Note: Descriptions are shown in the official language in which they were submitted.
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TITI.E QE THE INVENTI~N
VAR~A~L~ T~ANSP~RENC~ELFJC~RO-~PTICAL P~VIC~
The invention relates to a variable transparency
electro-optical device and more particularly to a device
of this type applied to devices for protecting the eyes
such as glasses comprising lenses composed of electro-
optical cells which possess particularly high light
absorption characteristics.
The field of application of the invention also
comprises conventional sun glasses or medical glasses, as
well as devices for protecting the eyes, such as welding
goggles or the like.
Variable transparency glasses are currently known and
are used notably as sun glasses and are of two types.
The first type comprises glasses provided with lenses
termed photochromic lenses. These latter include a
photochromic substance such as a silver salt and have the
property of passing, automatically and reversibly, from an
almost transparent state in the absence of sunlight to an
absorbent state in the presence of sunlight.
By virtue of their nature, these glasses generally
only permit absorption of a maximum amount of light of the
order of 70 to 80 % of the ambient light and then only
after a reaction time of about ten minutes.
Moreover the regeneration time after the lenses are
no longer irradiated is about 10 to 20 minutes and the
transmission of these lenses in the transparent state is
only about 80~, these lenses permanently retaining a
slightly coloured appearance.
In addition, these glasses are virtually ineffective
when used behind a glass surface since these polychromic
lenses are substantially sensitive to ultraviolet light.
These lenses are also very sensitive to temperature and
will be more absorbent at low temperature than at high
temperature to a similar quantity of ambient light.
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Finally, it is difficult, for the same degree of
absorption, to provide ~lasses ol this kind having lenses
less than several mm in thickness without the risk of
reaching the limit of solubility of the photochromic
substances and thus without the risk of causing the
precipitation thereof.
This results in glasses which react passively and
which are only relatively user f~iendly if it is desired
to obtain sun glasses that react immediately and
efficiently in a large number of situations and notably
behind a glass surface such as a windscreen of a motor
vehicle.
The second type comprises glasses, the lenses of
which are formed of electro-optical cells such as liquid
crystal cells, such as the glasses described in the patent
specification
No. PCT/IT87/00024.
This document relates to positive contrast liquid
crystal cells of the dichroic type associated with an
automatic or manual control circuit which makes it
possible to change the cells from a transparent state to
an absorbent state. In the transparent state, these cells
transmit about 80~ of the ambient light wherea~ they
transmit about 40% in the absorbent state. Although these
glasses constitute a major advantage over photochromic
glasses because they are active and their reaction time is
virtually zero after the control circuit has been
activated, it notably being possible to activate the
latter by ambient light detection means or by a simple
cut-out switch, they still present some disadvantages.
These glasses have on the one hand a rather low
coefficient of absorption with the result that the user is
poorly protected in the present of strong light and may be
inconvenienced. On the other hand, the plates of these
cells have to be subjected to an anti-ultraviolet
treatment to prevent premature degradation of the liquid
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crystal which, apart from the fact that it is costly,
gives a yellowish and not very aesthetic appearance to the
plates of the cells in their transparent state.
It is thus a main object of the invention to overcome
the disadvantages of the above-mentioned prior art by
providing a variable transparency electro-optical device
which combines the advantageous features of the use of
photochromic cells and of electro-optical cells.
The object of the invention is thus a variable
transparency electro-optical device comprising:
- at least one lens composed of an electro-optical
cell comprising a firstv plate and a second plate each
provided with a control electrode and a sealing frame
interposed between the two plates to form a sealed volume
enclosing an electro-optical material,
- voltage generating means connected to said
electrodes to apply a variable voltage to said material in
order to vary the transmission of the cell automatically
or manually as a function of the ambient light, this
device being characterized in that the cell comprises at
least one element charged with a photochromic substance
having a transmission which varies in reversible manner as
a ~unction of the intensity of the light impinging on said
cell.
These characteristics make it possible to produce a
variable transparency electro-optical device having two
successive absorption levels with two different reaction
speeds which permit immediate and effective reaction to
every light source and which, in addition, have a high
coefficient of absorption to ambient light.
It will also be noted that the element charged with a
photochromic substance advantageously forms a screen
against ultraviolet radiation so that the electro-optical
material is protected thereagainst.
In addition, dividing the absorption of the ambient
light between the electro-optical material and an element
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charged with a photochromic substance makes it possible
~or the latter to be charged either with a lesser amount
of said substance than that normally needed or to provide
lenses of very low thickness wh:ich advantageously reduces
the weight of the entire device.
It will be noted that especially in the application
of a device of this kind to means for protecting the eyes
such as glasses, the absorptive effect of the electro-
optical cells and the photochromic effect are
complementary in numerous common situations. ~hus for
example, in a motor vehicle in which the absorption of the
photochromic lenses is almost zero whereas that of the
electro-optical cells is normally effective. Moreover,
when the wearer passes outdoors from a sunny area to a
partially shaded area in which he wishes to be protected,
the electro-optical cells are easily deactivated, the
wearer then only benefiting from the absorptive effect of
the photochromic lenses.
According to a preferred embodiment, the element
charged with a photochromic substance is formed by one of
the plates of the electro-optical cell.
It is also an object of the invention to provide a
device of this type disposed in a supporting struc~ure to
form therewith a device for protecting the eyes.
Other advantages and features of the invention will
emerge from study of the following detailed description of
embodiments of the invention, given as non-limiting
example, in connection with the appended drawings, in
which:
- Figure 1 is a partially exploded perspective view
of an electro-optical device of the invention applied to a
pair of glasses; and
- Figure 2 is a diagrammatic section of an electro-
optical device of the invention associated with an
automatic circuit for controlling its transparency,
equipping the glasses of Figure 1.
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The following description of the electro-optical
device according to the invention will be made in the
context of its application in variable transparency
glasses, but it is understood that a device of this kind
can have other interesting applications, for example in
welding goggles or the like or also to produce glass for
windows.
Whereas the electro-optical material used in the cell
of the device of the invention described hereinafter is a ,
mixture of liquid crystals, other types of electro-optical
materials such as notably electrochromic materials may of
course also be used.
Figure 1 shows a pair of variable transparency liquid
crystal glasses according to the invention and having the
general reference 1. These glasses are generally designed
to protect the sight of their wearer against ambient light
of strong intensity. More specifically, these glasses
comprise a frame 2 into which are fitted in conventional
manner lenses 4, the absorption characteristics of which
vary reversibly as a function of the intensity of the
light impinging on the lenses.
These lenses 4 are formed of liquid crystal cells
which, as may be seen in particular from Figure 2, each
comprise a transparent front plate 6 and a transparent
back plate 8 connected together by a sealing frame 10.
These two plates 6, 8 and the frame 10 define a sealed
volume enclosing a mixture 12 of li~uid crystals and
dichroic dyestuffs doped with a chiral agent. -~ ,
In the embodiment described the plates 6 and 8 are
convex and each have on their inside face a transparent
electrode 14, 16 covered by an alignment layer (not shown)
which extends over the entirety of each plate. The
electrodes 14, 16 are composed for example of a mixture of
indium oxides and tin oxides and the alignment layer is
formed of silicon dioxide with a surfactant agent of the
octadecyltrialkoxysilane type fixed thereto.
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It should be noted that the drawings do not represen~
the exact thickness of the assembly formed in this manner,
this thickness being greatly exaggerated for sake of
clarity. More specifically, the distance between the two
electrodes is of the order of 5 to 9 ~m.
Figure 2 also shows that the cells 4 are associated
with an electrical control circuit 18 fed for example by a
battery or by solar cells integrated in the frame 2 (not
shown). This circuit is connected to ambient light
detection means 22 formed by one or several photosensitive
sensors which deliver an electrical control signal
representing the intensi~y of light which they receive.
This circuit also comprises two outputs connected to
electrodes 14 and 16 respectively in order to control the
degree of absorption of the cells.
In other words, the signal produced by the control
circuit 18 is applied to the electrodes of the cells to
vary the electrical field applied to the mixture 12 which
they contain. The effect of this variation is to modify
the transmission of the cell which results in variation in
its degree of light absorption.
In the embodiment shown, the electro-optical cells 4
and the sensor 22 are disposed in relation to one another
in such a way that incident light or ambient light reaches
said cells first. An arrangement of this type of the
sensor or sensors notably makes it possible, as will be
explained below, to obtain constant transmission in a
range of the level of light. It goes without saying that
in a different embodiment the detection means can be
disposed so as to be directly irradiated by the light rays
which the device of the invention is designed to absorb.
According to the invention each cell comprises at
least one element charged with a photochromic substance,
said element being disposed on the cell in such a manner
that the ambient light reaches this element first when the
glasses are worn by a user.
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In the example described, said element is formed by
the plate 6 of the cell 4. This plate is charged with a
photochromic substance such as a silver salt and can be
either a glass plate or of a synthetic material with
optical properties equivalent to those of the glass. A
plate of synthetic material made of polyvinyl pyrolidine
doped with microcrystals of AgCl is perfectly suitable.
This plate of synthetic material can also be made using
films of laminated polyester on a synthetic plate, for
example a plate sold under the reference C~ 39 by ESSILOR,
by means of an adhesive doped with polychromic
spirooxazines. p
The concentration of photochromic substance should
preferably not exceed 3% by weight and the thickness of
said element 6 is of the order of 1 mm.
In another embodiment of the invention, not shown,
said element can be formed of a supplementary plate
directly applied
to the plate 6 of the cells, for example by adhesion.
Accordin~ to another embodiment (not shown) of the
invention, the lenses 4 formed by an electro-optical cell
can be associated with a second electro-optical cell in
the same support structure to reinforce the absorptive
effect of the device for protecting the eyes such as
welding goggles. In this case the second cell is placed
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behind the lenses 4 in relation to the ambient light and
this cell can be an electro-optical cell of conventional
variable transparency.
The applicant has conducted comparative measurements
of the transmission (in % of light received) of a single
electro-optical cell in a switched and a non-switched
state, of a single photochromic lens and of the electro-
optical device of the invention in a switched and non-
switched state iII the presence of and in the absence of
light comprising W radiation for purposes of illustration
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and to better demonstrate the advantageous results of
combining the photochromic and e:Lectro-optical effects of
the invention.
The measurements in the presence of light were made
in the presence of a light source of ~OmW/cm2. The electro-
optical cell used is a liquid crystal cell comprising a
dichroic mixture sold by MERCK under reference ZLI 4282,
this mixture being doped with 0.85~ by weight of a chiral
agent sold by MERCK under reference S811.
The substrates of this cell are of glass. The
photochromic lenses used are standard photochromic lenses
which can for example be~obtained from Desag (Schott). It
will also be noted that the results of the transmission
measurements of the device of the invention were obtained
with a device in which the charged element of a
photochromic substance is applied to the electro-optical
cell by adhesion.
The results obtained are set out in the following
table:
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TRANSMISSION %
I
without light with light
I Photochromic
¦ lenses ¦ 75 to ~5% 1 20 to 30%
I
I Electro-optical ¦
I cell
I - non-switched I ~ 80%
¦ - switched ¦ 40%
Electro-optical I I ;
device of the I ¦
invention
¦ - non-switched ¦ 70% 1 19%
I - switched L 27% 1 9%
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This table shows that the device of the invention
presents a degree of absorption that can vary over a large '
range as a function of the intensity of the ambient light. I
In other words, the device of the invention can
become very absorbent in the event of great luminosity and
only transmit 9% of the ambient light~compared to 20 and
40% with a photochromic lens alone and respectively a
variable transparency electro-optical cell alone (in its
switched state).
The device of the invention can also become very
transmissive in the event of total interruption of the
luminosity and transmit 70% of the ambient light (in the
switched state of the electro-optical cell). Such a level
of transmission is substantially equal to the level of
transmission of the photochromic lenses and electro-
optical cells used on their own.
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If the electro-optical device of the invention is
interposed between detection means 22 and the light
source, the transparency of the device can regulate itself
so as to be substantially constant for periods of
transition notably corresponding to the transition of a
dark or poorly luminous medium to a clear or luminous
medium.
During transition of this type, the detection means
initially receive almost all the light in such a way that
they immediately control the darkening of the electro-
optical cell when the photochromic substance has no~ yet
reacted. ~he wearer thus has immediate protection.
In a second phase, that is a few minutes later, the
photochromic element, for example the photochromic
substrate o~ the cell, begins to react progressively and
in turn absorbs part of the ambient light. The
illumination of the detection means situated behind the
cell is then diminished while the latter trigger a control
signal which increases the transmission of the cell while
the photochromic element darkens.
It is thus possible to maintain a degree of constan~
transmission of the device of the invention both during
the transition of a dark medium to a luminous medium and
during slight variations in the luminosity of the medium.
n the opposite situation, i.e. in the case of
~ransition from a luminous medium towards a darker medium,
the cell deactivates imrnediately with the result that the
device rapidly becomes less absorbent, the photochromic
element then progressively loses its abillty to absorb
until it reaches a completely clear or sufficiently clear
state for the device to be able to readjust itself once
again as has been described above.
