Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2000-01-13
2001-11-06
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S013000, C359S443000, C359S460000
Reexamination Certificate
active
06313931
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a screen for laser rear projection which, selectively for one or several laser wavelengths, forward-scatters the incident, narrow-band laser radiation in a previously determined solid angle but simultaneously highly absorbs the stray broad-band ambient light.
The necessity of visualizing information is currently increasing enormously. For display technology, therefore, this opens up a market with many segments, high sales, and high growth rates. In the functional chain composed of image recording, transmission, and processing of information, considerable progress has been made in recent years. However, the quality of current display methods is no longer adequate for this high state of development. The displays in use today such as cathode ray tubes and liquid crystal displays are limited in their potential for improvement.
Laser display technology, in other words the sequential buildup of images by laser beam deflection without screen afterglow, offers an inherent potential for producing high-quality images. The image information is modulated serially with electro-optical or acousto-optical modulators suitable to the laser beam. The beam is deflected by mechanical mirror scanners, similar to conventional television tubes, linewise over the image surface. Laser sources are being developed by several projector manufacturers as a replacement for thermal radiators such as halogen lamps and discharge lamps in conventional light valve projectors with liquid crystal or micro-mirror matrices.
Basically, thermal radiators are limited in their brightness by their internal operating temperature (black-body radiation). The spectral distribution of the basic colors red, green, and blue (RGB) can be optimized only by appropriate filtration and intensity matching. In addition, a lamp of this kind radiates uniformly in all directions. Despite a rear mirror, only a portion of the total radiation is available for radiation in a particular projection direction.
On the other hand, with a laser any beam power can theoretically be produced and is available in a closely bundled beam with almost zero losses. In addition, as a result of the narrow spectral bandwidth, its power can be converted completely for the required basic color. In recent years, semiconductor lasers and solid-state lasers have been developed with a much higher efficiency (lumens per watt) and a much longer lifetime (more than 50,000 hours) than with conventional lamps.
The theoretical suitability of laser beams for display technology was recognized early on. Hitachi demonstrated color television large projection in 1970. General Electric, Texas Instruments, General Telephone and Electronic Labs, and others demonstrated similar systems in the 1970s. At that time, however, the available lasers (gas lasers) had a lower efficiency, were bulky, and were unsuited for economical mass production.
As a result of the clear technological advances in the development of laser sources, image modulators, and image scanners, completely new technical and economic opportunities are available today for laser display technology:
the possibility of creating the suitable basic colors, red, green, and blue, with high efficiency;
a very high quality of projection as regards brightness, contrast, and resolution, and
manufacturing economics by miniaturization and functional integration.
The laser is a coherent, monochromatically bundled light source. Any required beam density can theoretically be created. In addition, as a result of the narrow bandwidth of its power, the latter can be completely converted for the necessary basic color. With sequential pointwise image creation on the screen, the image is sharp at any distance and can be projected on sloping and curved surfaces as well.
With all the projectors, an image can basically be generated in two ways on the screen: by front projection and by rear projection. In the former case, the image is cast on the surface of the screen on which it is viewed. In this case, the screen must diffusely backscatter the incident light as much as possible. In the latter case, the image is projected on the opposite side of the screen (from the rear). In this case, the screen should allow the light to pass through as completely as possible, but at the same time must forward-scatter over a greater angle. The invention relates exclusively to this second method, rear projection.
Screens for rear projection with conventional image projectors (beamers) are commercially available in different sizes with different screen materials. The screen scatters the light beams directed at every image point on the rear by scattering at the surface or also by multiple scattering in the interior of a thin layer of the material. In this way, the image point on the viewing side is radiated outward diffusely from the screen as an expanded beam bundle. This phenomenon is well known for example from matte disks that are also used in the viewfinders of cameras.
Since the scattering of the light at the surface or within the image material results not only in the light being conducted farther in the forward direction but also to a certain extent in a backscattering of light from the projection surface, this light transmission is always subject to losses. A second disadvantage of these screens is that bright light from the viewing area also enters the screen and is not only conducted through the screen but is also backscattered to a certain degree. As a result, the screen always appears bright in an illuminated room, depending on the ratio of the forward-scattering intensity to the backscattering intensity, in various levels of gray.
In order to compensate for these losses and to reduce the brightness of the screen, so-called lenticular images or lenticular walls are frequently employed. With a fine lenticular pattern on the light exit surface to the viewer, the angle of forward scattering is narrowed. As a result, the image can be seen only in a narrow range of angles around the normal and brightened. At the same time, the recording angle of the projection wall for lateral stray light, for example originating from the lateral illumination of the room, which is directed toward the viewer, is reduced. All in all, these screens, despite illumination from the viewing area, appear darker and at the same time produce a brighter image. Of course, large screens of the latter type are very expensive to manufacture. However since they offer only limited improvements, projection can take place with the desired quality only in half-darkened rooms but not in bright rooms or in daylight.
In the case of front projection onto screens, the backscattering coating of the screen is designed so that the light from the projector is backscattered by this screen in a limited image angle, with the same advantage as in improved rear projection screens.
One major advantage of using lasers for projection is that, as a result of the resultant higher possible beam density by comparison with other light sources, the brightness of the image on the screen can be definitely increased. However, this advantage can be utilized only to a limited extent to increase image quality because in this case the same is true as in television technology: only by creating the image on a black screen can the image contrast be inherently created by the device, as well as the color saturation be transmitted undistorted to the viewer. This is because the minimum contrast perceived by the eye is proportional to the basic brightness of the image (Weber-Fechner Law), in other words the higher the brightness, the lower the perceptible contrast.
In color reproduction in bright rooms, the usually gray or white basic brightness of the screen is added to the color valence produced by the projector, and so the saturation of the color hues of the entire image changes at the same time that the contrast decreases.
This problem of high-quality image reproduction of contrast and color in bright rooms occurs both in front projection and in rear projec
Dausmann Guenther
Gnaedig Klaus
Gnaedig Zora
Halldorsson Thorstein
Crowell & Moring LLP
Daimler-Chrysler AG
Gnaedig Zora
Schuberg Darren
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