Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2000-10-16
2003-01-14
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C359S619000, C359S626000
Reexamination Certificate
active
06507441
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a directed reflector, such as a retroreflector and more particularly to, directed reflectors which utilize an array of lenslets in combination with a reflector such that incident (incoming) light of a wide range of acceptance angles is reflected therefrom at a generally constant return angle relative to the angle of incidence.
Directed reflectors are optical devices that reflect incident light in a direction which has an essentially fixed angle relative to that of an incident light beam, with little or no dependence on the angle between the light beam and the reflector surface.
FIGS. 1
a-c
, show a directed reflection effect of a directed reflector
20
. It is evident that, regardless of the angle of incidence of the incoming-light, the light is reflected at a constant angle with respect to the angle of incidence (&Dgr;&agr;).
Retroreflectors are directed reflectors which redirect light towards its originating source (i.e., &Dgr;&agr;=0). A review of various types of retroreflectors and their uses appears in reference [35] which is incorporated herein by reference. This advantageous property of retroreflectors has led to the widespread implementation thereof in planar array configurations which can be utilized in a variety of applications. For example, arrays of miniature retroreflector are often utilized in sheeting which are used for road signs in order to increase their visibility to motorists at night and for retroreflective safety devices used in vehicles and by other road users. Retroreflector arrays are also used with light barriers and with a beam scanning apparatus, such as the beam scanning apparatus used for generating light grids and light curtains.
One type of a retroreflectors array is constructed from a monolayer of microspheres embedded in a cover layer. Behind the rear surfaces of the microspheres, separated by a spacing layer, a reflective layer, e.g., vapor-coated aluminum, is disposed such that light penetrating through, and directed by, the microspheres is reflected out by the reflective layer. Such embedded microspheres array is also referred to in the art as embedded lens array, whereas each microsphere or lens in combination with its reflective surface is sometimes referred to as a cat eye retroreflector element. Nevertheless such microspheres typically have very poor lenticular quality, and as such lack a well defined focal length which is required by various reflector applications. Furthermore, such microspheres are characterized by high level of optical aberrations which result in loss of the ability to focus incident light, particularly when incident at a wide range of acceptance angles, on the reflective layer. Arrays of microsphere based retroreflectors have been observed in nature [16] and have found applications in-man made devices [1]. Such retroreflectors are thoroughly analyzed in the scientific and technical literature [see, for example, references 4, 11, 15, 18, 19, 22 and 51]. There are also several descriptions of fabrication methods for producing sheets containing many miniature cat-eye retroreflectors [9, 10, 17, 40, 42, 49, 50].
As described above, a prior art embedded lens array type retroreflector is typically a combination of crude lenses and a reflector surface (a mirror or a diffuse reflecting surface) that is located at the back focal plane of each lens. Any collimated beam of light that enters this structure is reflected back at the source (retroreflected), provided the reflection from the reflector re-enters the aperture of the same lens. However, embedded lens array retroreflectors utilizing flat, specular (optically smooth) reflectors suffer from a limited range of incidence angles acceptable for retroreflection (also referred to in the art as acceptance angles). This characteristic is demonstrated by
FIGS. 2
a-b
which depict cross sectional views of such retroreflecting arrays. As shown in
FIG. 2
a
, light entering prior art embedded lens array retroreflectors utilizing specular (smooth) reflecting surface, at an angle greater than the acceptance angle, is no longer returned to the same lenslet and as such is not retroreflected. In addition, when the reflecting surface is diffuse (optically rough or otherwise scattering) as specifically shown in
FIG. 2
b
, this diffusely reflective surface of such prior art lens arrays also leads to scattering of the reflected light over several lenses, resulting in loss of retroreflected light and low retroreflection coefficient.
Although this limited angular response range and low retroreflection coefficient of prior art retroreflectors is acceptable for some applications, there are, however, applications for which these effects are non-tolerable. In particular, applications which require accurate reflection of incident light, such as incorporation of retroreflectors in beam scanning applications, are extremely difficult to effect with arrays utilizing embedded microspheres; for these applications it is particularly important that the retroreflector and the light transmitter/receiver of the beam scanning apparatus are accurately aligned.
Since embedded lens arrays retroreflectors are not applicable in various applications such as, for example, beam scanning, interest has been centered, in the past, on cube-corner retroreflectors, which are products of high optical quality and as such can provide more accurate reflected beam of light. Cube-corner retroreflectors are trihedral structures which have three mutually perpendicular lateral faces meeting at a single corner, such as that configuration defined by the corner of a cube. The retroreflectivity typically achieved by cube-corner type reflecting elements is through triple reflection (often utilizing the principle of total internal reflection). A transparent cube-corner element receives a ray of incident light at an angle and sends it back in the same direction. To this end, see, for example, U.S. Pat. Nos. 3,924,929, 4,672,089, 4,349,598, and 4,588,258, EP 0 844 056 A1, which are incorporated herein by reference.
In order to overcome the relatively pronounced directional dependence which is associated with reflection at retroreflectors, attempts have been made to sub-divide a retroreflector consisting of triple reflectors into individual elements (see, for example, DE-PS No. 22 36 482, which is incorporated herein by reference). In this arrangement the individual retroreflecting elements are inclined to one another at increasing angles such that the scanning light beam of a light curtain impinges as closely as possible to normal incidence on the individual triple elements which are directionally dependent. An array configuration with retroreflecting elements inclined at increasing angles to one another is, however, only suitable for use with a sector-shaped scanning beam. Each individual retroreflector element must also be set at a predetermined position relative to the scanning beam and this makes it difficult, if not practically impossible, to manufacture such a retroreflector using cost-effective mass production techniques.
In addition, due to the functional design of cube-corner retroreflectors, fabrication or utilization thereof in sheeting results in the addition of undesirable thickness which limits their applicability.
While retroreflectors can and are manufactured by classical optical procedures (polishing, etc.), such devices are too expensive for most applications. Most retroreflecting surfaces in use today are stamped, molded, or otherwise replicated, arrays of corner-cube elements or cat-eye elements, or a suspension of very small glass or transparent plastic beads (microspheres) in paint, where the beads act as crude lenses and the paint as a reflector. Retroreflectors manufactured by classical methods are high precision instruments of very high performance, but they are too expensive and often too bulky for most applications. The standard molded/stamped retroreflectors of today and, more so, t
Arieli Yoel
Drori Avishai
Eisenberg Naftali P.
Glaser-Inbari Isaia
Karoubi Reouven
G.E. Ehrlich Ltd.
Mack Ricky
Optid, Optical Identification Technologies Ltd.
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