Patent Publication Number: US-2023152620-A1

Title: Systems With Adjustable Window Transmission and Haze

Description:
This application is a continuation of U.S. patent application Ser. No. 17/466,269, filed Sep. 3, 2021, which claims the benefit of provisional patent application No. 63/075,730, filed Sep. 8, 2020, which are hereby incorporated by reference herein in their entireties. 
    
    
     FIELD 
     This relates generally to structures that pass light, and, more particularly, to windows. 
     BACKGROUND 
     Windows such as vehicle windows sometimes include glass layers. To enhance privacy or block sunlight, windows may sometimes be tinted. 
     SUMMARY 
     A system such as a vehicle, building, or electronic device system may have a support structure with one or more windows. The support structure and window may separate an interior region within the system from a surrounding exterior region. Control circuitry and input-output devices may be mounted within the support structure. 
     During operation, the control circuitry may use the input-output circuitry to receive input. The input may be, for example, user input such as input from a vehicle occupant. 
     Based on the input, the control circuitry may adjust an alternating-current (AC) drive signal (e.g., an AC voltage) or other control signal for an adjustable layer in the window. The adjustments to the drive signal may be used to adjust the amount of light transmission exhibited by the adjustable layer and the amount of haze exhibited by the adjustable layer. The adjustable layer may be placed two or more different modes of operation such as a dark hazy mode, a dark non-hazy mode, a clear hazy mode, and a clear non-hazy mode. 
     The adjustable layer may be formed from a layer of polymer matrix material sandwiched between first and second transparent conductive electrode layers. The control circuitry can adjust the control signal applied to the adjustable layer by the electrodes to adjust the operating mode of the adjustable layer. 
     The polymer matrix material may include embedded guest-host liquid crystal cells. The guest-host liquid crystal cells may include a first liquid crystal material and dichroic dye. The polymer matrix material may also include embedded liquid crystal cells having a second liquid crystal material without dichroic dye. The first and second liquid crystal materials may have different properties (e.g., different values of dielectric anisotropy divided by elastic constant) so that the guest-host liquid crystal cells and the liquid crystal cells switch at different threshold voltage levels. The guest-host liquid crystal cells and liquid crystal cells may also be provided with properties that vary differently as a function of frequency. By adjusting the amplitude and/or frequency of the AC voltage signal applied to the polymer layer by the electrode layers, the operating mode and optical properties of the adjustable layer can be adjusted (e.g., the haze and transmission of the adjustable layer can be independently adjusted even when the guest-host liquid crystal cells and the liquid crystal cells are interspersed among each other within the same polymer matrix). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an illustrative system in accordance with an embodiment. 
         FIGS.  2  and  3    are cross-sectional side views of an illustrative guest-host liquid crystal cell in an adjustable window layer in accordance with an embodiment. 
         FIGS.  4  and  5    are cross-sectional side view of an illustrative liquid crystal cell without dichroic dye in an adjustable window layer in accordance with an embodiment. 
         FIG.  6    is a cross-sectional side view of an illustrative adjustable window layer that has guest-host liquid crystal cells and interspersed dye-free liquid crystal cells in a polymer matrix in accordance with an embodiment. 
         FIGS.  7 ,  8 ,  9     10 ,  11 ,  12 ,  13 ,  14 ,  15 ,  16 , and  17  are graphs of illustrative operations associated with illustrative adjustable window layers in accordance with embodiments. 
         FIG.  18    is a cross-sectional side view of an adjustable window layer formed from a pair of joined sublayers in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A system may have windows. The windows may include electrically adjustable layers. An electrically adjustable layer may have adjustable optical properties. An adjustable layer may, as an example, have guest-host liquid crystal cells and liquid crystal cells (i.e., non-guest-host cells) embedded in a polymer matrix that may be used to provide the adjustable layer with adjustable amounts of haze and light transmission. 
     Systems that that may be provided with windows and other transparent structures having electrically adjustable layers may include buildings, vehicles, electronic devices systems (e.g., head-mounted devices such as glasses with adjustable lenses), and other suitable systems. Illustrative configurations in which systems such as vehicles are provided with electrically adjustable window layers may sometimes be described herein as an example. This is merely illustrative. Adjustable layers may be formed in any suitable systems. 
     An electrically adjustable layer may be formed using a polymer layer (sometimes referred to as a matrix, polymer matrix, or polymer matrix layer) in which numerous cells of guest-host liquid crystal material and numerous cells of liquid crystal material have been embedded. Each guest-host cell may include liquid crystal material (the “host”) and dichroic dye (e.g., anisotropic dye molecules that serve as the “guest”). The dye molecules align with liquid crystals in the liquid crystal material so the orientation of the dye molecules can be controlled by controlling the orientation of the liquid crystals. Each liquid crystal cell may include liquid crystal material without dichroic dye. 
     Transparent electrodes may be used to apply a desired electric field through the polymer layer. For example, the control circuitry may adjust a voltage applied across the transparent electrodes to adjust the electric field in the polymer layer, thereby adjusting the operating mode of the adjustable layer. Control of the signal applied to the electrodes and therefore the signal applied to the polymer layer allows the alignment state of the liquid crystals in the guest-host cells to be adjusted and allows the alignment state of the liquid crystals in the liquid crystal cells to be adjusted. Various operating modes characterized by different amounts of light transmission and haze can be achieved. For example, an adjustable layer that includes both guest-host liquid crystal cells and non-guest-host liquid crystal cells can be placed in a dark hazy mode that offers a high level of privacy or a clear low-haze mode that offers good through-window visibility. These operating modes and/or other operating modes such as a low-haze low-light-transmittance mode and a high-haze high-light-transmittance mode, may be achieved by adjusting the drive signal amplitude and/or frequency (e.g., based on user input such as vehicle occupant input and/or other input). 
     An illustrative system of the type that may include adjustable windows is shown in  FIG.  1   . As shown in  FIG.  1   , system  10  may have a support structure such as support structure  12  that supports one or more windows such as window  16 . Support structure  12  and window  16  separate interior region  18  from exterior region  14 . The amount of haze and light transmission through window  16  may be adjusted during operation. During at least some operating modes, window  16  may be transparent to allow occupants of system  10  who are located within interior region  18  to view objects located in exterior region  14  through window  16 . 
     Structure  12  may form walls of a building, a vehicle body, an electronic device housing (e.g., a frame for a pair of glasses) or other supporting structures. In arrangements in which structure  12  forms a vehicle body, structure  12  may include a chassis to which wheels, propulsion systems, steering systems, and other vehicle systems are mounted and may include doors, trunk structures, a hood, side body panels, a roof, and/or other body structures. 
     System  10  may include control circuitry  20  and input-output devices  22 . Input-output devices  22  may include sensors (e.g., touch sensors, a microphone, buttons, etc.), audio components, displays, and other components for providing output to an occupant of system  10 , for making measurements of the environment surrounding vehicle  10 , and for gathering input from an occupant of system  10 . Control circuitry  20  may include storage and processing circuitry such as volatile and non-volatile memory, microprocessors, application-specific integrated circuits, digital signal processors, microcontroller, and other circuitry for controlling the operation of system  10 . In scenarios in which system  10  is a vehicle, control circuitry  20  may control the components of the vehicle based on user input and other input from input-output device  22  (e.g., to adjust the vehicle&#39;s steering, brakes, throttle, and other controls associated with driving the vehicle and/or to adjust optical properties for window  16  and/or other settings associated with operations other than driving the vehicle). If desired, system  10  may be an autonomously driven vehicle. Window settings such as window transparency and haze may be adjusted using voice comments, button input, touch screen input on a control panel or a touch sensitive window area, and/or other input (e.g., vehicle occupant input). 
     As shown in  FIG.  1   , window  16  may include multiple window layers  16 L. Window layers  16 L may include layers of transparent material such as transparent layers of glass, transparent layers of polymer, transparent semiconductor layers (e.g., transparent indium tin oxide layers or other transparent conductive layers), transparent polymer layers, and/or other transparent layers. These layers may include rigid and/or flexible materials. In some configurations, layers  16 L and window  16  are flat. In other configurations, some or all of window  16  is curved. As an example, illustrative window  16 X of  FIG.  1    may have a curved cross-sectional profile and may optionally exhibit areas with compound curvature (e.g., areas where window  16 X has non-developable surfaces). Illustrative arrangements in which window  16  has a planar shape may sometimes be described herein as an example. 
     Window layers  16 L may include one or more adjustable layers. Layers  16 L may also include one or more structural layers. As an example, window layers  16 L may include multiple structural glass layers. In some configurations, these layers may include an inner transparent structural layer (sometimes referred to as an inner glass layer) and an outer transparent structural layer (sometimes referred to as an outer glass layer). Optional additional layers may be included. The inner and outer layers of the window and/or other layers  16 L may include adjacent layers that are separated by an air gap and/or may include adjacent layers that are spaced apart by a gap that is filled with polymer, liquid, other dielectric, layers forming an adjustable light transmission device, etc. As an example, layers  16 L may include an outer window layer, an inner window layer, and an adjustable layer sandwiched between the outer layer without air gaps. 
     Layers  16 L (e.g., inner and/or outer structural glass layers surrounding an adjustable light transmission layer) may include single-layer glass layers (e.g., single layers of tempered glass) or, in some configurations, may include multi-layer structures formed, for example, from first and second glass layers that are laminated together. A laminated glass layer may have a polymer such as polyvinyl butyral (PVB) or a layer of another polymer that joins first and second glass layers to form a sheet of laminated glass. Multi-layer glass structures (laminated glass layers formed from two or more laminated glass layers with interposed PVB) and single-layer glass layers may include optional tinting (e.g., dye, pigment, etc.). Polymer layers in laminated glass layers (e.g., PVB layers) may also optionally be tinted. 
     Adjustable light transmission and haze may be provided using electrically adjustable guest-host liquid crystal material and non-guest-host liquid crystal material (sometimes referred to as liquid crystal material). To help avoid undesirable uniformity issues such as gravity-induced mura as well as undesired pressure sensitivity, the guest-host liquid crystal material and non-guest-host liquid crystal material may be formed in nanosized cells embedded within a polymer matrix layer. 
       FIGS.  2  and  3    show illustrative guest-host liquid-crystal cells  30 GH.  FIGS.  4  and  5    show illustrative liquid crystal cells  30 LC. Cells  30 GH and cell  30 LC may be spheres or droplets of other shapes that are formed within a polymer matrix. The size (e.g., the diameter) of cells  30 GH and  30 LC may be the same or may differ. In some illustrative configurations, cells  30 GH and cells  30 LC may have diameters of less than 200 nm, less than 150 nm, or other small size to help reduce light scattering. In some configurations, larger cells may be used. 
     As shown in  FIGS.  2  and  3   , cells  30 GH include a first liquid crystal material (liquid crystals) LC 1  and dichroic dye  34  (e.g., dye that exhibits anisotropic light absorption). There is typically more liquid crystal material in cells  30 GH than dye material (e.g., the dye may make up about 2-3% of cells  30 GH). The orientation of liquid crystals LC 1  can be adjusted by adjusting the electric field applied to liquid crystals LC 1 . The orientation of dye molecules  34  tracks that of liquid crystals LC 1 . The light transmission exhibited by cells  30 GH (and therefore the transmission of an adjustable layer formed from cells  30 GH) is high (e.g., at least 60% at least 75%, at least 85%, at least 90%, at least 95%, at least 99%, 100%, etc.) when a control signal (e.g., an AC drive voltage VON) is applied so that liquid crystals LC 1  and dye molecules  34  are aligned in a first state (e.g., parallel to the direction of incoming light rays such as illustrative light ray  36  in the example of  FIG.  2   ) and this light transmission is low (e.g., less than 50%, less than 25%, less than 15%, less than 10%, less than 5%, 0%, etc.) when the control signal (e.g., an AC drive voltage signal VOFF of 0V, a value less than 1V, or other low value) is applied so that liquid crystals LC 1  and dye molecules  34  are aligned in a second state (e.g., when liquid crystals LC 1  and dye molecules  34  are oriented randomly and are not aligned parallel to light ray  36 , as shown in the example of  FIG.  3   ). The low transmission state of cells  30 GH may be used to help block exterior sunlight and to help provide vehicle occupants with privacy. 
     In addition to incorporating cells  30 GH into a polymer matrix layer, the polymer matrix layer may be provided with liquid crystal cells  30 LC of  FIGS.  4  and  5    (e.g., nematic liquid crystal). As shown in  FIGS.  4  and  5   , cells  30 LC contain second liquid crystals LC 2 , but do not contain dichroic dye. Accordingly, the light transmission of cells  30 LC does not change between the on state (V=VON of  FIG.  4   ) and off state (V=VOFF of  FIG.  5   ). Nevertheless, the change in orientation of liquid crystals LC 2  changes the index of refraction n of cells  30 LC. In the example of  FIG.  4   , liquid crystals LC 2  are aligned along the Z axis and cells  30 LC are exhibiting a first refractive index value no. In the example of  FIG.  5   , drive signal V has been reduced in amplitude (V=VOFF, which may be, for example, 0V, less than 1V, etc.), so liquid crystals LC 2  assume a random orientation and the refractive index n of cell  30 LC becomes n_ave, which is different than no. The haze contribution of cells  30 LC can be varied in this way. When the refractive index of cells  30 LC matches that of the surrounding polymer matrix (e.g., within 0.1, within 0.05, or other suitable amount of index matching), light rays passing through the polymer matrix tend not to be scattered (e.g., haze has a low value such as a value less than 1%). When the refractive index of cells  30 LC differs from that of the surrounding polymer (e.g., by a mismatch of at least 0.1, at least 0.2, at least 0.3, or other index mismatch value), light passing through the polymer matrix will scatter from cells  30 LC and will exhibit haze (e.g., at haze value of at least 10%, at least 25%, at least 50%, at least 80%, or other suitable haze value). 
     Any suitable drive signal may be used in adjusting cells  30 GH and  30 LC. In an illustrative configuration, AC voltage drive signals are used (e.g., square wave voltages or other AC signals). The frequency of the AC drive signals may be at least 1 Hz, at least 10 Hz, at least 40 Hz, less than 480 Hz, less than 100 Hz, 10-100 Hz, or other suitable frequency. The peak-to-peak voltage of the drive signal (e.g., the voltage applied from one surface of the adjustable transmission layer to the other by a pair of transparent electrodes) may be at least 10 V, at least 20 V, less than 60 V, less than 40 V, 10-60 V, etc. (e.g., when V=VON), may be 0V, less than 1V, or other low value (e.g., when V=VOFF), and/or may have intermediate amplitudes. The frequency of the drive signal may also be varied, if desired. The drive signal can be adjusted by control circuitry  20  based on user input. 
     An adjustable light transmission layer may be formed by creating a layer of polymer matrix material that includes embedded guest-host liquid crystal cells  30 GH and liquid crystal cells  30 LC sandwiched between a pair of opposing conductive electrodes. Optional substrate layers may be used to help support the polymer matrix layer (e.g., during manufacturing). In an illustrative arrangement, guest-host liquid crystal material and liquid crystal material without dichroic dye may be provided with surfactant (e.g., surfactant that helps form a shell that maintains the spherical shape of cells  30 GH and  30 LC within the polymer matrix). These liquid crystal materials may be dispersed into a liquid polymer matrix solution (liquid polymer precursor material for the polymer matrix). High pressure and/or vibration then may be used to break the guest-host liquid crystal material and liquid crystal material into nanodroplets forming cells  30 GH and  30 LC. After cells  30 GH and  30 LC have been embedded throughout the matrix in this way, the liquid polymer of the matrix may be cured (e.g., by application of light such as ultraviolet light and/or high temperature), followed by baking to harden the matrix layer. 
     If desired, a pair of substrates each of which has been coated with a polymer matrix with embedded cells may be sandwiched together to form an adjustable light transmission layer. Configurations in which cells  30 GH and formed in a first sublayer and in which cells  30 LC are formed in a second sublayer and in which the first and second sublayers are subsequently joined may also be used. 
     An illustrative adjustable layer with interspersed cells  30 GH and  30 LC in polymer matrix layer  50  is shown in  FIG.  6   . As shown in the cross-sectional side view of adjustable layer  16 L of  FIG.  6   , polymer matrix layer  50  may be sandwiched between a pair of transparent conductive electrodes  40 . During operation, a drive signal (drive voltage V) may be applied across a pair of terminals that are coupled respectively to electrodes  40 . By applying the drive voltage across electrodes  40 , the electric field within layer  50  and the electric field applied to cells  30 GH and  30 LC can be controlled. 
     Electrodes  40  may, if desired, be supported by substrate layers  38 . Substrate layers  38  may be formed from rigid or flexible polymer films (e.g., layers of polyethylene terephthalate, cyclic olefin polymer, cellulose triacetate, polycarbonate, or other polymer materials). These materials and/or other polymers may also be used in forming polymer matrix  50 . The thickness of each substrate layer  38  may be, as an example, at least 1 micron, at least 10 microns, at least 100 microns, less than 3 mm, less than 500 microns, less than 150 microns, less than 30 microns, or other suitable thickness. Each electrode  40  may be formed from a transparent conductive layer such as a layer of indium tin oxide, a transparent conductive polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT), or other transparent conductive layer. The thickness of each electrode  40  may be, for example, at least 0.1 micron, at least 1 micron, at least 10 microns, less than 100 microns, less than 20 microns, less than 2 microns, or other suitable thickness. 
     Polymer matrix  50  and embedded guest-host liquid crystal cells  30 GH and embedded liquid crystal cells  30 LC, which are interspersed with cells  30 GH in a random fashion in the configuration of  FIG.  6   , may be formed by depositing liquid polymer precursor material for matrix  50  that contains guest-host liquid crystal material and liquid crystal material without dichroic dye onto electrodes  40  followed by application of pressure and/or vibrations to form cells  30  GH and  30 LC. The thickness of the layer of matrix  50  that is formed between electrodes  40  may be 4-20 microns (e.g., about 12 microns), at least 1 micron, at least 2 microns, at least 4 microns, at least 8 microns, at least 10 microns, less than 40 microns, less than 30 microns, less than 20 microns less than 15 microns, less than 9 microns, or other suitable thickness). 
     Layer  16 L may be formed from two sublayers that are joined together (each with its polymer matrix material facing the other). The sublayers may be joined using heat and/or pressure and/or may be joined using an interposed layer of adhesive (as examples). Following formation of window  16 , window  16  may be installed in a window opening in support structure  12  or other portion of system  10 . 
     The operating modes exhibited by adjustable layer  16 L depend on the properties of cells  30 GH and  30 LC. For example, characteristics of layer  16 L such as the sign (positive/negative), amplitude, and/or frequency dependence of the dielectric anisotropy of the liquid crystal materials used in forming liquid crystals LC 1  and LC 2 , the elastic constants of the liquid crystal materials, the indices of refraction of the liquid crystal materials in their aligned and random states relative to the refractive index of matrix  50 , the cell sizes of cells  30 GH and  30 LC, and other factors such as the placement of cells  30 GH, cells  30 LC, and electrodes  40  within layer  16 L, can be selected to tune the behavior of layer  16 L. 
     For example, the relative values of dielectric anisotropy divided by elastic constant of liquid crystals LC 1  (in cells  30 GH) and liquid crystals LC 2  (in cells  30 LC) can be selected to configure the switching thresholds associated with changes in light absorption and haze for layer  16 L as desired. The larger the dielectric anisotropy divided by elastic constant (Δϵ/K) of a given liquid crystal, the smaller the electric field needed to make that given liquid crystal respond to the electric field. If, for example, Δϵ/K for LC 1  is less than Δϵ/K for LC 2 , the electric field switching threshold for LC 2  (e.g., switching voltage threshold VT 2  across the adjustable layer) will be less than the electric field switching threshold (switching voltage threshold VT 1  across the adjustable layer) for LC 1 . 
       FIGS.  7 ,  8 ,  9     10 ,  11 ,  12 ,  13 ,  14 ,  15 ,  16 , and  17  are graphs of illustrative operations associated with illustrative adjustable window layers  16 L in various different configurations. In these examples, the polymer refractive index (polymer n) represents the index of refraction of polymer matrix layer  50 , Δϵ/K represents the values of dielectric anisotropy divided by elastic constant of the liquid crystal material in cells  30 GH (LC 1 ) and cells  30 LC (LC 2 ). The haze of the adjustable layer (layer  16 L in window  16 ) is represented by solid lines and the transmittance of the adjustable layer is represented by dashed lines. The voltages at which the liquid crystal materials switch (sometimes referred to as threshold voltages) are determined by values of dielectric anisotropy Δϵ divided by elastic constant K for each material. 
     In the example of  FIG.  7   , the Δϵ/K value of LC 1  is less than that of LC 2 , so cells  30 LC switch state at threshold voltage Vthreshold1 (sometimes referred to as VT 1 ) and cells  30 GH switch state at larger threshold voltage Vthreshold2 (sometimes referred to as VT 2 ). The drive voltage V can be set by control circuitry  20  to a value between 0V and VT 1  (operating mode I), a value between VT 1  and VT 2  (operating mode II), or a value above VT 2  (operating mode III). Polymer matrix layer  50  has an index n equal to index no of LC 2 , so cells  30 LC will not scatter light and haze will be low when V is greater than VT 1  (e.g., when LC 2  has become aligned as shown in  FIG.  4   ). Haze will be high when V is less than VT 1  and LC 2  is randomly oriented as shown in  FIG.  5   . Cells  30 GH are transparent when LC 1  is aligned as shown in  FIG.  2    (V&gt;VT 2 ) and absorb light to make layer  16 L dark when LC 1  are not aligned and are randomly oriented as shown in  FIG.  3    (e.g., when V&lt;VT 2 ). As a result, the adjustable layer of  FIG.  1    can be operated in three different modes: mode I (when 0V&lt;V&lt;VT 1 ), mode II (when VT 1 &lt;V&lt;VT 2 ), and mode III (VT 2 &lt;V). In mode I, light absorption is high (layer  16 L is dark) and haze is high (layer  16 L is hazy). The resulting dark and hazy appearance of layer  16 L may provide enhanced privacy. The high absorption of layer  16 L in mode I of  FIG.  7    provides a degree of privacy by blocking most (if not all) light. Additional privacy is provided by high haze. During mode II, the adjustable layer exhibits low haze and a dark appearance. During mode III, the adjustable layer is clear and exhibits low haze. The clear low-haze state of the adjustable layer in mode III allows a vehicle occupant to clearly view the exterior environment surrounding system  10  through window  16 . 
     In the example of  FIG.  8   , the index of matrix layer  50  matches the n_ave index value of LC 2  and the switching threshold VT 2  of LC 1  (cells  30 GH) is greater than the switching threshold of LC 2  (cells  30 LC). In mode I, the adjustable layer of  FIG.  8    is dark and exhibits low haze. In mode II, the adjustable layer of  FIG.  8    is dark and hazy. In mode III, the adjustable layer is clear and hazy, providing privacy while allowing light to pass through the adjustable layer. 
     In the example of  FIG.  9   , the refractive index of matrix layer  50  is matched to index no of LC 2 , the switching threshold of cells  30 GH is VT 1 , and the switching threshold of cells  30 LC is VT 2 . The adjustable layer of  FIG.  9    is dark and hazy in mode I, is hazy and clear (not dark) in mode II, and is clear and not hazy in mode III. 
     In the illustrative arrangement of  FIG.  10   , the refractive index of polymer matrix layer  50  is matched to index n_ave of LC 2  and the switching threshold VT 1  of cells  30 GH is lower than the switching threshold VT 2  of cells  30 LC. The adjustable layer of  FIG.  10    is dark with no haze in mode I, is clear with no haze in mode II, and is clear and hazy in mode III. 
     If desired, the diameters of cells  30 GH and  30 LC may be different. For example, cells  30 LC may have larger diameters than cells  30 GH. The larger size of cells  30 LC may help these cells scatter light when the index of cells  30 LC is switched to be different than the index of matrix layer  50  (e.g., when cells  30 LC are switched to place the adjustable layer in a high-haze state). With an illustrative arrangement, cells  30 GH may be less than 200 nm or less than 150 nm in diameter to help reduce light scattering, whereas cells  30 LC may have larger diameters (e.g., at least 210 nm, at least 250 nm, at least 300 nm, etc.) to promote haze formation when haze is switched on. Liquid crystal molecules with larger domain sizes tend to experience lower anchoring energy from surrounding portions of the polymer matrix, lowering their switching threshold voltage. As a result, the increase in size of cells  30 LC relative to cells  30 GH may tend to lower the switching threshold for LC 1 . This effect may be used in combination with differences in Δϵ to enhance the amount of the difference between the switching threshold for LC 1  and LC 2  (e.g., LC 2  may be provided with a larger Δϵ value in addition to placing LC 2  in larger cells).  FIGS.  11  and  12    illustrate possible configurations where cells  30 GH and  30 LC differ in size in this way. 
     In the examples of  FIGS.  11  and  12   , LC 2  has a higher Δϵ value than LC 1  and the diameter of cells  30 LC is larger than cells  30 GH. As a result, cells  30 LC switch at a lower threshold voltage value VT 1  than the switching threshold of cells  30 GH. In the  FIG.  11    example, the adjustable layer is dark and hazy in mode I, dark with no haze in mode II, and clear with no haze in mode III. In the  FIG.  12    example, in which matrix layer  50  has an index matched to n_ave rather than no, the adjustable layer is dark with no haze in mode I, dark and hazy in mode II, and clear and hazy in mode III. 
     In vertical electric field alignment configurations (see, e.g., cells  30 GH and  30 LC of  FIGS.  2  and  4   , respectively), a positive Δϵ value is used for switching, because negative Δϵ material will not exhibit changes in liquid crystal alignment under applied electric fields. In some configurations, the frequency-dependence of Δϵ may cause Δϵ to become negative for some of the liquid crystal material when operating at particular frequencies and may be taken into account in operating the adjustable layer. 
     Consider, as an example, the illustrative configuration of  FIGS.  13 ,  14 , and  15   . As shown in  FIG.  13   , the frequency dependence of Δϵ may be different for LC 1  and LC 2 . In the  FIG.  13    example, both LC 1  and LC 2  exhibit positive dielectric anisotropy at low frequency f_L. As a result, both LC 1  and LC 2  will exhibit switching at or close to the same threshold voltage (threshold voltage VT). At frequencies above crossover frequency f crossover such as at frequency f_H, the Δϵ value of LC 2  becomes negative, whereas the Δϵ value of LC 1  remains positive (and switching threshold VT is substantially unaltered in this example). As a result, both cells  30 GH (LC 1 ) and cells  30 LC (LC 2 ) will switch at VT when operated at f_L, but only cells  30 GH (LC 1 ) will switch at VT when operated at frequency f_H. This behavior is illustrated in  FIGS.  13  and  14   . In this example, the index of matrix layer  50  is equal to index no of LC 2 . When the drive signal V applied to the adjustable layer has frequency f_L ( FIG.  14   ), the state of the adjustable layer may be hazy with low transmittance (see, e.g., dark hazy mode I of  FIG.  14   , where V is less than switching threshold voltage VT) or may be clear (high transmittance) with low haze (see, e.g., mode II of  FIG.  14   , where V is more than VT). When it is desired to place the adjustable layer in a clear and hazy mode, the frequency of drive signal V may be set to f H and the amplitude of V may be set to be larger than VT (see, e.g., mode III of  FIG.  15   ). When V has a frequency f_H and a voltage below VT, the adjustable layer again operates in a dark and hazy mode (mode I′ of  FIG.  15   , which is similar to mode I of  FIG.  14   .). 
     In arrangements in which the liquid crystal materials LC 1  and LC 2  exhibit a frequency dependence of the type shown in  FIG.  13    and also exhibit different positive Δϵ/K values (and/or are formed in cells of different sizes to tailor their switching threshold values as described in connection with  FIGS.  11  and  12   ), four different operating states may be achieved (low transmittance with high haze, low transmittance with low haze, high transmittance with high haze, and high transmittance with low haze). A transmittance of less than 10% or less than 1% may be considered to be a low transmittance, a transmittance of greater than 90% or greater than 99% may be considered to be high transmittance, a haze of less than 10% or less than 1% may be considered to be a low haze, and a haze of more than 90% or more than 99% may be considered to be a high haze (as examples). Consider, as an example, the arrangement of  FIGS.  16  and  17   . In this example, cells  30 LC switch at VT 1  when operated at a low frequency, but do not switch as a function of changes in voltage V when operated at a high frequency (due to a negative Δϵ at the high frequency). Cells  30 GH switch at VT 2  at both low and high frequency drive conditions. As a result, when driven at the low frequency ( FIG.  16   ), the adjustable layer may be operated in mode I (e.g., a dark hazy state, sometimes referred to as a low transmittance high haze state) by selecting V&lt;VT 1 , mode II (a dark non-hazy state, sometimes referred to as a low transmittance low haze state) by selecting VT 1 &lt;V&lt;VT 2 , or mode III (a clear non-hazy state, sometimes referred to as a high transmittance low haze state) by selecting VT 2 &lt;V. When driven at the high frequency ( FIG.  17   ), the adjustable layer may be operated in mode IV (e.g., a clear hazy state, sometimes referred to as a high transmittance high haze state) by selecting VT 2 &lt;V. Operation at high frequency with V&lt;VT 2  is also possible (see, e.g., mode I′, which places the adjustable layer in a hazy and dark condition (low transmittance high haze state), as with mode I of  FIG.  16   ). 
     As shown in  FIG.  18   , adjustable layer  16 L for window  16  may be formed from a pair of sublayers such as first sublayer  16 L-T and second sublayer  16 L-B. These layers may be joined along bond line  52  using heat and/or pressure and/or using an optional layer of adhesive along line  52 . The cells of sublayers  16 L-T and  16 L-B may be the same (e.g., there may be intermixed cells  30 GH and  30 LC in layer  16 L-T and intermixed cells  30 GH and  30 LC in layer  16 L-B) or layer  16 L-T may have only a first type of cell (e.g., cell  30 LC) and layer  16 L-B may have only a second type of cell (e.g., cell  30 GH). If desired, an indium tin oxide electrode, conductive polymer layer electrode, or other conductive electrode (e.g., an optional third electrode in addition to electrodes  40 ) may be formed along line  52  to provide control circuitry  20  with the ability to independently control the voltage across layers  16 L-T and  16 L-B (e.g., to support operation in four different operating states). 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.