Optical: systems and elements – Projection screen
Reexamination Certificate
2002-02-07
2004-03-02
Mahoney, Christopher (Department: 2851)
Optical: systems and elements
Projection screen
C359S455000
Reexamination Certificate
active
06700702
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a screen apparatus that includes a microlens array and provides high transmission and high image contrast, as well as rejection of ambient light. The three fundamental components of the screen constitute a microlens array, e.g., a random microlens array, a substrate that supports the array, and a layer of light-absorbing material. The substrate and the microlens array can be separate components or a single unitary component.
The microlens array is preferably composed of generally anamorphic lens units that scatter and shape light according to distinct divergence angles along perpendicular directions. The use of random microlens units is also preferred since it eliminates the occurrence of image artifacts such as aliasing or moiré patterns. In addition, the size of each microlens unit in the array is preferably chosen small enough to provide high resolution. A typical microlens diameter or, more generally, maximum transverse dimension is between 20 and 120 microns.
The substrate, generally of a plastic material, provides support and mechanical rigidity to the whole screen. In the formation of the screen, an absorptive layer is added to the substrate, opposite to the microlens side, to provide ambient-light rejection and improve image contrast. Initially the light-absorbing material is added as a film layer, which is later exposed with aperture-forming illumination, e.g., ultraviolet illumination, to create apertures through which light propagates. The microlens array itself serves as the focusing element that creates the apertures.
A major contribution to the art of the present invention relates to the design of the microlens units to attain efficient focusing of the aperture-forming illumination throughout the array. Each lens element in the array is thus optimized to present a focus plane close to the absorptive layer so as to maximize the density of absorptive material while allowing the image-forming illumination to reach the viewer substantially unimpeded. The present invention can be used for general screen applications such as in rear-view projection televisions, computer screens, and general-purpose displays, among other devices.
BACKGROUND OF THE INVENTION
Display devices consist, in a broad sense, of an illumination system that provides luminous power (the light engine), relay optics such as lenses and mirrors, and a screen that projects information through images to the viewer in the so-called viewing space. The present invention relates to the screen component of display devices.
Desirable optical qualities for screens include high transmission, high contrast, rejection of ambient light, absence of image artifacts, high gain, and wide viewing angles. The basic elements of the screen responsible for these optical properties include a diffusing component, a supporting substrate, and an absorptive material. The diffusing component spreads the illumination in a controlled manner to direct the visual information to locations most likely occupied by the viewer. The absorptive material minimizes reflection of ambient light that reduces image contrast. Important examples of display devices include liquid crystal projection TV's; CRT projectors that use three distinct color sources in the light engine; flat-panel computer displays; and hand-held computing devices.
As the demand for high-quality displays increases, so does the necessity for adequate screen designs. For instance, the advent of HDTV (high-definition television) requires a considerable increase in image resolution, which implies that the screen must be able to resolve very fine features in the images being projected. To minimize power consumption and maximize brightness the screen must transmit a high fraction of the luminous power generated by the light engine. It is desirable that the transmission efficiency exceed 80%. On the other hand, to provide image contrast, the screen requires some sort of absorptive material that helps reject ambient light. If not properly designed, this absorptive material may lead to a significant decrease in transmission efficiency.
The diffusing component of screens used in virtually all commercial display systems to date rely on the use of lenticular arrays and/or random Gaussian surface diffusers. The lenticular arrays consist of cylindrical lenses with pitch between 300 microns and up to 700 microns. An example of this type of screen can be found in U.S. Pat. No. 5,870,224. More recently, improved lenticular screens with pitch down to 150 microns have been used. Because cylindrical lenses diffuse the illumination in a single direction (usually the horizontal) it must be combined with another diffusing component in the perpendicular direction, generally a random surface diffuser, if some wider viewing range is desired in the perpendicular direction.
The lenticular array and random Gaussian surface diffusers, while commercially available, present inherent disadvantages as follows. The lenticular array, because of its periodicity, may lead to diffraction effects as well as moiré fringing effects. Furthermore, lenticular arrays provide limited control over the distribution and shaping of light in the viewer space. Gaussian surface diffusers have the serious disadvantage of introducing speckle, which adds a grainy appearance to the image, unacceptable to the viewer. Also, Gaussian diffusers offer limited control over the scattering pattern. U.S. Pat. No. 4,666,248 discloses a screen geometry based on the use of a regular array of anamorphic lenses, in an attempt to obtain more control over the scattering profile. The regular nature of the array, however, does not avoid problems with diffraction and moiré fringing effects. Furthermore, the size of each microlens unit is between 300 and 500 microns, offering insufficient resolution for the increasingly high demands of visual display systems.
To provide a screen with ambient-light rejection capabilities and improved contrast it is necessary to introduce a light-absorbing component to the screen. A great majority of screens in commercial use add a bulk absorbing material (tint) to the body of the screen that, while adding contrast to it, also consumes a considerable fraction of the useful illumination originating in the light engine. In fact, transmission efficiency is typically below 60% and in many cases even less.
A more attractive approach relies on adding a layer of light-absorbing material to a screen that has focusing elements and has the absorbing material perforated in such a way that light is focused through the apertures, as illustrated in FIG.
1
. This is the approach described in U.S. Pat. Nos. 5,870,224; 4,666,248; 5,066,099; and 4,721,361. The main advantage of this scheme is that high transmission may be maintained even if the screen presents a high density of light-absorbing material.
Although this approach seems promising, there are some known difficulties such as quality of the apertures, mask alignment for aperture formation, and uniformity of the dark absorptive material. Also, the focusing array in the prior art has never been optimized to operate with the light-absorbing material, except for the use of a common focal distance for all focusing units. As a result, demonstration of the concept of a high-transmission screen through apertures perforated on a light-absorbing material has not been satisfactorily achieved by the prior art and is unavailable in current commercial display systems.
Some screens commercially available use focusing elements such as glass beads immersed on a light-absorbing material but this approach consumes a large fraction of the incident illumination and, therefore, cannot be considered satisfactory.
As a further aid in ambient-light rejection, U.S. Pat. Nos. 4,666,248 and 4,721,361 disclose a concept where a surface of the screen is structured with anti-reflection capabilities but this adds further complexity and may lead to image artifacts, unacceptable to the viewer. Furthermore, if the density of light-absorbing material in
Corning Incorporated
Klee Maurice M.
Mahoney Christopher
Paglierani Ronald J.
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