rely on the difficult step of cutting a curved sheet and subsequently flattening it. I am also aware of the disclosures on production of Bucky grids of U.S. Pat. Nos. 1,551,162, 2,336,926, 2,435,823 and 2,566,998. My procedure using adhered dimensionally stable sheets is quite different from all the above.

The initial sheets having transverse radiation-opaque louver-like elements are most conveniently made by the process of the aforementioned U.S. Pat. No. 3,524,789. They may therefore include substantially any plastic base although cellulose acetate butyrate is a particularly convenient one. Polyvinyl butyral is also desirable, but somewhat more difficult to handle because of its lower melting point. The rigid but flexible cover sheets employed in the first step of the process are usually thin metal which can be bent readily but is not so ductile that it is stretched under the process of the invention. Suitable commonly available materials include sheets of aluminum of the order of 0.01 to 0.04 inches (0.25 mm.) thick. The exact thickness of these sheets is, of course, not critical provided only that they are sufficiently strong to withstand the subsequent operation in which they are employed. The surfaces are preferably not glossy when very short focal lengths are being produced or when subsequent lamination steps are envisioned. The surfaces should never be exceedingly rough. Ferrotype sheets as used in photography are convenient and useful to provide highly polished surfaces. These sheets are not deformed in the process when the radius of curvature employed is more than about 25 cm. and they may be reused. Usually the radius of curvature is from about 25 to 250 cm. A satiny finish is quite satisfactory for many purposes.

Lamination of the plastic sheet between the two cover sheets is conveniently carried out in a press at pressures of 25 to 100 psi and above at temperatures sufficient to soften the thermoplastic polymer involved, for example, 300.degree. F. in the case of cellulose acetate butyrate. Suitable padding may be applied on either side of the sandwich or laminate being made in order to avoid possible adhesion to the platens, to provide greater uniformity of heating and, possibly, to moderate or distribute pressure more uniformly. The use of padding is not, however, essential to the process of the invention.

The hot sandwich is then deformed by bending over a suitable curved surface, for example, a section of a cylinder having a radius of about 24 inches. Smaller radii and larger radii are also useful from about 25 to 250 cm. Approximately the radius of curvature used is about twice the focal distance desired. A matching platen may be used or a sheet of fabric fastened at one edge of the curved surface may be drawn down taut. After the polymeric material has cooled, the arcuate thermoplastic sheet separates from the metal cover sheets as the result of differential expansion, that is, due to differences in expansion coefficients. The cooled sheet is normally somewhat less curved or arcuate than the cylinder around which it was formed to an extent depending upon a number of factors. The result is that it is not necessary to have a cylinder of large radius to obtain an arcuate thermoplastic sheet of that radius. When the thermoplastic sheet is of the order of 1 mm. or less in thickness, it may fequently be used directly because mounting means may exert sufficient force to serve as flattening means but not in the sense of achieving thermal flattening.

The cool arcuate sheet is thermally flattened by application of pressure and heating to about the extent needed for the initial lamination step and suitably while applying clear cover sheets. The cover sheets obviously can be colored if desired. This additional flattening step is necessary for thicker sheet materials of more than about 1 mm. thickness and optional for thinner sheet materials.

The invention is now further explained by the accompanying drawings which show the process of the invention in an essentially diagramatic manner and also products of the invention.

FIG. 1 shows the hot lamination step to give the hot laminate or sandwich.

FIG. 2 shows placing the hot laminate or sandwich in a press to provide curvature.

FIG. 3 shows the deformation of the laminate or sandwich of FIGS. 1 and 2.

FIG. 4 shows that after cooling, the laminate of FIG. 3 separates so that the metallic sheets are separated. In this and the following figures, the arcuate sheet is shown as fully formed around the cylinder.

FIG. 5 shows placing the deformed sheet of FIG. 4 in a heated flat press, and

FIG. 6 shows the distortion of the louvers after again pressing flat.

FIG. 7 shows a perspective view of the flat sheet of FIG. 6 and FIG. 8 shows a top view thereof.

FIG. 9 shows a laminate of two sheets as in FIG. 7 with axes at right angles and FIG. 10 shows the effect when that laminate is viewed from above.

Referring again to the Figures, thermoplastic lightcontrolling sheet 10 (suitably about 0.3 to 5 mm. thick) in FIG. 1 having louver elements 18 is laminated between 0.50 mm. thick sheets of aluminum 12 by applying pressure (means not shown) to heated plates 16 having non-adhering pads 14. Although some of the desired effect can be obtained by adhering only one cover sheet, it is preferred to use two as here described.

The laminate formed may be designated 12-10-12 and in FIG. 2 the still hot laminate 12-10-12 is placed between arcuate forming means 20,22 using adhesion preventing pad (not shown) if desired and pressure is applied as in FIG. 3 to deform the laminate. Because the angular arcs of the upper and lower aluminum sheets 12 are different although the widths as shown are the same, the apparent effect of this step is the vertical displacement of the louver elements 18 although a slight lateral motion of the upper edges may also occur in this step. At the same time as the laminate is deformed (FIG. 3), it is cooling because no heat is applied and adhesion of aluminum sheets 12 relaxes and they are freed as shown in FIG. 4 having the arcuate light-controlling sheet 30. It is within the scope of the invention to provide heat during the step and subsequently cool after deformation. As noted above, if the arcuate light-controlling sheet is sufficiently thin, of the order of 1 mm. or less, it may be used at this point relying on mounting means to provide sufficient flattening means to produce substantial divergence of the louvers.

The arcuate light-controlling sheet 30 is now placed between heated platens 16 and non-adhering pads 14 as shown in FIG. 5. The same apparatus as used in FIG. 1 is shown but, obviously, the exact piece of equipment is not necessary. Pressure and heat are applied as indicated in FIG. 6. Because of the way pressure is applied, there are forces which deform the arcuate sheet 30 into an optically flat sheet 40 having the louver elements variously and progressively inclined as shown. Approximately, the maximum angle of inclination will be a function of the angle in FIG. 5 between the lower platen 16 and the tangent to the lower surface of sheet 30 at the outermost edge. Clearly, also the smaller the radius of curvature of sheet 30, the greater will be the maximum inclination of louver elements 18 in sheet 40. The radius of curvature may be controlled by the bending means 20, 22 used for deformation in FIGS. 2 and 3.

FIG. 7 shows a single sheet 40 having convergent (from bottom to top) louver elements 18. The axis of the sheet is parallel to these elements. The top view in FIG. 8 shows top edges 50 of louver elements 18 as full lines and lower edges 52 as broken lines. Because the elements are opaque, it will be recognized that from above the sheet 40 will appear more opaque near the edges and more nearly transparent near the center when viewed at a distance. This view corresponds to any distance other than what may be designated the focal length which is the distance above the plane of the sheet at which extensions of all louver elements would meet. Only at this focal point will the eye see through the entire sheet; at all other positions there will be greater or less obscuring along the edges.

FIG. 9 shows a laminate (which may be further laminated with cover sheets, not shown, as may also sheet 40 of FIG. 7) of two sheets 42 and 44 at right angles. FIG. 10 shows a top view but because they would be confusing, only upper edges 50 and 60 of louver elements are indicated. The effect is to give a rather square hole in the middle and a focal point from which almost complete transparency is attained except for some distortion along diagonals. A laminate such as shown in FIG. 10 in which the louver elements comprise sufficient radio-opaque material such as red lead, serves to focus X-rays as a Bucky grid. Other uses will also be evident in signals, windows, and other articles depending on optical properties or effects. Thus, a grid of the type shown in FIGS. 9 and 10 may be used for viewing a television or cathode ray screen from one specific position which would not be visible or only limitedly so, from other positions.

Claims

What is claimed is:

1. In a process for the production of thermoplastic sheet material having diverging louver elements, the steps of

1. thermally bonding at least one thin sheet of metal to a flat sheet of radiation-transparent thermoplastic material having substantially parallel radiation-opaque louver elements at a uniform angle to its surface to form a hot laminate,

2. arcuately deforming said hot laminate forming an arcuate laminate having substantially parallel louver elements and cooling said laminates, and

3. separating each said sheet of metal from said arcuate laminate after cooling to provide an arcuate radiation-controlling sheet having substantially parallel louver elements.

2. The process according to claim 1 additional employing the step of applying flattening means for flattening said arcuate radiation-controlling sheet to form a light diverging film wherein the substantially parallel louver elements of steps (1), (2) and (3) are made mutually divergent.

3. The process according to claim 2 wherein the thermoplastic material is about 1 mm. thick or less and flattening means is applied by mounting so as to maintain an essentially flat configuration.

4. The process according to claim 1 wherein one sheet of metal is bonded to both surfaces of the thermoplastic material.

5. The process according to claim 1 wherein one sheet of metal is bonded to each surface of the thermoplastic material.

6. The process according to claim 1 wherein the hot laminate is arcuately deformed in Step (2) against a cylindrical surface with the lengths of louver elements parallel to the axis of the cylinder.

7. The process according to claim 6 wherein the thermoplastic sheet material is cellulose acetate butyrate.

8. The process according to claim 1 wherein the louver elements are radio-opaque.

9. A radiant energy diverging film of thermoplastic material of about 1 mm. or less in thickness, produced by the process of claim 1, and spontaneously flattened to have divergent radiation-opaque elements.

10. A radiant energy diverging film according to claim 9 having radio-opaque louver elements, comprised of radio-opaque substance and compatible thermoplastic binder.