Optics: image projectors – Composite projected image – Multicolor picture
Reexamination Certificate
1998-05-01
2001-02-06
Dowling, William (Department: 2851)
Optics: image projectors
Composite projected image
Multicolor picture
C349S022000, C359S197100
Reexamination Certificate
active
06183092
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates generally to devices for projecting pictures onto large viewing screens; and more particularly to such devices that project laser beams via reflective liquid-crystal light valves to form such pictures. The invention has its most important applications in such projection of moving pictures.
2. Related Art
a) Known potential of lasers—Since the advent of the laser, people have been trying to find new ways to use lasers in projecting pictures of one kind or another, for large audiences. This is both natural and reasonable, since lasers offer several important characteristics that are relevant in large-image projection.
As will be seen from the following recap of these characteristics, one would expect these characteristics to be responsible for a predominance of laser projection systems in large-screen displays for both video home use and theater-scale displays. Indeed, several powerful large international companies have attempted—at monumental cost—to develop such equipment for market.
Therefore, while reading the following discussion of laser advantages for large-screen projection sources, please bear in mind this overriding question—why are large-screen laser projectors not common in the marketplace?
(i) energy efficiency—All other things being equal, the amount of light needed to show any kind of picture on a projection medium (viewing surface) is proportional to the area to be covered by the picture. Optical energy is therefore of utmost importance in a large-format projection system, and it is necessary to pay for visible optical energy with electrical energy.
In such transactions it is well understood that some conversion inefficiency is unavoidably involved as a sort of tariff—in other words, that a sizable fraction of the electrical energy used will go into invisible forms of energy such as heat, or near-infrared and ultraviolet radiation. Normally there is relatively little objection to this price in itself, but the question does arise of just how sizable a fraction one can afford.
With nonlaser light sources, this concern is compounded when taking into account the additional surcharge for optical energy that is visible but goes off in directions other than into the collecting optics of a projector. Most nonlaser sources (incandescent hot-filament or arc lamps) radiate approximately equally in all directions. The amount of visible light that can be directly collected from such a source into an optical system is typically less than a tenth of the visible light produced.
It can be dismaying to pay for many times the amount of electrical energy used—even that which is directly used to make visible light, setting aside consideration of the conversion efficiency discussed above. Therefore it is common to provide reflectors behind the source, or more generally speaking to try to surround the source with reflectors to help capture a greater geometrical fraction of the visible energy. Such efforts, however, complicate and compound the management of heat thrown off due to those same conversion inefficiencies considered above.
A laser, though of course itself a costly article, greatly improves all these energy economics. Since its optical emissions are directional, essentially all the emitted light can be very easily captured for use.
Furthermore, to a significant extent the spectral components an be controlled so that minimal energy is wasted in infrared or ultraviolet radiation. A laser is therefore far more energy efficient than other sources—with respect to both raw conversion efficiency of electricity into visible light and geometrical capture of that visible light.
Lasers and their power supplies do give off heat, and this must be managed. In comparison with a typical arc lamp or like device, a laser is vastly more favorable with respect to the amount of heat, the temperature involved, and the difficulty of collection.
(ii) brightness—With most types of light sources, increasing the amount of light available calls for fabrication of a source that is scaled up in all three dimensions, more or less equally, and therefore greatly complicates the process of collecting the light and drawing off heat.
To make a brighter laser, it is necessary in essence to make a laser which is just like one that has various desirable known properties, except with a bigger tube. Over a small range of brightness increases, furthermore, what is needed is only a longer tube. Heat management with a longer tube reduces to using the same hardware, but more of it, as with a shorter tube. Even if brightness requirements do call for increased diameter too, the elongated character of most laser structures tends to distribute and thus mitigate the problems of power and heat management.
With a bigger laser, all the greater amount of optical flux can be made to go in essentially the same direction and into essentially the same projection system as the corresponding smaller laser. These oversimplifications of course slight some practical considerations such as design of power-supplies, cooling, and lasing modes, but summarize an important way—for purposes of image formation—in which lasers differ from other light sources.
(iii) contrast—Several properties of lasers tend to enhance contrast in a projected image. The simplest of these is once again inherent directionality, which facilitates both collection of input illumination and handling of an image, with minimum crosstalk between different portions of the beam or the image.
Contrast is enhanced by avoiding such crosstalk—or in other words preventing the spill of a cast over an entire image frame, from bright image areas. Such undesired spill corrupts areas that should be dark. Further enhancement of just the same sort arises from the inherent collimation of a laser beam.
Equally or more important, when most modern image-modulation devices are taken into account, is the inherent monochromaticity of a laser beam. Other sources emit light over the entire visible spectrum, requiring subdivision into spectral segments, and physical separation into distinct beams that can be separately modulated and then recombined to give full-color images.
In either type of system, laser or nonlaser, the final optical stage—i.e., the projection lens—is preferably broadband since it preferably carries all the colors in a common beam; for this purpose a high-quality achromat is desired. The benefits under discussion apply to all earlier optical stages, where the functions being performed are much fussier and complicated than the final projection stage.
With such other sources, each distinct beam carrying a separated spectral segment is already broadband, either complicating or degrading the effectiveness of all optical effects or manipulations. These include everything from perturbation of simple focusing (chromatic aberration) to the operation of sophisticated image-modulating devices (see below).
Since operation of lenses, polarizers, prisms, dichroics and image modulators are all wavelength-dependent, the operation of virtually all optical components in a projection system using such other sources tends to scatter light away from the precise bright-region positions where it should be. The result is to create a kind of halo about such positions—or, again, depending on the brightness contours of a particular image, even to produce a filmy bright cast over much of a scene that should be darker.
Also of great importance is the inherent polarization of a laser beam. Many large-screen projectors of the present day employ an image-writing stage that controls a high-intensity light beam by spatial modulation of the beam.
As discussed more fully in later sections of this document, almost all such modulators rely upon formation of a latent image in polarization state (or as it is sometimes called, a “phase object”). This image is later developed by passage through some form of polarization analyzer.
Other projectors intensity-modulate a scanning spot of a high-intensity beam; here too, the phenomenon most commonly exploit
Ashen & Lippman
Dowling William
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