Patent Application
20210263346
Neutral density filters are used routinely in still and video image capture applications, particularly outdoors where brightness can be extremely high. High quality passive neutral density filters can perform very well in terms of uniformity in transmission/color over incidence angle. Filter kits are available and filters can be stacked using various fixtures, providing a broad range of discrete density levels. However, the user must make a “best guess” as to the appropriate filter for the environment. The environment can of course have a high degree of temporal variation (e.g., clouds temporarily occluding the sun). The mechanism for changing such a filter is manual, and thus cumbersome, slow, and inconvenient. Moreover, changing filters in a dynamic setting where the camera is not easily accessible (e.g., a drone or action video camera) can be particularly problematic.
Variable neutral density (VND) filters typically include a pair of polarizers, where one is rotated with respect to the other to control the transmission level. This has the benefit that a single filter unit can be made to provide analog transmission over a broad range of densities. Moreover, an electromechanical tuning mechanism can enable closed-loop density reconfiguration for remote and hands-free operation. Polarizer-based VNDs are limited in reconfiguration speed by the tuning mechanism (e.g., a motor). Two-polarizer configurations typically have angular transmission artifacts (due to geometric rotation of the polarizer absorption axes) which become worse at shorter focal lengths and/or when the density is increased. A new version of this (Sharp and McGettigan, US 20180259692, the contents of which are incorporated herein by reference), which uses multiple stages that can open in a “fan” arrangement mitigates this artifact, allowing much better performance over wide angle and dynamic range.
Another form of VND filter is electro-optical, using (e.g.) liquid crystal devices to change density. Functionally, LC devices are typically used to introduce a voltage-controlled phase-difference (aka retardation) as a means of changing the state-of-polarization upstream of an analyzing polarizer.
Alternatively, they may use (e.g.) a guest-host LC material to behave as a variable polarizer (Osterman, U.S. Pat. No. 9,933,631). In some applications (e.g., intra-frame modulation of density for temporal apodization, e.g., Davis, U.S. Pat. No. 10,187,588), rapid tuning and the lack of moving parts may be advantageous relative to electro-mechanical means. Significant progress has been made in recent years to produce LC VND filters with angle-insensitive density/color and sufficient range in tuning density. However, the ability to maintain uniform transmission and color over a very wide angle and over a significant range of densities, can be extremely challenging for LC devices operated in an analog mode. This may constrain the physical location of the filter in the optical train, for example between the lens and the image capture device where light is more collimated. This may not be practical in some instances, and it may also introduce performance tradeoffs.
There remains a need for a rapidly reconfigurable ND filter that operates over a wide dynamic range (e.g., >5 stops) and wide acceptance angle (e.g., >80° angle of view (AOV)) without introducing significant image artifacts.
It is against this background that the present invention has been created.
The invention uses switchable polarization control elements (e.g., liquid-crystal (LC) devices) to enable rapid reconfiguration of a neutral density filter. The described digital neutral density (DND) filters abandon the flexibility of analog modulation for the purpose of obtaining high-performance over wide-angle for a discrete set of density settings. As such, the invention could be considered analogous to a set of ND filters that can be swapped/stacked in the conventional way, but with a single monolithic unit, no moving parts, and the benefits of rapid switching. The described filter architecture avoids situations in which the switch(es) can introduce transmission nonuniformity artifacts when operated over the full range of densities, particularly for the shortest focal-length lens requirement.
The invention recognizes that single-domain LC devices operated in an analog mode (e.g., anti-parallel nematic electrically controlled birefringence (ECB), and vertical alignment (VA)) can exhibit poor angular performance. This applies particularly to voltage settings in which the director-field (i.e. the distribution of anisotropic molecules in the LC volume) is substantially oblique (i.e. intermediate between in-plane and normal to the substrate). In a worst-case scenario, the LC device may exhibit a first-order (i.e. linear) shift in retardation with respect to incidence angle. In such cases, there can be a high degree of asymmetric angular nonuniformity in density that can render a particular setting useless in practice. The invention seeks to leverage switching between director field distributions that are either substantially in-plane, or substantially normal to the cell substrates, and that both states exhibit second-order dependence of retardation on incidence angle. The invention thus uses the two voltage states in which the polarization optic delivers the best performance in terms of stability of the polarization transformation with respect to incidence angle.
For applications requiring the highest angle-of-view (AOV), the invention further recognizes the value of using LC device configurations that exhibit zero phase-difference (retardation) in the thickness direction (proportional to thickness direction pathlength difference, Rth) in both voltage states. Such devices are described in co-pending application Sharp, US 20190353948, the contents of which are hereby incorporated by reference.
The invention recognizes that the burden of full analog modulation from a single LC VND stage over a wide density range can be excessive, with performance over angle typically suffering. In order to achieve multiple density settings using the invention, the filter architecture can employ filter stages with switches operated independently in each stage. The invention can distribute the system-level density over a plurality of lower-density digital filter stages, thereby relieving the requirements from a particular stage. This can create an opportunity to optimize the AOV performance using a stage operated at a reduced density. For instance, a three-stage filter may have Stage 1 switching between 0-stops and 1-stop of attenuation, Stage 2 switching between 0-stops and 2-stops of attenuation, and Stage 3 switching between 0-stops and 4-stops of attenuation. The result is a DND filter that can switch between 0-stops and 7-stops of attenuation in 1-stop increments.
Some of the vocabulary used in this specification is unique to this type of filter. Additionally, there may be different interpretations of the precise definition of some more common industry terms. In the interest of clarity, the following is a list of definitions for important terms and metrics used throughout the specification.
Neutral Density Filter: A neutral density filter is an optical component that transmits light with a prescribed level of attenuation and with wavelength insensitivity. Attenuation is quantified herein in terms of “stops”, consistent with terminology used in image capture. The attenuation in stops is calculated as the base-two logarithm of the inverse of the power transmission function. Thus, 1-stop of attenuation is equivalent to 50% transmission, 2-stops of attenuation is equivalent to 25% transmission, 3-stops of attenuation is equivalent to 12.5% transmission, and so-on. In the context of visible filters, the attenuation is given by assuming a flat input spectral-power-distribution (SPD) and calculating the ratio of lumens transmitted by the filter to that in the absence of the filter.
The term “neutral” is somewhat subjective and application-specific, referring to an allowed degree of wavelength dependence in attenuation over a defined spectral range of interest. In the context of the visible spectrum, the term “achromatic” may be applied equivalently. An ideal neutral density refers to an attenuation that is spectrally flat; a solution that is most broadly applicable in practice. The term neutral may be expanded to include filters that tolerate an incremental color-shift at normal incidence and/or a shift in color that depends upon incidence angle. The latter is defined below as color-nonuniformity.
Filter Stage: In this context, a filter stage is a stand-alone unit capable of providing two or more density values. Filter stages interact on a power basis, such that the composite transmission is given by the product of the transmission function of each filter stage. A variable neutral density (VND) filter can be implemented in a single filter stage. A digital neutral density (DND) filter of the invention may preferably use two or more stages to generate more than two density values. The analyzing polarizer from one stage typically serves as the input polarizer for the subsequent stage. A DND with more than two density states can be realized with a single filter stage containing two or more digital electro-optic devices. In the latter case (e.g., Sharp US 20190353948, the contents of which are incorporated herein by reference) the state of polarization resulting from the plurality of switches is converted to a plurality of density values using a single polarization analyzer.
Variable Neutral Density (VND) Filter: A VND filter is a neutral density filter with analog tunability over a prescribed range of densities. A VND filter may use mechanical (or electro-mechanical) tuning, or electro-optic tuning. The former may involve rotation of one polarizer with respect to another, and the latter may refer to the application of an electric field to one or more electro-optic devices.
Digital Neutral Density (DND) Filter: A DND is a neutral density filter that switches between discrete density states, using digitally driven electro-optic devices. In an arrangement of the invention there are two or more filter stages used to create multiple states, where each stage has one (or more) electro-optic devices switched between two states. A DND filter may have two or more electro-optic switching devices, where the number of density values can scale as large as 2N, where N is the number of independent electro-optic switches.
Angle-of-View (AOV): In this context, AOV refers to the full-cone angle of light incident on the filter (with respect to the surface normal) in which performance is deemed acceptable over all azimuth angles. In the context of a VND or DND, it further includes the full range of filter densities. The AOV is subject to acceptance criteria for density/color nonuniformity. For instance, it may be stated that a particular VND has a 90° AOV, subject to a 0.5-stop density nonuniformity and an allowed maximum shift in color coordinates, or color nonuniformity. The desired AOV in image capture is typically determined by the shortest anticipated focal-length of the lens and the diagonal dimension of the image-capture device. For instance, a full-frame image sensor used with a 24 mm focal length lens may have a desired AOV of 84°.
Insertion Loss: This is the transmission loss of a VND or DND when set to the lowest density value, specified in terms of stops. For example, in a two-polarizer VND, the insertion-loss is measured when the polarizer absorption axes are parallel. Given that the input is virtually unpolarized, the example VND has a 1-stop loss associated with creating a polarized state, with (typically) an additional 0.5-stop loss associated with absorption/reflection by elements of the VND (e.g., absorption orthogonal to the polarizer absorption axes and Fresnel reflections). It is therefore common for an exemplary polarizer-based VND to have a net 1.5-stop insertion loss.
Dynamic Range: This refers to the (normal incidence) difference between the maximum density setting and the minimum density setting of a VND or DND, specified in terms of stops. For example, in a two-polarizer VND that can be rotated between hard-stops associated with parallel polarizers (0-Stops), and polarizers with absorption axes at 82.8° (6-stops), the dynamic range is 6-Stops. Note that the dynamic range is independent of insertion loss, though the actual densities produced by a VND/DND are given by adding the density setting to the insertion loss (in stops). In this example, the 82.8° setting corresponds to an attenuation of 7.5-Stops when insertion loss is included.
Density Nonuniformity: This is defined as the absolute value of the difference between the minimum transmission (in stops) and the maximum transmission (in stops) for any light incident within the AOV. It can be calculated using the transmission at a single wavelength, or it can be calculated by integrating the photopically-weighted transmission over the input SPD. As described above, the density is given as the ratio of lumens transmitted by the filter to that incident, subject to an input SPD. The density nonuniformity is obtained by computing the latter (assuming a flat-top input SPD) with a representative sampling of AOI and azimuth angles that resolves the features, and then locating the high and low values for density nonuniformity calculation.
Color Nonuniformity: This is defined in terms of the maximum rms difference between any two color coordinates (e.g., a*b*, u′v′, etc) associated with the transmission function of the VND/DND over the AOV, assuming a flat-top input SPD. In an active device, each density state may have a unique color nonuniformity, where the AOV may be limited by the filter state exhibiting the highest angular sensitivity in transmitted color. Note that color nonuniformity does not account for the actual tint of a density state, only the shift with respect to angle. The filter tint is captured by the specification for neutrality, as discussed above.
Polarizer: There are several types of polarizers, but the most common is a stretched-PVA iodine/dye-stuff type. These are linear polarizers with a uniaxial absorption axis in the stretching direction (in-plane), so they are termed o-type polarizers because they transmit the ordinary wave. Liquid-crystal based polarizers have also been demonstrated that are e-mode type, so they transmit the extraordinary mode.
Retarder: A retarder is also called a phase-difference film. It is an anisotropic (uniaxial or biaxial, positive or negative anisotropy) dielectric, such as a stretched polymer, a cross-linked reactive mesogen, or an active liquid-crystal device. Most stretched polymer retarders have their refractive indexes in the principal coordinate system of the film (i.e. two in-plane refractive indexes and one thickness-direction refractive index). Uniaxial retarders can be produced by uniaxial in-plane stretching (aka a positive A-Plate), where the slow-axis typically corresponds to the stretching direction. A negative uniaxial retarder has an optic axis representing the fast-axis. Biaxial retarders can be produced by biaxial stretching (e.g., orthogonal in-plane stretching or in-plane stretching combined with thickness direction stretching). For instance, biaxial in-plane stretching can produce zero in-plane pathlength-difference (Re) resulting in only thickness direction pathlength-difference (Rth) (aka a negative C-Plate). Retarders have dispersion, which typically introduces additional chrominance, though there are more achromatic retarders exhibiting “reverse dispersion” in phase-difference. For the latter, pathlength difference increases with wavelength.
Liquid crystal devices (active or RM) can have additional degrees of freedom relative to stretched films. Because liquid crystal devices are generally inhomogeneous (at least in the thickness direction), they are typically analyzed as a stack of thin layers, each with a homogeneous director distribution. Various types of RM and active LC alignments are common, including splays, bends, and twists about the surface normal. As in active devices, pretilts can be introduced when needed. In this context, retarders are used to transform a state-of-polarization, where an analyzer (polarizer) is ultimately needed to convert SOP to a density.
Oblique Anisotropy: This refers to optically uniaxial (or biaxial) material with an optic axis tilted with respect to the surface normal. A uniaxial retarder with substantially intermediate tilt (e.g., 45°) can introduce first-order retardation with respect to incidence angle. This can result in relatively large asymmetric transmission of a filter stage with respect to incidence angle. An oblique polarizer (e.g., guest-host LC) may have an absorption axis which is tilted with respect to the surface normal with related issues. Oblique anisotropy can exacerbate angular variations in (wavelength dependent) phase-difference and projected optic axis orientation, which is generally undesirable in the present invention.
In-Plane Pathlength Difference: Denoted “Re”, this refers generally to the net optical pathlength difference projected onto the substrate plane. Retardation is the ratio of Re to wavelength, multiplied by 2π. In the context of preferred anisotropic materials used for exemplars of the invention, it refers to anisotropic material aligned with the principal axes of the retarder film. With this constraint, the in-plane retardation is the phase-difference resulting from the two in-plane refractive indexes. A “composite Re” can be associated with a retarder stack. Note that the situation is more complex when using (e.g.) twisted LC structures where the optic axis orientation is inhomogeneous.
Thickness Direction Pathlength Difference: Denoted “Rth”, this refers generally to the optical pathlength difference projected onto the substrate normal. In the context of preferred anisotropic materials used to demonstrate the invention, Rth refers to anisotropic materials aligned with the principal axes of the retarder film. Rth in this context is given by the difference between the thickness direction refractive index and the average of the in-plane refractive indexes, multiplied by thickness. Each polarization optic can have an Rth value, and a stack of two or more polarization control elements (e.g., retarders and LC devices) can have a “composite Rth” which represents the accumulation of Rth.
Note that the composite Rth is not always obtained by adding the Rth of each element, since the azimuthal distribution of retarders can affect the accumulation of Rth (see for example Sharp US 20190018177, the contents of which are incorporated herein by reference). According to the invention, it is a general objective to drive the composite Rth to zero for all states of a DND.
Geometric Compensation: Denoted GC, this refers to one or more passive retarder films that only perform a polarization transformation for light incident off-normal. GC is effectively a polarization rotation (or polarization reflection) that counteracts a geometric rotation experienced by off-normal rays. For example, the orientation of a vector in one reference frame (e.g., the absorption-axis of a polarizer/slow-axis of a retarder in the substrate plane) changes when it is projected onto another reference frame (e.g., the plane normal to the ray-vector (or k-vector)). An ideal GC preserves the normal-incidence function of the in-plane arrangement of polarization optics for off-normal rays in the presence of geometric rotation.
Intra-Stage Compensation: This refers to an arrangement of elements in a stage that collectively minimizes transmission nonuniformity and color nonuniformity over angle. That is, intra-stage compensation endeavors to preserve the normal-incidence transmission function at large angles of incidence. Since this represents the interaction of light with elements within the stage on a field-basis, it is synonymous with stabilizing Re and minimizing composite Rth and geometric effects over incidence angle. These are key elements of the invention for producing building blocks that individually perform well over angle.
Inter-Stage Compensation: This refers to an arrangement of filter stages that collectively minimizes transmission nonuniformity and color nonuniformity within the AOV. Since stages interacted on a power transmission basis, this refers to a complementary arrangement of two (or more) stage transmission functions, such that the AOV is optimized. Optimization may refer to transmission uniformity and/or color uniformity over angle.
ON/OFF-State: There are many configurations of electro-optic devices, which can be grouped into two categories. In one category, the device delivers a relatively small Re value when unenergized (e.g., VA), and a relatively large Re value when energized. In another category, the device delivers a relatively large Re value when unenergized (e.g., TN, ECB, pi-cell), and a relatively small Re value when energized. In some preferred arrangements, the devices are driven such that one Re value is virtually zero, while the other Re value converts light from one SOP to the orthogonal SOP (e.g., a half-wave). For the sake of clarity, the ON-State will be considered the state with relatively large Re and the OFF-State will be considered the state with relatively small Re. The latter applies both to single LC devices and to the LC-pairs of the invention.
Definition of Problem
The ability to adjust the light level reaching an image sensor independent of the aperture is highly valuable for optimizing still/video image quality, particularly in outdoor settings with a short focal length. The most common solution is to carry a set of neutral density filters, each with a unique density. Density changes can be accomplished by manually unthreading a filter from the lens housing and replacing it with one more suited to the ambient luminance and the desired result. Alternatively, filter tray fixtures allow filters to be easily inserted/removed in combinations. Other density values can thus be obtained by stacking filters (e.g., a set of three filters can in principle produce eight outputs. In this example, the lowest density omits filtration, and the highest density stacks all three filters. The latter represents the dynamic-range of the filter. In the case of a geometric series of densities (e.g., a set including 1, 2 and 4 stop ND filters), the dynamic range can be 7-stops, in 1-stop increments. This example can be considered a crude digital neutral density (DND) filter in the sense that the scheme permits switching between a discrete set of fixed densities. It is obviously slow, cumbersome, requires manual access to the lens and filter set during any density changes, and does not allow analog density setting. But because of the high quality available with passive ND filters, it is often preferred because it can be used at any focal length without concern for artifacts associated with angular variations in transmission and color. Furthermore, the settings are well defined by the fixed density values.
An analog (e.g., manual) version of this is the VND filter using a pair of neutral polarizers. As in the above example, reconfiguration is slow and requires manual access to the lens. The common two-polarizer version enables analog tunability, though it trades this convenience for angle artifacts (e.g., the “dreaded-X”) which precludes high densities particularly when shooting at shorter focal lengths. This situation is improved when using the “fan” arrangement of polarizers, described in US 20180259692, including two or more filter stages using inter-stage compensation for angular artifacts.
There are also non-mechanical versions of the VND/DND. Generally, these use some form of electro-optic (e.g., liquid-crystal (LC)), piezo-electric, or electro-chromic devices), or optically-addressed devices (e.g., photo-chromic materials) to reconfigure the density. For rapid low-voltage reconfiguration, LC-based ND filters are desirable. They can be broken into two categories; Type 1, which uses active polarization control (e.g., an active retarder) in combination with a passive polarization analyzer, and; Type 2 which merges these functions such that an LC device is itself an active polarizer (e.g., guest-host LC). A simple Type-1 filter stage may include an input polarizer, an appropriately oriented analyzing polarizer, an active LC device that can manipulate the input SOP, and one or more passive phase-difference films as required by the design. Regardless, most active LC devices (e.g., nematics) tend to have an analog response to an applied electric field, and therefore can in principle enable a VND function. For example, a single nematic LC device (TN, VA, ECB, pi-cell) between crossed linear polarizers may perform sufficiently well as a VND when used in collimated light. But in image capture, an ND filter threaded onto a camera lens may have an AOV requirement of 40°, 60°, 80°, and even greater than 100°. The simple LC VND as described has limited practical utility even at the smallest of these angles.
The expectation of using one or more analog driven LC devices to produce an analog ND filter may be unrealistic in ultra-wide AOV situations due to the interferometric nature of Type 1 filtering. That is, AOI-dependent changes in phase-difference, amplitude-splitting, and analyzer orientation can distort the transmission and color relative to the normal-incidence transmission. An engineered normal-incidence composite Re value can be highly distorted when a ray travels at an oblique angle through the filter due to various phenomena that are considered in this disclosure. In some cases, it may be possible to characterize the behavior of a reconfigurable ND filter in each density setting, and over angle, in order to correct for angular artifacts in post-processing. However, there are limits to the range over which this can be accomplished. In an extreme example, the transmission of one or more primary colors may be completely blocked due to such a distortion; a case that clearly cannot be fixed in post-production.
It should further be noted that the interferometric nature of Type 1 filter stages may produce undesirable transmission characteristics even under the best of circumstances (i.e. at normal-incidence). This is because of the phase-difference dispersion of typical LC devices between polarizers, owing to inverse-wavelength dependence and reduced birefringence with wavelength. Transmission spectra may exhibit a shift toward the blue or amber that may be acceptable in some electronic capture applications, so long as the color is angle-stable.
Because of the nonlinear behavior of AOV-limiters with incidence angle, a particular phenomenon that has no material impact on performance at lower incidence angles can become the performance limiter at higher incidence angles if not properly attended to. It is essentially the “onion-peeling” problem, where the designer is granted visibility into the next underlying issue limiting AOV as a reward for addressing higher-significance issues. Once observable, the challenge is to identify a technique for peeling the next layer of the onion. The following describes a specific prioritization for addressing such issues, and ways in which the invention addresses them.
The invention leverages materials and devices that were developed for the flat-panel display industry. Over the years, the level of sophistication and performance of these materials and devices has improved because performance expectations for televisions and monitors has increased. Some of the specifications that are sought, such as high contrast over angle, are relevant to both neutral-density filters and display devices. While solutions are at times relevant, there are departures that tend to make the neutral-density problem more difficult.
In display, the objective is typically to maintain high-contrast, particularly to extreme viewing angles in the horizontal. In ND filtering, it is desirable for the transmission function to have minimal dependence on both AOI and azimuth. Because LC devices are not achromatic in their switching, emphasis in display is given to minimizing transmission in the dark state over angle. The relatively poor spectral performance of the bright-state is less consequential due to the availability of RGB color balancing. A similar function can be provided by post-processing imagery from an image-capture device.
Contrary to optimizing LCD display design, there may be no particular rationale for AOV preference of one DND filter state over another. In a multi-stage DND, the output may be the product of spectra representing stages in both unfiltered and filtered states. As such, a chromatic output can be equally impactful regardless of the state. To the extent that each state represents a valid output, the overall DND AOV may be limited to that associated with the lowest performing state. And because a DND is typically multi-stage, distortions can be compounded if the design is not robust.
Current LCDs achieve uniform neutral gray-levels over angle using spatial methods that may not be relevant to active ND filtering. In one example, a VA device coupled with a −C-plate compensator can have virtually zero Re/Rth, giving a very dark off-state over a broad AOV (particularly when geometric-compensation is employed). Additionally, azimuthal homogenization of a gray level may be accomplished using multiple domains, a spatial technique that may not be viable for ND filters due to scatter and diffraction artifacts. In a second example, an in-plane switch (IPS) mode is used that minimizes oblique anisotropy, where again, GC can be used to maintain high contrast over angle. However, the IPS-mode requires a sophisticated electrode structure that is practical for pixelated applications, though likely not for ND filters. In summary, both of these spatial methods yield high-performance displays, though their implementation presents challenges for active ND filters. The invention thus seeks architectures using homogeneous materials/devices to provide quality wide-angle all-pass and filtered states.
Filter Examples and Analysis
For the sake of simplicity and consistency of performance comparison in modelling, the filter stage designs are constructed using only exemplary A-Plates and C-Plates. The ordinary/extraordinary refractive indexes of anisotropic media are taken to be 1.50/1.51, respectively, and birefringence dispersion is not included. Polarizers are assumed ideal, in the sense that transmission along the absorption axis is zero and that along the orthogonal direction is unity. Practically speaking, suitable adjustment in parameters is required to obtain an accurate performance prediction using real materials. For instance, a C-Plate retardation may need to be increased to account for a higher index, which decreases the angle in the film. While birefringence dispersion is not included in the model, the wavelength dependence of phase retardation is included, and hence, so is the wavelength dependence of filter transmission. Any accurate performance predictions for (e.g.) an LC-based DND would require inserting actual refractive indexes and LC director profiles.
For benchmark purposes, and for illustrating the benefits of DND filter architectures, consider a simplified prior-art single-stage shutter shown in
Conversely, the slow-axis index remains fixed in the 90° azimuth, while the increased pathlength causes a red-shift of the peak wavelength. As before, there is a loss in transmission due to the geometric rotation of the polarizers. This spectral shift presents a problem for both transmission and color nonuniformity.
The dominant AOV issue with the above prior art shutter is that the Rth values in both states are non-zero. As disclosed in the invention by Sharp (US 20190353948, the contents of which are incorporated herein by reference), a dual-LC cell can be used to provide a digital-switch with a constant Rth in both states. This prior art “self-compensating liquid-crystal retardation switch”, illustrated as a light-shutter, is shown in
In the previous examples, nothing was done to mitigate the leakage associated with counter-rotation of the polarizer absorption axes.
Four illustrative examples of shutter designs were presented. Factor 1 of the AOV limiters was eliminated from all examples by only considering A-Plate/C-Plate stacks, Factors 2 and 3 were considered in these examples, and Factor 4 will be considered in multi-stage DND configurations. To summarize,
The invention seeks to identify DND filter structures that are angle stable and ideally provide quasi-neutral transmission states. The objective of a DND stage is to act as an all-pass filter in one state, and a filter with a wavelength/angle stable prescribed transmission in the other state. In the context of a Type 1 filter, a stage contains an input polarizer, an analyzing polarizer, one or more active LC devices, and additional retarder films as needed. There are a number of methods for producing a filtered-state in a Type 1 filter stage, including; (1) Introducing a passive bias retarder (e.g., via the A-Plate); (2) Introducing a bias retardation via the LC switches; (3) Controlling the angle between the polarizers, and; (4) Using one or more partial-polarizers. The best spectral performance from an all-pass filter occurs when the two polarizers are parallel and the structure between them vanishes. This has the additional advantage that no specific state-of-polarization is required for the filtered-state, so long as the projection onto the analyzer yields the desired transmission. For example, a one-stop filtered state can be produced by circular polarization, linear±45° polarization, or any SOP lying in the S1/S2 plane of the Poincare sphere.
In the parallel-polarizer configuration, the filtered state relies upon the quality of the polarization conversion by the switching structure. The required polarization conversion is substantial, being nominally equivalent to rotating a polarizer by 45° (1-Stop), 60° (2-Stops), or 76° (4-Stops), and therefore dispersion can be large if phase-difference is used to accomplish it. Specifically, the polarization conversion by the structure is likely to be wavelength-dependent, leading to a wavelength-dependent transmission. There are an infinite number of polarization transformations that can produce the desired projection of the electric field onto the analyzing polarizer. A 45°-oriented linear retarder requires the least modulation in retardation, with a reduction/increase in orientation angle requiring a larger modulation in retardation. More complex twisted structures may also produce the same end-point on the Poincare sphere. The following contains some illustrative examples using retardation switching.
An alternative to the above approach is to use crossed polarizers, where the unfiltered-state requires conversion of all visible wavelengths (ideally) to the orthogonal SOP. If accomplished using a zero-twist retarder, the transmission peaks at the half-wave wavelength, with significant roll-off at shorter/longer wavelengths. Typically, some color balance between blue and red is sought, which tends to result in a green-tinted transmission. The filtered state is thus produced by a relatively small Re value, which can have the benefit of showing lower wavelength-dependence relative to the parallel-polarizer case. The design equations for the crossed-polarizer case can be given by
Where ΓLC, ΓA, ΓU, ΓF are the retardation values for the (matched) liquid-crystal devices, the passive −A-Plate retarder, and the composite retardation in the unfiltered-state and the filtered-state, respectively. These equations were used to generate the 4-Stop design shown in
While the parallel and crossed DND stage configurations presented show good spectral stability with respect to angle, the tint can be problematic particularly in multi-stage configurations. For instance, a three-stage crossed-polarizer configuration has a maximum density given by the product of three blue-rich spectra. The compounding effects of this can produce a nonuniformity in transmission between blue and red wavelengths that may be deemed unacceptable. If, for example, the filter has 7-stops of attenuation in the green, with 4 in the blue and 10 in the red, it is likely that a correction in post-production is not a viable option. Alternatively, a parallel-polarizer stage could be combined with a crossed-polarizer stage, such that the highest density state is the product of a red-rich spectrum with a blue-rich spectrum, therefore mitigating the compounding effects. This can be regarded as an example of inter-stage compensation to improve color performance. A better solution, of course, would be to have DND stages that produce more neutral output spectra.
One of the challenges in DND design involves dealing with the wavelength dispersion of retarders. If it were feasible to make active LC devices with the proper reverse birefringence dispersion, filter transmission spectra could be relatively flat in both unfiltered and filtered states. Currently, it is not practical for a variable birefringence LC device to switch the same phase-retardation for all wavelengths of the visible, and consequently, one or both states of a DND stage may show some chromatic behavior. It is known that retarders can be combined to effectively engineer a reverse-dispersion in a polarization transformation. Koester (C. Koester, “Achromatic combinations of half-wave plates”, J Opt Soc Am, 49, 4, 405-409 (1959)) showed that a pair of half-wave retarders can produce a linear polarization transformation with lower wavelength-dependent than that from a single half-wave retarder. It is also known that there can be an accumulation of Rth in such designs, and as such, the AOV can be diminished for the sake of achromatizing a polarization transformation. One of the requirements of the self-compensating LC switch is that it requires an independent pair of LC devices for constant Rth, but the pair can be leveraged to serve a second purpose: achromatizing the ON-State transformation.
The invention recognizes that the pair of LC devices driven anti-phase and digitally, is functionally an in-plane switch with a constant Rth. The configuration allows arbitrary switching between two in-plane orientations and two retardation values. The prior art shows that an in-plane half-wave retardation switch (ferroelectric liquid crystal, FLC) combined with passive A-Plate retarders can function as compound elements with engineered reverse-dispersion. In one configuration (Sharp, U.S. Pat. No. 5,870,159, the contents of which are incorporated herein by reference), the FLC is oriented with slow-axis crossed with that of a passive+A-plate, yielding zero composite Re. When switched to the other orientation, the structure behaves as an achromatic (Koester) rotator. Similarly, an FLC device flanked by +A-plates (Sharp U.S. Pat. No. 5,658,490, the contents of which are incorporated herein by reference) can function as an achromatic compound retarder with switchable slow-axis orientation. The FLC devices can be replaced by a pair of nematic switches according to the invention, enabling structures with zero Rth and an achromatic transformation.
In a general achromatic rotation, the angles of the two+A-Plate retarders for a rotation angle of a can be given by
respectively, where ε is a small angle that can be used to adjust the coverage and flatness of the spectrum. In general, the above shows that the polarization rotation given by a pair of half-wave retarders with orientation α1 and α2 is 2(α2−α1). In the case where the passive retarder is a negative A-Plate, an additional 90° of rotation is needed for that element
Consider the specific example where there is only one switch per stage, and each stage conforms to the design shown in
The architecture with the best performance, shown in
Normal-incidence transmission spectra for filtered and unfiltered outputs were generated for each of the states of the previous example, using the design of
According to the teachings herein, the optic axis orientation for each of the retarders of a zero Rth switch, including two LC devices and a passive retarder, is arbitrary. The anti-phase switching gives a constant Rth which is removed as needed with film compensation, per the invention. The film compensation can be done with the addition of a C-plate retarder, or the associated Rth can be incorporated into a biaxial film that also includes the required Re. This can be illustrated by examples.
The previous example illustrated a zero-Rth switch that provides either an isotropic state or an achromatic rotator state. But the switch need not be isotropic in one state, and could for example be an achromatic rotator in both states. The previous equations give the slow-axis orientations for an achromatic polarization rotation of α. Consider the case where a first liquid crystal half-wave retarder (LC1) and a passive half-wave+A-Plate retarder have orientations, α1=θ1/4 and, α2=3θ1/4, respectively, where θ1 is a first achromatic polarization rotation angle. When LC1 is driven to a C-Plate state, a second polarization transformation is produced by the action of the passive A-plate retarder in combination with the second liquid crystal half-wave retarder (LC2), which is now in the A-Plate state per the invention. The passive A-Plate retarder can now be considered to form the first retarder required for a second achromatic rotation, where the second rotation angle is θ2=3θ1. Per the above equations, the orientation of LC2 is therefore, α3=9θ1/4. Functionally this device has a bias rotation angle of θ1, which when switched, further rotates the polarization by 2θ1. This could be used as-is, or in combination with one or more passive retarders that removes the bias rotation.
Consider the example of the previous achromatic rotator switch receiving a linear polarization along 0° that is required to switch the output polarization between linear polarization states with an orientation of +45° and −45° (=135°). According to the design equations, allowing for small orientation changes (i. e. ε) to modify the spectral coverage, this can be accomplished using slow axis orientations of α1=12.0°, α2=33.4°, and α3=−80.0°. Assuming dispersionless half-wave retarders with a 500 nm center wavelength, this gives two transformations, with ideal achromatic rotations of ±45° at two wavelengths. These spectra overlap at wavelengths of approximately 440 nm and 580 nm. As in the design with a zero-Re state, this switch has zero intrinsic Rth when the passive retarder is a positive A-Plate, and requires the addition of a passive −C-Plate half-wave when the passive retarder is a negative A-Plate.
A device that switches between ±45° linear polarization, with wide acceptance angle, and wavelength insensitivity can be used for applications such as sequential stereoscopic 3D with linear polarizer eyewear. Also, this switch can be combined with other passive polarization functional elements to produce other outputs. For example, following this switch with an achromatic quarter-wave retarder with zero-orientation gives a wide-angle achromatic circular polarization (left/right) handedness switch, as shown in
As indicated previously, there are other ways to create a desired filtered state from a Type 1 filter stage. Previous examples showed a bias retardation introduced via a difference between identical retardation LCs and the passive A-Plate. It could also be introduced via a difference between the retardation of the LC devices, with similar performance results. A DND filter can also be realized using stages with partial polarizers. A partial polarizer has a prescribed absorption along the absorbing axis, such that when crossed with a high contrast, a desired filtered state can result. Stages with different partial polarizer absorption values can enable multiple density states, much like the examples given. However, an increase in transmission may occur because of the reduced projection along the absorbing axis off-normal in one azimuth. Similarly, a decrease in transmission may occur in the orthogonal direction due to increased pathlength. This may be mitigated by providing a partial polarizer with absorption along the thickness direction.
Filters of the invention can also be realized using Type 2 structures. In this case, an active polarizer (e.g., a guest-host LC device) can be used to replace the combination of a passive analyzer and a polarization switch. Some of the same factors described to maximize the AOV apply to a Type 2 structure. Oblique anisotropy is minimized by digitally switching between an in-plane polarizer and a thickness-direction polarizer. Geometric compensation can then be introduced as described previously to improve the AOV in the filtered state.
A DND filter and the zero-Rth achromatic switches of the invention can be used in a number of communications, avionics, military, medical, and consumer applications. Aperture sizes can be small or very large, owing to the use of display materials and devices. The sensor may be a single-pixel device, an array sensor, or the human eye. For the latter, the system can be near-eye (head mounted), in-vehicle (e.g., an automotive heads-up display), direct-view, or a virtual cockpit (e.g., viewing the camera feed from a drone using a virtual reality goggle). The system may operate in the UV, visible, near-infrared, or infrared spectra. Basically, the DND of the invention is appropriate for any application requiring agile changes in transmission with no moving parts. It is particularly well suited to producing a plurality of density outputs that are stable over a very broad range of incidence angles.
More generally, an exemplary DND stage shown in
This application claims priority to U.S. Provisional Application No. 62/980,877 filed Feb. 24, 2020, the contents of which are incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62980877 | Feb 2020 | US |
