WO1995034006A1

WO1995034006A1 - Optical microrelief retroreflector

Info

Publication number: WO1995034006A1
Application number: PCT/SE1995/000644
Authority: WO
Inventors: Lars Danielsson; Staffan Niemann
Original assignee: Industrimateriel Ab
Priority date: 1994-06-06
Filing date: 1995-06-02
Publication date: 1995-12-14
Also published as: SE9402969D0; SE9402969L; SE503429C2

Abstract

The present invention concerns a retroreflective device that is composed of a number of optical microrelief elements. The retroreflector includes a first layer (12) consisting of a number of densely packed coplanar microrelief lenses, a second layer (14) consisting of a number of densely packed coplanar microrelief mirrors, coated with a reflecting layer (16) on the side opposite to the microrelief lenses, and a third layer (17) in-between the first and the second layer. The first and the second layer are oriented so as to exhibit a multitude of element pairs, each in the form of a microrelief lens (12) and a microrelief mirror (14) with essentially coinciding optical axes. The distance between a microrelief lens (12) and the corresponding microrelief mirror (14) is such that the microrelief mirror is located essentially in the focal plane of the microrelief lens. The microrelief mirror has its equivalent centre of curvature essentially in the centre of the corresponding microrelief lens.

Description

OPTICAL MICRORELIEF RETROREFLECTOR

Description The present invention concerns a retroreflective device that is composed of a number

5 of optical microrelief elements, where optical microrelief elements is taken to have reference to either diffractive zone plates, refractive Fresnel elements or hybrids composed of diffractive zone plates and refractive Fresnel elements.

Such a retroreflector is intended to be used for road signs, vehicle plates, reflexes, 10 etc.

Retroreflective devices are previously known (cf. for example American patents No 2,555,191 and 4,511,210) that are based on spherical glass balls acting as lenses. These glass spheres have a refractive index in relation to the ambient media that is

15 large enough to make its focal plane coincide with the image side of the sphere. This side, possibly with an addition of a separating layer of optional thickness, is coated with a totally reflecting layer with the same centre of curvature as the glass spheres. Typical diameters of the glass balls are of the order of 100 micrometers. Basically this gives a ray structure where light from a distant source is reflected antiparallel

20 with the incident light.

These known retroreflectors are expensive to manufacture. The requirements are high on shape and refractive index for the glass spheres, which basically are bad lenses. The spherical aberration so large that the spheres are in fact functioning only 25 for central light rays.

* An alternative optical design is based upon retroreflecting prisms in the form of corner cubes. The prisms are arranged in a densely packed matrix on a plastic layer

_* where the size of the prisms is of the order 100 micrometers. A retroreflector of this 30 type is substantially easier and cheaper to manufacture than the one based on spherical glass balls. However, the light output decreases considerably faster with increasing incidence angle and special arrangements have to be employed in order to make the device independent of the azimuthal orientation in relation to the normal of the layer.

The main objective with the present invention is now to accomplish a retroreflector that is cheap to manufacture and has a large light output at the same time as it is possible to achieve acceptable incidence angles.

The intended objective is accomplished with a retroreflector of the kind indicated in the introductory paragraph above. This retroreflector includes 1. a first layer consisting of a densely packed array of coplanar microrelief lenses, i.e. microrelief elements of lens type, 2. a second layer consisting of a densely packed array of coplanar microrelief mirrors, i.e. microrelief elements of mirror type, coated with a reflecting layer, and 3. a third layer in-between the first and the second layer where

• the first and the second layer are oriented so as to exhibit a multitude of element pairs, each in the form of a microrelief lens and a microrelief mirror with essentially coinciding optical axes, • the thickness of the third layer is such that the distance between a microrelief lens and the corresponding microrelief mirror is such that the microrelief mirror is located essentially in the focal plane of the microrelief lens, and where

• the microrelief mirror has its equivalent centre of curvature essentially in the centre of the corresponding microrelief lens.

Preferred types of design of the retroreflector are specified in the subordinate patent claims.

Description of the preferred embodiment The invention is further described below with reference to the attached figures:

Fig. 1 shows in perspective a substrate for a retroreflector composed of a number of microrelief lenses and microrelief mirrors; Fig. 2 shows schematically the ray structure in an optical element pair which is composed of a zone plate of lens type and a* zone plate of mirror type; and Fig. 3 shows the location of the focal point in media with different index of refraction.

As exhibited in Fig. 1. the retroreflector depicted in this example consists of a square layer substrate 10. This consists of a suitable acrylic glass, for example polycarbonate. The side of the substrate 10 facing the viewer exhibits a large number of identical squares 12. Each square 12 is representing a microrelief lens. Opposite such a microrelief lens 12 there is on the other side of the substrate 10 a corresponding microrelief mirror 14. The microrelief lens 12 and the microrelief mirror 14 have essentially coinciding optical axes and constitute together an optical element pair.

The optical element pairs, i.e. the microrelief lenses 12 and the microrelief mirrors

14 may be manufactured by means of replication from a mother matrix, which in turn can be manufactured by means of laser or electron beam lithography. Other ways of manufacturing the mother matrix for replication, which have been long known, are by means of holographic methods or interference methods. The replication should be performed synchronously on the two sides of the substrate 10.

The side of the substrate 10 opposite to the microrelief lenses 12 is coated with a reflecting layer 16, as shown in Fig. 2. This layer 16 may for example be an aluminum coating which may be applied directly on to the substrate 10 by means of vaporization or sputtering.

Thus the substrate 10 is composed of a first layer of densely packed square microrelief lenses 12 and second layer of likewise square, focusing microrelief mirrors 14. In-between these two layers there is another layer 17 of an optical distance material. In the presently illustrated shape all these layers are integrated in the same substrate 10, where the first and the second layer have the same matrix shape relative to the optical elements. The distance between a microrelief lens 12 ^•and the corresponding microrelief mirror 14 is such that the microrelief mirror 14 is situated in the focal plane of the microrelief lens 12 with due consideration of the refractive index of the medium between the optical elements. The microrelief mirror 14 shall have its equivalent centre of curvature in the centre of the microrelief lens 12. This implies that the focal distance of the microrelief mirror 14 shall be half of that for the microrelief lens 12 with due consideration of the refractive index of the intermediate third layer 17.

The arrangement illustrated in Fig. 2. shows an example on a large scale, of the bending (diffraction in this case) of light rays in an optical element pair including a zone plate 12 of lens type and a zone plate 14 of mirror type, the latter with a reflecting layer 16 on the back side.

In the present example the refractive index of air is unity, and the refractive index of the medium of the substrate - polycarbonate - is 1.6. The focal distance of the zone plate 12 is f=480 μm in the medium and the length of its square side is D=200 μm.

The zone plate 12 includes approximately 40 annular zones (Fresnel zones). The focal distance of the zone plate 14 is 240 μm and the length of its square side is the same as that of the zone plate 12, i.e. 200 μm.

It should be noted that the value of the F-number, i.e. the numericaraperture =f/D, of the lens elements is governing the achievable incidence angle. In the arrangement of the example above, where the F-number is f/D= 1.5 (1.1 counting the corners), the maximum incidence angle is 27 degrees.

The refractive index of the substrate is governing its thickness. This is illustrated in

Fig. 3. which shows the location of the image points p_j and P2 when the intermediate layer is air, with refractive index unity, and when the intermediate layer is polycarbonate with refractive index 1,6.

A retroreflector with the data specified above seems to have a considerably better reflectivity than one that is based on spherical glass balls. The span of incidence angles of the new retroreflector is estimated to be about the same as for an arrangement based on glass balls, when the lens elements have an F-number of unity.

A retroreflector according to the ideas of this invention, can be designed for zero spherical aberration. However, if diffractive elements are chosen, it will exhibit a considerable chromatic aberration, i.e. strict retroreflection will only be achieved for one colour (wavelength) at a time. This problem could be alleviated by making the mother matrix for replication of the optical microrelief elements consist of series of elements, optimized for a spectrum of different wavelengths. It's likewise easy to adapt a retroreflex substrate for different colours of the final product. The chromatic aberration may also be reduced if different kinds of elements are chosen for the lens and mirror respectively, for example a diffractive element for the lens and a refractive element for the mirror.

An important consideration in designing the microrelief elements for a retroreflector is also that the function of diffractive elements is substantially less subject to humidity and impurities than refractive elements. Sometimes it might be advantageous to let the lens and mirror layers face each other with an air layer building up the proper distance between them.

It may be convenient to supply one or both the external sides of the retroreflecting substrate, i.e. the lens side as well as the mirror side, with a protection layer. This is not shown in the figures. A colour filter layer may also, when convenient, be applied to the lens side.

The shape of the microrelief elements may of course be modified, for example it may be convenient to make them hexagonal.

For industrial production of the new retroreflector, it may be convenient to employ cylindrical rollers with the lens and mirror layers imprinted or stricken into the surface of roller. This will make it possible to roll the two microrelief layers on to the two sides of the substrate. The manufacturing of small size reflectors may also be accomplished by means of injection moulding according to some process which has similarities to that for manufacturing of CD's.

Claims

1. A retroreflective device that is composed of a number of optical microrelief elements, where optical microrelief elements is taken to have reference to either diffractive zone plates, refractive Fresnel elements or hybrids composed of diffractive zone plates and refractive Fresnel elements, characterized by a first layer consisting of a number of densely packed coplanar microrelief lenses (12), i.e. microrelief elements of lens type, a second layer consisting of a number of densely packed coplanar microrelief mirrors (14), i.e. microrelief elements of mirror type, coated with a reflecting layer (16) on the side opposite to the microrelief lenses (12), and a third layer (17) in-between the first and the second layer where the first and the second layer are oriented so as to exhibit a multitude of element pairs, each in the form of a microrelief lens (12) and a microrelief mirror (14) with essentially coinciding optical axes, the distance between a microrelief lens (12) and the corresponding microrelief mirror (14) is such that the microrelief mirror (14) is located essentially in the focal plane of the microrelief lens (12), and where the microrelief mirror (14) has its equivalent centre of curvature essentially in the centre of the corresponding microrelief lens (12).

2. A retroreflective device according to Claim 1, characterized by microrelief lenses (12) as well as microrelief mirrors (14) of hexagonal form.

3. A retroreflective device according to Claim 1, characterized by microrelief lenses (12) as well as microrelief mirrors (14) of square form.

4. A retroreflective device according to Claims 1-3, characterized by lens elements with a numerical aperture adapted so as to allow acceptable and desired incidence angles.

5. A retroreflective device according to Claims 1-4, characterized by three layers of the same material and integrated in the one substrate.

6. A retroreflective device according to Claims 1-5, characterized by a reflecting layer (16) that is composed of a metallic sheath.

7. A retroreflective device according to any of the Claims 5 and 6, characterized by a protecting layer on one or both outer sides - the lens side and/or the mirror side - of the substrate (10).

8. A retroreflective device according to any of the Claims 5-7, characterized by a substrate (10) that consists of polycarbonate (acrylic glass).

9. A retroreflective device according to any of the Claims 1-4, characterized by microrelief mirrors (14) embodied in a reflecting substrate and microrelief lenses (12) embodied in a transparent substrate and where said substrates are fitted together.

10. A retroreflective device according to any of the Claims 1-9, characterized by a substrate where the first and the second layer embodies microrelief elements, of different kinds, viz. one consisting of diffractive elements and the other consisting of refractive elements.