WO1997001115A1

WO1997001115A1 - Fresnel lens

Info

Publication number: WO1997001115A1
Application number: PCT/NL1996/000238
Authority: WO
Inventors: Paul L. Csonka; Zoltán Horváth; Norbert KROÓ
Original assignee: Van Der Beek, Janco, Jan, Wigbold
Priority date: 1995-06-13
Filing date: 1996-06-13
Publication date: 1997-01-09
Also published as: AU6017896A; HU9501804D0

Abstract

The invention concerns a Fresnel lens comprising at least one grooved surface, whereby at least a portion (9, 10, 11) of said grooved surface is covered with a layer (12) avoiding scattering light of frequencies being within the frequency range of light prone to be scattered by the Fresnel lens. The production of undesirable scattered light is inhibited by covering the surfaces (9) and edges (10, 11) and by a lightabsorbing layer (12), either completely or partially.

Description

FRESNEL LENS

The invention concerns a Fresnel lens.

In particular the invention concerns a Fresnel lens which is adapted to alter the spatial distribution and to reduce the magnitude of scattered light generated in such a Fresnel lens.

Although most of the light incident on a Fresnel lens along the lens axis is generally concentrated by the lens near it's focal point, the remainder of the- light energy is scattered into points other than near the focal point as is clearly shown in figure 1.

The presence αf this scattered light is often un¬ desirable.

For example, when viewing the surroundings through a Fresnel lens fastened to the rear or side window or the rear view mirror of a car, this type of scattered light manifests itself as a background glare. That glare reduces the quality of the perceived picture and is especially disturbing during darkness.

Another example concerns overhead projectors utilizing a Fresnel lens; in these such scattered light appears as a brilliant background, not only in the projected image, but also in the eyes of the person who uses the projector. That can interfere with the per- formance of the speaker. Although farther from everiday experience, but representing a potentially very important group of applications, are the Fresnel lenses, Fresnel prisms and other various predominantly flat surfaces used in micro- optical computer linkups. These include the surfaces employed to match optical fibers to lightsources and signal receptors. On all these surf ces undesirable scattered light of the type here discussed does in general appear.

The aim of the invention is to provide a Fresnel lens wherein the intensity of the undesirable scattered light is decreased and the spatial distribution of the scattered is amended, so that the detrimental effects of such scattered light are reduced.

This aim is reached in that at least a portion of said grooved surface is covered with a layer avoiding scattering light of frequencies being within the frequency range of light prone to be scattered by the Fresnel lens.

The production of undesirable scattered light is inhibited by covering the surfaces and edges and by a lightabsorbing layer, either completely (Figure 3a) or partially (Figure 3b) .

Subsequently, the invention will be elucidated with the help of the drawings, which show: figure 1: a cross section of a prior art Fresnel lens, to explain the disadvantages thereof; figure 2a: a more detailed cross section of a part of the Fresnel lens depicted in figure 1; figure 2b: a more detailed cross section of another part of the Fresnel lens depicted in figure 1; figure 3a: a detailed view of a part of a Fresnel lens processed according to the invention; figure 3b: a detailed view of another part of a Fresnel lens processed according to the invention; figure 4a: a detailed view of a part of a Fresnel lens processed according another embodiment of the in¬ vention,- figure 4b: a detailed view of another part of a Fresnel lens processed according said other embodiment of the invention. Figure 1 shows a prior art Fresnel lens 1, its axis 2, and its focal point 3, lightbeams 4 incident on Fresnel lens 1, and parallel to its axis 2, lightbeams 5 concentrated near the focal point 3, and scattered light¬ beams 6. Figure 2a and figure 2b display enlarged sections of a prior art Fresnel lens 1, that is its flat surface 7 as well as on a grooved surface sections, the gently in¬ clined side surfaces 8, the steeply inclined side surfaces 9, concave edges 10 and convex edges 11. Also visible are the points of origin of some scattered light¬ beams 6.

One can imagine Fresnel lenses both of whose surfaces are grooved, however, for the sake of simplicity we do not show such lenses in figure 2.

It is clear from the figures that a large fraction of the undesirable scattered lightbeams 6 is generated on the side surfaces 9 and on the concave edges 10 and convex edges ll. Indeed, these surface components do not directly participate in useful image forming, or focussing, but play a role in the generation of unwanted scattered light.

The main feature of our invention is illustrated by Figure 3. The production of undesirable scattered light is inhibited by covering the side surfaces 9, and edges 10 and 11 by a lightabsorbing layer 12, either completely (Figure 3a) or partially (Figure 3b) .

The spatial distribution of scattered light can be altered (Figure 4) by appropriately treating all of

(Figure 4a) or part of (Figure 4b) the side surfaces 9, and edges 10 and 11 in a manner which causes them to diffusely reflect light, so that any sufficiently small element of a surface treated such, and that can be con-sidered so small, that it is a sufficiently good approximation of a flat surface, will scatter a lightbeam normally incident on it into a solid angle being essentially IT with an approximately uniform intensity distribution. If the intensity distribution of scattered light from an untreated Fresnel lens is not like this, then with such treatment one can achieve a more uniform diε¬ tribution of the scattered light, that is, one can de¬ crease the scattered light intensity in those directions which receive more of the scattered intensity from the untreated lens. Figure 4a and 4b show the so treated surfaces 13. The lightabsorbing layer 12 indicated in Figure 3 can be realized, for example, by applying a paint cover to the appropriate sections of side surfaces 9 and edges

10 and 11. The treated surfaces shown in Figure 4 can be realized in several ways. According to one method the appropriate sections of side surfaces 9 and edges 10 and

11 are first chemically treated with an appropriate agent, then the agent is rinsed off and finally the surface is dried. If the surface to be treated is glass, the chemical agent may be fluorine.

According to another method, the appropriate sur¬ faces are furbished or ground. If the surface in question is made of glass, then it can be ground using diamond powder, or by sandblowing. If the surface in question is made of a material softer than glass, then it can usually be ground using a powdered material softer than diamond. If the surface to be treated is readily deformed at room temperature, then it is advisable to perform the furbishing or grinding at low temperatures.

By covering sections of the grooved surface of a Fresnel lens with a protective layer, one can achieve that said lightabsorbing layer covers only the non¬ protected areas of the surface, or that a diffusely scattering surface is only formed where said protective layer does not cover the surface. Such lightabsorbing layer can be useful, even if it strongly absorbs light only over part of the spectral region of the undesired scattered light. We have experimentally demonstrated the feasibility of our invention on commercially available non-rigid Fresnel lenses. In the course of these experiments the lenses were illuminated by monochromatic as well as white light. As a result of covering the appropriate surfaces, we achieved a position dependent decrease in scattered light intensity of at least 50 %, even when the side surfaces 9 and concave edges 10 were essentially left uncovered and only convex edges 11 were covered by a lightabsorbing surface.

Having illustrated and described the principles of our invention with reference to preferred embodiments, it should be apparent to those persons skilled in the art that the invention may be modified in arrangement and detail without departing from the invention. We claim as our invention all such modifications as come within the true spirit and scope of the following claimε.

Claims

1. Fresnel lens, comprising at least one grooved εurface, characterized in that at least a portion of said grooved surface is covered with a layer avoiding scattering light of frequencies being within the frequency range of light prone to be scattered by the Fresnel lenε.

2. Fresnel lens according to claim 1, characterized in that the layer is adapted for absorbing light of frequencies being within the frequency range of light prone to be scattered by the Fresnel lens.

3. Fresnel lens according to claim 1, characterized in that the layer iε a diffuεe reflecting layer.

4. Fresnel lens according to claim 3, characterized in that the diffuse reflecting layer is a layer with such properties, that, if a lightbeam whoεe frequency lieε within the frequency range of light prone to be εcattered by εaid Freεnel lenε, the lightbeam is normally incident on a sufficiently small εurface of εaid diffuse reflecting layer, said layer reflects said lightbeam into a solid angle of the order of magnitude of it radials with an intensity distribution that is substantially uniform over εaid εolid angle, said surface being sufficiently small whenever said area can be adequately approximated by a flat area.

5. Fresnel lens according to one of the claims 1-4, characterized in that the portion compriseε at leaεt partε of thoεe parts of the grooved surface that extend substantially parallel to the axis of the Fresnel lenε.

6. Fresnel lens according to one of the claims 1-5, characterized in that the portion comprises at least parts of the convex edges of the grooved surface.

7. Fresnel lens according to one of the claims 1-6, characterized in that the portion compriseε at leaεt partε of the concave edges of the grooved surface.

8. Fresnel lens according to claim 5, characterized in that the portion compriseε adjacent parts of the adjacent edges of the grooved surface.

9. Method for preparing a Fresnel lens according to one of the preceding claimε, characterized in that at leaεt a portion of the grooved εurface of the Freεnel lenε is covered by a layer avoiding scattering light of frequencies being within the frequency range of light prone to be scattered by the Freεnel lens.

10. Method according to claim 9, characterized in that the portion of the grooved surface of the lenε iε covered with light abεorbing material.

11. Method according to claim 9, characterized in that the portion of the grooved εurface is covered with a diffuεe reflecting layer.

12. Method according to one of the claimε 9-11, characterized in that the areas to be covered are preprocessed before the layer is applied.