Method of making an array of aberrated optical elements

A method of making an array (18) of aberrated optical elements (20). The method comprises the steps of providing a substrate having a first surface with forming elements thereon, and controlled working a localized region on the first surface of the substrate. The controlled working is of a magnitude sufficient to aberrate one or more of the forming elements in an affected site surrounding the localized region.

FIELD OF THE INVENTION

This invention relates generally to a method of making an array of aberrated optical elements.

BACKGROUND OF THE INVENTION

Retroreflective sheeting is used to reflect light from a source point back to an observation point. For example, in a highway safety application (e.g., a highway sign, a pavement marker), light from a vehicle headlight is reflected back to the eyes of the driver of the vehicle. When the light is reflected back to the observation point, the divergence angle α can range from 0° to more than 3° at any given rotational angle ϵ.

The value of the divergence angle α and the rotational angle ϵ in any given situation depends on the geometry of the source (e.g., the headlight) and the observer (e.g., the driver), and the distance from source/observer (e.g., the vehicle) to the retroreflective sheeting. For example, the divergence angle α for a large truck's right headlight and its driver at a distance of about 40 meters from a road sign will be approximately 3°, while the divergence angle α for an automobile's left headlight and its driver at a distance of about 600 meters from a road sign will be approximately 0.05°. The value of the rotational angle ϵ will be different for left and right headlights of a vehicle, and will also depend on the vehicle and driver geometry and the position of the road sign.

Ideally, retroreflective sheeting used in road signs will produce a pattern of retroreflected light having sufficient intensity over an adequate range of divergence angles α and a wide range of rotation angles ϵ. For example, even a non-urban retroreflective highway sign should retroreflect light through a divergence angle α of about 1° which corresponds to the value of the divergence angle α from a large truck's right headlight back to its driver at a distance of about 120 meters from the road sign. Also, if the sheeting is in random orientation on a road sign, retroreflectance is required at every value of the rotational angle ϵ.

To increase the mean geometric divergence of a retroreflective material, it is known to introduce intentional aberrations into the retroreflective elements. For example, with microcubes, aberrations can be introduced which result in dihedral angles deviating slightly from 90°. Historically, these aberrations have stemmed from geometries introduced into the master substrate during tooling. (See e.g., U.S. Pat. Nos. 3,712,706, 4,775,219, 4,938,563, 6,015,214, US2003/007815A1, etc.). U.S. Pat. No. 6,871,966 discloses a method of introducing aberration-producing geometries by controlled working of the master substrate after tooling or by controlled working of copy substrates made from the master substrate. (This patent is assigned to the assignee of the present invention and its entire disclosure is hereby incorporated by reference.) Specifically, the substrate is worked at localized regions on its non-element-carrying surface (i.e., the surface opposite the surface on which the forming elements are carried) at a magnitude sufficient to change the geometry of the optical elements on the opposite element-carrying surface.

SUMMARY OF THE INVENTION

The present invention provides a method of producing an array of aberrated retroreflective elements wherein the element-carrying surface of a substrate is controlled worked. These and other features of the invention are fully described and particularly pointed out in the claims. The following description and drawings set forth in detail certain illustrative embodiments of the invention which are indicative of but a few of the various ways in which the principles of the invention may be employed.

DETAILED DESCRIPTION

Referring now to the drawings, and initially toFIGS. 1 and 2, retroreflective sheeting10according to the present invention is shown. The retroreflective sheeting10comprises a clear resin (e.g., acrylic, polycarbonate, vinyl, etc.) sheet material12having a front surface14and a rear surface16on which an array18of aberrated retroreflective elements20are formed. The illustrated retroreflective elements20are cube corner elements and, more particularly, microcubes. As is best seen by referring additionally toFIG. 3, each retroreflective element20comprises three mutually perpendicular faces22that meet an apex24and that intersect with each other at edges26forming three dihedral angles. However, other retroreflective elements are possible with, and contemplated by, the present invention. Moreover, the elements20need not be retroreflective, as they can be any type of optical elements (including micro-optical elements) wherein aberrations are intentionally introduced for the purpose of improving an optical property.

When discussing the optical properties of the array18and/or the retroreflective elements20, it is helpful to use an analogous array18′ as a basis for comparison. The analogous array18′, shown inFIG. 4, has the same array pattern as that of the array18and has retroreflective elements20′ that are all identical to the retroreflective elements20prior to aberration. For example, in the illustrated cube corner embodiment, the retroreflective elements20′ of the analogous array18′ could each include three dihedral angles that are each exactly (or almost exactly) 90°. The analogous non-aberrated array18′ would possess certain overall optical qualities including a mean geometric divergence and a total retroreflectance.

According to the present invention, the aberrations in the retroreflective elements20are of a sufficient magnitude to cause the array18to have a greater mean geometric divergence (e.g., at least 0.2°, at least 0.5°, at least 1°, and/or at least 0.1°) than the non-aberrated array18′ (e.g., 0° to 0.5°). Additionally or alternatively, the array18can have at least 90%, at least 94% and/or at least 98% preservation of total retroreflectance when compared to the non-aberrated array18′. The aberrations in the retroreflective elements20are preferably such that face smoothness and/or edge sharpness is substantially the same as the non-aberrated elements20′.

Referring now toFIG. 5, a method of making the retroreflective sheeting10is schematically shown. In this method, a blank substrate100is tooled to produce a master substrate110having a surface116carrying forming elements120. The forming elements120can be of the same gender or the opposite gender as the retroreflective elements20and, in either event, have a geometry corresponding to the desired geometry of the non-aberrated elements20′. In the illustrated embodiment, the master substrate110includes male elements120.

The master substrate110is then used to generate (e.g., by electroforming) one or more original copy substrates110. The copy substrates110each have a surface116carrying forming elements120, which are opposite in gender as the forming elements120of the master substrate110. The one or more original copy substrates110can be assembled into an assembling substrate210. The assembling substrate210can be used to generate (e.g., by electroforming) next order copy substrates110, which each have a surface116carrying forming elements120, which are opposite in gender as the forming elements220of the assembled substrate210. The second order copy substrates110can be assembled together into a second order assembling substrate210. The second order assembling substrate can be used to generate (e.g, by electroforming) the third order substrates110and the copy sequence could be continued in a similar manner until the production copy substrates M10are produced. The production tool M10will have an arrayed surface M16carrying forming elements M20, which are of the opposite gender as the retroreflective elements20. The production tool M10can then used to form (e.g., by embossing, casting, compression molding, etc.) the retroreflective elements20on the surface16of the plastic sheet material12to produce the retroreflective sheeting10.

As is shown schematically inFIG. 6, controlled working is performed on at least one localized region130/230on the first surface116/216of the master substrate110, a copy substrate110, and/or an assembly substrate210. The controlled working step can be performed at one or more stages of the copying/clustering steps. Prior to the controlled working step, the geometry of the forming elements120/220on the substrate110/210would correspond to the non-aberrated elements20′ in the analogous array18′. After the controlled working step, the forming elements120/220on the worked substrate110/210, and the forming elements120/220on substrates110/210copied from the worked substrate, would correspond to the aberrated retroreflective elements20in the array18.

The controlled working can be performed on the master substrate110, the copy substrates110, or the assembling substrates210.

The substrates110/210can be made from a metal or a plastic. In the illustrated embodiment, for example, the substrates110/210can be made from electroformed nickel. Because the element-carrying surface116/216is worked (as opposed to the opposite surface), the present invention can accommodate a variety of substrate thicknesses, even those greater than about 10.00 mm. That being said, the substrate110/210can have a thickness in the range of about 0.01 mm to about 2.0 mm and/or in the range of about 2.0 to about 10.0 mm. The master substrate110can be thicker than the other substrates (i.e., greater than about 10.0 mm).

The working of the substrate surface116/216is intended to either add, remove, or modify material in the localized regions on this surface, or to simply apply local pressure, temperature, or other disturbance. The working can be of a degree sufficient to cause a change in the stress of the material resulting in, for example, a minute change of one or more dihedral angles. Preferably, the working on the arrayed surface116/216is of a magnitude sufficiently small that it will not damage the smoothness of faces and/or the sharpness of edges of the forming elements120/220and/or elements120/220adjacent thereto. As shown inFIG. 7, the working on the surface116/216of a substrate110/210can be performed while an electroformed copy110/210is still attached thereto.

The working of the arrayed surface116/216can be accomplished by a variety of means, including the application of energy, chemicals, or pressure to the second surface. Energy can be applied, for example, as either electrical energy or focused heat, such as by an infra-red laser. Pressure can produce localized distension involving the movement of material which preserves material mass while creating stress. Particle impingement (e.g., very light sand-blasting) can also be employed.

As is shown schematically inFIG. 8, when a controlled working is performed on a localized region130/230on the first surface of the substrate110/210, the aberrations effects not only the forming element(s)120/220in this region, but also those in an affected site140/240including and surrounding the localized region130/230. The change in the forming elements120/220in the site140/240will usually not be uniform and will change depending upon, for example, the distance from the localized region130/230. For example, in the illustrated cube corner example, the forming elements120/220close to the localized region130/230could have a greater change in their dihedral angles than the forming elements120/220remote from the localized region130/230.

As was indicated above, controlled working is performed on at least one localized region130/230. Typically, a controlled working step will be performed on several localized regions130/230of the substrate110/210. The location of these regions130/230on the substrate surface116/216and/or relative to each other may be in predetermined pattern or semi-random (i.e., a distribution that is under statistical control, but not controlled in full detail). Additionally or alternatively, the magnitude of the controlled working can be the same or different in each localized region130/230.

One may now appreciate that the present invention provides a method of producing an array18of aberrated optical elements20. Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalent and obvious alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification. The present invention includes all such alterations and modifications and is limited only by the scope of the claims.

GLOSSARY

An optical element is an element having one or more light-interacting faces.

An array is an arrangement of a large number of the optical elements.

A microoptical element is an optical element having dimensions less than or equal to about 1 mm.

A cube corner element is an element comprising mutually intersecting faces (e.g., three faces) having dihedral angles which are each approximately a predetermined value (e.g., 90°).

A microcube is a corner cube element having a total cube area of less than about 1 mm2. The total cube area is the area enclosed by the cube shape defined by the projection of the perimeter of the cube corner element in the direction of the principal refracted ray.

Retroreflection is reflection in which reflected rays are preferentially returned in directions close to opposite the direction of incident rays, this property being maintained over wide variations of the direction of the incident rays. A retroreflector is a surface or device that produces retroreflection.

A retroreflective element is an optical element that produces retroreflection.

A retroreflective material is a material that has a continuous layer of retroreflective elements. Retroreflective sheeting is a retroreflective material preassembled as a thin film.

The illumination axis is the half-line from the retroreflector center through the source point. The source point is the location of the source of illumination. The retroreflector center is the point on or near a retroreflector that is designated to be the location of the device.

The entrance angle β is the angle between the illumination axis and the retroreflector axis. The retroreflector axis is a designated half-line from the retroreflector center in a direction centrally among the intended directions of illumination.

The observation axis is the half-line from the retroreflector center through the observation point. The observation point is the location of the observer.

The divergence angle α is the angle between the illumination axis and the observation axis. This angle is also called the observation angle.

The mean geometric divergence is the average of all divergence angles α for all rays with a given direction of illumination retroreflected by a retroreflector.

The rotational angle ϵ is the angle in a plane perpendicular to the retroreflector axis from the observation half-plane to the datum axis measured counterclockwise from a view point on the retroreflector axis. The observation half-plane is the half-plane that originates on the line of the illumination axis and contains the observation axis. The datum axis is a designated half-line from the retroreflector center perpendicular to the retroreflector axis. The datum half-plane is the half plane that originates on the line of retroreflector axis and contains the datum axis.

Total retroreflectance is the percentage of light retroreflected by the optical device.

A forming element is an element used to form, or used to form forming elements to form, an optical element whereby the forming element will have faces corresponding to the faces of the to-be-formed optical element.

A substrate is a material having a surface which carries one or more optical elements or one or more forming elements.

A male optical element is an optical element wherein the faces of the element project outward from first surface of the substrate.

A female optical element is an optical element wherein the faces of the element recess into the first surface of the substrate.

A male forming element is a forming element wherein the faces of the element project outward from first surface of the substrate.

A female forming element is an optical element wherein the faces of the element recess into the first surface of the substrate.

The substrate thickness for a substrate carrying male elements is the thickness of material from which the elements project. The substrate thickness for a substrate carrying female elements is the total thickness of the material.

Copying is the reproduction of forming elements from one substrate onto another substrate by, for example, electroforming, casting, molding, embossing, etc.

The master substrate is the substrate in which the forming elements are first formed in a non-copying manner such as, for example, cutting or ruling.

The chain of copying is the series of copies stemming from an original entity (e.g., a master substrate) by producing a copy (or copies) of the original entity, then producing one or more copies from this copy (or these copies) of the original entity, and then producing one or more copies of the copy (or copies) of the copy (or copies) of the original entity, and so on.

The first order copy is a substrate having forming elements directly copied from the master substrate.

The nth order copy is a substrate having forming elements that are copies from the nth link in a chain of copying containing n−1 intermediate copy links between the original entity and the nth order copy. For example, a 3rd order copy is a copy of a 2nd order copy, which was a copy of a 1st order copy, which was a direct copy of the original entity.

The production tool is the tool carrying the forming elements used to form the optical elements.

An aberration refers to a small change in the geometry of an optical element. For cube corner elements, it would refer more specifically to a small change in one or more of the dihedral angles of the element, this change being sufficient to cause a change in the mean geometric divergence of the optical element or an optical element formed therefrom or from copies thereof.

The angular change produced by the aberration will usually be less than 1°. The verb aberrate means to form an aberration.

An aberrated element is an optical element or a forming element having an aberration.

Controlled working is adding, removing, modifying, distorting, deforming, or disturbing in a controlled manner as opposed to, for example, in a random and/or unintentional manner.

A localized area is the very small area on the array at which the controlled working is performed (e.g. where the energy, chemical, or pressure is actually applied).

An affected site is an area on the area including and surrounding the localized area wherein the elements are aberrated as a result of controlled working in the localized area.