GB2332638A

GB2332638A - Process for producing a doubly convex lens screen

Info

Publication number: GB2332638A
Application number: GB9827821A
Authority: GB
Inventors: Peter Schlechtingen; Michael Helker
Original assignee: ITOS GmbH; Gebrueder Merten GmbH and Co KG
Current assignee: ITOS GmbH; Gebrueder Merten GmbH and Co KG
Priority date: 1997-12-24
Filing date: 1998-12-17
Publication date: 1999-06-30
Anticipated expiration: 2018-12-17
Also published as: GB9827821D0; GB2332638B; DE19757836C1

Abstract

For producing a doubly convex lens screen (10; figures 1,2) comprising a plurality of Fresnel lenses (13; figures 1,2), a mould core 16 is used which is assembled from at least one ring 17a,17b,17c, which may be either open or closed. The or each ring 17a,17b,17c is assembled by joining together segments 22 which are provided with a Fresnel structure 30 on their doubly convex curved exterior faces. These segments 22 are preferably prepared from rods, with the Fresnel structures 30 being formed by machining the exterior face of each segment 22. The mould core 16 is used in the formation of the lens screen (10; figures 1,2) using injection moulding. The mould core 16 may be assembled using segments 22 having different Fresnel structures, each of the segments 22 being readily replaceable in the or each ring 17a,17b,17c. The exterior surface of the mould core 16 is preferably spaced at a substantially constant distance from the interior surface of a hollow mould, disregarding the effect of the Fresnel microstructure, so that the lens screen formed in the injection moulding process has a substantially constant wall thickness (see figure 5). In this manner, a uniform radiation absorption behaviour is achieved.

Description

Process for producing a doubly convex lens screen The invention relates to a process for producing a doubly convex lens screen comprising multiple lens elements consisting of Fresnel lenses.

Fresnel lenses, also called stepped lenses or zone plates, consist of a flat plate of transparent material in one side of which saw-toothed grooves are formed, the working edges of which produce the same result as a lens of an altogether spherical exterior surface. It has been known to produce lens screens comprising numerous Fresnel lenses as a plane plastic plate out of which the user can cut individual lens elements.

Such lens elements can be joined to form desired combinations.

Motion detectors comprising an infrared sensor are also equipped with lens screens comprising Fresnel lenses determining the detection field of the motion detector. To ensure a detection field as large and complete as possible, panorama lens screens are required which allow for a 360" detection in a horizontal plane, on the one hand, and also for a complete detection in a vertical plane, on the other hand.

To ensure a correct optical representation in both planes, it is necessary to employ a doubly convex lens screen. While a lens element with a simple convexity, i. e. a cylindrical element, can be produced by bending a plane lens, such a production is not possible in the case of doubly convex lens elements.

From GB 902 535, an individual lens element in the shape of a Fresnel lens is known, which is curved in a doubly convex manner, the thickness of the material across the stepped Fresnel structure being substantially the same in any place on the lens. However, such an individual lens element is unsuitable for motion detectors. These require a lens screen with numerous lens elements. If such a lens screen is produced by an injection moulding process, very complex moulding tools are required. If it turns out that a lens screen produced by means of these moulding tools does not meet practical requirements, it is practically impossible to change the moulding tool. In these cases, a very expensive new moulding tool has to be produced.

Accordingly, it is desirable to provide a process for producing a doubly convex lens screen by means of which lens screens of high quality and precision can be produced and varied in a relatively simple manner.

According to an embodiment of a first aspect of the present invention, there is provided a process for producing a doubly convex lens screen comprising lens elements consisting of multiple Fresnel lenses, comprising the following steps: a) producing segments having a doubly convex exterior surface with a Fresnel structure provided in the exterior surface, b) joining the segments to form an open or closed ring, c) using the ring as a mould core of an injection tool by means of which the lens screen is formed.

In a process embodying the present invention, a mould core is used consisting of numerous separate segments being joined to form the mould core. The segments each have a double convex exterior surface in which the corresponding Fresnel structure is formed. The segments form an open or closed ring in the form of a strip or stripe. The production of the Fresnel structures on the exterior surfaces of the segments is made separately from segment to segment. Thereby, it is possible to apply the same or different Fresnel structures to the segments. If the lens screen is to be changed, it is only necessary to change individual segments. In this manner, the optical characteristics of the lens screen to be produced by means of the mould core can be changed. Even if the mould core has been damaged by scratches or the like, it is only necessary to replace the damaged segments.

The production of the Fresnel structure on the segments can be made by machining and subsequent polishing. Machining by means of spark erosion or a laser beam is also possible.

For practical reasons, the segments are desirably produced by at first producing segment blanks of rod material, for example out of round rods. These segment blanks have spherical smooth exterior surfaces on the face sides thereof. The Fresnel structures are formed into these exterior surfaces. Then the segments are produced by producing or machining the segment lateral surfaces of the segment blanks in such a manner that the segments can be joined to form a doubly convex mould core.

The exterior surfaces of the individual segments forming the ring or a structure consisting of multiple rings are preferably shaped such as to form a continuous steady overall surface. This means that the exterior surface of the ring or the annular structure does not have any breaks or kinks, apart from the Fresnel structure of the exterior surface, of course, which forms a microstructure. The continuous overall structure of the exterior surface of the mould core, together with a correspondingly formed hollow mould, result in the production of a lens screen having the same wall thickness in any place thereof, so that no local variations in thickness occur. Such variations in thickness would cause the absorption behaviour of the lens screen consisting of plastics to differ locally so that zones of different sensitivity would be formed.

According to an embodiment of a second aspect of the present invention there is provided a mould core for producing a doubly convex lens screen comprising multiple lens elements consisting of Fresnel lenses, having a mould core with at least one open or closed ring in the form of a stripe of exchangeably joined segments with a doubly convex exterior surface, the exterior surfaces, provided with Fresnel structures, of the segments continuously merging into each other at the edges thereof.

Finally, according to an embodiment of a third aspect of the present invention there is provided a lens screen with doubly convexly curved lens elements consisting of Fresnel lenses and forming an open or closed ring, the ring comprising a substantially constant wall thickness across all the lens elements.

In the case of a presence of immediately adjacent multiple rings, the wall thickness is also constant across the rings - apart from the microstructure of the Fresnel steps, of course. Thereby, a low absorption that is uniform in all directions may be achieved so that a motion detector equipped with a lens screen embodying the present invention has the same sensitivity in all directions.

Reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 shows a cross-sectional view through a lens screen along line I-I of Fig. 2, Fig. 2 shows a top plan view of the lens screen according to Fig. 1, Fig. 3 shows an exploded representation of a mould core consisting of three segmented rings, Fig. 4 shows one of the segments of the mould core, and Fig. 5 shows a cross-sectional view through a mould for producing the lens screen.

The lens screen 10 represented in Figs. 1 and 2 is a panorama lens screen for a motion detector. It comprises a dome-shaped or hemispherical cap 11 which is doubly convexly curved and, for example, has a spherical shape.

The lens screen 10 comprises an integral plastic piece of transparent polyethylene. An edge 12 is formed on the cap 11.

While the exterior side of the cap 11 is smooth, the interior side is provided with numerous lens elements 13 which are formed as Fresnel lenses. Each lens element 13 is also doubly convexly curved.

In the embodiment represented, the lens elements 13 are arranged in the form of three concentric rings surrounding a central area 14. In the central area 14, some lenses are formed in the shape of spherical lenses 15.

In the interior of the lens screen, an infrared sensor (not represented) is positioned. Each of the lens elements 13 is focussed onto the infrared sensor. Thus, each lens element guides radiation from an angular supervision area associated therewith onto the sensor. According to individual requirements, the lens elements each arranged on the same ring may be identical.

Fig. 3 shows the mould core 16 of an injection mould by means of which the lens screen 10 can be produced. This mould core comprises three rings 17a, 17b, 17c each comprising a disc cut out of a sphere, the upper and lower lateral surfaces 18, 19 extending parallel to each other and the circumferential surface 20 being curved spherically. Each ring comprises a hole 21 extending therethrough in the interior.

Each ring is divided into numerous segments 22, each comprising a negative Fresnel structure in the shape of concentric saw-toothed grooves on the exterior surface 23 thereof. Each of the segments 22 has right and left lateral surfaces 27 converging towards the axis of the ring. The exterior surface 23 is limited by lateral edges 28 and upper and lower terminal edges 29. Adjacent segments 23 of the same ring abut onto each other at the lateral edges thereof, but the transition is continuous, i. e. without kinks, so that the spherical exterior surface is not interrupted or disturbed. The same is true for the terminal edges 29. At the terminal edges 29, two adjacent rings 17a, 17b or 17c, 17d abut onto each other, without the continuity of the exterior surface being interrupted.

Each segment is formed out of a rod-shaped segment blank. The segment blank consists of a cylindrical or angular rod material with a spherical exterior surface on a face side. The curve of this exterior surface corresponds to the curve of the subsequent lens screen. At first, the exterior surface is smooth. The segment blank is then clamped into a machine tool, and the Fresnel structure is formed into the spherical exterior surface 23 by machining. The resulting segment blank comprises an exterior surface 23 which is too large, i. e. oversized, and further comprises a rod-shaped body. Then the lateral surfaces 18, 19 and 27 of the rod material are produced in the manner in which they are needed, so that the segments of identical shape form the corresponding ring 17a, 17b, 17c.

The flanks of the Fresnel structure 30 are clearly shown in Fig. 4. Each working flank consists of a spherical surface formed by machining the exterior surface 23.

In the case of the present embodiment in Fig. 3, all the segments of a ring are similar to one another. The segments are joined to form the corresponding ring. For this purpose, they can comprise interlocking engagement portions (not repre sented), for example groove and tongue. However, it is also possible to connect the segments by adhesion. If required, it is possible to release an individual segment and replace it by another segment.

In the present case, the mould core 16 is composed of the rings 17a, b, c.

Fig. 5 shows a mould in which the mould core 16 of Fig. 3 is used together with a hollow mould 32. The hollow space 33 of the mould is filled with plastics material, which then cures to form the lens screen.

The saw-toothed Fresnel structure 30 can be recognized on the exterior side of the mould core 16. The width of the hollow space 33 of the mould is dimensioned such that the envelope 35 connecting the valleys of the Fresnel structure has the same distance from the interior surface 34 of the hollow mould 32 in any place thereof so that the wall thickness of the lens screen - apart from the Fresnel microstructure 30 - is the same in any place thereof, even across multiple lens elements.

In the case shown in Fig. 5, all the peaks of the Fresnel structure 30 have the same distance to the interior surface 34 of the hollow mould. Deviations from this are possible. In any case, it is ensured that the lens screen has substantially the same wall thickness and thus the same absorption behaviour in any place thereof.

Claims

Claims 1. Process for producing a doubly convex lens screen comprising lens elements consisting of multiple Fresnel lenses, comprising the following steps: a) producing segments having a doubly convex exterior surface with a Fresnel structure provided in the exterior surface, b) joining the segments to form an open or closed ring,

c) using the ring as a mould core of an injection tool by means of which the lens screen is formed.
2. Process according to claim 1, wherein the step (a) of producing the segments comprises the following substeps: al) producing segment blanks of rod material with a spherical exterior surface on the face side thereof, a2) forming Fresnel structures into the spherical exterior surfaces of the segment blanks, a3) producing the segment lateral surfaces.
3. Process according to claim 1 or 2, wherein multiple rings of segments are produced with a doubly convex exterior surface, the exterior surfaces of adjacent rings continuously merging into each other.
4. Process according to any one of claims 1 to 3, wherein the Fresnel structures are formed by machining and subsequent polishing.
5. Process according to any one of claims 1 to 4, wherein during the formation of the lens screen, a hollow mould is used, the interior profile of which follows the exterior profile of the mould core - regardless of the Fresnel microstructure at a substantially constant distance.
6. A process, for producing a doubly convex lens screen, substantially as hereinbefore described with reference to the accompanying drawings.
7. Mould core for producing a doubly convex lens screen comprising multiple lens elements consisting of Fresnel lenses, having a mould core with at least one open or closed ring in the form of a stripe of exchangeably joined segments with a doubly convex exterior surface, the exterior surfaces, provided with Fresnel structures, of the segments continuously merging into each other at the edges thereof.
8. Mould core according to claim 7, wherein at least two rings are provided being joined with the exterior surfaces thereof continuously merging into each other.
9. A mould core, for producing a doubly convex lens screen, substantially as hereinbefore described with reference to the accompanying drawings.
10. Lens screen comprising lens elements having a doubly convex curve and consisting of Fresnel lenses, multiple lens elements forming an open or closed ring in the form of a stripe, wherein the ring provides a substantially constant wall thickness - regardless of the Fresnel microstructures - across all the lens elements.
11. A lens screen substantially as hereinbefore described with reference to the accompanying drawings.