Optics: measuring and testing – By light interference – Having polarization
Reexamination Certificate
2000-05-04
2003-10-21
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having polarization
C356S504000
Reexamination Certificate
active
06636322
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and device for measuring the cell gap of a liquid crystal display, a liquid crystal display, and a retardation plate used for measuring the cell gap of a liquid crystal display, which enable accurate measurement of the cell gap.
DESCRIPTION OF THE RELATED ART
Heretofore, various display devices using liquid crystal materials, such as active-driven-type TFT (thin film transistor) display, PALC (plasma address liquid crystal) display, and duty-driven-type STN (super twisted nematic) display, are known. Especially, the PALC elements are switching elements utilizing plasma discharge, which are expected to be applied to very large-sized displays. Unlike TFT, the PALC panel does not involve semiconductor processes during manufacture, and therefore, it may be manufactured at low cost. PALC also draws attention for its possibility to be applied to a large-size LCD with low energy consumption.
The above-mentioned liquid crystal displays are characterized for example by their lightweight, thin form, and low energy consumption. These advantages of the liquid crystal display hold the key to the upcoming multimedia society, and studies related to application of the LCD to various office automation devices and audio visual equipment are now under way. The liquid crystal displays often utilize a TN (twisted nematic) display mode where the nematic liquid crystal molecules are twisted for approximately 90 degrees near both electrode substrates, or an STN display mode where the liquid crystal molecules are twisted for over 180 degrees.
Japanese Patent Application Laid-Open No. 10-186330 discloses a liquid crystal display with a wide viewing angle and a good display quality. According to the disclosed LCD, the cell gap size is closely related for example to the display color, the response speed, and the orientation stability, so there is a need of a more accurate method for measuring the cell gap. Even further, the accurate information of the cell gap value is very important for designing and evaluating the LCD.
Heretofore, cell gap measurement devices in the market were used to compute the cell gap by measuring the retardation of the liquid crystal layer. According to the prior art, the gap of an empty cell with no liquid crystal material injected thereto is measured using optical interferometry, and the gap of a liquid crystal-injected cell is measured using a crystal rotation method.
However, it was difficult to obtain a highly accurate cell gap value according to the empty cell gap measurement using optical interferometry, since the liquid crystal cells were equipped with ITO transparent electrodes, an orientation film, a color filter and so on, which caused multiple interference. The separation of the measured value was complex, and the obtained value was not very accurate.
Further, when liquid crystal is filled inside the liquid crystal cell, the refractive index difference between the liquid crystal layer and the substrate is small. Therefore, hardly any interface reflection occurs at the substrate, and no interference fringe appears, which makes it almost impossible to measure the cell gap of a cell filled with liquid crystal material in principle.
On the other hand, according to the crystal rotation method, it is capable of measuring only Np liquid crystal cells where the dielectric anisotropy of the normally-white liquid crystal is positive. It could not be used to measure Nn liquid crystal cells where the dielectric anisotropy of the cells have negative dielectric anisotropy.
When the liquid crystal being sandwiched between two parallel substrates is uniaxially and horizontally oriented to the substrate, the light coming in from the substrate surface is separated into two components by the refractive index anisotropy of the liquid crystal. Therefore, the cell gap of the liquid crystal cell may be measured by the incident angle dependence of the transmission light intensity.
Japanese Patent Application Laid-Open No. 3-115804 discloses a light-weight small-size cell gap measurement device characterized in superposing a lower transparent insulative substrate of the measurement device and an upper transparent insulative substrate of the liquid crystal panel to be measured, so that the rubbing direction of the upper substrate is orthogonal to the rubbing direction of the lower substrate. Further, two polarizing sheets are mounted to both sides of these panels so that the absorption axis orientation of one sheet is orthogonal to the absorption axis orientation of the other sheet.
Further, Japanese Patent Application Laid-Open No. 4-80641 discloses comparing the value of spectrum transmitted through a liquid crystal cell being mounted between two polarizers, with the value of spectrum transmitted therethrough with the angle of the polarizers being varied by 90 degrees. According to the invention, the optical retardation, the cell gap and the liquid crystal birefringence of the liquid crystal cell may be measured.
Japanese Patent Application Laid-Open No. 4-184207 discloses detecting the transmitted light energy while varying the impressed voltage, and obtaining the voltage value where the transmission is minimum, in order to perform an easy and accurate measurement.
Moreover, Japanese Patent Application Laid-Open No. 5-71924 discloses measuring the gap of a liquid crystal cell including Np liquid crystal with positive dielectric anisotropy, while applying thereto either high voltage which is more than 50 times the threshold voltage, or high magnetic field which is more than fifty times stronger than the magnetic field.
Japanese Patent Application Laid-Open No. 9-90371 discloses a method for manufacturing a liquid crystal display. The invention involves pressurizing an empty cell so as to maintain the inner pressure of the empty cell to be equal to the pressure provided to the cell after filling the cell with liquid crystal components, radiating monochromatic beam to the cell, and observing the distribution of the interference fringes. According to the method, the uniformity of the gap inside the empty cell could be inspected.
Further, Japanese Patent Application Laid-Open No. 10-232113 notices that when ultraviolet light or near-ultraviolet light is radiated to an orientation film, the orientation film generates fluorescence. The invention discloses radiating a spot light to an empty cell, and receiving the fluorescence generated at the surf ace of both orientation films by CCD sensor elements, in order to measure the gap of an empty cell.
As explained, many methods for measuring the cell gap of a liquid crystal cell are proposed.
However, according to the conventional methods, including the optical interferometry method using light source generating visible radiation capable of measuring only empty cells, and the crystal rotation method for measuring the incident angle dependence of the transmission rate with retardation generated in the inner-plane orientation parallel to the substrate when polarized light is radiated to the substrate of an Np liquid crystal cell with molecules oriented horizontally when no voltage is impressed, it was impossible, without impressing electric field or magnetic field thereto, to measure the incident angle dependence of the transmitted light energy, in order to obtain the retardation of the liquid crystal cells having the following characters. That is, according to the conventional method, it was impossible to measure the cell gap of a liquid crystal cell equipped with a color filter, having an uneven color filter surface or an uneven substrate surface for liquid crystal driving elements, having molecules oriented perpendicularly when no voltage is impressed, and including an Nn liquid crystal material having negative dielectric anisotropy. In other words, it was impossible to measure the cell gap when the liquid crystal molecules were oriented perpendicularly to the substrate, since the cell will not have any optical retardation (optical path) in the inner plane orientation of the substrate (i
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Turner Samuel A.
LandOfFree
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