MXPA99005268A - Retroreflective sheet - Google Patents

Abstract

A retroreflective sheet (10) capable of exhibiting an excellent decorative effect during both daytime and nighttime, and particularly an excellent rainbow-colored decorative effect. The sheet comprises, in order, a binder (3) in the back side of which a plurality of microspheres (2), arranged in a monolayer, are partially embedded, a focusing layer (4) underlying some of the microspheres, and a reflective layer (5) underlying the focusing layer (4) and underlying microspheres (2) which do not have a focusing layer therebehind. The focusing layer has a film thickness selection coefficient x in the range of 0.16 to 0.95 as defined by equation (II) of equations (I) and (II), wherein f is the coefficient for the optimum layer thickness of the layer, d is the diameter of the microspheres (12), h is the thickness of the focusing layer (4), nb is the refractive index of the binder layer (3), ng is the refractive index of the microspheres (2), and ns is the refractive index of the focusing layer (4).

Description

RETRORREFLEJANTE SHEET FIELD OF THE INVENTION The present invention relates to a included retroreflective lens sheet that is particularly suitable for decorative purposes, for example, labels, decals, emblems and related products, and advertising signs.

BACKGROUND In the past, many different varieties of retroreflective sheets (sometimes referred to as "light retroreflective" or "laminated" for a variety of purposes) have been proposed and sold Retroreflective sheets are perhaps the most commonly used for security purposes. specifically for signs on roads and as decals, emblems and medallions for vehicles, for example, bicycles and automobiles and personal items, for example, garments, bags, etc. In such applications, the sheet is typically desired to provide a retroreflective brilliance very strong for particular observers, for example drivers of approaching vehicles, as a result, such REF .: 30379 sheets are typically constructed to minimize light scattering so that incident light is reflected only in a narrow range in the direction of the light source, ie, retroreflection. Because of this, such leaves are easily illuminated by vehicles approaching with the lights on during operation, making them more easily visible to the occupants of these vehicles, but not illuminated as effectively by other light sources. For example, when the light source is the sun, an exterior lamp or interior lighting, retroreflective light illuminating the sheet is not easily recognized or detected by most observers except for those who happen to be placed substantially adjacent to it. the light path from where the light source is towards the blade. This problem is particularly relevant in the case of advertisements on the streets, shop windows and window displays, station signs and other diverse decorations and related purposes for security purposes. A decorative film suitable for such uses as signs and traffic safety equipment is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-45507. As shown in Figure 1, this decorative film comprises a protective layer 36, spheres or glass microspheres 32 partially embedded in the protective layer 36 and a binder 31, a thin reflective film 33, a support layer 35, a layer 37 adhesive and a release liner 38. The thickness of the binder 31 varies. The glass spheres 32 have a diameter of approximately 500 micrometers ("μm") or less, a refractive index of 2.0 or greater and a ratio of embedded in the anchoring binder 31 from about 10 to about 80% of their diameter. The thin reflective film 33 consists of a thin deposited film of metallic vapor which contacts the glass spheres 32 in the depressions 3. These decorative sheets have the effect of changing the color tone - depending on the angle observed under constant irradiation of light, and they shine beautifully in a rainbow-like scattering spectrum. However, the sheets are limited to retroreflection of light which is incident at angles within a relatively narrow angle of incidence, and therefore it has been difficult to obtain this excellent decorative effect at other angles of incidence. The examples of retroreflective sheets with effects Similar decorative ones are the retroreflectors described in Japanese Unexamined Patent Publication (Kokai) No. 2-54922 and the retroreflective sheet material described in U.S. Patent No. 5,503,906. Both references describe a decoration with a pattern that is to be formed by providing a transparent or colored resin layer over portions of the front or rear side of the reflective sheet with the exposed spheres. Although such sheets can provide retroreflection of colors, do not provide a rainbow type decoration. Light reflected in pattern or selective form can be obtained by the method to produce a retroreflective pattern described in Japanese Unexamined Patent Publication (Kokai) No. 60-128401. According to this approach, the reflective coating side of the reflective sheet is bonded by contact to an adherent substance partially coated with an adhesive in a pattern and only those sections serve as retroreflective sections. However, as with the technique described above, a rainbow type decoration can not be obtained. In addition, the vehicle reflector described in the unexamined utility model publication (Kokai) No. 6-78076, the sheet with a retroreflective side described in Japanese Unexamined Patent Publication (Kokai) 2-140703, and the method to produce a retroreflective sheet described in Japanese Unexamined Patent Publication (Kokai) No. 48-72290 represent techniques for obtaining wide-angle retroreflection, ie the retroreflective capacity even when the incident angle in relation to the sheet is relatively acute. That is, these techniques allow retroreflection through a wide range of angles by causing the light to be retroreflected by the sheet 10 when the incident at an acute angle? Lf?: With respect to the sheet 10, as shown in FIG. Figure 2 attached. U.S. Patent No. 3,801,183 describes retroreflective films having retroreflective legends and backgrounds comprising, in part, a discontinuous lacquer layer. EP-A-404539 discloses a retroreflective sheet constituted of a monolayer of microspheres embedded in a binder layer. Behind the microspheres is a separating layer and a partially transmitting reflective layer of light, and behind some of the microspheres is a layer of lacquer. The discontinuous coating of lacquer alters the optical efficiency of the article in those portions so that the retroreflective efficiency of the article varies across its surface, ie, there is a retroreflective legend and retroreflective background areas. There is a need for retroreflective sheets that provide an effective retroreflective effect over a wide variety of angles of incidence and that also provide decorative effects that are visible under a variety of observation conditions.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a retroreflective sheet that shows a desirable combination of these properties. Briefly, the retroreflective sheets of the invention comprise, in order, a binder on the back side of which a plurality of microspheres, arranged in a monolayer, are partially embedded, a focus layer underlying some of the microspheres in a desired manner, and a reflecting layer underlying the focusing layer and adjacent microspheres which do not have a focus layer below them. The focusing layer has a coefficient x of film thickness selection in the range of 0.16 to 0.95, preferably 0.32 to 0.80, as defined by equation (II) of the following equations (I) and (ID: ng -2nb) = (I) 2fnb (ns- ng) + ns (nb - ng) J »- < »> where f is the coefficient for the optimum layer thickness of the layer, d is the diameter of the microspheres, h is the effective thickness of the focus layer, nb is the refractive index of the binder layer, ng is the index of refraction of the microspheres, and ns is the refractive index of the focusing layer. Although the term "focusing layer" has been used in the present application, it will be recognized by those familiar with the art that the term "separating layer" could have been used instead of the previous one, without departing from the meaning of the present invention. The sheets of the invention show an excellent decorative effect regardless of the time of day or night when presenting a different appearance that depends on the observation angle / The sheets of the invention produce this decorative effect ~ by providing a decorative retroreflective effect which it changes the color tones while it shines pleasantly in a spectrum of dispersion type rainbow, on a wide interval of angles of incidence. This provides a distinctive or characteristic effect that is of particular utility for decoration or advertising purposes. The sheets of the invention are capable of displaying effective retroreflectivity over a wide observation angle, ie, for light irradiated within a wide range of incident angles, and also capable of being recognized without retroreflectivity depending on the angle of observation.

DESCRIPTION OF THE DRAWING The invention will be further explained with reference to the drawing, wherein: Figure 1 is a cross-sectional view of an example of a retroreflective sheet of the prior art. Figure 2 is a schematic illustration showing the wide reflection angle with a retroreflective sheet. Figure 3 is a cross-sectional view of a portion of a retroreflective sheet illustrative of the invention. Figure 4 is a cross-sectional view of the area around a microsphere in the retroreflective sheet shown in Figure 3. Figure 5A shows a plan view, and Figure 5B shows a schematic cross-sectional view of a retroreflective sheet of the invention used to explain the decorative effect of a retroreflective sheet according to the present invention.

Figure 6 is a schematic drawing illustrating the distribution of light reflected by the retroreflective sheet of Figure 5. Figures 7A and 7B are plan views illustrating the decorative effect observed based on the distribution of light reflected in the figure 6. These figures, which are idealized, are not to scale and are intended to be illustrative only and not limiting.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE MODALITIES Figure 3 is a cross-sectional view of a preferred embodiment of a retroreflective sheet of the invention. As shown in this drawing, the retroreflective sheet 10 comprises a binder layer 3 on the back side of which a plurality of microspheres 2, arranged in a monolayer, are partially embedded, a focus layer 4 being underlying at least some of the microspheres 2, and a reflective layer 5 is below the focusing layer 4. The sheet 10 further comprises on its rear or underlying side an optional adhesive layer 6 covered by a release liner 7 and further comprising, on its front side, an optional cover film 1 and an optional transparent cover 8.

The binder layer 3 can be substantially transparent and can, if desired, be colored. It typically comprises resin and preferably shows good adhesion to the other components of the sheet with which they are in contact, especially the microspheres and the focusing layer. Illustrative examples of suitable polymeric materials include, but are not limited to, urethane resins, epoxy resins, and acrylic resins. The thickness of the delayed binder layer is based on numerous factors including the diameter of the microspheres embedded therein and the total thickness of the retroreflective sheet. Typically, the microspheres will be embedded in about 30 to 70%, and preferably about 40 to 60%, of their diameter. The microspheres (sometimes referred to as "avalorios") and typically are of glass or ceramic material, and may be made of polymer or other suitable material, as is known to those familiar with the art. Preferably they are substantially spherical of a substantially uniform size. Typically they have, preferably, a refractive index of about 2.20 to 2.30. Illustrative examples of suitable glass microspheres include, for example, those formed from, for example, glass based on BaO-ZnO-TiO. The diameter of the microspheres will differ based on numerous factors including the thickness of the binder layer and the thickness of the reflective sheet, but will typically be about 30 to 120 μm, and preferably about 50 to 100 μm. The focus layer 4 typically comprises resin and will be substantially transparent, and may be colored, if so desired. In addition, it preferably shows good adhesion to other components of the sheet with which it is in contact, for example, the microspheres, the binder layer and the reflective layer. It is formed in a pattern that depends on the desired decorative effect, and is not placed on one side adjacent to the consecutive glass spheres 2 or in a continuous manner as in conventional retroreflective sheets. The thickness of the focusing layer is determined by the formula represented by the equations mentioned above (I) and (II) which define the optimum thickness of the focusing layer so that the resulting sheet only reflects in the direction of the source of light. According to the invention, the coefficient x of film thickness selection in equation (II) is in the range from 0.16 to 0.95. Typically, the focusing layer has a refractive index of about 1.35 to 1.65. Illustrative examples of suitable polymeric materials that can be used to form focus layers according to the invention include, but are not limited to, polyvinyl butyral resins, urethane resins and epoxy or epoxy resins.

As mentioned before, the focusing layer is formed in a desired pattern, which is formed behind only one of the microspheres in a desired manner similar to image. This selective formation of the focusing layer can be carried out by any of a variety of printing and coating techniques. An illustrative method is stencil printing, for example, screen printing. Based on the desired pattern, the stencil can be easily changed and your mesh size can also be easily controlled. The retroreflective sheets of the present invention provide an excellent graphic function and therefore a decorative function, especially a decorative effect similar to rainbow, which is not possible with the prior art, by using the focusing layer with patterns. The thickness of the focusing layer can be selected within a prescribed range to allow recognition of the resulting retroreflection under both day and night observation conditions, and the effective observation angle range can be increased to a wider range. further, by making parts of the focusing layer thinner than the focus layers of the previously known lens systems, it is possible to enlarge the retroreflective light observation angle, while in the portions without a focus layer, the light The incident produces a scattering spectrum similar to rainbow in a direction of 42 ° with respect to incident light. Having both functions within the same sheet provides a decorative effect so the appearance changes based on the angle of observation. On the back side of the focusing layer 4 as well as on the exposed rear sides of the microspheres 2 and the binder layer 3 in those portions not covered by the focusing layer 4, a reflective layer 5 is provided. The reflective layer 5 can be manufactured by any suitable method, using a variety of reflective materials that are well known to those familiar with the art. The reflective layer is preferably formed as a vapor deposition coating or a metal film, or a metallic spray paint coating, particularly a resin which contains metallic powders. In the latter case, the reflective layer can be formed by stenciling of the metallic spray paint, in a manner similar to that performed for the formation of the focusing resin layer described above. The thickness of the reflective layer can vary within a wide range, based on the composition and properties of the reflecting layer, the properties of the bonding layer of adjacent glass spheres and the focusing resin layer, as well as other factors , but typically it is between about 2 and 20 μm, and preferably between about 5 and 15 μm. When the reflecting layer of a metal vapor deposition film is formed, the vapor deposition can be carried out by a common method of vapor deposition of a metal such as aluminum or tin. A deposited film of metal vapor such as aluminum or tin shows good addition with the focusing layer and the adhesive layer when manufactured with a film thickness of 300 Angstroms ("A") or greater, and also shows a high refractivity. When the reflective layer is formed of a metallic powder paint, it is advantageous to use a paint which is a mixture of a metallic powder such as aluminum and a resin material such as polyvinyl butyral resin or urethane resin powder which can be show good addition with the focus layer and the adhesive layer. Such a paint can be easily applied using the stencil printing method, such as the one mentioned above. Typically, the retroreflective sheets of the invention additionally comprise an optional adhesive layer 6 and optionally a release liner 7 at the rear thereof, for example, on the rear side of the reflective layer 5, as shown in the embodiment in Figure 3. The adhesive should provide a desired adhesion to the sheet and to the final substrate or adhered substance (eg metals, plastics, wood, resin-painted surfaces and the like) to which the sheet is to be applied. In addition, it is preferably capable of suitable forms of application and preferably does not degrade the other components of the sheet or of the adhering substance. Illustrative examples include pressure sensitive adhesives, chemically activated adhesives, heat activated adhesives, etc. An example of a suitable class of adhesives are adhesives based on acrylic resin. The thickness of the adhesive layer is not particularly restricted, but is typically from about 10 to 50 μm, and preferably from about 20 to 40 μm. Often it may also be desired to provide a cover film or support layer 1 on the front side of the binder layer 3. Such a film can impart improved strength and robustness to the resulting sheet as well as providing a means of protection for the underlying components of the sheet., for example, as a means in which ultraviolet absorbent substances are dispersed. The cover film is preferably substantially transparent, may have a color if desired, and should adhere well to the underlying binder layer. In many embodiments, the characteristics of the cover film will largely determine the flexibility and elasticity of the resulting retroreflective sheet.

Illustrative examples of materials which can be used as a cover film include polymeric materials such as polyvinyl chloride (PVC), polyurethane, polyethylene terephthalate (PET), polymethylmethacrylate, and the like. The thickness of the cover film is not particularly restricted and depends to a large extent on the physical properties which it is desired to obtain. Typically, the thickness of the cover film is from about 20 to 150 μm, and preferably from about 40 to 100 μm. In some embodiments, the sheet 10 will comprise a transparent coating 8 on the front surface thereof, for example on the cover film 1, if present, or on the front surface of the binder layer 3. The transparent coating 8 can be used to obtain the desired environmental resistance and water resistance, provide a means to protect the graphic signs applied to the front surfaces of the cover film 1 and / or the binder layer 3, to provide a suitable surface for application of graphic signs to it, impart a bright appearance, etc. The transparent coating 8 can be substantially transparent and can have some color if desired. Illustrative examples of suitable clear coating materials include those with excellent adhesion to the underlying sheet components such as paints and dyes which include thermosettable urethane paints, ultraviolet light curable paints and fluororesin paints. The thickness of the transparent coating is not particularly restricted, but is usually from about 10 to 100 μm, and preferably from about 20 to 50 μm. The above explanation does not describe in detail the method of forming the layers constituting the retroreflective sheet of the light of the invention, but unless otherwise specified, the same techniques commonly used in the technical field can be used or can be used. modify as necessary for the invention. In addition to stencil printing which has been mentioned as being advantageous for the formation of the focusing resin layer, suitable techniques also include other printing methods as well as coating methods such as knife coating and spray coating. It will now be shown that when the refractive index of the ng microspheres and the diameter d, the refractive index of the binder layer nb and the refractive index of the ns focus layer of the retroreflective sheet of light of the invention are known, then the thickness of the focusing layer can be calculated from the equations (I) and (II) given above. This is explained with reference to Figure 4. In Figure 4, the microspheres 2 have a refractive index ng and a diameter d and are embedded in the binder layer 3 having a refractive index nb, while the layer 4 of focusing with a refractive index ng surrounds the rear side thereof at a prescribed thickness h with a reflective layer 5 behind it. As shown by the tracing of light rays (arrows) in the drawing, the incident light in the reflecting sheet 10 and penetrating through the microspheres 2 in this way can. be retroreflected. The cover film 1 is essentially irrelevant to the calculation of the film thickness selection coefficient x insofar as it has a substantially uniform thickness. As an example, if d (diameter of microspheres) is 80 μm, nb (refractive index of the binder layer) is 1487, ng (refractive index of microspheres) is 2,175 and ns (refractive index of the binder layer). approach) is 1.472, then you can use equation (I) to derive the following equation to obtain f (optimal focus layer thickness coefficient) = 0.2857. 1. 472 (2,175-2 (1,487)) / = 2 [1,487 (1,472-2,175) + 1,472 (1,487-2,175)] 1,472x0,799 / = • -4,1162 J 4.1162 Furthermore, since h = d • f and d (the diameter of the glass spheres) is 80 μm as stated above, h (the thickness of the focusing resin layer) can in turn be calculated by the following equation. h = d • f = 0.080 x 0.2857 0.0229 (mm) In other words, the optimum thickness of the focusing layer is approximately 23 μm. According to the present invention, the film thickness selection coefficient x of the focusing layer which controls the outward radiation of the retroreflected light, which is defined based on the conditions specified for the conventional retroreflective sheet typical used for safety purposes, or specifically by the equations (I) and (II) mentioned above, if it becomes smaller than 1.00 and therefore much smaller than according to the prior art, that is, a thinner focusing resin layer, and in this way retroreflective light recognition is allowed at a wider viewing angle. In addition, the enlargement of the retroreflected light expansion angle allows the recognition of retroreflected light even under daylight, while the design of the focus resin layer allows an excellent rainbow-like decoration which can be combined with the brilliance , since the sections without focusing layer in contact with the underside of the microspheres act as a prism. In fact, as the thickness of the focusing layer is reduced, the reflected light radiates outward at an increasing angle, and when the thickness reaches zero, the reflected light can produce a rainbow-like scattering spectrum in a direction of approximately 42 ° with respect to incident light. The brightness of the reflected light is reduced as it is radiated at a wider angle, which makes it more difficult for observers to recognize it, while the narrower angles cause the incident light from the light sources to be blocked by the observer, which makes it difficult for the reflected light to be recognized, particularly when the source of light is the sun, a lamp in the street, interior lights, etc. However, based on the findings of the present inventors, when the coefficient x of film thickness selection is in the range of 0.16 to 0.95 in equations (I) and (II) which define the thickness of the film layer. By focusing, the reflected light can easily be recognized even under light sources such as the sun. Furthermore, it also becomes clear that the decorative effect mentioned above and the wide-angle observation effect is shown most effectively when a printing method is used, and especially stencil printing for forming the focusing layer and, if it is necessary, for the formation of the adjacent reflecting layer. When a retroreflective sheet of light according to the invention is used, a single reflective sheet is able to combine the retroreflected light, the rainbow-like light and the lack of color, and is therefore highly suitable for decorative purposes. The decorative effect and the wide-angle observation effect of the retroreflective sheet of light of the invention will be more easily understood with reference to the attached figures 5 to 7. Here, as shown in Figure 5A, a pattern of the focusing layer 4 of the retroreflective sheet 10 is placed within a rectangle only in the central section A of the sheet 10. As seen in Figure 5B (not shown) simply show the microspheres), a focus layer is not formed in the perimeter section B of the reflecting sheet 10.

As shown in Figure 6, when the light is irradiated from a light source on the surface of the retroreflective sheet illustrated in Figure 5, the light is reflected through a wide angle as indicated by the arrows, and different patterns of this light reflected by an observer over a wide angle can be recognized. In the case illustrated in this drawing, different patterns can be recognized in the three different regions which are in the central region closest to the light source 11, the ring area b around it and the outer region c outside From the ring. A strong refraction is observed in the central region a of the central section A of the reflecting sheet 10, while the rainbow-like refraction is observed in the ring area b for the perimeter section B. The light source is interior lighting in the case illustrated here, but the same retroreflective effect will be obtained with daylight, street lamps or other light sources. Therefore, in the central region a of Figure 6, the central section A of the sheet 10 appears bright while the section B of the perimeter appears dark, as seen in Figure 7A. The ring area b of Figure 6, the central section A of the reflecting sheet 10 appears dark while the perimeter section B appears as a rainbow, as shown in Figure 7B. However, in the outer region c of Figure 6, the brightness / darkness sections or the rainbow or other regions can not be observed, and therefore appear colorless. Finally, the findings of the present inventors can generally be summarized by stating that for a retroreflective sheet of lens type included as described above, improvements in retroreflective properties require that particular attention be paid to the following factors: (1) ) selection of high quality microspheres with a uniform size (a variation in size results in fewer focusing entities on the surface of the reflective layer) and (2) an accurate control of the thickness and shape (pattern) of the film of the Focus layer. The additional details of the invention are defined in the features of the claims.

EXAMPLES The invention will be further explained by the following illustrative examples which are intended to be non-limiting.

Example 1 A cover film is formed from the polyvinyl chloride resin, as follows. A plasticized vinyl chloride resin containing a substance that absorbs ultraviolet light and a thermostabilizer is applied in the form of an organosol solution to the surface of a paper coating precoated with a release agent based on an alkaladic substance, and dried . The resulting vinyl chloride film has a thickness of 68 μm. A solution of polyurethane resin, FL510 (commercial name of Sumitomo 3M Co.) is applied on the surface of the support layer to form a layer of binder is iseco. Subsequently transparent microspheres with an average diameter of 71 μm and a refractive index of 2.26 are dispersed in a cascade flow as a single layer on the surface of the semi-dry binder layer. The spheres are embedded in the already formed binder layer to a size of approximately 50% of its diameter. After the dispersion of the glass spheres, a solution of polyvinyl butyral resin is applied with a bar coater over the binder layer at varying thicknesses, as indicated, and dried so as to adapt to the shape of the microspheres. , as shown. The drying conditions are adjusted based on the thickness of the film. As shown in Table 1 below, a total of nine (9) different transparent focus layers are formed which have a film thickness that increases, in increments of 3 μm within a range of 0 to 24 μm.

Aluminum vapor is then deposited on the back surface of the focusing layer and the rear surfaces of the microspheres are exposed to a film thickness of 600 A to form a reflective layer. An adhesive layer is then formed on the back surface of the resulting reflective layer by applying and drying an adhesive consisting of an iso-octyl acrylate-acrylic copolymer (copolymerization ratio: 90:10, average molecular weight: 400,000) in a paper coating coated with silicone resin as a release layer, and this is laminated on the back surface of the reflective layer. After formation of the adhesive layer, the paper coating on the coated film is peeled off and the exposed surface of the coated film is coated with a clear coating using a composition containing a polyester polyol and a polyisocyanate curing agent. The coated paint solution is allowed to dry to provide a clear coat with a thickness of 20 μm. Each of the nine (9) retroreflective light sheets illuminate an incident angle of 90 ° with light rays from a light source (incandescent lamp of 60) located a distance of 2 m from the reflecting sheet. An observer at a distance of 2 m from the reflecting sheet moves around the light source in the center, and a record is made of the maximum angle of observation (degrees) with respect to the angle of incidence, in which it is visually detected retroreflected light. The results of the measurements are included in Table 1 below. This measurement of the retroreflected light observation angle is repeated after changing the light source to daylight. However, in this case, the recognition capacity of the reflected light is confirmed at a distance of 2 m from the reflecting sheet with the single light coming from the rear of the observer. The results of the measurement are included in table 2.

Example 2 It is repeated in the process described in Example 1, except that a 270 screen screen printing machine is used instead of a bar coater for forming the focus layer. In addition, the formation of the reflective layer is subsequently carried out, for example, by deposition of aluminum vapor, printing a mixture of polyvinyl butyral resin and aluminum powder (SAP2171N, trade name, product of Showa Aluminum Po der Co .) at a solids ratio ratio of 1: 9, to a thickness of 2 μm using a 270 mesh screen printing machine. The results of the measurements are shown in Tables 1 and 2.

Table 1 With all the reflective sheets included in Table 1, an observation angle of 45 ° or greater does not allow observation of retroreflected light and rainbow-like colors, under any conditions.

Table 2 As discussed above, the invention provides a retroreflective decorative sheet that exhibits excellent decorative effect regardless of the time of day or night. In addition, the decorative effect of this retroreflective sheet can be obtained regardless of the angle of incident light, to provide color change tones and at the same time it provides a decorative appearance as a bright and pleasant rainbow scattering spectrum. In addition, the retroreflective sheets of the invention also have a wide viewing angle within which the decorative effect can be observed. For example, as the angle of observation gradually widens almost to the light source, the sections with the focus layer strongly reflect in the regions of small observation angle, which causes the sections with the focus layer to appear brightly retroreflected. As the viewing angle expands approaching 42 °, the retroreflected light disappears from the sections with the focus layer, and the sections without the focus layer reflect rainbow colors. In this case, sections without the focus layer appear as bright rainbow colors. Various modifications and alterations of this invention will become apparent to those familiar with the art without departing from the scope and spirit of this invention. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects or products to which it refers.

Claims (5)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A retroreflective sheet comprising, in order, a binder layer on the rear side of which a plurality of microspheres, arranged in a monolayer, partially embedded, an underlying focus layer to part of the microspheres, and a reflecting layer underlying the focus layer and the underlying microspheres which do not have a focus layer behind them, wherein the focus layer has a coefficient x of film thickness selection in the range of 0.16 to 0.95, as defined by equation (II) of the following equations (I) and (II): ns (ng -2nb) / = (I) 2 [nb (ns - ng) + ns (nb - ng) J X = (II) where f is the coefficient for the optimum layer thickness of the focusing layer, d is the diameter of the microspheres, h is the thickness of the focus layer, nb is the refractive index of the binder layer, ng is the refractive index of the microspheres, and ns is the refractive index of the focusing layer.
2. 2. The sheet according to claim 1, characterized in that it further comprises a cover film on the front surface of the binder layer.
3. 3. The sheet according to claim 1, characterized in that it also comprises an adhesive on the back side thereof.
4. The sheet according to claim 1, characterized in that the reflective layer is a metal vapor deposition coating or a coating of a resin containing metal powders.
5. 5. The retroreflective sheet according to any of claims 1 to 4, characterized in that the pattern of the focusing layer is one formed by the screen printing or stencil printing method.