Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1991-06-20
1994-06-07
Gross, Anita P.
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359 65, G02F 1133, G02F 11335
Patent
active
053194780
DESCRIPTION:
BRIEF SUMMARY
The invention relates to light control systems consisting of a combination of liquid crystal cells and polarizers.
In conventional arrangements with liquid crystal cells, they are usually operated with one or more linear polarizers. Where circularly-polarized light was available and had to be controlled, it was converted to linearly-polarized light, e.g. with a .lambda./4 plate and, if required, was re-converted to circularly-polarized light after the liquid crystal cell again with a .lambda./4 plate. The rotary or TN (=Twisted Nematic) cell, in particular, requires linearly-polarized light for its mode of operation in which the direction of polarization of the incident light is rotated with the helix structure in the switched-off state (wave-guiding mode).
It has now been found that there is one type of operation possible and in many respect advantageous, i.e. direct operation of liquid crystal cells with circularly-polarized light, the circularly-polarized light being produced either with conventional circular polarizers or with monomeric or polymeric cholesteric liquid crystals.
The invention is accordingly characterised in that light control systems of the type referred to above are provided with means for supplying circularly-polarized light to liquid crystals.
The invention is explained hereinafter with reference to exemplified embodiments illustrated in the drawings wherein:
FIG. 1 shows an arrangement with a TN-cell in the conventional mode of operation.
FIG. 2 shows an arrangement with a TN-cell in operation with circularly-polarized light in transmission mode.
FIG. 3 is a similar arrangement to FIG. 2 but in reflection mode.
FIG. 4 shows an arrangement with a parallel-oriented cell in reflection mode with circularly-polarized light.
FIG. 5 shows an arrangement with a vertically oriented DAP cell in operation with circularly-polarized light.
The arrangement shown in FIG. 1 comprises a light source 1, whose light is converted into dextrorotatory circular light with a circular polarizer 3. The latter is followed by a .lambda./4 plate 4, which converts the circularly-polarized light into linearly-polarized. The direction of polarization is indicated by a double arrow. The .lambda./4 plate 4 is followed by a TN cell 5 which, in the switched-off state, rotates the direction of polarization of the light through 90.degree. in known manner. This rotated light can pass without attenuation through a linear polarizer 6 following the TN cell 5 and turned through an angle of 90.degree. with respect to the direction of polarization before the cell. If voltage is applied to the cell 5, the rotatory effect is eliminated and the light, which is now not rotated, cannot pass the polarizer 6. The TN cell 5 thus operates in the conventional mode, in which the direction of polarization of the incident light is rotated in the switched-off state (wave-guiding mode).
In order further to increase the polarization quality after the .lambda./4 plate in this arrangement, a low-polarization linear polarizer can be incorporated between the .lambda./4 plate and the TN cell to improve the linear polarization without appreciably affecting the brightness of the system.
FIG. 2 shows another mode of operation. The dextrorotatory circularly-polarized light produced as described above is fed directly to a TN cell 7. For this mode of operation, the cell must have a minimal optical path difference of .delta.=.DELTA.n.d=.lambda./2 between the switched-off and switched-on states. If this condition is satisfied, the direction of rotation of the circularly-polarized light changes as it passes, i.e. in this case from dextrorotatory (+) to levorotatory (-). The TN cell 7 is followed by another circular polarizer 8 which is in this case transparent to the opposite direction of rotation to the first circular polarizer 3, i.e. levorotatory. When the TN-cell 7 is switched on and thus becomes optically uni-axial, it does not affect the state of polarization of the light, so that it is blocked by the circular polarizer 8.
Any known form of circular
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Funfschilling Jurg
Schadt Martin
Gould George M.
Gross Anita P.
Hoffmann-La Roche Inc.
Johnston George W.
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