Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
2000-05-25
2003-11-18
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
Reexamination Certificate
active
06650383
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to lightvalves and, more particularly, to improved reflective twisted nematic liquid crystal (LC) lightvalves and systems employing same.
BACKGROUND OF THE INVENTION
Reflective lightvalves are becoming widely used in projection displays. Such lightvalves can be decreased in size without incurring a penalty in pixel aperture, allowing a corresponding shrinkage in size and cost of the entire projection system. Reflective lightvalves based on twisted nematic liquid crystal (TNLC) layers, such as the 45° (degree) twist or 54° twist modes, make use of well developed LC technology, and with relatively modest driving voltages can provide a reasonably satisfactory optical response when reproducing black, white or intermediate grayshaded image regions. A TNLC layer of twist angle &agr; and birefringence &Dgr;n which is set to a thickness d satisfying:
d=
(&lgr;/&Dgr;
n
){square root over (
m
2
−(&agr;/&pgr;)
2
)} (1)
will provide reasonably high contrast within a band of wavelengths centered at &lgr;; in most cases this band is wide enough to project a single primary color (i.e., red, green, or blue), allowing the full image to be projected from three lightvalves. Minimum suitable thickness is obtained by setting m=1 in equation (1); the choice m=1 thus minimizes the drive voltage that must be supplied when projecting image regions of maximum brightness.
Though the intrinsic contrast of the TNLC lightvalve is in itself usually acceptable over a single color band, the contrast of the full projection system is almost always poorer than that of the lightvalve alone, due to an interaction between the TNLC lightvalve and the projection optics, discussed further below. In demanding applications, even the intrinsic contrast of the TNLC lightvalve in isolation may be marginal; however there is a known method for improving the intrinsic crossed polarizer transmission of the TNLC layer. U.S. Pat. No. 4,408,839, issued to Wiener-Avnear and entitled “Twisted Nematic Liquid Crystal Light Valve with Birefringence Compensation,” the disclosure of which is incorporated by reference herein, discloses a TNLC corrector layer which the illumination traverses before reaching the TNLC lightvalve; this corrector compensates the black state of the lightvalve. The twist within the TNLC corrector is chosen in such a way that the LC molecules along the exit surface of the corrector are perpendicular to the LC molecules at the adjoining entrance surface of the lightvalve. These two monomolecular sublayers then have parallel birefringences of opposite sign, and so cancel. The two TN layers are given opposite twists of equal magnitude, which means that if one considers successive additional pairs of sublayers (one from each LC layer, selecting the two sublayers to be at equal distances from the adjoining exit and entrance faces), the two sublayers in successive pairs continue to cancel each other in the above fashion (if d&Dgr;N for the two layers is the same). The Wiener-Avnear patent discloses that the intrinsic TNLC black state intensity is thus made zero at all wavelengths. Other two layer transmission lightvalves have been disclosed in which the d&Dgr;N product or twist angle is not the same in the two layers.
However, while the imperfect intrinsic black state of the TNLC lightvalve is correctable by the known double layer techniques, the prior art reflective lightvalve exhibits a number of other limitations. Examples of these limitations will be described below in the subsections (1) through (8).
(1) The spectral width of the high contrast zone is not adequate to project all three color bands from a single lightvalve, hence three lightvalves are required. While use of three lightvalves has the advantage of maximizing image brightness, there are applications where adequate brightness could be achieved at lower cost from a single lightvalve (or two lightvalves), if a single lightvalve were capable of projecting all three colors.
(2) A particular method for projecting multiple colors from a single lightvalve is to sequentially project each color component at a sufficiently high rate that the eye perceives the three components to be simultaneous. However, it is difficult to switch TNLC lightvalves rapidly enough to achieve a flicker free image in sequential mode. Switching time is approximately quadratic in the cell gap d. The equation (1) contrast condition sets a minimum attainable thickness d for the TNLC lightvalve, and hence a minimum switching time. Once an LC is chosen with the largest possible &Dgr;N, and the twist angle a is set to a sufficiently large value that high bright state reflectivity is obtained together with an adequately broad band of high contrast, the cell gap d of the TNLC lightvalve is then fixed by equation (1).
(3) Different cell gaps must be assigned to the red, green, and blue lightvalves, as per equation (1). Since switching speed is then different in the three color channels, the projected image of a moving object of mixed color (e.g., white) will exhibit different colors in its leading and lagging edges.
(4) These constraints on voltage and cell gap indicate that an LC with the largest possible birefringence &Dgr;N should be chosen. This may rule out the use of LCs with other desirable properties, such as fluorinated LCs, which have low sensitivity to UV (ultraviolet) light.
(5) Small errors in cell gap thickness d can degrade the quality of the black state. The optical effect is governed by changes in the dimensionless parameter &bgr; defined as:
β
≡
π
d
Δ
n
λ
.
(
2
)
In principle, a small error in cell gap can be compensated by rotating the lightvalve slightly. The orientation of minimum black state reflectivity is given by:
Θ
′
=
a
tan
γ
γ
(
3
)
where
&lgr;≡{square root over (&agr;
2
+&bgr;
2
)}. (4)
Unfortunately, errors in the cell gap are often the result of quasi-random process variations, and cannot be determined until the lightvalve is actually fabricated. At that point, it is no longer possible to rotate the lightvalve because it must remain aligned with the desired image orientation and with the lightvalves of the other two color channels.
(6) Even in the absence of cell gap errors, the TNLC lightvalve does not provide ideal contrast. Most commonly, these LVs (lightvalves) are used in projection systems where the LV is illuminated through a polarizing beamsplitter (PBS) and quarterwaveplate (QWP); in such systems the intrinsic black level is given by:
B
System
=
(
2
a
β
sin
2
γ
γ
2
)
2
+
NA
2
n
2
(
β
sin
2
γ
γ
)
2
,
(
5
)
where NA is the numerical aperture of the projection optics (at the lightvalve), and n is the PBS substrate refractive index. The cell gap d is usually chosen to satisfy equation (1) at a single (usually central) wavelength in the color band. At this central wavelength, both terms in equation (5) are zero, but at other wavelengths, both are non-zero, and projected black regions of the image are not completely dark. When displaying darker shades of a single color (i.e., shades where one color is set to a reflectivity slightly above black state while the other two are set nominally at zero), the residual reflectivity of the two black state lightvalves causes the image color in the driven channel to be significantly desaturated, i.e., to be washed out.
As discussed above, the first term in equation (5) is intrinsic to the TNLC lightvalve itself. The second term arises from a birefringence-like effect in the TNLC layer. The QWP phase shift precisely eliminates compound angle depolarization at the single wavelength satisfying equation (1) above, but at other wavelengths the TNLC layer introduces its own phase shift which is not matched by the QWP. This phase offset is linear in small wave
Lu Minhua
Rosenbluth Alan Edward
Yang Kei-Hsiung
International Business Machines - Corporation
Kim Robert H.
Rude Timothy L
Trepp Robert M.
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