WO2000052527A1

WO2000052527A1 - An optical element screen

Info

Publication number: WO2000052527A1
Application number: PCT/US2000/005211
Authority: WO
Inventors: John R. Filipe; John A. Futhey
Original assignee: Fresnel Optics, Inc.
Priority date: 1999-03-01
Filing date: 2000-02-29
Publication date: 2000-09-08
Also published as: TW418341B

Abstract

A projection screen is provided which includes a diffuser (24) having a first side and a second side and an integral optical element (22) on the first side. A front surface layer (26) is disposed on the second side of the diffuser. The front surface layer (26) is laminated to the second side of the diffuser.

Description

AN OPTICAL ELEMENT SCREEN

BACKGROUND OF THE INVENTION

Projection screens for use in rear-type imaging systems are typically used to view an image of an object projected on the rear surface and diffused to the front surface. Examples of rear-type imaging systems include rear projection big screen television receivers, rear-type slide projectors, and rear-type microfilm readers.

In prior art systems, as shown in Figure 1 , a separate sheet with a Fresnel lens 10 is placed between a projection mechanism, positioned on an input side 11, and the imaging screen 12. The primary purpose of the Fresnel lens is to enhance center to edge brightness uniformity. As understood in the art, Fresnel facets of the lens 10 gather and collimate incoming light into the imaging screen 12. The screen 12 can include a bulk diffuser 14 which spreads the light into a specific gain profile. A front surface layer 16, typically comprising acrylic or glass, is disposed on the viewing side of the screen 12 to protect the diffuser 14.

Such an arrangement can cause "ghost images" on the screen because of secondary internal reflection and refraction effects in the Fresnel lens. Such ghost images are most noticeable in high resolution systems. This phenomenon is explained in more detail with reference to Figure 2. A ghost image is a secondary and dimmer image of the desired image shown on the imaging screen. It is especially noticeable, for example, when the screen background is dark and a few widely spaced text characters are being displayed in the corners of the screen. A secondary or ghost image of these characters, being dimmer and offset towards the center of the screen, is visible. A light ray passing through point A intersects the piano or flat side of the Fresnel lens 10 at point B, and is refracted to point C. At point C, a majority of the light is refracted through the Fresnel facets to point D, which produces the primary and desired image on the diffusion screen. However, at point C, a significant portion of the light reflects off of the Fresnel facet, and is directed back inside the Fresnel lens 10 to point E. Because the angle of incidence at point E is greater than the critical angle, all of the energy in the light ray is reflected by total internal reflection. This ray passes through a draft surface 18 of the Fresnel lens 10, and exits towards point F. This secondary ray is dimmer and offset from the primary ray at point D, and is the source of the ghost image.

Prior art configurations have also suffered from the deleterious effect of having the imaging screen scrape against the Fresnel lens, for example, during shipping. The faceted optical surfaces of the Fresnel lens are very fragile and sharp. Vibration during shipping and moving can cause the Fresnel lens to rub against the screen to damage one or both components. The lens and screen can waφ in a non- uniform manner when exposed to temperature and/or humidity extremes which can result in cosmetic damage.

SUMMARY OF THE INVENTION

A projection screen is provided which includes a diffuser having a first side and a second side and an integral optical element on the first side. A front surface layer is disposed on the second side of the diffuser. In one embodiment, the front surface layer is laminated to the second side of the diffuser.

The optical element can include a Fresnel lens, a lenticular array, prisms, or the like, which collimates or redirects impinging light rays. Preferably, the light rays pass through the optical element and through the diffuser.

The diffuser includes a polymeric carrier substrate and polymeric lenslet particles dispersed throughout the earner substrate. The lenslet particles can be spherical, elliptical, or have other desired geometries which directly influence the gain profile. Preferably, the lenslet particles and the carrier substrate have a different refractive index. In one embodiment, the lenslet particles have a refractive index between about 1.55 and 1.60, and the carrier substrate has a refractive index of less than about 1.54.

The front surface layer can include an anti -reflective surface or a hardcoat thereon. The front surface can also include a matte or anti-glare treatment. The front surface layer can includes neutral density tints for contrast enhancement. In an alternative embodiment, the front surface layer includes a reflective surface which can redirect light rays passing through the optical element and the diffuser back therethrough. The reflective surface can include linear or cube-corner prisms.

The projection screen of the present invention can be used with front or rear- type image projection systems. The projection screen of the present invention substantially reduces or eliminates Fresnel ghosting. The projection screen also has the advantage of being a compact, light weight unit which requires less assembly steps than prior art systems, resulting in lower costs.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Figure 1 is a side view of a prior art imaging screen for use in a rear-type imaging system.

Figure 2 is an enlarged view of area "A" of Figure 1 illustrating a ghosting effect.

Figure 3 is a side view of an imaging screen for use in a rear-type imaging system in accordance with the present invention.

Figure 4 is an enlarged view of area "B" of Figure 3. Figure 5 is an isometric view of an imaging screen illustrating a first reference point for measuring the light output from the screen.

Figure 6 is a graph illustrating the gain distribution as viewed from horizontal and vertical angles from the center of an imaging screen. Figure 7 is an isometric view of an imaging screen illustrating a second reference point for measuring the light output from the imaging screen.

Figure 8 is a graph illustrating the gain distribution as viewed from the outside edge of an imaging screen.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows. Figure 3 illustrates a preferred embodiment of an imaging screen 20 for use in projection systems, such as the front and rear-type. The screen 20 includes an optical element 22 integrally formed on a diffuser 24 which is preferably laminated to a front surface layer 26. The optical element 22 can include a Fresnel lens, a lenticular array, prisms, or the like, which collimates or redirects impinging light rays. Preferably, the light rays pass through the optical element 22 and through the diffuser 24.

A projection system on the input side 28 of the screen 20 forms an image thereon which is collimated by optical element 22. The collimated image is diffused by diffuser 24 into a specific gain profile to an output side or viewing side 30. The front surface layer 26, which can include acrylic, glass, or the like, can include neutral density tints for contrast enhancement. The layer 26 can also include an anti- reflective surface which can include subwavelength structures such as moth eye structures as disclosed in U.S. Patent 4,013,465, issued to Clapham et al. on March 22, 1977, the teachings of which are incoφorated herein by reference. The layer 26 can further include a hardcoat treatment coating, preferably on the outermost layer toward the viewing side 30. The layer 26 can also include a matte or anti-glare treatment. In an alternative embodiment, the front surface layer 26 includes a reflective surface which can redirect light rays passing through the optical element 22 and the diffuser 24 back therethrough. In one embodiment, the reflective surface can include linear prisms, as taught in U.S. Patent 3,846,012, issued to Brown on November 5, 1974, and U.S. Patent 4-260,220, issued to Whitehead on April 7, 1981 , the teachings of both references being incoφorated herein by reference. In another embodiment, the reflective surface includes cube-corner prisms, as taught in U.S. Patent 3,712,706, issued to Stamm on January 23, 1973, U.S. Patent 3,684,348, issued to Rowland on August 14, 1972, and U.S. Patent 3,689,346, issued to Rowland on September 5, 1972, the teachings of each reference being incoφorated herein by reference.

Figure 4 illustrates the optical element 22 formed on the diffuser 24 in accordance with the present invention. In this embodiment, the optical element facets face the incoming rays from the projection system, such as the ray passing through point A. The light passing through point A impinges upon the optical element 22 at point B, where the lens refracts the ray into a collimated direction. This ray then strikes the diffuser 24 at point C wherein the light is dispersed over a wider solid angle in the output space D. The optical element 22 slopes do not originate any ghost images. The drafts or risers, of the optical element 22 do create some light loss and light scattering, but this light is disorganized and does not give rise to any secondary images.

The diffuser 24 can be formed from materials such as disclosed in International Publication Number WO 96/20419, published under the Patent

Cooperation Treaty on July 4, 1996, the teachings of which are incoφorated herein by reference. More particularly, the diffuser 24 contains suspended particles in a host material. The particles are of a different refractive index than the host material, and behave as lenslets. To form the diffuser 24, polymeric particles or lenslets are dispersed throughout a second polymer, which can be referred to as the host or earner substrate/material. Preferably, the lenslet particles have a different index of refraction than the carrier substrate. In one embodiment, the lenslet particles have diameters in the range between about 10 and 100 microns. The lenslets can be spherical, elliptical, or have other desired geometries which directly influence the gain profile. The lenslet particles in one embodiment are formed from polystyrene which has a refractive index in the range between about 1.55 and 1.60 based on the D542 ASTM (American Society for Testing and Materials) scale. The carrier substrate can be formed from polypropylene, polyethylene, polycarbonate, or the like, which have a refractive index different from the lenslet particles. Preferably, the refractive index of the carrier substrate is less than about 1.54 based on the D542 ASTM scale. The lenslet particles can be dispersed throughout the carrier substrate in many ways. For example, the lenslet particles are mixed with a pre-polymerized monomer liquid. The liquid is polymerized to form a solid carrier having the lenslet particles throughout. In a case where the polymer melt temperatures of the lenslet particles and the carrier substrate are significantly different and the carrier substrate melts at a lower temperature, solid lenslet particles or pellets are mixed with the carrier substrate. The carrier is melted or liquified, for example, through extrusion or injection without melting the lenslets. Alternatively, the lenslet particles can be sprayed in solid form onto a backing layer mixed with a liquid adhesive. The liquid adhesive becomes the earner substrate.

The optical element 22 is preferably compression molded or embossed into the diffuser 24. There are various other methods for precisely replicating the Fresnel microstructure into the diffuser 24. In the preferred methods, the diffuser and the Fresnel lens tooling are subjected to heating and cooling phases under clamp pressure.

In the heating phase, the diffuser material is softened or elevated to a temperature greater than or equal to the glass transition temperature (Tg) of the diffuser substrate which, in one embodiment, is between about 49 and 191 degrees Celsius (120 to 375 degrees Fahrenheit). Clamping pressure, preferably in the range of about 689.5 to 17,926.4 kPa (100 to 2,600 psi) is used to keep the diffuser material and tooling in contact during the entire replication process and also to force the diffuser substrate to fill or faithfully replicate the micro structured Fresnel lens facets. The final phase, cooling, reduces the temperature of the diffuser substrate and Fresnel tooling to less than the glass transition temperature of the diffuser and permits release of the lens from the Fresnel tool surface.

The diffuser 24 can be tailored to supply symmetric or asymmetric profiles. For example, for a home theater rear projection television application, it is desirable to have a very wide horizontal viewing cone so that a viewer can sit at any point in the room. It is further desirable to provide a very bright image to provide sufficient contrast in well lit ambient conditions. It is not desirable to provide a wide vertical viewing cone because the light striking the floor and ceiling is essentially wasted. In this example, the diffuser is tailored so that the peak gain and horizontal viewing angle are enhanced at the expense of a tight vertical angle. Asymmetric gain performance can be achieved, for example, through stretching the resulting diffuser 24 in a preferential direction using extrusion or blow molding processes or pressing the material and letting it flow preferentially in one direction by constraining the flow in the other direction. Other methods of providing asymmetric gain performance include using non-spherical lenslets and selectively orienting them through gravity, centrifugal, or other means. Additional microstructured surfaces, such as prisms and lenticulars, can be used with the imaging screen 20 to provide tilt compensation such as asymmetric performance within a viewing plane and other customized gain distributions.

Example 1

The following symmetric and asymmetric gain profiles have been obtained.

Example 2

A Fresnel lens was molded into a diffuser created from a polystyrene bead suspended in polypropylene. Light output was measured at various horizontal (H) and vertical (V) viewing angles using a goniophotometer, with zero degrees defined as normal (n) to the screen. First, the light output was measured at the screen center as seen in Figure 5. It is noted that at the center of the screen, the Fresnel lens has little effect. The horizontal and vertical symmetric gain profiles are plotted in Figure 6. The edge of screen was measured as seen in Figure 7. The result is shown in Figure 8 illustrating two cases: an imaging screen with a Fresnel lens and an imaging screen without a Fresnel lens. At the edge, without the Fresnel lens, the light exits the screen at an angle that is not normal to the screen. Thus, a viewer who is sitting normal to the screen would see a dark corner at the edge of the screen viewing the screen head-on. With the addition of the Fresnel lens, light is collimated by the lens so that it enters the diffuser normal to the screen. The result is a light output distribution that is centered in the direction normal to the screen. The practical benefit is an increase in brightness uniformity over the screen. In technical terms, this is measured as decreased "center-to-edge fall off.

Figure 9 illustrates an exemplary rear projection screen television 32 which includes a projection system 34 which projects an image through a lens system 36. The image is reflected to the imaging screen 20 by mirrors 38 and 40.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A projection screen, comprising: a diffuser having a first side and a second side and an integral optical element on the first side; and a front surface layer disposed on the second side of the diffuser.

2. The projection screen of Claim 1, wherein the optical element includes a Fresnel lens.

3. The projection screen of Claim 1, wherein the optical element includes a lenticular array.

4. The projection screen of Claim 1, wherein the optical element includes prisms.

5. The projection screen of Claim 1, wherein the front surface layer is laminated to the second side.

6. The projection screen of Claim 1 , wherein the front surface layer includes a reflecting surface which can redirect light rays passing through the optical element and the diffuser back therethrough.

7. The projection screen of Claim 6, wherein the reflecting surface includes linear prisms.

8. The projection screen of Claim 6, wherein the reflecting surface includes cube-corner prisms.

9. The projection screen of Claim 1, wherein the projection screen is used in a rear-type image projection system.

10. The projection screen of Claim 1, wherein the projection screen is used in a front-type image projection system.

11. The projection screen of Claim 1 , wherein the diffuser includes a polymeric carrier substrate and polymeric lenslet particles dispersed throughout the carrier substrate.

12. The projection screen of Claim 11 , wherein the lenslet particles are spherical.

13. The projection screen of Claim 11, wherein the lenslet particles are elliptical.

14. The projection screen of Claim 11, wherein the lenslet particles and the earner substrate have a different refractive index.

15. The projection screen of Claim 11, wherein the lenslet particles have a refractive index between about 1.55 and 1.60.

16. The projection screen of Claim 11, wherein the carrier substrate has a refractive index of less than about 1.54.

17. The projection screen of Claim 1, wherein the front surface layer includes an anti-reflective surface thereon.

18. The projection screen of Claim 1, wherein the front surface layer includes a hardcoat thereon.

19. The projection screen of Claim 1 , wherein the front surface layer includes a matte treatment.

20. The projection screen of Claim 1, wherein the front surface layer includes neutral density tints for contrast enhancement.

21. A proj ection screen, comprising: a diffuser having a first side and a second side; an optical element disposed on the first side; and a front surface layer disposed on the second side; wherein the diffuser, the optical element, and the front surface layer are laminated together.

22. The projection screen of Claim 21, wherein the optical element is selected from the group consisting of a Fresnel lens, lenticular arrays, and prisms.

23. The projection screen of Claim 21, wherein the diffuser includes a polymeric carrier substrate and polymeric lenslet particles dispersed throughout the carrier substrate.

24. The projection screen of Claim 23, wherein the lenslet particles are spherical.

25. The projection screen of Claim 23, wherein the lenslet particles are elliptical.

26. The projection screen of Claim 23, wherein the lenslet particles and the carrier substrate have a different refractive index.

27. A method for forming a projection screen, comprising: forming an integral optical element on a diffuser; and laminating the diffuser to a front surface layer.

28. The method of Claim 27, further comprising the step of forming lenslet particles within the diffuser for diffusing the incoming light.