WO2019012304A1

WO2019012304A1 - Optical rejection filter with angular invariance of the rejected wavelengths

Info

Publication number: WO2019012304A1
Application number: PCT/IB2017/001052
Authority: WO
Inventors: Eva OTON; Estelle NETTER
Original assignee: Nikon Corporation; Essilor International
Priority date: 2017-07-12
Filing date: 2017-07-12
Publication date: 2019-01-17

Abstract

The invention relates to an optical rejection filter (2) comprising a liquid crystal cell, comprising: - a liquid crystal mixture (22) comprising cholesteric liquid crystals, - two alignment layers (20, 21) encasing the liquid crystal mixture, the liquid crystal mixture being capable of reflecting rays of light incoming on a surface of the cell by Bragg reflection, the wavelength of the reflected rays depending on the angle of incidence of the rays on the surface of the cell and on the pitch of the helices of the liquid crystals molecules, wherein for each point of coordinates (x,y) of the cell is defined a pitch of the helices of the liquid crystal molecules, and the pitch varies along at least one direction within the cell, such that the cell comprises at least two points reflecting the same wavelength of two rays of light incoming on the respective points of the surface with different angles of incidence.

Description

OPTICAL REJECTION FILTER WITH ANGULAR INVARIANCE OF THE

REJECTED WAVELENGTHS

Field of the invention

The present invention relates to optical rejection filters based on Bragg reflection, in particular to optical rejection filters formed of cholesteric liquid crystals.

Background of the invention

Rejection filters based on Bragg reflection are highly selective and efficient filters. Such rejection filters can be formed, for example, of cholesteric liquid crystals forming helices that are twisted around a main direction between two plates of a liquid crystal cell. The helices are formed by stacking liquid crystal molecules with a slight variation of orientation between two juxtaposed molecules. A pitch is defined along said main direction, by the distance needed for the slight variations of orientation of the stacking of liquid crystals molecules to combine to a full 360° variation of orientation with respect to an initial orientation. Depending on the value of the pitch, a different wavelength is reflected.

Cholesteric liquid crystals, when combined with transparent electrodes, are used as switchable selective rejection filters in liquid crystal displays, for example. In the absence of an electrical field, the cholesteric liquid crystal form helices and the pitch of the helices define the wavelength that is reflected and thereby rejected by the filter. When a voltage is applied on the transparent electrodes placed on the plates of the liquid crystal cell, the orientation of the liquid crystal molecules is modified and the light incoming on the liquid crystal cell is no longer reflected.

However in such rejection filters the wavelength of light reflected, and thus rejected, by filters based on Bragg reflection is dependent of the angle of incidence of the light incoming on the filter, following the Bragg condition:

where n is the average refractive index of the liquid crystal cell, Θ is the angle of incidence of the rays of light, measured from the surface on which the rays are incoming, λ is the reflected wavelength and d is the distance between two consecutive rows of helices of the liquid crystals.

What is called "Bragg wavelength" corresponds to the wavelength that is reflected under normal incidence.

However, when rays of light are incoming on a cholesteric liquid crystal cell at an angle which is different from normal incidence, for those rays of light, the cell has an apparent pitch of the helices which is different from the actual pitch of the helices. Therefore the Bragg wavelength for this angle of incidence is different.

In particular, when the angle of incident light is lower than a reference angle, the pitch of the helices appears larger than the pitch perceived for said reference angle, and there is a blue shift of the reflected/rejected wavelength, which means that the wavelength of the Bragg reflection increases. In particular, when compared to the actual pitch, for which the reference angle is the normal angle, when the angle of incident light is different, the perceived pitch is always larger than the actual pitch.

Thus, when such rejection filters are placed on an ophthalmic lens for example, the light reflected by the surface of the lens has a different color depending on the angle of observation of the reflected light. From the point of view of the wearer of the glasses, the rejected light that is not transmitted by the ophthalmic lens varies with the angle of observation of the wearer and thus with the gaze direction. The response of the filter is thus not uniform and depends on the angle of observation.

There is thus a need to develop optical rejection filters based on Bragg reflection in which the wavelength of the light that is reflected, and thus rejected, by the filter is independent of the angle of observation.

Presentation of the invention

In view of the above, one aim of the invention is to alleviate at least part of the inconveniences of the prior art.

In particular, one aim of the invention is to provide an optical rejection filter based on Bragg reflection that is independent of the angle of incidence of the light incoming on the rejection filter and thus of the angle of observation of the light rejected by the filter.

To this end, an optical rejection filter is disclosed, comprising a liquid crystal cell, the liquid crystal cell comprising:

- a liquid crystal mixture comprising cholesteric liquid crystals,

- a first and a second alignment layers encasing the liquid crystal mixture, the cholesteric liquid crystals defining a plurality of helices extending between the alignment layers, each helix having a pitch,

the liquid crystal mixture thereby being capable of reflecting rays of light of a determined wavelength incoming on a surface of the cell defined by one of the first and second alignment layers by Bragg reflection, the wavelength of the reflected rays of light depending on the angle of incidence of the rays of light on the surface of the cell and on the pitch of the helices,

wherein for each point of coordinates (x,y) of the cell is defined a pitch p(x,y) of the helices of the liquid crystal molecules at this point of the cell, and the cell is configured such that the pitch p(x,y) of the helices varies along at least one direction comprised within a plane formed by one of the first and second alignment layers, such that the cell comprises at least two points capable of reflecting the same predetermined wavelength λ of two rays of light incoming on the respective points of the surface with different angles of incidence.

In embodiments, the overall variation of the pitch p(x,y) within the cell is λ

smaller than— , wherein n_a is the mean value of an ordinary refractive index n₀ and n a

an extraordinary refractive index of n_e of the cholesteric liquid crystal.

In embodiments, the first and second alignment layers define a first and a second orientations of the cholesteric liquid crystal molecules adjacent with the first and the second alignment layers respectively, and the distance d(x,y) between the first and second alignment layers defines a thickness of the liquid crystal cell, and: the thickness d(x,y) of the liquid crystal cell varies over at least one portion of the liquid crystal cell as a function of the pitch according to d(x,y) = k p(x,y), wherein k is a positive integer, and

the angle Aa(x,y) between the first and the second orientations is constant over the at least one portion of the liquid crystal cell.

In other embodiments, the first and second alignment layers define a first and a second orientations of the cholesteric liquid crystal molecules adjacent with the first and the second alignment layers respectively, and the distance d(x,y) between first and second alignment layers defines a thickness of the liquid crystal cell, and:

the first and the second alignment layers are parallel to each other and the thickness do of the liquid crystal cell is defined according to do=k po_, with po λ

being a nominal pitch defined as p₀ =— , n_a being the mean value of the ordinary refractive index n₀ and the extraordinary refractive index n_e of the cholesteric liquid crystal, and with k being a positive integer corresponding to a number of nominal pitches comprised within the thickness of the cell, and for each point (x,y) of the cell is defined an angle Aa(x,y) between the first and the second orientations of the cholesteric liquid crystals molecules located at this point of the cell, wherein the angle varies according to the following equation :

A (x, y) = 360xd₀ 1 p(x, y) - 360 .

In that embodiment, at least one of the first and second alignment layers may be formed of a photopolymerized polymeric layer defining a corresponding first or second orientation of the liquid crystals adjacent said layer, and wherein the angle variation between the first and second orientations is obtained by varying one of the first or second orientation while the other orientation remains constant.

In some embodiments, an observation point is defined with reference to the cell, at a distance thereof, and the cell is configured such that any ray of light of a predetermined wavelength λ incoming on the surface of the cell and directed towards the observation point is reflected by the liquid crystal mixture.

In that case, each point (x,y) of the cell may be associated to a corresponding incidence angle (Θ) of a ray of light incident on this point and directed towards the observation point,

and the pitch p of the helices varies according to the following equation : ρ(θ) =

n_a x cos(arcsin(sin(#) / n_a ))

said calculated pitch value ρ(θ) corresponding to the pitch value p(x,y) of the corresponding point of coordinates (x,y).

A tunable rejection filter is also disclosed, comprising an optical rejection filter according to the above description, and

wherein the tunable rejection filter further comprises two electrodes configured to be connected to a voltage source, said voltage source being configured to control the orientation of the cholesteric liquid crystals between the first and second alignment layers.

An optical device comprising the tunable rejection filter according the above description is also disclosed, mounted in series with an optical element or affixed to said optical element so that a light ray goes in series through the tunable rejection filter and the optical lens for the wavelength which are not rejected by the tunable rejection filter.

In embodiments, the optical device is an ophthalmic lens.

A method for producing an optical rejection filter or a tunable rejection filter according to the above description is also disclosed, wherein the method comprises encasing a liquid crystal mixture comprising cholesteric liquid crystals between two alignment layers to form a liquid crystal cell, wherein the cholesteric liquid crystals form a plurality of helices, each having a pitch p,

wherein the liquid crystal cell is configured such that the pitch of the helices is variable along at least one direction comprised within a plane formed by one of the first and second alignment layers, such that the cell comprises at least two points capable of reflecting the same predetermined wavelength of two rays of light incoming on the respective points of a surface of the cell with different angles of incidence.

In embodiments, the first and second alignment layers define a first and a second orientations of the cholesteric liquid crystal molecules adjacent with the first and the second alignment layers respectively, and the method further comprises: rubbing the first and second alignment layers such that the angle between the first and second orientations of the liquid crystal molecules is constant over at least a portion of the cell,

calculating a thickness over the at least one portion of the liquid crystal cell as a function of a position in (x,y) coordinates according to : d(x,y) = k p(x,y), wherein k is a positive integer, and

positioning the first and second alignment layers relative to one another such that the distance between the layers corresponds to the calculated thickness.

In embodiments, the first and second alignment layers define a first and a second orientations of the cholesteric liquid crystal molecules adjacent with the first and the second alignment layers respectively, and the method further comprises:

- calculating a nominal pitch po of the liquid crystal cell helices according to:

λ

Po =— > wherein λ is the rejection wavelength and n_a the mean value of the ordinary refractive index n₀ and the extraordinary refractive index n_e of the cholesteric liquid crystals, determining a thickness do of the liquid crystal cell according to: do=k po, wherein k is a positive integer,

positioning the first and second alignment layers relative to each other such that the distance between the layers corresponds to the calculated thickness, - defining for each point (x,y) of the cell an angle Aa(x,y) between the first and the second orientations of the cholesteric liquid crystals located adjacent the point of the cell, and

processing at least one of the alignment layers such that the angle Aa(x,y) varies over at least a portion of the cell according to:

A (x, y) = 360x d₀ / p(x, y) - 360x k .

In that case, the processing step may be performed by rubbing one alignment layer to set one of the first or second orientation of the molecules to be constant, by exposing the other alignment layer to a polarization interference pattern, to set the other of the first or second orientation of the molecules to vary spatially.

In embodiments, the method further comprises the steps of:

defining an observation point relative to the cell, at a distance thereof, associating to each point of coordinates (x,y) of the cell an incidence angle Θ of a ray of light incoming on said point (x,y) and directed towards the observation point,

for each incidence angle Θ, calculating a pitch value according to λ

Ρ(θ) = ,

n_a x cos(arcsin(sin(#) / «_a ))

wherein λ_Β is a wavelength intended to be rejected by the rejection filter for any ray of light incoming on the surface of the cell and directed towards the observation point, said calculated pitch value ρ(θ) corresponding to a pitch value p(x,y) of the corresponding point of coordinates (x,y), and

processing the cell such that the pitch of the helices corresponds to the calculated pitch value. According to the invention, the rejection filter comprises a spatial variation of its pitch that compensates the angular dependency of the response of rejection filters based on Bragg reflection.

The smooth variation of the pitch along the rejection filter enables to compensate the blue shift in the reflected wavelength for a wide variety of angles of incidence and thus to compensate the blue shift in the rejected wavelength for a wide variety of angles of observation.

The present invention can be used in all kinds of optical devices and elements, such as optical filters, optical lenses and devices or ophthalmic elements and devices. Non-limiting examples of ophthalmic elements include corrective and non-corrective lenses, including single vision or multi-vision lenses, which may be either segmented or non- segmented, as well as other elements used to correct, protect, or enhance vision, including without limitation contact lenses, intra-ocular lenses, magnifying lenses and protective lenses or visors such as found in spectacles, glasses, goggles and helmets.

Description of the drawings

Other features and advantages of the invention will be apparent from the following detailed description given by way of non-limiting example, with reference to the accompanying drawings, in which:

Figure 1 represents schematically a tunable rejection filter comprising an optical rejection filter according to an exemplary embodiment of the invention,

- Figure 2a schematically shows an example of an optical rejection filter according to an embodiment of the invention, in a lateral view, Figure 2b schematically shows an example of an optical rejection filter according to an alternative embodiment of the invention in a perspective view and in a lateral view,

- Figure 2c schematically shows an example of an optical rejection filter according to another embodiment of the invention, Figure 3a shows the transmittance curves as a function of the wavelength of an optical rejection filter as shown in figure 2a for different incidence angles in the air for unpolarized light.

Figure 3b shows the transmittance curves as a function of the wavelength for an optical rejection filter as shown in figure 2b for different incidence angles in the air for unpolarized light.

Figure 4 schematically represents the main steps of a process according to an embodiment of the invention. Detailed description of at least one embodiment of the invention

Tunable rejection filter

With reference to figure 1, is shown a tunable rejection filter 1 which comprises an optical rejection filter 2 and two electrodes 3, which are preferably made of ITO (Indium-Titanium Oxyde) to be transparent and electrically conductive. The electrodes are connected to a voltage source 4.

The optical rejection filter 2 is composed of a liquid crystal cell which comprises a first alignment layer 20, a second alignment layer 21, and a liquid crystal mixture 22 filling the liquid crystal cell between the first and second alignment layers 20, 21. The alignment layers 20, 21 are plates made of a transparent material over the visible spectral range. A surface 22 of the cell is also defined, which is a surface on which rays of light are incident. This surface is formed by a surface of one of the alignment layers opposite the surface of contact in contact with the liquid crystal mixture. In the following, an example will be taken in which the surface 22 is formed as a surface of the second alignment layer.

In embodiments, micro sized spacers (not shown) may be included in the cell so as to ensure a predetermined distance between the alignment layers.

The electrodes are arranged respectively on each of the first and second alignment layers, and are positioned opposite one another. The voltage source 4 is able to selectively apply a difference of potential between the electrodes.

The liquid crystal mixture 22 comprises cholesteric liquid crystals, and preferably at least one chiral dopant. The cholesteric liquid crystals molecules, encased between the alignment layers, define when no voltage is applied (so-called OFF state), a plurality of helices extending between the plates, and twisting around an axis which is substantially perpendicular to the plates or, if the plates are not strictly parallel, to at least one plane formed by one of the alignment layers.

At the ends of an helix, one can define a first orientation which is the orientation of the liquid crystal molecule of the helix which is adjacent the first alignment layer, and a second orientation which is the orientation of the liquid crystal molecule of the same helix which is adjacent the second alignment layer.

Each helix formed by liquid crystal molecules self-organizes between the molecule in contact with the first alignment layer and the molecule in contact with the second alignment layer, and exhibits a helical pitch which is the distance than is needed for an helix to complete a twist of 360°.

As already explained, the cholesteric liquid crystals are able to reflect specific wavelengths of incident rays of light according to the pitch of the helices, by Bragg reflection, such that :

where λ_Β is the wavelength reflected by Bragg reflection, p is the pitch of the helices, and n_a is the average refractive index in the medium formed by the cholesteric liquid crystal mixture, which is defined by:

n₀ + n_e

n_n =

Where n_Q is the ordinary refractive index and n_e is the extraordinary refractive index of the medium.

When a voltage is applied by the voltage source on the electrodes 3, (so-called ON state) the liquid crystal layer exhibits homeotropic state or orientation, in which the liquid crystal molecules are aligned perpendicularly to the alignment layers, and thus do not produce any Bragg reflection. Variable pitch of the liquid crystal cell

As shown in figure 1, each point of the liquid crystal cell 2 is associated to coordinates (x,y) along axes x and y which extend within a plane formed by one of the alignment layers.

Moreover, for each point (x,y) of the cell is defined a pitch p(x,y) of the helix which is located at this point of the cell.

In the liquid crystal cell, the pitch of the helices p(x,y) varies spatially along at least one direction comprised within a plane formed by one of the alignment layers, over at least a portion of the cell. For instance, the pitch can vary along the x axis, or along the y axis, or along both axes.

The variation of the pitch of the helices within the liquid crystal cell is such that the cell comprises at least two points (x₀, yo), (xi, yi) capable of reflecting the same wavelength λ from two rays of light incoming on the surface of the cell at the respective points (xo, yo), (xi, yi), with different angles of incidence θο, Qi .

In an embodiment, for the point (x₀, yo), the angle of incidence θο is the normal to the plan of the liquid crystal cell. In a further embodiment there are multiple point (xi, yO, each capable of reflecting the same wavelength λ from rays of light incoming on the surface of the cell at the respective points (xi, yO, with an angle of incidence 9j.

Therefore, the adjustment of the pitch along the cell allows controlling the wavelength that is reflected by the optical filter.

Preferably, the overall pitch variation within the cell is smaller than the pitch Po which allows reflecting the wavelength λ under normal incidence: P₀= λ/η_α.

Indeed, a small variation of the pitch within the cell is often sufficient in obtaining this result and it is not necessary to make the pitch double or triple within the cell, as will be shown in the experimental results described hereinafter.

A spatially varying pitch over at least a portion of the cell 2 can be achieved by different ways. With reference to figure 2a, according to a first embodiment, the pitch p(x,y) can be made to vary by varying the thickness d(x,y) between the alignment layers 20, 21, while ensuring that the angle Aa(x,y) between the first and the second orientation of the liquid crystal molecules adjacent respectively the first and second liquid crystal layer is constant within the portion of the cell. An example of the angle Aa(x,y) is shown in figure 2b.

For instance, the alignment layers 20, 21 are preferably formed of polyimide and then rubbed to induce a first orientation and a second orientation of the liquid crystal molecules adjacent respectively to the first and second alignment layers, which are constant over the whole surface of respectively each orientation layer.

As the angle between the first and second orientation of the liquid crystal molecules is constant, a variation of the thickness of the cell implies a variation of the pitch of the helices, as shown schematically in figure 2a.

According to an exemplary embodiment shown on figure 2a, the liquid crystal cell can be of the form of a wedge cell exhibiting a thickness varying linearly along one direction. This wedge cell may comprise, on one end, a spacer 24 of a first size to provide the cell with a first thickness, and on an opposite end, a spacer 24' of a second size to provide the cell with a second thickness greater than the first.

In that case, the pitch of the helices increases smoothly from the first end to the second end, and the same wavelength λ is reflected for incoming rays of light of gradually greater incidence angles towards the second end.

According to an alternative embodiment, the cell may comprise a single spacer on one end, and no spacer on the opposite end, such that the increase in thickness along the direction between the two ends equals the thickness of the spacer.

With reference to figure 2b, another embodiment for making the pitch of the helices vary will be described.

According to this embodiment, the first and/or the second orientations of the liquid crystal molecules adjacent respectively the first and second alignment layers are tuned by photoalignment so that the angle Aa(x,y) between the first and second orientation varies spatially from helix to helix along at least one direction of the cell. The portion of the cell in which the angle varies spatially is of constant thickness.

Preferably, one of the first or second orientations is tuned to remain constant along said direction of the cell, while the other of the first or second orientation is tuned to vary spatially along said direction, to make the angle vary accordingly.

To do so, one alignment layer, which is preferably made or polyimide, is rubbed according to one direction so that the orientation of the liquid crystal molecules adjacent said layer is constant, and in particular the molecules extend along this direction. This is first alignment layer shown in figure 2b.

The other alignment layer is subjected to a photoalignment process to make the orientation of the molecules adjacent this layer vary spatially along said direction, as shown in the second alignment layer represented in figure 2b.

This alignment layer is preferably made of a photopolymerizable polymeric layer, which is exposed to a polarization interference pattern defining the spatial change of the orientation of the molecules.

As the helix formation of the liquid crystal molecules is self-organized, the helices are formed in order to comply with the first and second orientations, and with the constant thickness of the cell. Therefore as the angle Aa(x,y) varies spatially within the cell, the pitch p(x,y) is adapted accordingly.

More specifically, in that case the thickness d(x,y) of the cell is constant and noted do, and do is preferably chosen according to do=k po_, with k an integer corresponding to a number of pitches within the thickness do, and po a nominal pitch λ

under normal incidence defined as p₀ =^■

n average

A twist of a helix can be defined as:

d₀

twist (x, y) = ^x. y) + 360.

p(x, y)

where ^χ, γ) in the angle between the first orientation of the molecules and a reference direction.

Then the angle between the second orientation of the molecules and the same reference direction ct₂ (x, y) can be defined as: d_Q

¾ (^χ) Υ) ⁼ twist(x, y)— 360.—

Po

And therefore:

Aa(x, y) = ct₂ (x, y)— a x, y) = 360

Preferably, as the overall variation of the pitch within the cell must not exceed one pitch p₀ under normal incidence, the overall variation of the angle Δα over the cell must not exceed 360°.

With reference to figure 2c, according to a preferred embodiment, an observation point O is defined relative to the liquid crystal cell, at a distance thereof. For instance, if the liquid crystal cell is used as an optical lens for eyeglasses, the observation point O is selected at the position of the eye of the wearer, in particular at the position of what is known as the eye rotation center point. According to another example, if the liquid crystal cell is used as part of an optical instrument, the observation point O can be selected at the position of the sensor of the instrument. In both examples the observation point O can be considered to be positioned on or close to an optical axis of the lens.

According to this embodiment, the pitch p(x,y) of the helices of the liquid crystal molecules varies within the cell such that the same predetermined wavelength λ is reflected by the liquid crystal cell - and thus rejected by the optical filter - for any ray of light incoming on the surface of the cell and directed towards the observation point O. As an schematic example, three rays of light are shown in figure 2c, impinging on different points of the cell with corresponding angle Θ1, Θ2 and Θ3, and the same wavelength λ is reflected for all three rays of light.

Said otherwise, for the example of an optical lens, or ophthalmic lens, for an eyeglass, the pitch p(x,y) of the helices varies within the cell such that the same wavelength is reflected whatever the gaze direction of the eye. This is possible because the observation point for each gaze direction can be known beforehand. Accordingly the pitch for each point of the optical lens can be calculated or determined beforehand so as to enable to manufacture the optical lens correspondingly. As described below, the pitch for each point may be determined or calculated based on determining in a first place an apparent pitch, or directly knowing the angle of gaze direction or of the incident light ray to consider on each point. Further in the case of eyeglasses, if the eye rotation center is used as observation point, its position is fixed with regard to the eyeglass whatever the gaze direction.

For each point of coordinates (x,y) of the cell corresponds an angle of incidence 9(x,y), measured with reference to the normal to the cell, of a ray of light incoming on the cell at the point (x,y) and directed towards the observation point.

Thus, a pitch of the helices p(x,y) associated to a point (x,y) is also associated to the corresponding angle 9(x,y), such that one can write p(9)=p(x,y).

In order to ensure that the same wavelength λ can be reflected for any angle of incidence Θ directed towards the observation point, the pitch ρ(θ) must satisfy the following equation:

This equation is the application of Bragg' s reflection law where the element arcsin (sin9/n_a) represents the angle of the ray of light refracted inside the liquid crystal cell, and the cosinus of the angle expresses the fact that the apparent pitch as seen by an incoming ray of light is not the same as the actual pitch of the helix as seen from normal incidence and generally organized as rotating along the normal incidence.

Therefore for every point (x,y) of the cell is associated a defined pitch p(x,y)=p(0).

According to the embodiment of the invention introduced with reference to figure 2a, in which the cell exhibits a varying thickness and a constant angle between the first and the second orientation, the thickness of the cell d(x,y) is preferably such as

d (x, y) = k. p (x, y) Where k is an integer and corresponds to a number of pitch values of the helices within the thickness. The overall twist of each helix of the liquid crystal cell is therefore 360k.

In order to achieve a varying thickness of the cell, one alignment layer may be a flat plate and the other alignment layer may be curved and exhibit a specific shape such that the distance between the alignment layers varies according to the required thickness.

A simulation has been performed of the mapping in thickness of a wedge cell according to the incidence angle Θ. For this particular example, the number of pitch values (k) has been set to 20. Moreover, the determined wavelength λ to be reflected is set equal to 485 nm, and the average refractive index of the liquid crystal mixture is equal to 1.54.

In the table below, Bmedium corresponds to the angle, measured relative to the normal of the alignment layer, of a ray of light refracted by the alignment layer, i.e. 9medium= arcsin (sin9/n_a).

Θ (°) Θ medium (°) pitch p(6)(nm) thickness d(9) (μητι)

0 0.00 314.48 6.29

5 3.24 314.98 6.3

10 6.48 316.50 6.33

15 9.73 319.06 6.68

20 12.97 322.71 6.45

25 16.21 327.50 6.55

30 19.45 333.51 6.67

35 22.69 340.87 6.82

40 25.94 349.70 6.99 In figure 3a are shown the transmittance curves as a function of the wavelength for a wedge cell such as defined in Table 1 for different incident angles in the air, until 40°, for unpolarized light.

One can notice that the wavelengths that are rejected by the optical filters are independent of the incidence angles.

Accordingly, as described below, for manufacturing an optical lens having the reflected wavelength being constant when perceived from a fixed observation point, it is sufficient to use the data from the table 1 and manufacture a cell so that the respective thickness are used in points which correspond to the respective angle as calculated from axis corresponding to an axis normal to the surface and going through the observation point.

As an alternative, the embodiment introduced hereinabove with reference to figure 2b, in which the angle Aa(x,y) varies spatially within the cell can also be implemented with the definition of the pitch which allows reflecting a wavelength which is independent of the angle of incidence: p {x, y) = ρ {θ)

( . (sindw

n_a x cos ^arcsin—— J J

A simulation has been performed of the mapping in angular orientation of the liquid crystal molecules of a cell according to the incidence angle Θ. The values k, λ and n_a are the same as in the preceding example.

Θ Θ medium (°) pitch p(6)(nm) Δα(θ) (°)

(°)

0 0.00 314.48 0

5 3.24 314.98 11.5

10 6.48 316.50 46

15 9.73 319.06 103.5 20 12.97 322.71 184

25 16.21 327.50 286

28 18.16 330.95 359

As the angle Aa(x, y) must remain under 360°, in this example the angular compensation of the reflected wavelength can occur until an incidence of 28°.

Figure 3b shows the transmittance curves of an optical filter comprising a cell such as defined in Table 2, as a function of the wavelength for different incidence angles in the air for unpolarized light.

One can see that the wavelengths rejected by the filter are independent of the incidence angle until 28° incidence.

Accordingly, as described below, for manufacturing an optical lens having the reflected wavelength being constant when perceived from a fixed observation point, it is sufficient to use the data from the table 2 and manufacture a cell so that the respective angle Aa(x, y) between the first orientation and the second orientation for each helix are used in points which correspond to the respective angle as calculated from axis corresponding to an axis normal to the surface and going through the observation point.

Manufacturing process

With reference to figure 4, the main steps of a process for designing and manufacturing an optical filter according to alternative exemplary embodiments are shown.

The process comprises a step 500 of encasing a liquid crystal mixture comprising cholesteric liquid crystals between two alignment layers to form a liquid crystal cell, wherein the liquid crystal cell has been configured prior to this step such that the pitch of the helices of the liquid crystal molecules in the cell is variable, as explained hereinbefore.

The process therefore comprises a step 100 of computing the pitch p(x,y) of the helices to be obtained. In an embodiment, the pitch p(x,y) is computed as:

Where an observation angle is defined relative to the cell, and Θ is the incidence of a ray of light incident on a point (x,y) of the cell and directed towards the observation point.

According to a first embodiment, the pitch of the helices is variable because the thickness of the cell is variable and the angle between the first and second orientation of the molecules adjacent the first and second alignment layers is constant.

Therefore the process may comprise a step 200 of computing a thickness d(x,y) of each point (x,y) of the cell corresponding to the distance between the alignment layers, said thickness varying spatially within the cell, based on the computed pitch p(x,y).

The process then comprises a step 300 of rubbing two alignment layers 20, 21 according to a constant direction to ensure that the angle Aa(x, y) will remain constant over the cell.

Then the process comprises a step 400 of positioning the alignment layers 20,

21 at a distance corresponding to the thickness d(x,y) computed in step 100, prior to implementing the step 500.

According to another embodiment, the pitch of the helices is variable because the angle Aa(x, y) between the first and second orientation of the molecules adjacent the first and second alignment layers varies spatially while the thickness of the cell is constant.

In that case the process may comprise a step 200' of computing the angle Aa(x, y) of each point (x,y) of the cell, between the first and second orientations of the liquid crystal molecules, based on the computed pitch p(x,y), and setting a constant thickness do of the cell.

The process then comprises a step 300' of processing alignment layers 20, 21 to obtain the computed angle. For instance, a layer may be rubbed to set a constant first orientation of the molecules and the other layer may be treated with photoalignment, i.e. exposed to a polarization interference pattern, to set a varying second orientation of the molecules.

Then the process comprises the steps 400 of positioning the layers at a distance corresponding to the thickness d₀ of the cell and 500 of encasing the liquid crystal mixture between the alignment layers.

Claims

1. Optical rejection filter (2) comprising a liquid crystal cell, the liquid crystal cell comprising:

- a liquid crystal mixture (22) comprising cholesteric liquid crystals,

- a first and a second alignment layers (20, 21) encasing the liquid crystal mixture (22),

the cholesteric liquid crystals defining a plurality of helices extending between the alignment layers, each helix having a pitch,

the liquid crystal mixture (22) thereby being capable of reflecting rays of light of a determined wavelength incoming on a surface of the cell defined by one of the first and second alignment layers by Bragg reflection, the wavelength of the reflected rays of light depending on the angle of incidence of the rays of light on the surface of the cell and on the pitch of the helices,

2. Optical rejection filter (2) according to claim 1, wherein the overall variation of

λ

the pitch p(x,y) within the cell is smaller than— , wherein n_a is the mean value of an

n a

ordinary refractive index n₀ and an extraordinary refractive index of n_e of the cholesteric liquid crystal.

3. The optical rejection filter (2) according to claim 1 or 2, wherein the first and second alignment layers (20, 21) define a first and a second orientations of the cholesteric liquid crystal molecules adjacent with the first and the second alignment layers respectively, and the distance d(x,y) between the first and second alignment layers defines a thickness of the liquid crystal cell,

and wherein :

- the thickness d(x,y) of the liquid crystal cell varies over at least one portion of the liquid crystal cell as a function of the pitch according to d(x,y) = k p(x,y), wherein k is a positive integer, and

4. The optical rejection filter according to claim 1 or 2, wherein the first and second alignment layers (20, 21) define a first and a second orientations of the cholesteric liquid crystal molecules adjacent with the first and the second alignment layers respectively, and the distance d(x,y) between first and second alignment layers defines a thickness of the liquid crystal cell,

and wherein:

the first and the second alignment layers (20, 21) are parallel to each other and the thickness do of the liquid crystal cell is defined according to do=k po_, λ

with po being a nominal pitch defined as p₀ =— , n_a being the mean value of the ordinary refractive index n₀ and the extraordinary refractive index n_e of the cholesteric liquid crystal, and with k being a positive integer corresponding to a number of nominal pitches comprised within the thickness of the cell, and

for each point (x,y) of the cell is defined an angle Aa(x,y) between the first and the second orientations of the cholesteric liquid crystals molecules located at this point of the cell, wherein the angle varies according to the following equation :

A (x, y) = 360xd₀ 1 p(x, y) - 360 .

5. Optical rejection filter according to claim 4, wherein at least one of the first and second alignment layers (20, 21) is formed of a photopolymerized polymeric layer defining a corresponding first or second orientation of the liquid crystals adjacent said layer, and wherein the angle variation between the first and second orientations is obtained by varying one of the first or second orientation while the other orientation remains constant.

6. Optical rejection filter according to any of the preceding claims, wherein an observation point (O) is defined with reference to the cell, at a distance thereof, and wherein the cell is configured such that any ray of light of a predetermined wavelength λ incoming on the surface of the cell and directed towards the observation point is reflected by the liquid crystal mixture.

7. The optical rejection filter according to claim 6, wherein each point (x,y) of the cell is associated to a corresponding incidence angle (Θ) of a ray of light incident on this point and directed towards the observation point,

and wherein the pitch p of the helices varies according to the following equation : ρ(θ) =

n_a x cos(arcsin(sin(#) / n_a ))

8. Tunable rejection filter (1) comprising an optical rejection filter (2) according to any one of the preceding claims, and

wherein the tunable rejection filter further comprises two electrodes (3) configured to be connected to a voltage source (4), said voltage source being configured to control the orientation of the cholesteric liquid crystals between the first and second alignment layers.

9. Optical device comprising the tunable rejection filter of claim 8, mounted in series with an optical element or affixed to said optical element so that a light ray goes in series through the tunable rejection filter and the optical lens for the wavelength which are not rejected by the tunable rejection filter.

10. Optical device according to claim 9, the optical device being an ophthalmic lens.

11. Method for producing an optical rejection filter according to any of claims 1 to 7 or a tunable rejection filter according to claim 8, wherein the method comprises encasing (500) a liquid crystal mixture (22) comprising cholesteric liquid crystals between two alignment layers (20, 21) to form a liquid crystal cell, wherein the cholesteric liquid crystals form a plurality of helices, each having a pitch p, wherein the liquid crystal cell is configured such that the pitch of the helices is variable along at least one direction comprised within a plane formed by one of the first and second alignment layers, such that the cell comprises at least two points capable of reflecting the same predetermined wavelength of two rays of light incoming on the respective points of a surface of the cell with different angles of incidence.

12. Method according to claim 11, wherein the first and second alignment layers (20, 21) define a first and a second orientations of the cholesteric liquid crystal molecules adjacent with the first and the second alignment layers respectively, and the method further comprises:

rubbing (300) the first and second alignment layers such that the angle between the first and second orientations of the liquid crystal molecules is constant over at least a portion of the cell,

- calculating a thickness (200) over the at least one portion of the liquid crystal cell as a function of a position in (x,y) coordinates according to : d(x,y) = k p(x,y), wherein k is a positive integer, and

positioning the first and second alignment layers (400) relative to one another such that the distance between the layers corresponds to the calculated thickness.

13. Method according to claim 11, wherein the first and second alignment layers (20, 21) define a first and a second orientations of the cholesteric liquid crystal molecules adjacent with the first and the second alignment layers respectively, and the method further comprises:

calculating a nominal pitch po of the liquid crystal cell helices according to: λ

Po =— > wherein λ is the rejection wavelength and n_a the mean value of the ordinary refractive index n₀ and the extraordinary refractive index n_e of the cholesteric liquid crystals,

determining (200') a thickness do of the liquid crystal cell according to: do=k po, wherein k is a positive integer,

positioning (400) the first and second alignment layers relative to each other such that the distance between the layers corresponds to the calculated thickness,

defining (200') for each point (x,y) of the cell an angle Aa(x,y) between the first and the second orientations of the cholesteric liquid crystals located adjacent the point of the cell, and

processing (300') at least one of the alignment layers such that the angle Aa(x,y) varies over at least a portion of the cell according to:

A (x, y) = 360x d₀ 1 p(x, y) - 360xk .

14. Method according to claim 13, wherein the processing step (300') is performed by rubbing one alignment layer to set one of the first or second orientation of the molecules to be constant, by exposing the other alignment layer to a polarization interference pattern, to set the other of the first or second orientation of the molecules to vary spatially.

15. Method according to any of claims 11 to 13, further comprising the steps of: defining an observation point (O) relative to the cell, at a distance thereof, associating to each point of coordinates (x,y) of the cell an incidence angle Θ of a ray of light incoming on said point (x,y) and directed towards the observation point,

for each incidence angle Θ, calculating a pitch value according to λ

Ρ(θ) = ,

n_a x cos(arcsin(sin(#) / n_a ))

processing the cell such that the pitch of the helices corresponds to the calculated pitch value.