FR2910140A1 - Overhead projector screen, has Fresnel lens for redirecting imaging beam, lens element provided on output surface of screen, and flat strips positioned in front of lens element and located parallel to refracted rays of imaging beam - Google Patents

Abstract

The screen has a Fresnel lens (53) for redirecting an imaging beam and a lens element (32) provided on an output surface of the screen, where the element comprises a black matrix. Flat strips (540) are positioned in front of the lens element and behind a Fresnel surface of the Fresnel lens. The strips are located parallel to refracted rays (350, 351) of the imaging beam and placed in a curtain film (54) attached to the Fresnel lens.

Description

1 Projector screen and corresponding overhead projector. 1. Domain

  of the invention. The invention relates to the field of image projection.

  More specifically, the invention relates to a screen for an overhead projector to limit the parasitic rays. 2. State of the art According to the state of the art, an overhead projector illustrated schematically with reference to FIG. 1, comprises an imaging source 10, a folding mirror 11 and a screen. The imaging source 10 emits an imaging beam to the mirror 11 which reflects the beam back to the screen. The screen itself includes a Fresnel lens 12 and a lenticular element 13. The Fresnel lens 12 collimates the beam towards a spectator. The collimated beam passes through the lenticular element 13 which makes it possible to widen the angle of vision. This technique has the disadvantage of producing an imaging beam with stray lights. Thus, a ray 141 of the imaging beam is subdivided, at the entrance of the Fresnel lens 12, into a ray transmitted to a spectator and a parasitic ray 150 which is returned by the mirror 11 to the Fresnel lens 12 and Similarly, a ray 140 of the imaging beam passes through the Fresnel lens and subdivides, at the exit of the Fresnel lens 12, into a ray transmitted to a spectator and a parasitic ray 150 which is returned in another direction and is returned after one or more reflections to the lenticular element 13 and the viewer. The lenticular element with vertical linear patterns, black bands (or black matrix) absorb certain parasitic rays. Nevertheless, despite this filtering, parasitic rays can cross the screen. 3. Summary of the invention. The invention aims to overcome these disadvantages of the prior art. More particularly, the invention aims to reduce or eliminate stray lights.

To this end, the invention provides a screen for an overhead projector, comprising a Fresnel lens for redirecting an imaging beam and a lenticular element on the exit face of the screen. In order to reduce or even eliminate the parasitic lights, the screen comprises lamellae positioned before the lenticular element and substantially parallel locally to the rays of the imaging beam. According to an advantageous characteristic, locally, the angle between the lamellae and the radii of the imaging beam is less than or equal to 2.

According to a particular characteristic, the lamellae are placed after or before the Fresnel surface of the Fresnel lens. Advantageously, the slats are flat or cylindrical. According to a particular characteristic, the slats are placed in a film. According to a specific feature, the film is attached to the Fresnel lens. According to another characteristic, the slats are placed in the Fresnel lens. The invention also relates to an overhead projector comprising an imaging source and a screen as mentioned above according to the invention. 4. List of figures. The invention will be better understood, and other features and advantages will become apparent on reading the following description, the description referring to the appended drawings in which: FIG. 1 illustrates an imaging system known per se; FIG. 2 is a very schematic diagram of a retro-projector according to one embodiment of the invention; Figures 3, 5, 6 and 8 show an exploded and perspective view of the screen of the overhead projector of Figure 2 according to various embodiments of the invention; Figures 4, 7 and 9 illustrate a schematic side view of screens shown in Figures 3, 5, 6 and 8; Figure 10 shows a front view of an element of the screens of Figures 6 and 8 according to an alternative embodiment of the invention; and Fig. 11 illustrates a Fresnel lens according to a variant of the invention. 5. Detailed description of the invention. The general principle of the invention is therefore based on lamellae placed before a lenticular element to eliminate most or all of the parasitic rays created by reflection or refraction of the imaging beam on a Fresnel lens. In fact, the parasitic rays having angles different from the direct rays of the image intended for a spectator, slats (for example in a curtain film or film with lamellae or micro louver in English) placed before the lenticular element, filter angularly the imaging beam. According to a variant of the invention, the lenticular element is replaced by a diffusing screen that can be tinted. FIG. 2 is a very schematic diagram of a retro-projector 2 according to a first embodiment of the invention. The projector 2 comprises: an imaging source 20 producing an imaging beam 25 having an axis 26; a rear projection screen 24 illuminated by the beam 25; and two folding mirrors 22 and 23 folding the beam 25 and reducing the depth P of the projector 2.

The imaging source 20 and the mirrors 22 and 23, as well as their arrangement are well known to those skilled in the art and are not further detailed. According to the invention, the screen 24 comprises a Fresnel lens, a lenticular element 32 and a curtain film placed before the lenticular element 32 in the path of the imaging beam. The Fresnel lens collimates the beam towards a spectator. The collimated beam passes through the lenticular element 32 which makes it possible to widen the angle of vision. Preferentially, the lenticular element 32 comprises black bands (or black matrix) which absorb the outer rays and any parasitic rays coming from the imaging beam, not absorbed by the curtain film.

FIG. 3 presents an exploded and schematic view of a part of the screen 24 according to an embodiment comprising a film 34 with horizontal strips 340, placed behind a refractive Fresnel lens 33. According to this view, the beam imaging (including radii 350 near the optical axis 26 and 351 farther from the optical axis 26) has been shown unfolded, the fold mirrors being omitted. The screen 24 also comprises a lenticular element 32 at the output. FIG. 4 schematically shows a side view of the main optical elements (imaging source 30 and folding mirror 31) and of the screen 24 corresponding to the embodiment of FIG. 3. The curtain film comprises lamellae 340 Absorbing the rays striking them, transparent areas 341 (eg an acrylic material, plastic or glass) between the lamellae 340 passing through the incident rays of the imaging beam. Absorbent lamellae 15 include, for example, a black ink. Thus, the rays 350 and 351 emitted by the source 30 are reflected by the mirror 31 and then pass through the Fresnel lens 33 and are refracted at the outlet of the lens 33 in a horizontal direction. The refracted rays 350 and 351 then pass through the film 34 and then the element 32 towards a viewing zone of an image, where a spectator is located. According to the illustrated embodiment, the slats are horizontal, which facilitates the manufacture of the curtain film. Preferably, the lamellae 340 are equidistant and separated by a distance h. Preferably, the slats 340 extend from one face to the other of the film 34 (alternatively, some or all of the slats do not extend from one side to the other of the film 34). Preferably, the distance h and the width e of the lamellae is such that the ratio e / h is greater than the tangent of an angle of incidence a limit beyond which all the parasitic rays are absorbed by a lamella. Advantageously, a is less than or equal to 5; thus, the ratio w / h is preferably greater than or equal to 11.5. Advantageously, the distance h is between 0.8 times and 5 times the distance h1 (or pitch in English) between two consecutive prisms of the lens 33 (The pitch is generally between 50 and 200 m). The larger the h, the less the desired rays of the imaging beam are absorbed, which improves the optical efficiency of the film. Thus, advantageously, the distance h 2910140 5 is between 40 m and 1 mm. According to advantageous embodiments, the distance h is equal to an integer of times the pitch. According to an alternative embodiment, the lamellae are not flat and horizontal but are cylindrical and centered on the optical axis 26 of the system; the lamellae are concentric and the center of the circle which they describe is on the axis 26. Advantageously, in this embodiment, the lamellae are located in the shadow zones, that is to say, not illuminated by the imaging beam; in the embodiment described, the lamellae are thus opposite the non-active optical faces 331 of the lens 33 (as opposed to the faces 330 which refract the imaging beam) According to FIG. passing through the axis 26, this variant, the slats 340 are located between two circular surfaces separated by a distance h2 and which pass through the boundaries between the faces 330 and 331.

In general, according to the invention, the slats may have any shape in a plane perpendicular to the axis 26, while remaining parallel to the rays of the imaging beam passing through the curtain film. According to a particular embodiment of the invention, a curtain film 100 includes the imaging beam. According to a variant, as illustrated in FIG. 10 which represents a front view of the curtain film, only a central zone 102 comprises lamellae 101 (curves or not). This zone 102 is delimited by two planes 1021 and 1022 passing through the axis 26 and perpendicular to this axis. Advantageously, the planes 1021 and 1023 form an angle 13 of between 20 and 60. FIG. 5 shows an exploded and schematic view of a portion of the screen 24 according to a variant of the invention, comprising a film 54 with horizontal strips 340, placed behind a reflective Fresnel lens 53. According to this view, the Imaging beam (comprising radii 350 and 351) has been shown unfolded, fold mirrors being omitted. The screen also includes a lenticular element 32 output. The spokes 350 and 351 and the element 32 are similar to those of Figure 3 and have the same references. FIG. 6 shows an exploded and schematic view of a portion 35 of the screen 24 according to a variant of the invention, comprising a film 61 with curved slats 60, placed behind a reflective Fresnel lens 53. More precisely the lamellae 60 are circular in a plane perpendicular to the optical axis 26 of the system; the lamellae are concentric and the center of the circle they describe is on the axis 26. According to this view, the imaging beam (including radii 350 and 351) has been shown unfolded, the fold mirrors being omitted. The screen also includes a lenticular element 32 output. The spokes 350 and 351, the lens 53 and the element 32 are similar to those of Figure 5 and have the same references.

FIG. 7 schematically shows a side view of the main optical elements (imaging source 30 and folding mirror 31) and the screen 24 corresponding to the embodiment of FIG. 5. The curtain film 54 comprises lamellae 540 absorbing rays striking them, transparent areas 541 between the lamellae 540 allowing the incident rays of the imaging beam to pass. Thus, the rays 350 and 351 emitted by the source 30 are reflected by the mirror 31 and then pass through the Fresnel lens 53 and are refracted on one of the faces 731 of the lens 53 and then reflected by other faces 732 of the lens 53 to be returned to the film 54 in a horizontal direction.

The refracted rays 350 and 351 then pass through the film 54 and then the element 32 towards a viewing zone of an image, where a spectator is located. According to the illustrated embodiment, the slats 540 are horizontal. The film 54 is similar to the film 34. The embodiments of the film 54 are also similar to those of the film 34. In the same way as the embodiment of FIG. 3, a parasitic ray 70 is refracted by one the input faces of the lens 73 and is absorbed by a plate 540. FIG. 7 which can also represent a vertical section passing through the axis 26 of the variant presented with reference to FIG. 540 of FIG. 7) are located between two cylindrical surfaces separated by a distance h2 and which pass through the boundaries between the faces 730 and 731. Advantageously, in this embodiment, the lamellae are located in the zones of FIG. shadow; in the embodiment described, the lamellae are therefore opposite the refractive faces 731 of the lens 73.

In general, the lamellae of the film placed behind the reflective Fresnel lens 73 may have any shape in a plane perpendicular to the axis 26, while remaining parallel to the rays of the imaging beam.

According to a variant illustrated with reference to FIG. 11, lamellae 111 are integrated with the Fresnel lens 112 (and not in a film). The optical functions of the Fresnel lens 112 are the same as those of the combination of the film 61 or 54 and the lens 53, the input faces of the lens 112 being similar to that of the lens 53 and the characteristics of the lenses. 10 lamellae 111 are the same as those of the lamellae of the film 61 or 54, or more generally of any variant of these films. FIG. 8 is an exploded and schematic view of a part of the screen 24 according to a variant of the invention, comprising a film 80 with locally curved strips 340 parallel to the incident rays of the imaging beam, placed in front of a refractive Fresnel lens 53. According to one variant, the Fresnel lens is refractive. According to another variant, the lamellae are integrated with the refractive Fresnel lens (in a manner similar to the lens of FIG. 11). According to FIG. 8, the imaging beam (comprising radii 350 and 351) has been shown unfolded, fold mirrors being omitted. The screen also includes a lenticular element 32 output. The spokes 350 and 351 and the element 32 are similar to those of Figure 3 and have the same references. FIG. 9 schematically shows a vertical section passing through the axis 26 of the main optical elements (imaging source 30 and folding mirror 31) and the screen 24 corresponding to the embodiment of FIG. curtains 80 comprises strips 800 absorbing striking rays, 802 transparent areas between the lamellae 800 passing the incident rays of the imaging beam.

Thus, the rays 350 and 351 emitted by the source 30 are reflected by the mirror 31 and then pass through the film 80 and the Fresnel lens 33 and are refracted on one of the faces of the lens to be returned in a horizontal direction. The refracted rays 350 and 351 then pass through the member 32 to a viewing area of an image, where a viewer is located.

The lamellae 800 are located between two conical surfaces which preferentially pass through the boundaries between the faces of the lens 33. In general, the lamellae are locally parallel to the rays of the imaging beam (the angle 13 that locally an incident ray of the imaging beam is the same as a lamella adjacent to this ray, the angle 13 varying as a function of the distance to the axis 26). Preferably, the distance h and the width e of the lamellae 800 is such that the ratio e / h is greater than the tangent of a limiting angle of incidence (with respect to the considered lamella) beyond which all the parasitic rays are absorbed by a coverslip. Advantageously, a is less than or equal to 5; thus, the ratio w / h is preferably greater than or equal to 11.5. Advantageously, the distance h is between 0.8 times and 5 times the distance h1 (or pitch in English) between two consecutive prisms of the lens 33 Advantageously, the distance h is between 40 m and 1 mm.

According to advantageous embodiments, the distance h is equal to an integer number of times the pitch. Of course, the invention is not limited to the embodiments described above. According to embodiments, the lamellae are not exactly parallel locally to the (useful) rays of the imaging beam but are substantially parallel (for example, for tolerance rays in manufacture). Thus, locally, the angle between the lamellae and the rays of the imaging beam is advantageously less than or equal to 2 and even more advantageously to 1.

In particular, the invention is compatible with overhead projectors having a structure different from that described above. In particular, the invention is particularly well suited to shallow projectors with high imaging beam incidence angles on the Fresnel lens. It is also compatible with deeper projectors with a single or no fallback mirror. The invention is, moreover, compatible with transparent, rigid or flexible optical elements made of any material (for example glass or plastic (for example acrylic)).

The person skilled in the art can also adapt the invention with systems using additional optical elements, in particular field lenses, mirrors, optical filters. According to embodiments described above, horizontal lamellae are advantageously used with a lenticular element whose lenses of vertical axis filter the parasitic rays having a non-zero horizontal angle. Thus, the filtering performed by the lamellae and that effected by the lenticular element are complementary. More generally, the lamellae placed after the Fresnel lens are advantageously perpendicular to the lens axis of the lenticular element. Advantageously, a constant distance between the slats makes it easier to manufacture. According to variants of the invention, the spacing between the slats is not constant. 15

Claims (10)

  1. Screen (24) for an overhead projector (2), comprising a Fresnel lens (33, 53) for redirecting an imaging beam and a lenticular element (32) on the exit face of the screen, characterized in that the screen comprises lamellae (340, 540, 60, 800, 111) positioned before the lenticular element and substantially parallel locally to the rays (350, 351) of the imaging beam.   2. Screen according to claim 1, characterized in that, locally, the angle between the lamellae and the rays of the imaging beam is less than or equal to 2.   3. Screen according to any one of claims 1 and 2, characterized in that the slats (340, 540, 60, 111) are placed after the Fresnel surface of the Fresnel lens.   4. Screen according to claim 3, characterized in that the slats (340, 540) are flat.   5. Screen according to claim 3, characterized in that the lamellae are cylindrical (60).   6. Screen according to any one of claims 1 and 2, characterized in that the slats are placed (800) before the Fresnel surface of the Fresnel lens.   7. Screen according to any one of claims 1 to 6, characterized in that the slats (340, 540, 60, 800) are placed in a film.   8. Screen according to claim 7, characterized in that the film is attached to the Fresnel lens.   9. Screen according to any one of claims 1 to 6, characterized in that the slats (111) are placed in the Fresnel lens. 30 2910140 11   An overhead projector (2) comprising an imaging source (20) and a screen (24) according to any one of claims 1 to 9.