Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2001-07-31
2003-08-12
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S013000, C359S599000, C362S558000
Reexamination Certificate
active
06606173
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates to projection displays and more particularly an improved method of homogenizing and formatting the light from a light source to produce higher uniformity and efficiency in the projected range.
2. Description of Related Art
Illumination systems used for image projectors are designed to generate a spatially uniform plane which can be used to illuminate an imaging device film or other media. The reflected or transmitted light from the imaging device is then projected onto a screen for viewing. The brightness and spatial brightness uniformity should be within certain limits for each particular application to be considered acceptable to the viewers.
Image projectors including film movie projectors, slide projectors, electronic liquid crystal and micro-electro-mechanical (mem) projectors, microfilm and overhead projectors all require a high degree of spatial light uniformity in the image to produce a pleasing image. This has always been a challenge for projection system designs due to the fact that the light sources available for these systems all have very disorganized light output and therefore require complex optical systems to organize the light. Additionally, high degrees of magnification in short distances (which often occur in these optical systems) cause a problem which is well known in the optical field—the cosine
4
roll off of power in the image as you move radially away from the center of the image. This effect is most predominant at the corners of the image. Another problem is that light sources tend to produce round or elliptical gaussian beam profiles, while most images are rectangular in format. Typically, the light beam is spatially truncated (i.e., the portions of the beam which fall outside a rectangular profile that corresponds to the image are blocked). This leads to another problem, which is maximizing the brightness of the illumination—when the light is truncated to change its geometry, the truncated light is obviously wasted.
Many optical methods have been used in the prior art to try to minimize the variations in uniformity which are due to the particular characteristics of the available light sources as well as to maximize the brightness of the illumination.
The optical method used depends somewhat on the light source used. Many different types of light sources are in common use today. Some types are electric filament, and arc lamps including metal halide arc, low and high pressure mercury arc, xenon arc, carbon arc, as well as solid state Light Emitting Diode (LED) sources, and Lasers. Not all of these light sources, however, are suitable for displays using prior art technologies.
Two of the most common types of light sources in use in commercial applications are metal halide arc lamps and high pressure mercury arc lamps.
These arc lamps are usually configured in an optical illumination system which employs an elliptical or parabolic reflector to gather and direct the light to a focal point or collimated beam respectively, as shown in FIG.
1
. Both of these types of systems produce highly non-uniform beams. Some systems use reflective tunnels or light pipes through which the source light is channeled in order to create a scrambled, hence more spatially uniform bundle of light rays as shown in FIG.
2
.
Lenslet arrays are also sometimes used to increase the uniformity of the light. Some versions of these lenslets are described in U.S. Pat. Nos. 5,098,184 and 5,418,583. The lenslet arrays function essentially in the following manner. Two lenslet arrays are separated by a distance equal to the focal length of the individual elements. The elements of the first array form an image of the source in the aperture of the elements of the second array. In the case of a laser, the source image is a diffraction pattern. The elements of the second array then form an image of the aperture of the elements of the first array on the illumination plane. The aperture is chosen to match the aspect ratio of the device (film gate, or LCD) to be illuminated. A field lens in close proximity to the second array focuses the chief rays of each element to the center of the illumination plane so that the subsets of the beam sampled by all elements of the arrays are superimposed at the illumination plane and an averaging process thus occurs that causes the illumination plane to have more uniform irradiance. A second field lens is often required at the illumination plane to ensure that the light is telecentric as most often required by projection imaging optics.
In this manner a beam with non-uniform irradiance may be sampled by arrays composed of many elements and converted to a uniform beam with a different geometry (generally rectangular).
The lenslet array optical system which is used in an illumination system has design characteristics that must be adjusted to ensure that the illumination and imaging systems are compatible. If they are not, then light is wasted. For example, the geometry of the illumination should be the same as the geometry of the imager. The numerical aperture of the illumination system should also be compatible with the imaging system. The ratio of the footprint of light incident on the first array to the distance to the illumination plane determines the numerical aperture of the illumination light. Thus the focal length of the array elements and the field lens focal lengths are adjusted to ensure that the illumination numerical aperture matches the imaging numerical aperture.
At first blush, laser light appears to have enormous potential for being the illumination source in projection display systems. The light is well behaved and organized (ie: it is collimated), it is chromatically pure, and with a minimum of three wavelengths (Red, Green, and Blue) a high color space or gamut can be created, and high power low cost lasers are becoming available. There are, however, several problems with laser-based illumination systems.
First, the coherency of laser light leads to speckle, which is a fine-grained non-uniformity. The speckling effect is increased with the use of so-called holographic diffusers as proposed in this invention. The net effect is often a high frequency mottling effect sometimes called worminess. Another problem is that the laser light is collimated and, as such, it is difficult to create a cone or numerical aperture which will allow an image to be projected onto a screen, as with a projector. Yet another problem is that the laser light typically has a gaussian intensity profile and it may have a wide range of diameters, depending upon the particular laser source which is used. This can, and often does, lead to a non-uniform light distribution on the final screen or projected image surface.
Another problem is that currently available lasers typically do not have enough power to provide sufficient illumination in some display devices. Further, using prior art methods, it is difficult to combine the beams of multiple lasers to obtain sufficient illumination for this purpose.
Another problem with the use of laser light as a display illumination device is that the beam generated by a laser may be astigmatic in its divergence. In other words, the divergence in the beam”s cross section may be greater in one axis than another. This causes additional processing problems compared to a circularly symmetric diffraction limited beam.
Yet another problem with the use of laser light in a display illumination device is that, if laser light is diffracted in an optical system, a certain amount of light passes through the diffracting device without being diffracted. This effect is referred to as zero-order light leak. Zero-order light leak may prevent the resulting diffraction pattern from conforming to a well-defined, desired function. Another problem with using laser light sources for illumination is that they are monochromatic. Since it is desirable to have a source of white light, it may be necessary to combine laser light beams of several different wavelengths (e.g., red, green and blue.)
Fischer Robert E.
Kappel David
Tadic-Galeb Biljana
Boutsikaris Leo
Gray Cary Ware & Freidenrich LLP
Riake Corporation
Robinson Mark A.
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