Patent Application
20060274276
1. Field of Invention
The present invention relates to compensation of light. The invention is more particularly related to compensating residual retardation and skew rays in a light channel with a waveplate of greater than ¼ lambda retardation where lambda is a central wavelength of light in the light channel.
2. Discussion of Background
In a conventional LCoS kernel quarter waveplates are positioned between the LCoS microdisplays and the Polarizing Beam Splitters (PBS). Their purpose is to increase the contrast ratio of the video image projected by the kernel. An example in the prior art is Berman et al., US Publication 2002/0001135.
The present inventor has realized the need for more precise compensation of both skew ray and residual retardation in optical systems. Roughly described, the present invention uses a greater than ¼ lambda waveplate, which is rotated so that retardation of the waveplate along a skew ray axis is equal to ¼ lambda and retardation along a second axis has a magnitude equivalent to a residual retardation on the second axis.
In one embodiment, the present invention provides a prism assembly having a light modulator and a waveplate of greater than ¼ lambda configured to provide ¼ lambda retardation on a skew ray axis of light entering the microdisplay. The invention further provides that the configuration of the greater than ¼ lambda waveplate also provides an amount of retardation along a residual retardation axis of the microdisplay to compensate for residual retardation of the microdisplay.
In another embodiment, the present invention is a waveplate configured to compensate for first and second deviations in light rays within an optical system, wherein the waveplate is positioned in the optical system so that, a lambda value of the waveplate compensating the first deviation is ¼ lambda along an optical axis of the first deviation, and a lambda value of the waveplate compensating for the second deviation along an optical axis of the second deviation has a magnitude approximately equal a magnitude of the second deviation.
In yet another embodiment, the present invention is a microdisplay package configured for use in a lambda wavelength light channel, comprising, a microdisplay comprising an optical face, and a waveplate positioned along the optical face of the microdisplay and having an amount of retardation of greater than ¼ lambda along a main optical axis of the waveplate.
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts, and more particularly to
Positioning of the waveplate 100 between a microdisplay and optics in a kernel can increase a contrast ratio of a video image projected by the kernel. The positioning method works by making the dark state of the microdisplay blacker (darker). The dark state is increased through two different mechanisms.
The first mechanism is to correct geometrical depolarization of off axis light, generally caused by light rays (skew rays) transmitted off axis by the optics (e.g., a Polarizing Beam Splitter (PBS)). This is called skew ray compensation. The requirements for this mechanism to work properly are that the retardation of the waveplate be ¼ λ (λ being a central wavelength of light in a light channel in which the waveplate 100 is placed) and that the retardation axis of the waveplate be parallel to the axis of linear polarization of light rays transmitted normally by the optics (e.g., non skew rays transmitted normal to an output face of the PBS.
The second mechanism is to compensate for residual retardation in a dark state of a microdisplay operating in conjunction with the waveplate. The axis of residual retardation of a microdisplay is typically oriented 45 degrees to the mechanical edges of the microdisplay and the amount of residual retardation is “small” often on the order of several hundredths of a λ. The retardation required to compensate this residual retardation must be of a similar magnitude and oriented + or −90 degrees to the 45 degree oriented axis of residual retardation of the microdisplay.
A method for aligning the quarter waveplates in a reflective LCoS kernel is to insert quarter waveplates between the optics (e.g., PBSs) and the microdisplays set in a dark state. The waveplate are accurately fabricated having ¼ λ retardations corresponding to the center wavelength of the spectral bands utilized in each of the 3 LCoS light channels. To determine the proper orientation of the waveplate axis, light is input to the kernel and the intensity of the reflected light from each channel is observed at the output face. The axis of each waveplate is rotated until a maximum dark state is observed. The purpose and function of rotating the waveplate is to adjust the relative magnitudes of two vector components of the retardation. The first vector component is along the axes required for optimal skew ray compensation (the skew ray axis) and the second vector component is along the axis required for optimal residual retardation compensation (the residual retardation axis of compensation).
In practice it is found that system contrast ratio is a sensitive function of the waveplate rotation angle. One reason for this is that the “quality” of skew ray compensation is a sensitive function of the magnitude of the retardation along the skew ray axis. Even a small deviation from ¼ λ reduces system contrast substantially. Unfortunately, rotation of the waveplate to develop the residual retardation component does just that: reduces the magnitude of the retardation of the skew ray compensating component. This is illustrated in
The present invention is not only an improved but also a practical means of compensation.
Again, referring to
The R vector is also shown as resolved onto the skew ray axis and being ¼ lambda (projection 305).
One way to calculate the R and the amount of waveplate rotation is to:
(1) Start with ¼ lambda on the skew ray axis;
(2) Determine an amount/approximate amount X of desired retardation along the rr axis of compensation;
(3) Vectorily add X to the skew ray axis' ¼ lambda to produce R; and
(4) The angle between the skew ray axis and R is the amount of rotation.
In one embodiment, the present invention takes the form of prefabricated microdisplays wherein an optical window of the microdisplay is a greater than ¼ lambda waveplate rotated according to the present invention. Alternatively, a standard microdisplay is packaged with a greater than ¼ lambda rotated waveplate.
In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the present invention is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner. Furthermore, the inventors recognize that newly developed technologies not now known may also be substituted for the described parts and still not depart from the scope of the present invention. All other described items, including, but not limited to microdisplays, waveplates, kernels, prism assemblies, and beam splitters, etc should also be consider in light of any and all available equivalents.
The present invention may suitably comprise, consist of, or consist essentially of, any of element and their equivalents as described herein. Further, the present invention illustratively disclosed herein may be practiced in the absence of any element, whether or not specifically disclosed herein. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
