FRONT PROJECTION SCREEN WITH LENTICULAR FRONT SURFACE

Abstract: A front projection screen is made up of a front portion and a rear portion, the front portion having a front surface defining a front lenticular lens array and light scattering particles forming a diffusion region between the front and rear surfaces, and the rear portion having a reflective surface spaced apart from the diffusion region. In use, an incoming ray of projected light traverses the diffusion region and the space between the diffusion region and the reflective surface, before being reflected back through these regions again. Thus, the ray traverses different portions of the diffusion region in different directions, increasing the likelihood that the ray will encounter at least one scattering particle, thus reducing the incidence of speckle. A stiffening layer of a glass plate with an anti-reflection coating is bonded to the front surface of the screen.

Description: Front projection screen with lenticular front surface. Background of the invention This invention relates to front projection screens, and more particularly relates to such screens having a front projection screen having a front portion and a rear portion, each portion having a front surface and a rear surface, the front portion having light scattering particles defining a diffusion region between the front and rear surfaces, the front surface of the front portion defining a front lenticular lens array, the array comprised of mutually parallel lenticules extending longitudinally across the screen, and the rear portion defining a reflective surface in spaced apart relationship to the diffusion region of the front portion. The development of small display devices such as LCDs (liquid crystal displays) and DMDs (deformable mirror devices) suitable for reproducing video images, has given rise to the need for small aperture projection optics to project these images, and for front projection screens to display the projected images with adequate contrast over a wide viewing angle. Various front projection screens are known which employ either a rear surface lenticular lens array or a linear fresnel lens array, together with light scattering particles to spread the image into the audience field. See, for example, US patents 4,767, 186; 4,964,695; and 4,911,529. These screens may also employ light-absorbing areas between the lens elements to absorb ambient light and thus improve image contrast. One problem encountered with such small aperture systems is that light may pass through the screen without encountering any of the light scattering particles, giving rise to a phenomenon known as scintillation or speckle, which is the random occurrence of bright spots on the screen. Such scintillation or speckle is obviously undesirable in that it reduces definition, and is distracting to the viewer. Obiects and summary of the invention Accordingly, it is an object of the invention to provide a front projection screen in which the incidence of scintillation or speckle is reduced. It is a further object of the invention to provide a front projection screen, in which a reliable stable construction is realized. It is another object of the invention to provide a front projection screen, in which influences of the ambient, such as heat and/or humidity, on the stability of the construction of the front projection screen are reduced drastically.. In accordance with the invention, a front projection screen is characterized in that the front projection screen comprises a front transparent stiffening layer, the stiffening layer comprising a plate having a front and a rear surface, the rear surface of the plate in contact with the front surface of the front portion defining the lenticular lens array, and the front surface of the plate having an anti-reflection layer. In use, an incoming ray of projected light traverses the diffusion region and the space between the diffusion region and the reflective surface, before being reflected back through these regions again. Thus, the ray traverses different areas of the diffusion region in different directions, increasing the likelihood that the ray will encounter at least one light scattering particle, and thereby reducing the incidence of speckle. The stiffening front transparent layer provides a stable construction with increased stiffness, which results in better performance due to better optical properties as a consequence of lower mechanical deformations of the screen. Further the stiffening front transparent layer provides a longer stable performance of the screen, because influences from the ambient have been shielded. As a consequence a likelyhood of partial delamination of the front and rear portions of the screen, which may be possibly caused by head and/or humid ambient is reduced drastically. As used herein, the term "lenticular lens array" means an array of mutually parallel elongated lens elements or lenticules, extending longitudinally across the screen in one direction, and having a defining contour in the transverse direction. The diffusion region can extend to the rear surface of the front portion e.g., across the entire thickness of the front portion from the front surface to the rear surface, with the rear portion providing the space between the diffusion region and the reflecting surface. Instead, the diffusion region may be spaced apart from the rear surface of the front portion, e.g. may be confined near the front surface, in which case the remaining region between the diffusion region and the rear surface of the front portion can function as a spacer region between the diffusion region and the reflecting surface. The reflecting surface may then be located at the rear surface of the front portion. In addition, the rear surface of the front portion may define a second lenticular lens array, parallel to the first array, with each lenticular lens element or lenticule of the first array having a corresponding lenticule in the second array. In accordance with another embodiment of the invention, the front and/or the rear lenticular array is made up of lenticules having steep sidewalls, so that at least some portion of the light entering these lenticules is totally internally reflected by the sidewalls and concentrated in the tip region of the lenticule. This total internal reflection (TIR) is described in more detail in the above-referenced US patents 4,767,186 and 4,964,695, the specifications of which are incoφorated herein by reference. In any of the above embodiments and variations of embodiments, a pattern or matrix of light-absorbing material may be present between the lenticules of the front and/or rear lenticular lens array, in order to enhance the contrast of the image displayed in the presence of ambient light. Brief description of the drawing The invention will now be described in more detail with reference to a number of specific embodiments, as illustrated in the accompanying drawing, in which Figure 1 is a schematic cross section of one embodiment of a screen having a front portion with a front lenticular array, a light absorbing structure associated with the array, and bulk diffusion, and a rear portion having a rear reflective array; Figure 2 is a schematic cross section of another embodiment of a screen similar to that shown in Figure 1 , except that the front portion also has a rear lenticular lens array; Figure 3 is a schematic cross section of another embodiment of a screen, similar to that of Figure 1, except that the diffusion region is confined near the front surface, and the rear portion has a front reflecting surface; Figure 4 is a schematic cross section of still another embodiment of a screen, similar to that of Figure 3, except that the front portion also has a rear lenticular lens array; Figure 5 is a schematic cross section similar to that of Figure 4, except that the reflective layer follows the contour of the rear lenticular array; Figure 6 is a schematic cross section similar to that of Figure 5, except that the reflective layer is confined to the tip regions of the rear lenticules; Figure 7 is a schematic cross section similar to that of Figure 2, except that a spacer region is present in the front portion; Figure 8 is a schematic cross section of a front projection screen similar to that of Figure 2, except that it lacks the spacer layer; Figure 9 is a schematic cross section of a front projection screen similar to that of Figure 2, except that the light absorbing structure is associated with the rear lenticular lens array; and Figure 10 is a schematic cross section of a front projection screen similar to that of Figure 1 , including a stiffening front transparent layer. Description of the preferred embodiments Referring to now to Figure 1, screen 10 is composed of front portion 12 and rear portion 14. Front portion 12, of a light transmissive material, such as polymethyl methacrylate, has a front surface 16 defining a lenticular lens array 17, made up of individual lenticules oriented parallel to one another and extending longitudinally across the screen. Figure 1 is a cross section taken transverse to the longitudinal direction, showing the contour of the lenticules. The front surface also has a light absorbing structure including raised ribs 18, extending between the lenticules, and alight absorbing layer 20 on top of these ribs. This layer 20 is intended to absorb a large portion of light striking the screen at relative large angles (e.g. ambient light) while absorbing only a small portion of the light striking the screen at small angles (e.g. projected light). The first portion 12 has light scattering particles dispersed throughout the bulk of the portion, to form diffusion region 22. The second portion 14 of screen 10 is made up of spacer layer 24 of a light transmissive material, such as glass, and layer 26, forming a light reflective surface 28. Layer 28 may be of any suitable mirror-forming material, such as silver of aluminium. The function of spacer layer 24 is to provide a space between the diffusion region 12 and the reflective surface 28. Thus, an incoming light ray 29 passes through diffusion region 22, then through spacer layer 24, then is reflected back through these layers again before exiting the screen. This arrangement results is significantly less speckle than is seen in the front projection screen shown in Figure 8. Figure 2 shows another embodiment of a screen, similar to the embodiment of Figure 1 , except that front portion 32 of screen 30 has a second lenticular array 50 on the rear surface. This lenticular array is also composed of mutually parallel, longitudinally oriented lenticules, each lenticule opposed to a corresponding lenticule of the front surface. Thus, the pitch P (distance between adjacent lenticules of an array) is the same for both the front and the rear arrays, while the contour of the lenticules of the second array may be different from the counter of the first array. As in the embodiment of Fig. 1, a ray 52 traverses the bulk diffusion region 42 twice, the second traverse being after passage through spacer layer 44 and reflection from surface 48, resulting in significantly less speckle than is seen in the screen of Fig. 8. Fig. 3 shows another embodiment of a screen, similar to the embodiment of Fig. 1, except that the light scattering particles of screen 60 are confined to a diffusion region 72 near the front surface of front portion 62, and that rear portion 64 is composed of layer 74, which forms a light reflective surface 76 where it meets front portion 62. In this configuration, the spacer layer 24 of Fig. 1 is replaced by the spacer region 78 in front portion 62, which region 78 does not have any light scattering particles. Thus, ray 79 upon entering the screen, traverses diffusion region 72, then spacer region 78, then is reflected from surface 76, to again traverse spacer region 78 and diffusion region 72, resulting in a reduced incidence of speckle when compared to the screen of Fig. 8. In the Fig. 3 embodiment, the layer 74 may be of substantial thickness, to provide mechanical support for the front portion, or may be a relatively thin layer, for example, of a coated or vapour deposited material, on the rear surface of the front portion. Fig. 4 shows yet another embodiment of a screen, similar to the embodiment of Fig. 3, in which the front portion 82 of screen 80 has a second lenticular surface 84, similar to lenticular surface 50 of Fig. 2. In this embodiment, incoming ray 86 traverses diffusion region 88, then spacer region 90, then is reflected from surface 92, then again traverses spacer region 90 and diffusion region 88 before exiting screen 80, resulting in a reduced incidence of speckle when compared to the screen of Fig. 8. Figs. 5 and 6 show two different variations of the embodiment of Fig. 4. In Fig. 5, screen 100 is composed of front portion 102, similar to front portion 82 of Fig. 4, and rear portion 104, consisting simply of a layer of material 106 which forms reflective surface 108 conforming to the contour of second lenticular surface 110. Layer 104 may, for example, be a vapour deposited layer of silver or aluminum. Thus, ray 112 upon entering screen 100, traverses diffusion region 114 and spacer region 116, is reflected at surface 108, and again traverses spacer region 116 and diffusion region 114, before exiting the screen, resulting in a reduced incidence of speckle when compared to the screen of Fig. 8. In Fig. 6, screen 200 is composed of front portion 202, similar to front portion 102, and rear portion 204, now reduced to separate layers of a material 206 confined to the tip portions 212 of the lenticules 220 of second lenticular surface 210. Material 206 may be, for example, silver or aluminum, vapour deposited through a mask, or coated onto the tips with an applicator such as a roller, having a much larger dimension than that of the lenticules. Thus, ray 216 upon entering screen 200, traverses diffusion region 218 and spacer region 220, is reflected at surface 208, and again traverses spacer region 220 and diffusion region 218, before exiting the screen, resulting in a reduced incidence of speckle when compared to the screen of Fig. 8. In Fig. 7, screen 300 is composed of front portion 302 and rear portion 304, similar to front and rear portions 32 and 34 of Fig. 2, except that in front portion 302, the diffusion region is confined to a region 306 near the front surface, leaving a spacer region 308 which is free of light scattering particles. Rear portion 304 is similar to rear portion 34, composed of spacer layer 310 and layer 312 forming reflective surface 314. Thus, incoming ray 316 traverses diffusion region 306, spacer region 308 and spacer layer 310, before being reflected by surface 314, to re-traverse these regions and layer before exiting the screen, resulting in a reduced incidence of speckle when compared to the screen of Fig. 8. The embodiments of Figs. 2, 4, 5, 6 and 7 are particularly advantageous in that commercially available rear projection screens having surface or bulk diffusion can be employed as the front portion of the front projection screen. Such rear projection screens are described, for example, in JEI, December, 1993, pp. 27, 33. Fig. 8 shows a front projection screen 400, having a front portion 402 of such a prior art rear projection screen, having a front lenticular array 404, raised portions 406 with light-absorbing layer 408, rear lenticular array 410 and bulk diffusion 412. The rear portion consists of minor 403. Thus, the screen lacks the spacer layer 24 of Fig. 1. The screens of Figs. 2 and 8 were visually compared for focus, moirέ and speckle, for different thicknesses of the spacer layer 44. Results are shown in the following Table. Table Spacer layer Focus Moirέ Speckle none (Fig. 8) best high high 3/32 inch good medium low 1/8 inch moderate low low 1/4 inch poor low lowest Fig. 9 shows a front projection screen 500, having a front portion 502 and a rear portion 504, similar to the front and rear portions 32 and 34 of Fig. 2, except that in front portion 502, the light absorbing structure is associated with the rear lenticular lens array 510, instead of the front anay 507. Thus, ribs 512 and light absorbing layers 514 are located between lenticules 511, and any light reaching such layers 514, such as ray 524, is absorbed. - Fig. 10 shows a front projection screen 1000. having front and rear portions 12 and 14, identical to those in Fig. 1 , and a stiffening front transparent layer 27, including a plate 13, and an anti-reflection layer 15. Plate 13 may be of a plastic or glass composition, for example, while layer 15 may be a 1/4 wave thick MgF 2 , for example, where the wavelength is the central or dominant wavelength of the projected light. Such a layer 27 tends to prevent the delamination of front and rear portions 12 and 14, caused for example, by a humid ambient. It is understood that the anti reflection layer 15 is preferably applied so as to prevent the occurrence of "gost images" by reflections (shown as outgoing light ray 29 A) of the incoming light ray 29 at the entrance of transparent layer 27. As will be appreciated, the front and rear portions 12 and 14 may be replaced by the front and rear portions of any of the other embodiments described herein. The invention has been described in terms of a limited number of embodiments and variations of embodiments. Other embodiments and variations will become readily apparent to those skilled in the art, and are intended to be encompassed within the scope of the invention as defined in the appended claims.

Claims: o CLAIMS 1. A front projection screen having a front portion and a rear portion, each portion having a front surface and a rear surface, the front portion having light scattering particles defining a diffusion region between the front and rear surfaces, the front surface of the front portion defining a front lenticular lens anay, the anay comprised of mutually parallel lenticules extending longitudinally across the screen, and the rear portion defining a reflective surface in spaced apart relationship to the diffusion region of the front portion, characterized in that the front projection screen comprises a front transparent stiffening layer, the stiffening layer comprising a plate having a front and a rear surface, the rear surface of the plate in contact with the front surface of the front portion defining the lenticular lens anay, and the front surface of the plate having an anti-reflection layer. 2. The front projection screen of claim 1 in which the plate is glass and the anti- reflection layer is MgF . 3. The front projection screen of claim 1 or 2, in which the thickness of the anti- reflection layer is 1/4 of the central wavelength of the projected light.

Inventors: BAKER SCOTT H, MAGOCS STEPHEN, BAKER, SCOTT, H., MAGOCS, STEPHEN
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: G03B21
IPC codes: G03B21