RETROREFLECTING MEANS

Abstract: A retroreflecting means has for its objective to provide a simple and inexpensive means thereby to avoid the need for such expensive means as triple screen mirrors or corner cube reflectors, and yet provide a degree of clarity of a reflected image or a degree of accuracy in the retroreflective direction previously obtainable only when employing such expensive means. The objective is met by a construction comprising a fine pitch retroreflective sheet (2) overlaid by an array of optical lenses (5).

Description: RETROREFLECTING MEANS This invention relates to retroref lecting means. Re tr or ef le c ting means are used in a variety of applications, such as hazard or direction retroref lectors, in cameras, warnings and indicators, measurement systems, guidance systems, and in retroref lecting units that of themselves have a variety of applications. In certain circumstances, considerable clarity is required in a retr or eflected image or considerable accuracy over the retroreflective direction with either the total elimination of any in-built deviation in the retroreflective direction, or with an in-built deviation of a specified degree. This has led to the development of retroref lecting means such as triple mirror screens, and corner cube screens, that can be manufactured to provide the desired clarity and accuracy but which are expensive. The object of the present invention is to provide a retroref lecting means that is simple and relatively inexpensive to produce and yet has high retroreflection efficiency and high accuracy of retroreflective direction. According to the present invention, a r et r or ef 1 ect i ng means comprises a f ine pitch retroreflective sheet overlaid by an array of optical lenses. The array of optical lenses can be in a fixed position in relation to the fine pitch retroreflective sheet to focus incoming radiation on to or close to the screen surface, which enhances the retrodirectivity of the combination or can be mounted such that the lens to screen distance can be varied thereby affording a means of controlling the degree of spread of the ray bundle with the maximum spread being dictated by the in-built deviation in the retrodirecti e layer. The array of optical lenses may be formed by locating individual lenses in appropriate apertures in a support plate, but to eliminate any "dead" areas, and to increase the efficiency of retroreflection, a closely packed lens array can be provided, such as for example by forming the lenses of hexagonal shape and directly securing adjacent lenses to each other. In some applications, it may be advantageous to position each lens at the end of a chamber extending towards the screen to prevent any cross-talk or overspill of the images formed by any one lens and the retroreflection of that image from the screen. As an alternative, cross-talk or overspill of the images can be prevented by positioning a field aperture plate at a prescribed distance in front of the array of optical lenses. The effect of the invention in any of its possible forms is that ray bundles focussed by a particular lens on to the screen are retroreflected from the screen back through the same segment of the same lens. Consequently any defects induced by any inaccuracy in the lens and in that part of the screen are effectively self -cancelled by the retroflexion of the rays through the lens that focusses those rays on the screen. The result is that the combined use of a relatively inexpensive, fine pitch retroreflective sheet » and relatively inexpensive lens is permitted, with cost advantages thus provided, and yet a retroreflecting means with directivity and retroflexion efficiency matching those of high q.uality and expensive retroreflectors such as corner cube reflectors is provided, with the added advantage that each unit retrofocusses point for point all incoming radiation, as is required in imaging systems used for example in photography, and particularly in the production of three-dimensional images and metrological processes. When an adjustment of the lens to screen distance is provided for, control is provided over the degree of spread of retr odi r ected radiation. " This is of considerable importance when the retroreflecting means is used as a hazard or direction ' retroref lector, e.g. an airport landing indicator. Several embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a retroreflecting means according to the invention; Figure 2 is a section on the lines II-II of Figure 1; Figure 3 corresponds to Figure 2 but shows a second embodiment of the retroreflecting means according to the invention; Figure 4 corresponds to Figure 2, but shows a third embodiment of the retroreflecting means according to the invention; TVO 88/03659 _ 4 _ PCT/GB87/00806 Figure 5 shows part of a lens array of alternative construction to that of Figure 1; Figure 6 is a schematic representation showing the positioning of a field aperture plate in front of the 5 lens array of the retroreflecting means of the invention; and Figure 7 is a schematic representation of the use of the retroreflecting means of the invention in an imaging system. 10 In the drawings a retroreflecting means 1 is formed by a fine pitch retroreflective sheet 2, such as a beaded sheet or the like, that overlays, or itself forms the base of a housing 3, the top face 4, of which is constituted by an aperture plate with a lens 5 secured in 15 each aperture. As is depicted in Figure 2, the apertured sheet and its lens array are located on side walls 6 of the housing, at a predetermined distance from the retro eflective sheet, to focus incoming radiation on the retroreflective sheet. Thus, incoming radiation is 20 gathered into "bundles" by the lenses, with the result that each "bundle" of rays is retroreflected from the screen back through the same lens, and more than that with individual rays retroreflected back through the same segment of its lens. Consequently, any defects induced 25 by any inaccuracy of the lens and of that part of the screen are self cancelled, resulting in considerable accuracy in the retroreflective direction and considerable clarity of the retroreflected image, without ( the need to resort to expensive retroreflective means such as known corner cube screens or precision triple mirror screens. In certain applications such as for example in imaging systems, it is advantageous to prevent any overspill of the image formed by any one lens and the retroreflection of that image from the screen. Therefore, as is shown in Figure 3, it is preferred to locate each lens 5 at the end of a chamber 7, thereby to guarantee that incoming radiation focussed onto the screen by a lens is guaranteed to be retroreflected from the screen back through the same lens. Because the retroreflective surface is formed by a relatively inexpensive fine pitch retroreflective sheet, it is inevitable that there is an in-built, deviation in the retroreflected radiation with then an inevitable degree of spread of retrodirected radiation. To provide for the control over the degree of spread of retrodirected radiation and the elimination of any spread in connection with a usage of the invention where retroreflected radiation is to travel back along the exact path of the incoming radiation, the invention proposes, as is shown in Figure 4 that an ability for the adjustment of the lens to screen distance is provided for. Thus, as is illustrated in Figure 4 and with the retroreflecting means of circular configuration, the retroreflective sheet 2 can be provided with an outermost flange 8 internally threaded to engage external threads on the sidewall 6. It will however, be readily understood that the array of lenses and the apertured plate can similarly be in screw threaded relationship with the sidewall, and it will further will be readily understood that any other relatively simple means of adjustably connecting either the screen or the apertured plate and lenses can be employed. In certain applications it is advantageous that there is the limination of any "dead" areas in the lens array such as for example in the use of the invention in imaging systems and in other systems where an increase in the efficiency of the retroreflector is highly desirable. Thus, as is shown in Figure 5 instead of locating the lenses in an apertured plate, the lenses 5A can be closely packed such as by forming them of hexagonal shape with adjacent lenses secured directly to each other, and with, the array of lenses located in a support frame 9. As an alternative to the construction shown in Figure 3, when there is the need to eliminate any cross¬ talk or overspill of i ' mages formed by any one lens and the retro eflection of that image from the screen, an apertured plate 10 can be located in advance of the array of lenses 5, 5A, as shown in Figure 6, and distanced from the lenses in accordance with the formula -t = D d T where D = the diameter of a circular aperture or maximum cross sectional dimension of a hexagonal or other non circular aperture; d = the included angle of incoming radiation passing through the aperture and lens array; T = the distance of the apertured plate from the lens array; and t = the distance from the front surface of the lens array to the surface of the retroreflective screen. By providing such a field aperture plate 10 any cross-talk or overspill of the images formed by any one lens and the retroreflection of that image from the screen is eliminated without the need for the provision of the chambers 7 illustrated in Figure 3. It will of course be understood that the cross-sectional shape of the aperture in the plate 10 is to conform to the particular lens array with which the plate is associated. Illustrated in Figure 7 is an example of a usage of the retroreflecting means of the invention. Here, the retroreflective means 1, as has been described above, is used in conjunction with a beam splitter 11 whereby retroreflected radiation is directed to -an integral recording medium 12 sensitive to retroreflected radiation, -formed by a sensitive surface 13 overlaid by a two dimensional array of micro-lenses 14. The result is a one step, one exposure, integral spacial image of the scene in front of the retroreflective means 1 recorded on the integral recording plane formed by sensitized sheet. This image will have a 1:1 scale, and point for point imaging with continuous parallax in all directions, limited only by the size of the retroreflecting means 1.

Claims: CLAIMS 1. A retroreflecting means characterised by a fine pitch retroreflective sheet overlaid by an array of optical lenses. 2. A retroreflecting means as in Claim 1, characterised in that the array of optical lenses are in a fixed position in relation to the fine pitch retroreflective sheet to focus incoming radiation on to or close to the screen surface. 3. A retroreflecting means as in Claim 1, characterised in that the lens to screen distance is adjustable. 4. A retroreflecting means as in any of Claims 1 to 3, characterised in that the array of optical lenses is formed by locating individual lenses in appropriate apertures in a support plate * 5. A retroreflecting means as in any of Claims 1 to 3, characterised in that a closely packed lens array is provided. 6. A retroreflecting means as in any of Claims 1 to 4, characterised in that each lens is positioned at the end of a chamber extending towards the screen to prevent any cross-talk or overspill of the images formed by any one lens and the retroreflection of that image from the screen. 7. A retroreflecting means as in any of Claims 1 to 5, wherein cross-talk or overspill of the image is prevented by positioning a field aperture plate at a prescribed distance in front of the array of optical lenses. 8. An imaging system i corporati g a etroreflecting means as in any of Claims 1 to 7, characterised in that the retroreflecting means is associated with a beam splitter whereby retroreflected radiation is directed to an integral recording medium sensitive to the retroreflected radiation.

Inventors: DAVIES NEILL, MCCORMICK MALCOLM, DAVIES, NEILL, MCCORMICK, MALCOLM
Applicants: UNIV SHEFFIELD, THE UNIVERSITY OF SHEFFIELD
Application year: 1987
Main classification: G02B5
IPC codes: G02B3, G02B5