Tech Note 2 |
Effects of a Cut Glass Edge on Light Polarization
and Tiled-LCD Quality
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J. M. Cohen
4Physics, Harrisburg, PA
M. R. Cohen
Physics Dept., Shippensburg University, Shippensburg, PA
Abstract
Controlling polarization across the seam is critical to tiled AMLCD image quality. If polarization is spatially modulated by a structural feature, a visible artifact could be produced. Depolarization produced by a cut glass edge in tiled AMLCD and its effect on image quality are examined.
Introduction
Tiled-AMLC displays1 use several AMLC tiles as building blocks
for a larger display. Many factors in the construction of individual tiles can
be accurately controlled, while the influence of other, uncontrolled variables
must be ameliorated by alternate means. Plane polarized light, produced by the
entrance polarizer of the AMLCD stack, may suffer a polarization change when
scattered from the cut tile edges in the seam region of adjoining tiles. This
results in altered transmission intensities after the exit polarizer.
Random, diffuse scattering from the imperfect edge can be viewed through the
correlation between surface roughness and the total integrated scatter.2
To some extent, surface roughness issues can be approached by altering sawing
and finishing techniques, or through improved refractive index matching of the
adhesive used between tiles.
Larger microfacets and striations, however, can be produced in the cutting and
polishing procedures with some measure of angular correlation. Depending on
the orientation of the facet and the relative polarization of the incident light,
the resulting reflection and transmission coefficients3 can produce
significant light intensity with spatially modulated polarization at undesirable
locations around the seam region. Attempts to reduce the correlations within
this surface microstructure may be hindered by material and geometric constraints
of the AMLCD.
The nature and extent of these end effects is considered here. Aside from incidence
angle considerations, angular dependencies will not be addressed in this study.
The angle-integrated results given are termed depolarization factors.
Results
The depolarization factor was measured for light scattering from edge sections of the production level AMLC tiles used in display assembly. As the light of interest covers a broad wavelength range through the primary colors, the photometric instrumentation accessed significant wavelengths in each of these regions: 488 nm, 543.5 nm, 594.1 nm, and 632.8 nm. A sketch of the photometer appears below.
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The level of depolarization for light scattered from the cut edge was measured
by observing the light intensity for the tile sample between crossed linear
polarizers. The sample assembly, shown above, consisted of a glass plate with
an adhered linear polarizing film, followed by the AMLC tiles, and then ending
with another linear polarizing film on a glass plate. A number of effects other
than depolarization need to be normalized out of the final intensities.
Light intensity variations must be accounted for that arise from multiple reflections
from the glass plates, variation in crossed polarizer transmission efficiency
with incident light angle, and back reflection from the second polarizing film.
To this end, measurements were made of reflected light intensities vs. incidence
angle for model glass samples and for transmitted and reflected light intensities
vs. incidence angle for model crossed linear polarizers. Sample data of this
type, obtained at 488 nm, appears below.
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Once such factors have been accommodated, it is possible to evaluate the depolarization following scatter from the cut edge. Again, representative data acquired at a wavelength of 488 nm is shown below.
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Acknowledgments
The authors gratefully acknowledge the technical and material support of the Rainbow Displays optical team under the direction of D. P. Seraphim. We particularly wish to thank R. G. Greene of Rainbow Displays for his helpful analyses and discussion.
References
1. R.G. Greene, J.P. Krusius, D.P. Seraphim, D. Skinner, and B. Yost, SID 2000 Digest, 30.3.
2. J. C. Stover, Optical Scattering, McGraw-Hill, New York, 1990, pp. 17 - 20.
3. Lipson, Lipson, and Tannhauser, Optical Physics, 3rd ed., Cambridge University Press, Cambridge, 1995, pp. 106-117.
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