Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-08-03
2003-04-15
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S175000, C349S176000
Reexamination Certificate
active
06549254
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a Bragg reflection optical device for reflecting at least one predetermined wavelength band present in an incident light, the device including at least one liquid crystal film of the cholesteric type. The invention also concerns methods for manufacturing such optical devices.
It will be recalled that a cholesteric type liquid crystal only reflects, by Bragg reflection, light which has circular polarisation having the same rotational direction as that of the liquid crystal. It will be noted that, in the following description, the reflection coefficient is equal to 1 when a circular polarisation light is completely reflected.
The characteristic of certain cholesteric liquid crystals is having a helical periodic structure having a pitch which can be adjusted. This helical structure causes Bragg reflections whose reflection band, i.e. the wavelength range which it can reflect, can easily be modified by selecting other values for the helical pitch and/or the liquid crystal birefringence.
From such crystals, one can make optical devices, in particular for display, by introducing, between two plates or substrates, several cholesteric liquid crystals having one more pitches adjusted to reflect respectively a wavelength corresponding to a determined colour.
One problem which is commonly encountered when such devices are made lies in the fact that the reflected colour, in particular the colour red, has a dull or faded appearance.
In order to explain this phenomenon, reference will be made hereinafter to
FIG. 1
, which shows a curve
1
illustrating the reflection spectrum of an optical device with cholesteric liquid crystals adjusted to reflect the colour red. It is to be noted that reflection of wavelengths &lgr; corresponding to the colour red by such a liquid crystal is imperfect. Indeed, curve
1
can be broken down into a main band A, corresponding to reflection of the colour red, and into two lateral bands B and C on either side of main band A. The effect of the presence of lateral bands B and C is that the colour red reflected by the optical device is not pure, i.e. it is not sufficiently saturated, nor sufficiently brilliant.
It will be recalled that saturation is linked to the limitation of the wavelength spectrum of the colour red, and that brilliance is linked to the whether the reflection coefficient is close to 1 or not.
Moreover, the characteristic of the human eye will accentuate the undesirable effect of these lateral bands on the purity of the red colour which it sees.
FIG. 1
shows a curve
2
illustrating the response of the human eye as a function of the wavelength &lgr; of the light which the eye receives, i.e. for all the colours of the visible spectrum (this curve also being called the photopic curve). It will be noted that the human eye is most sensitive in day vision (photopic vision) to the wavelengths &lgr; closes to wavelength 555 nm, which corresponds to the peak of curve
2
.
FIG. 2
shows a curve
3
illustrating the eye's perception of the colour red reflected onto the optical device having the feature shown in curve
1
. In other words,
FIG. 2
shows the reflection spectrum of the colour red multiplied by the human eye's response, as a function of wavelength &lgr;. It will be noted in
FIG. 2
that the effects of lateral band C of the low wavelengths of the colour red are amplified by the human eye, which adversely affects the colour red and gives it a dull appearance; it then becomes orange-red.
It has been observed that similar phenomena occur with the colour blue. However, the effects of the lateral bands are more amplified for the colour red than for the colour blue, because of the photopic curve of the human eye.
In order to overcome this problem of the purity of colour emitted by an optical device of the aforementioned type, there exist several types of solution in the prior art.
A first solution to this purity problem is described in the work entitled “Liquid Crystal in Complex Geometries” by Taylor and Francis, published in 1996, page 257, and consists in doping the liquid crystal with a dye which is intended to absorb the undesired parts of the reflection spectrum.
One drawback of the first solution is that the optical effect obtained is not optimum. Indeed, it is possible for the light reflected by the liquid crystal not to have met molecules of dye or to only have been modified by a few dye molecules, so that the colour is unsaturated, or, in other words, is not pure.
Another drawback of this solution is that it requires the mixture of the liquid crystal and the dye to be physically separated from other liquid crystals which reflect respectively green and blue, in order to avoid diffusion of the dye molecules in the neighbouring liquid crystals of different colours. This has the effect of increasing the complexity of the device.
Another drawback lies in the fact that the dye has residual absorption for the wavelengths of the main band, which has the effect of reducing brilliance.
Another drawback lies in the fact that this solution involves absorption of the transmitted light, which means that a stack of several liquid crystal cells cannot be used to combine optical effects, for example different colours.
Another drawback lies in the poor chemical stability of the molecules forming the dye, in particular in the presence of ultraviolet rays (UV), which reduces the reliability and lifetime of the display device.
A second solution to the aforementioned purity problem is described in European Patent Application No. EP 0 872 759, in the case of a liquid crystal display device (LCD). This solution consists in providing the LCD device with a filter able to absorb the visible wavelengths different to that corresponding to the colour that the crystal has to reflect. This filter eliminates the effect of the lateral bands described above from the spectrum reflected by the liquid crystal, so as to make the reflected colour more pure.
This second solution also has various drawbacks. It requires the complex arrangement of the absorbent filter, which goes against the usual industrial concerns as to cost, compactness and rationality. Moreover, it requires the arrangement of equalising layers to allow a constant thickness of the liquid crystals to be assured over the entire surface of the cell, which increases the complexity of such a device.
This solution also has the drawback of involving absorption of the transmitted light, which means that a stack of several liquid crystals cannot be used to combine their optical effects.
SUMMARY OF THE INVENTION
An object of the present invention is thus to provide an optical device which overcomes the aforementioned drawbacks, in particular an optical device able to reflect or transmit with optimum purity a predetermined colour having a wavelength comprised within the visible range, for example the colour red, or outside such range, for example infrared rays.
Another object of the present invention is to provide a device able to reflect or transmit a predetermined colour with optimum saturation.
Another object of the present invention is to provide a device able to reflect or transmit a predetermined colour the brilliance of which is optimum.
Another object of the present invention is to provide an optical device allowing a plurality of colours to be reflected to transmitted.
Another object of the present invention is to provide an optical device answering the usual concerns in the industry as to cost, compactness and rationality.
According to the invention, an optical device of the type indicated in the preamble is provided, characterised in that the liquid crystal film has, in at least part of its thickness, a birefringence gradient as a function of the depth in said film.
One advantage of the birefringence gradient liquid crystal of such an optical device is that it can limit the reflection spectrum to a wavelength band having very clear limits with neighbouring wavelengths, and it makes the reflection coefficient close to 1. As a result, the ban
Bohley Christian
Grupp Joachim
Herzig Hans Peter
Kipfer Peter
Klappert Rolf
Asulab S.A.
Kim Robert H.
Nguyen Hoan
Sughrue & Mion, PLLC
LandOfFree
Bragg reflection optical device and methods for the... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Bragg reflection optical device and methods for the..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Bragg reflection optical device and methods for the... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3037530