Patent Description:
Providing multiple focal planes, or discrete steps of focus adjustment, is useful for a number of applications. It can be part of creating a more realistic three dimensional display, as well as the ability to capture three dimensional data. In the prior art, multiple focus capture utilized mechanical movement such as gears or liquid lenses. Such mechanisms are expensive, slow, and relatively fragile. Another prior art method of capturing multiple focal lengths uses multiple mirrors and lenses. This is like having multiple cameras; it is bulky and expensive. Because of the bulk and expense, it also limits the number of focal lengths that can be simultaneously captured. A large beam splitter has also been used in the prior art to create two light path lengths. However, this is also a bulky solution.

Such prior art solutions are some combination of large, expensive, and slow. Liquid lenses are expensive and slow, and large beam splitters are large. This makes them difficult to use, and not useful for size or cost constrained systems, particularly portable or worn devices.

<CIT> discloses an optical space switch accommodating a plurality of input light paths and output light paths. The optical space switch comprises a plurality of polarization control optical switches, each consisting essentially of: polarization control means having elements, one for each input light path, for rotating through <NUM>° the polarizing direction of light information incident from each input light path or otherwise retaining the polarizing direction thereof for output; and a light path routing element for routing the light path for the light information output from the polarization control means in accordance with the polarizing direction of the light information. These polarization control optical switches are arranged in a matrix pattern or coupled in cascade to implement a polarization control optical space switch. <CIT> discloses an optical path length adjuster (<NUM>) that enables electro-optical control of a physical path length between two optical elements, suitable for use in the adjustment of an optical path length within three dimensional display devices that generate a virtual image within a defined imaging volume. The adjuster varies an optical path length between an input optical path and an output optical path and includes: a plurality of first optical (<NUM>) elements and second optical elements arranged in alternating sequence along an optical path, each first optical element (<NUM>) for determining a polarisation state of a light beam passing through that element and each second optical element for selectively transmitting or reflecting a light beam incident on that element depending on the selected polarisation state of the incident light beam, wherein the optical path length traversed by an input beam on the optical path can be varied by is selecting a particular second optical element at which reflection of the input beam is to occur, the reflected input beam emerging along the output optical path. <CIT> discloses an optical system that can be electronically controlled using switchable wave plates in conjunction with polarized light. <CIT> discloses an illumination device including a solid-state light source device, a pickup lens unit on which light from the solid-state light source device is incident, and a polarization conversion element. A lens constituting the light exit surface of the pickup lens unit is an aspherical lens having a light exit surface shaped like a rotationally symmetric shape centered on a light axis when viewed from a direction of the light axis, and having a cross-section of an aspherical shape when cut with a plane parallel to the light axis.

A digital light path length modulator is described. The digital light path length modulator includes an optical path length extender (OPLE) and a polarization modulator, and can be used to adjust the path length of light. In one embodiment, light with state <NUM> polarization travels through a longer path in the OPLE than light with state <NUM> polarization. This can be used to create two focal planes. An OPLE is made up of one or more plates with a plurality of polarization sensitive reflective elements. A plurality of digital light path length modulators create a modulation stack.

In one embodiment, using a modulation stack the number of focal planes can be increased. This provides the capacity to build a system that can meet the physiological requirements of human vision, by creating display in which the 3D indicia of overlap, focus, and vergence match. This produces a better quality 3D display and can prevent the headaches associated with 3D displays.

This mechanism in one embodiment can also be used for image capture, and various other uses in which light waves or other waves in a similar spectrum are either projected or captured, including but not limited to cameras, binoculars, 3D printing, lithography, medical imaging, etc. Creating a simple, easy to manufacture digital light path length modulator is like the step from vacuum tubes to transistors, it enables more complex, cheaper, and much more dense digitally controlled elements, which can become building blocks for a wide range of uses.

The following detailed description of embodiments of the invention makes reference to the accompanying drawings in which like references indicate similar elements, showing by way of illustration specific embodiments of practicing the invention. Description of these embodiments is in sufficient detail to enable those skilled in the art to practice the invention. One skilled in the art understands that other embodiments may be utilized and that logical, mechanical, electrical, functional and other changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.

<FIG> is a block diagram of one embodiment of the digital light path length modulator. The digital light path length modulator <NUM>, includes an optical path length extender (OPLE) <NUM> and a polarization modulator <NUM>. The polarized or unpolarized light impacts the polarization modulator <NUM>. The polarization modulator <NUM> may rotate polarization, leave the polarization unchanged, and/or filter the light. The output of the polarization modulator <NUM> enters the OPLE <NUM>. In one embodiment, the polarization modulator <NUM> is digital, thus providing a digital control to select light path length by turning on and off the polarization modulator <NUM>. In one embodiment, the switching speed of the polarization modulator <NUM> is adjustable, and switching speed may be under <NUM> milliseconds. The combination of the polarization modulator <NUM> and OPLE <NUM> enables the digital light path length modulator <NUM> to selectively lengthen the light path.

<FIG> is a block diagram of one embodiment of the digital light path length modulator. The digital light path length modulator <NUM> includes an OPLE <NUM> and a polarization modulator <NUM>. In this instance, the polarization modulator <NUM> is placed after the OPLE <NUM>. The polarization modulator <NUM> can act as a filter, to remove a portion of the light. Either configuration of the digital light path length modulator, shown in <FIG> may be utilized. In one embodiment, a digital light path length modulator may include a polarization modulator on both sides of the OPLE.

The OPLE may comprise one or more plates with a plurality of polarization sensitive reflective elements. The OPLE does not need to be flat, and may in one embodiment be curved to provide additional optical features. The polarization sensitive reflective elements in the OPLE are substantially parallel in one embodiment. In another embodiment, they may not be parallel, but may be angled to provide other optical characteristics. The polarization sensitive reflective elements in the OPLE may be evenly spaced in one embodiment. In another embodiment, the spacing of the polarization sensitive reflective elements may vary along the length of the OPLE.

<FIG> is a block diagram of one embodiment of a system in which the digital light path length modulator may be used. The system is for display. The light source <NUM> provides the light for display. The light source <NUM> may be a spatial light modulator.

In one embodiment, there may be a digital correction system <NUM>, which adjusts the output of the light source to compensate for the predicted difference in the location of light of different polarizations coming out of the digital light path length modulator <NUM>. By pre-adjusting the light, the resulting light regardless of its path length is properly positioned when it is displayed.

The digital correction system <NUM> spatially shifts the image elements entering the digital light path length modulator <NUM> which would be shifted by the digital light path length modulator <NUM>, to place them in the correct location upon exit from the digital light path length modulator <NUM>. The spatial shifting may include lateral shift correction, and correction for other artifacts of the system. Such pre-calculation of the output of a digital display system is known in the art. Digital correction systems <NUM> are utilized to correct for lens warping, color separation, and other issues. The digital correction system <NUM> creates an output which is in the "rendering state" such that the perceived image by the user is correct.

In one embodiment, the optical path length extender (OPLE) <NUM> may be configured to be self-aligned. That is, spatial shift between the light that travels the longer and the shorter path through the OPLE <NUM> may be eliminated, or may be set to an intentional spatial shift. The creation of such a self-aligned OPLE is discussed below.

In the embodiment of <FIG>, the light from light source <NUM> is polarized by polarizer <NUM>. The polarizer <NUM> may be eliminated if the light source <NUM> outputs polarized light, or may be integrated into the light source <NUM>. The output of the polarizer <NUM> is light with one polarization.

The digital light path length modulator <NUM> includes a polarization modulator <NUM> and an OPLE <NUM>. The polarization modulator <NUM>, in one embodiment, is an electronically controlled element which can rotate the polarization of beams of light between two orthogonal states, state <NUM> and state <NUM>, by selectively modulating the polarization of some or all of the light. In one embodiment, the orthogonal states are S-polarized and P-polarized light. The polarization modulator <NUM> may also be a filter which selectively filters light.

In one embodiment, the polarization modulator <NUM> is an electronically controlled liquid crystal device (LCD). In another embodiment, the polarization modulator may be a Faraday modulator, a switchable birefringent crystal (i.e. LiNO3), or another modulator, which can selectively modulate a portion or all of the light impacting it. In one embodiment, the polarization modulator <NUM> may selectively polarize the light based on other factors, such as color, wavelength, etc..

The polarization modulator <NUM> may modulate a subset of the light that impacts it, in one embodiment. In another embodiment, the polarization modulator <NUM> may modulate all of the light, and switch modulation in time sequential slices. Time sequential slices means that light impacting at time T is not modulated, while light impacting at T+x is modulated. Because the image perceived by a human user is constructed of a series of time sequential slices of data, in one embodiment, these slices are perceived as components of a single image. This is referred to as "biological real time," which is perceived as being concurrent by a human viewer, even though it is time sequential in processing.

The polarized or selectively polarized light impacts the OPLE <NUM>. The OPLE <NUM> includes one or more plates, each plate having a plurality of polarization sensitive reflective elements, which reflect light having a first polarization, and pass through light with a second polarization. The reflected light bounces between the polarization sensitive reflective elements two or more times, before exiting the OPLE <NUM>. This increases the path length of the light having the first polarization, compared to the light having the second polarization which passes directly through the OPLE <NUM>. In one embodiment, the light exits the OPLE <NUM> at the same angle that it entered the OPLE <NUM>.

Use of this system, alters the relative light path length of the light with the two polarizations, because the light with a first polarization travels through a longer path than the light with the second polarization.

Utilizing a plurality of digital light path length modulators <NUM> allows for a multitude of digitally selectable path lengths. Having the various selectable path lengths enables the creation of multiple focal lengths of light exiting the digital light path length modulator <NUM>, since the light appears to be at different distances from the user, based on the length of the light path. In one embodiment, image elements formed by the light that has a longer light path appear further from a user.

In one embodiment, the OPLE <NUM> is self-aligned so that light exiting the OPLE <NUM> is not spatially shifted, or intentionally spatially shifted, regardless of polarization. The specific configuration of the OPLE <NUM>, and its manufacture, is discussed in more detail below.

The OPLE <NUM> and polarization modulator <NUM> make up the digital light path length modulator <NUM>. A digital light path length modulator <NUM> creates two or more light path lengths. Although only a single digital light path length modulator <NUM> is shown in <FIG>, the system may include a modulation stack with a plurality of digital light path length modulators <NUM>, to create an increasing number of light path lengths. This may be used to create more focal planes, to create a perception of a hologram. The system thus provides slices of a hologram at two or more focal planes. As the number of focal planes is increased, the output provides 3D cues that approach the limits of human perception. By utilizing a number of focal planes perceived by a user, the perception or recording of a digital hologram can be created.

The output of the digital light path length modulator <NUM> is displayed via display element <NUM>, or through some other means. The display element <NUM> may provide a component for a three-dimensional display, with image elements displayed in different focal planes.

<FIG> is a block diagram of another embodiment of the system in which the digital light path length modulator may be used. In this embodiment, rather than displaying light/images/data, the system captures light/images/data. In one embodiment, the initial image or data enters a lens <NUM>. Polarizer <NUM> polarizes the light, if it is not already polarized when it is captured.

The polarized light is then selectively modulated by polarization modulator <NUM>, and passed through OPLE <NUM>. As noted above, within the OPLE <NUM>, the differently polarized light has different path lengths. In one embodiment, a portion of light may be polarized so that a portion of an image embodied in the light goes through a longer light path than another portion. In one embodiment, all of the light may have the same polarization, and the changes in polarization and thus focal length may be varied in time sequential slices. In one embodiment, the system may combine concurrent and time-based light path adjustment.

Imager <NUM> captures or displays the image. The imager <NUM> may be an electronic image sensor, such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) sensor. The imager <NUM> may be another capture element, such as film, binoculars, scope, or any mechanism to capture or display an image. In one embodiment, a digital correction system <NUM> may be used to correct the captured or displayed image to account for any spatial shift between the light beams, because of the path they took. As noted above, in one embodiment, the OPLE <NUM> is self-aligned.

The OPLE <NUM> and polarization modulator <NUM> together form a digital light path length modulator <NUM>. In one embodiment, although only a single digital light path length modulator <NUM> is shown, the system may include a modulation stack with a plurality of digital light path length modulators <NUM>.

<FIG> and <FIG> are diagrams of one embodiment of a digital light path length modulator in a near eye display (NED) system. The light modulator <NUM> outputs polarized light, both state <NUM> and state <NUM> polarized light, in one embodiment. Polarizing filter <NUM> removes the state <NUM> polarized light, and passes through state <NUM> polarized light only. The polarization modulator <NUM> in <FIG> is "off," leaving the state <NUM> polarized light to pass through. In this context, the term "on" refers to a setting in which the polarization modulator <NUM> rotates the polarization of light, while the term "off" refers to the setting in which the polarization modulator <NUM> does not alter the polarization of light.

OPLE <NUM> has a plurality of polarization sensitive reflective elements, which reflect state <NUM> polarized light, while passing through state <NUM> polarized light. Here, state <NUM> polarized light is transmitted straight through (having the shorter light path. ) The output in one embodiment is transmitted to near eye display (NED) projection optics <NUM>. Of course, though it is not shown, additional optical elements may be included in this system, including lenses, correction systems, etc..

<FIG> is a diagram of the digital light path length modulator of <FIG> with the polarization modulator "on. " Here, again, the polarizing filter passes only state <NUM> polarized light. However, here, the polarization modulator <NUM> modulates the light, and outputs state <NUM> polarized light. The state <NUM> polarized light is reflected by the polarization sensitive reflective elements in OPLE <NUM>. Thus, this light goes through a longer light path.

A comparison of <FIG> and <FIG>, shows that the state <NUM> polarized light has a longer light path than the state <NUM> polarized light. In this way, a digital light path length modulator <NUM> can change the light path length. While only a single digital light path length modulator <NUM> is shown here, a plurality of digital light path length modulators <NUM> may be stacked to provide a larger number of light path lengths.

<FIG> and <FIG> show a time sequential embodiment, in which all of the light entering the digital light path length modulator <NUM> has one polarization, and is either modulated or not modulated by polarization modulator <NUM>. In this example, the system switches between the states shown in <FIG> and <FIG>, in time. The polarization modulator <NUM> may selectively modulate the polarization of a subset of the light, in one embodiment. In one embodiment, modulation may be based on location, time, color, wavelength, and optionally other differentiable factors.

<FIG> and <FIG> are diagrams of one embodiment of a digital light path length modulator in a camera system. The camera lens <NUM> captures the image data, and transmits it through the polarizing filter <NUM>. The polarization modulator <NUM> selectively polarizes the light, and sends it through OPLE <NUM>. The output from OPLE <NUM> goes to image sensor <NUM>. The polarization modulator <NUM> is "off" in <FIG>, and the state <NUM> polarized light is not modulated. The OPLE <NUM> does not reflect state <NUM> polarized light, and thus the light passes straight through the digital light path length modulator <NUM>. The light coming out digital light path length modulator <NUM> impacts the image sensor <NUM>. Of course, though it is not shown, additional optical elements may be included in this system, including lenses, correction systems, etc..

<FIG> shows the same system when the polarization modulator <NUM> is on, and modulates the light to state <NUM> polarization. The state <NUM> polarized light goes through a longer light path, because it is reflected by the polarization sensitive reflective elements in the OPLE <NUM>. This will cause objects at a nearer distance to come into focus without moving any elements of the imaging lens. Also, focus can be changed as fast as the polarization modulator can change states, which can be under <NUM> milliseconds. The OPLE <NUM> and polarization modulator <NUM> form a digital light path length modulator <NUM>. In one embodiment, the system may include a plurality of digital light path length modulators <NUM>.

<FIG> illustrates one embodiment of an optical path length extender (OPLE). The OPLE <NUM> includes a plurality of polarization sensitive reflective elements also referred to as a polarized beam splitters (PBS) <NUM>, which selectively reflect light beams of one polarization state, while passing through light with the other polarization state. The two polarizations are referred to as state <NUM> and state <NUM> polarization. The polarization sensitive reflective elements <NUM> are evenly spaced, parallel to each other, a distance t1 apart from each other, in one embodiment. In another embodiment, the polarization sensitive reflective elements <NUM> may not be parallel, or evenly spaced.

The thickness of the OPLE <NUM>, t2, in this example, is designed to have the reflected light beam bounce twice before exiting the OPLE <NUM>. Angle "a" defines the angle of the polarization sensitive reflective elements <NUM> with respect to the face of the OPLE <NUM> where light enters. The relationship of t1, t2, and angle a define the path length of the light with the polarization state that is reflected. These values also define the position (location and angle) of the reflected polarized light exiting the OPLE <NUM>.

As can be seen, in this example, state <NUM> polarized light passes straight through the OPLE <NUM>, and state <NUM> polarized light is reflected through the polarization sensitive reflective element <NUM>, and thus takes a longer light path.

This figure also illustrates that not all of the light impacts the OPLE perpendicular to the face of the OPLE <NUM>. Because light spreads (when it is not collimated light), some of the light impacts as angled light beams <NUM>. <FIG> shows that the OPLE <NUM> provides path lengthening for light impacting at an angle. The change in angle as light enters the OPLE and is refracted is not shown in this figure or similar figures for simplicity.

<FIG> illustrates another embodiment of an OPLE <NUM>, in which the OPLE <NUM> is thicker. In this example, the state <NUM> polarized light bounces multiple times within the OPLE <NUM>. As can be seen from <FIG> and <FIG>, the thickness of the OPLE impacts how much the light path is lengthened.

<FIG> illustrates one embodiment of a self-aligned OPLE <NUM>, which includes two plates <NUM>, <NUM>, that adjust the spatial shift of the reflected light. The self-aligned OPLE <NUM> can be used to spatially realign the reflected and passedthrough light. In one embodiment, the two plates <NUM>, <NUM> are matched. As shown in <FIG>, they may be matched by having the same thickness, t2, and mirror image polarization sensitive reflective elements <NUM>. In one embodiment, two plates may be matched by having the spatial shift created by one of the plates <NUM> with a particular thickness, and angle of the polarization sensitive reflective elements be matched by a second plate <NUM> with a different thickness and/or angles, which is designed to provide a matching spatial shift.

Using the self-aligned OPLE adjusts the spatial shift between the light going through the shorter and longer light paths. In one embodiment, for a self-aligned OPLE <NUM> with two plates having mirrored <NUM> degree angled polarization sensitive reflective elements, the relative positions of the two light beams exiting the OPLE <NUM> is identical to their relative position entering the OPLE <NUM>. This is useful to eliminate the need for correction of a spatial shift in the input or output. In one embodiment, the self-aligned OPLE <NUM> may also be used to intentionally set a particular spatial shift. The effective thickness of OPLE <NUM> is the cumulative thickness of the plates <NUM>, <NUM> making up the OPLE <NUM>, in this case <NUM>*t2.

As shown in <FIG>, there may be a gap between the two plates <NUM>, <NUM> of OPLE <NUM>. In one embodiment, the size of the gap is irrelevant, since light travels straight through the gap. In one embodiment, other optical elements, including other OPLEs, polarization modulators, etc. may be positioned between the two plates <NUM>, <NUM>. The plates of the self-aligned OPLEs <NUM> do not require spatial proximity, in one embodiment.

<FIG> illustrates another embodiment of an OPLEs <NUM>. In this example, there is no gap between the two plates <NUM>, <NUM> of the OPLE <NUM>. Furthermore, in this configuration, the polarization sensitive reflective elements <NUM> of the two plates <NUM>, <NUM> are not aligned. However, they do have the same distance (d) between the polarization sensitive reflective elements. As can be seen, such shifts between the positioning of the parallel polarization sensitive reflective elements <NUM> do not alter the functioning of the OPLE <NUM>.

While the self-aligned OPLEs shown in <FIG> and <FIG> show plates of thickness t2, it should be understood that the actual thickness of a plate is not limited. <FIG> illustrates a self-aligned OPLE <NUM> in which the plates are much thicker, causing more lengthening of the light path between state <NUM> polarized light and state <NUM> polarized light. It should also be noted that the thickness of the two plates used in a self-aligned OPLE need not be matched.

<FIG> illustrates one embodiment of an OPLE <NUM> which includes a plurality of plates. A single OPLE <NUM> may include one or more plates with polarization sensitive reflective elements. In the example illustrated in <FIG>, the OPLE <NUM> includes five plates. The effective thickness of OPLE <NUM> is the thickness of the plates making up the OPLE <NUM>, in this case <NUM>*t3+t2+<NUM>*t2.

<FIG> illustrates one embodiment of the effect of using an OPLE, with a light source for non-reflected light. As can be seen, in this example, for state <NUM> polarized light which is not reflected by the polarization sensitive reflective elements, the real light source and the "virtual" or perceived light source are in the same position. This figure additionally shows that for a real light source, light travels in a cone, rather than a straight light as is usually illustrated for simplicity.

<FIG> illustrates one embodiment of the effect of using the OPLE of <FIG>, with a light source for reflected light. In this illustration, state <NUM> polarized light is reflected. Thus, the user's perceived "virtual light source" is spatially shifted from the real light source. The lengthening of the light path shifts the virtual source vertically, while the movement of the light caused by the bounce shifts the apparent light source horizontally. In the example shown, the virtual light source is shifted to the right and back. The virtual rays show that the user's perception tracks back the light, to perceive the virtual source.

<FIG> illustrates one embodiment of the effect of using a self-aligned OPLE, with a light source, for reflected light. As can be seen, by using the self-aligned OPLE, the virtual light source appears to further away (e.g. lengthening the focal length) but not shifted in position. Although the self-aligned OPLE shown in <FIG> has no gap between the plates, as discussed above a gap, including a gap with additional optical elements could continue to provide this self-alignment feature. Furthermore, while the two plates in the shown self-aligned OPLE are matched, they need not have an identical thickness or angle for the polarization sensitive reflective elements, as discussed above.

<FIG> shows the effect of light extension, on a perceived image. The illustration shows three degrees of light extension side by side, for comparison. The first one passes the light without any lengthening, so the image is perceived at the focal plane of the display. The second one lengthens the light path, which causes the user to perceive the image at a different focal plane. The third one lengthens the light path further, which causes the user to perceive a virtual image at a third focal plane. Thus, by controlling the length of the light extension, a system can create virtual images and image elements at various focal points. Using the digital light path length modulator, the system can adjust the light path digitally, and selectively position virtual images at various focal lengths.

<FIG> shows the effect of light extension on capturing image data, at different real planes. Because of the light extension, the virtual images appear co-planar, even when the real objects are at different planes.

<FIG> illustrate embodiments of an OPLE combined with a digital light path length modulator, showing some examples of the various light path lengths which may be created. <FIG> show a digital light path length modulator <NUM> and an OPLE <NUM>.

In the example shown in <FIG>, the polarization modulator <NUM> is "off," and thus the state <NUM> polarized light goes straight through both OPLEs <NUM>, <NUM>, while the state <NUM> polarized light is reflected in both OPLEs <NUM>, <NUM>. Thus, in this example, the light that is state <NUM> polarized entering the system is not extended at all, while the light that is state <NUM> polarized when it enters the system is extended, in this example by <NUM>*d.

In contrast, in <FIG> the polarization modulator <NUM> is "on," reversing polarization between the two OPLEs <NUM>, <NUM>. Therefore, the state <NUM> polarized light passes through the first OPLE <NUM>, then is changed to state <NUM> polarized light, and is reflected in the second OPLE <NUM>. The originally state <NUM> polarized light is reflected in the first OPLE <NUM>, is changed to state <NUM> polarized light, and is passed through the second OPLE <NUM>.

When the angle of the polarization sensitive reflective elements is at <NUM> degrees, as shown in <FIG>, the length of the incoming state <NUM> polarized light is extended by d in the second OPLE <NUM>, while the length of the incoming state <NUM> polarized light is extended by 2d, in the first OPLE <NUM>. Thus, by utilizing a set of digital light path length modulators, a number of different light path lengths may be created.

<FIG> show another digital light path length modulator combined with an OPLE. In this example, the distance between the polarization sensitive reflective elements in the first OPLE <NUM> is different from the distance between the polarization sensitive reflective elements in the second OPLE <NUM>. Thus, the extension of the beams is based on an addition of d1 (the distance between the polarization sensitive reflective elements of the first OPLE <NUM>) and d2 (the distance between the polarization sensitive reflective elements of the second OPLE <NUM>). Note that the reason the extension is by d+d2 is because the angle of the polarization sensitive reflective elements for both OPLEs <NUM>, <NUM> is <NUM> degrees. If the angle is altered, the length of the light path extension may also be altered.

In this way, by tuning the light with the polarization modulator, and optionally varying the thickness of the OPLEs in a series of digital light path length modulators, the system can adjust the light path extension in discrete steps. That is, by adjusting the polarization of a light beam, while it passes through a plurality of OPLEs, the total path length of the light can be adjusted incrementally between a shortest length (where the light passes through all OPLEs without reflection) and a longest length (where the light is reflected by all of the OPLEs.

In one embodiment, for OPLEs having different light path lengths - caused by one or more of different thickness, spacing or angle of the polarization sensitive reflective elements -- N OPLEs produce <NUM>^N light path lengths. For identical OPLEs, N OPLEs produce N+<NUM> light path lengths. In one embodiment, they may produce more light path lengths than N+<NUM>, or <NUM>^N. In one embodiment, the thickness of an OPLE may range from <NUM> microns to <NUM>, though it may be larger or smaller.

<FIG> illustrate one embodiment an OPLE combined with a digital light path length modulator. In this example, the OPLE <NUM> in the digital light path length modulator <NUM> has the polarization sensitive reflective elements at a different angle than the other OPLE <NUM>. As can be seen, the light path length adjustment in such an instance would be larger.

<FIG> illustrates one embodiment of a modulation stack including three digital light path length modulators. Each of the digital light path length modulators <NUM>, <NUM>, <NUM> includes a polarization modulator and an OPLE. In this example, two of the OPLEs <NUM>, <NUM> are self-aligned OPLEs.

In various embodiments, one or more of the following variations may be made: the effective thickness of the OPLEs may vary, as may the angles of the polarization sensitive reflective elements, and the OPLE may include one, two, or more plates. The effective thickness of the OPLE is defined as the cumulative thickness of the plates which are parts of the OPLE. Thus the effective thickness of OPLE <NUM> is different than the thickness of OPLE <NUM>, even though the individual plates in the two OPLEs <NUM>, <NUM> are identical.

With the shown set of three different OPLEs, the system can create up to eight, <NUM><NUM> focal lengths by selectively modulating the polarization, as follows:.

<FIG> illustrates one embodiment of manufacturing plates to be used as OPLEs. One of the advantages of the present system is the ability to easily and affordably manufacture consistent OPLEs. Note that the manufacturing creates plates, one or more of which form an OPLE. As discussed above, an OPLE may include one or more such plates.

The OPLEs, in one embodiment, are made with a plurality of base substrates <NUM> stacked. The substrates <NUM> are made of material that is optically transparent to the wavelengths being modulated. In one embodiment, the substrate <NUM> is made of glass. Alternatively, another material that is optically clear to the wavelengths used, such as plastic, transparent ceramic, silicon, sapphire, or other materials may be used. In one embodiment, each substrate <NUM> is between <NUM> microns and <NUM> thick. The thickness of the substrate defines the spacing of the polarization sensitive reflective elements in the final OPLEs.

Deposited on each substrate <NUM> is a polarization sensitive reflective element <NUM>, also referred to as a polarized beam splitter (PBS). In one embodiment, the polarization sensitive reflective element <NUM> is a dielectric film. The dielectric film may be deposited in various ways. In one embodiment, the dielectric film comprises oxide/nitride stacks which are spun, vapor deposited, or annealed, or evaporated onto the substrate. The polarization sensitive reflective element in another embodiment is a wire grid, nanoscale wire lines forming a closely spaced grid. In one embodiment, the sizing of the wire grid is wires that are <NUM> wide, with a <NUM> pitch. The spacing of the grid lines is smaller than the wavelength of the light.

The substrates <NUM> with the polarization sensitive reflective elements <NUM> are then attached to each other. The attachment may use adhesion, fusing, or other methods of securing together the substrates <NUM>, in one embodiment. In one embodiment, the adhesive used is optically clear glue. In one embodiment, the substrates <NUM> may be attached via a scaffolding, in which the substrates <NUM> are spaced apart using a support structure, rather than adhered or otherwise directly attached. Other methods of securing substrates together may be used. At a minimum, three substrates <NUM> with polarization sensitive reflective elements <NUM> are stacked to form an OPLE, while at a maximum the number of substrates <NUM> may be in the hundreds.

Once the adhesive or other attachment mechanism <NUM> is fully secured, the OPLEs are cut apart, at saw lines <NUM>. The angle of the saw lines <NUM> is between <NUM> and <NUM> degrees, and defines the angle of the polarization sensitive reflective elements <NUM> in the OPLE <NUM>. In a preferred embodiment, the saw lines are at a <NUM> degree angle. In one embodiment, the thickness of the OPLEs is between <NUM> microns and <NUM>. <FIG> illustrates a perspective view of an OPLE, having a length and width, and a thickness t2.

Returning to <FIG>, the process shown produces consistent OPLEs <NUM> with polarization sensitive reflective elements <NUM> at an angle between <NUM> to <NUM> degrees. These plates may be used for an OPLE which provides a staircase effect for lengthening the light path, which may be controlled by digitally modulating the polarization of the light impacting the OPLE. The OPLE and the digital light path length modulator is easily and consistently manufactured, and takes up very little space.

The digital correction system <NUM> in one embodiment changes the brightness of the light having a particular polarization through the digital light path length modulator <NUM>, to correct for the loss of brightness due to the OPLE. The digital correction system <NUM> in one embodiment spatially shifts the image elements entering the digital light path length modulator <NUM> which may be shifted by the digital light path length modulator <NUM>, to place them in the correct location upon exit from the digital light path length modulator <NUM>.

The corrections from digital correction system <NUM> may include brightness, lateral shift, and correction for other artifacts of the system. Such pre-calculation of the output of a digital display system is known in the art. Digital correction systems <NUM> are utilized to correct for lens warping, color separation, and other issues. The digital correction system <NUM> creates an output which is in the "rendering state" such that the perceived image by the user is correct.

In one embodiment, the optical path length extender (OPLE) <NUM> may not produce any spatial shift between the light that travels the longer and the shorter path through the OPLE <NUM>. In one embodiment, the OPLE <NUM> may produce a spatial shift or may be set to an intentional spatial shift.

The digital light path length modulator <NUM> includes a polarization modulator <NUM> and an OPLE <NUM>. The polarization modulator <NUM>, in one embodiment, is an electronically controlled element which can rotate the polarization of beams of light between two orthogonal states, state <NUM> and state <NUM>, by selectively modulating the polarization of some or all of the light. In one embodiment, the orthogonal states are clockwise and counterclockwise circularly polarized light. In one embodiment, the two orthogonal states are S-polarized and P-polarized linearly polarized light. The polarization modulator <NUM> may also be a filter which selectively filters light.

The polarized or selectively polarized light impacts the OPLE <NUM>. The OPLE <NUM> reflects light having a first polarization, and passes through light with a second polarization. The reflected light bounces, before exiting the OPLE <NUM>. This increases the path length of the light having the first polarization, compared to the light having the second polarization which passes directly through the OPLE <NUM>. In one embodiment, the light exits the OPLE <NUM> at the same angle that it entered the OPLE <NUM>.

Use of this system alters the relative light path length of the light with the two polarizations, because the light with a first polarization travels through a longer path than the light with the second polarization.

In one embodiment, light exiting the OPLE <NUM> is not spatially shifted, or intentionally spatially shifted, regardless of polarization. The specific configurations of an OPLE <NUM>, and its manufacture, is discussed in more detail below.

Imager <NUM> captures or displays the image. The imager <NUM> may be an electronic image sensor, such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) sensor. The imager <NUM> may be another capture element, such as film, binoculars, scope, or any mechanism to capture or display an image. In one embodiment, a digital correction system <NUM> may be used to correct the captured or displayed image to account for differences in brightness/light level or spatial shift between the light beams, because of the path they took.

<FIG> is a diagram of one embodiment of a first type of OPLE, referred to as a transverse OPLE. The OPLE includes one or more plates, each plate having a plurality of polarization sensitive reflective elements, which reflect light having a first polarization, and pass through light with a second polarization. The reflected light bounces between the polarization sensitive reflective elements two or more times, before exiting the OPLE. This increases the path length of the light having the first polarization, compared to the light having the second polarization which passes directly through the transverse OPLE.

<FIG> is a diagram of one embodiment of a second type of OPLE, referred to as a longitudinal OPLE not according to the claimed invention. The OPLE includes a reflective element on the bottom surface, which reflects light having a first polarization. The light in turn bounces back from the top of the OPLE, before exiting the OPLE through the bottom surface. This increases the path length of the light having the first polarization, compared to the light having the second polarization which passes directly through the longitudinal OPLE.

<FIG> illustrate one embodiment of the light paths in a cross-sectional view of one embodiment of an optical path length extender (OPLE) not according to the claimed invention. The OPLE includes an entry surface, coated by a partially reflective coating <NUM>, a quarter wave plate <NUM>, and a wire grid polarizer <NUM>. In this example, the light polarization is defined in the direction of propagation. For example, in one embodiment:.

Of course these particular polarizations are merely exemplary, and the actual polarizations are two sets of orthogonal polarizations. One of skill in the art would understand that the polarizations may be altered without changing the invention.

For light with polarization type two, here C1 (circular polarization type <NUM>), the light passes through the partially reflective coating <NUM>, passes through the quarter wave plate <NUM>, and exits through wire grid polarizer <NUM>. The quarter wave plate <NUM> alters the C1 polarization to an L1 polarization, so the exiting light is L1 polarized. This may be input to another digital light path length modulator.

<FIG> show the path taken by light with polarization C2 (circular polarization of type <NUM>), as it impacts the OPLE. It is changed to polarization L2 by the quarter wave plate <NUM>. Light with polarization L2 is reflected by the wire grid polarizer <NUM>. <FIG> shows the path of the reflected light, returning through the quarter wave plate, which re-converts it to C2 polarization. It then impacts the partially reflective coating <NUM>.

The partially reflective coating <NUM> reflects a portion of the light, as C1 polarized light, and permits the rest of the light to pass through, as C2 polarized light. The now C1 polarized light passes through the quarter wave plate one more time, before exiting through the wire grid polarizer. Thus, the path of the light entering with the C2 polarization is three times the length of the path of light entering with the C1 polarization, since it reflects back up through the OPLE, and down through the OPLE a second time, before exiting. However, there is no lateral shift of the virtual source during this process.

<FIG> illustrates another embodiment of an OPLE not according to the claimed invention. OPLE <NUM> includes a first layer <NUM> with a partially reflective coating <NUM>, a middle layer <NUM> comprising a quarter wave plate <NUM>, and a third layer <NUM> including a wire grid polarizer <NUM>. OPLE <NUM> in one embodiment utilizes spacers <NUM> between each of the layers. In another embodiment, the layers may be attached to each other.

The partially reflective coating <NUM> is applied to a first layer <NUM>. The partial reflective coating <NUM> is one embodiment a thin layer of a reflective metal or dielectric, in the <NUM>-<NUM> angstrom thickness. In one embodiment, material is aluminum or silver. In one embodiment, partially reflective coating <NUM> is applied to a bottom of the first layer <NUM>. In one embodiment, the middle layer <NUM> is entirely made of quarter wave plate <NUM>, or may have a quarter wave plate portion. The quarter wave plate may be mica, or a polymer plastic. There is no limitation on a size of the quarter wave. The bottom layer includes a wire grid polarizer <NUM>, which may be applied to the top of the third layer. Each of the layers is made of a material clear to the type of light that is used with the OPLE. The material may be a glass, plastic, sapphire, or other material. The thickness of the OPLE is selected to optimize the value of the light path lengthening. In one embodiment, the reflective elements may be shaped, rather than flat.

<FIG> illustrates a perspective view of one embodiment of the elements of an OPLE. As can be seen the layers may be separately made and then either attached using spacers, or using intermediate layers of glass or other materials. The height of the OPLE is defined by the size of the layers, including intermediate layers or spacers. The height controls the lengthening of the optical path.

<FIG> is a flowchart of light path extension through one embodiment of an OPLE not according to the claimed invention. The process starts at block <NUM>. At block <NUM>, circularly polarized light is received, with polarization C1. The light passes through the quarter wave plate, changing polarization to linear (L1). The light hits the wire grid polarizer at block <NUM>.

At block <NUM>, it is determined whether the light of polarization L1 will be reflected by the wire grid polarizer. If the L1 polarized light is not reflected, at block <NUM> the L1 polarized light is passed through the wire grid polarizer, and exits the longitudinal OPLE.

If the L1 polarization is reflected, as determined at block <NUM>, at block <NUM> the light is reflected back through the OPLE. At block <NUM>, the reflected light passes through the quarter wave plate again, changing the polarization from the L1 to C1.

At block <NUM>, the partially reflective coating reflects back a portion of the light. The reflected portion of the light changes polarization to C2. The twice reflected light passes through the quarter wave plate again, changing the polarization from C2 to L2.

At block <NUM>, the L2 polarized light passes out of the longitudinal OPLE. The process then ends.

<FIG> is one embodiment of a modulation stack including a plurality of OPLEs. The modulation stack includes four digital light path length modulators. Each of the digital light path length modulators <NUM>, <NUM>, <NUM>, <NUM> includes a polarization modulator and an OPLE. In this example, the first OPLE <NUM> is a longitudinal OPLE, while the other OPLEs are transverse OPLEs. One of the transverse OPLEs <NUM> is a self-aligned OPLE.

With the shown set of four different OPLEs, the system can create up to sixteen, <NUM><NUM> focal lengths by selectively modulating the polarization, as follows:.

In one embodiment, because the light exits from both sides of a longitudinal OPLE, the longitudinal OPLE <NUM> is preferentially a first OPLE in a modulation stack <NUM> that includes longitudinal OPLEs. In one embodiment, the number of longitudinal OPLEs <NUM> is limited by the level of light loss for each longitudinal OPLE.

<FIG> is a flowchart of one embodiment of manufacturing a longitudinal OPLE. The process starts at block <NUM>.

At block <NUM>, two optically transparent sheets of material are used. In one embodiment, the sheet is made of glass. Alternatively, another material that is optically clear to the wavelengths of the system, such as plastic, transparent ceramic, silicon, sapphire, or other materials, may be used.

At block <NUM>, a partially reflective coating is applied to the first surface of the first sheet. In one embodiment, this first surface is the "top" surface of the sheet, which will form the entry surface of the OPLE.

At block <NUM>, the second surface of the first sheet is attached to the quarter wave plate. In one embodiment the adhesive used is optically clear glue. In one embodiment, the substrates may be attached via spacers, in which the substrates are spaced apart using a support structure, rather than adhered or otherwise directly attached. Other methods of securing substrates together may be used. The quarter wave plate is made of a birefringent material, for which the index of refraction is different for different orientations of light passing through it. The quarter wave plate may be a bulk material, such as mica, quartz, calcite, or plastic. The quarter wave plate may be a film applied to an optically clear material. The quarter wave plate converts circularly polarized light into linear polarized light, and vice versa.

At block <NUM>, the second sheet is attached to the other side of the quarter wave plate. The quarter wave plate is now sandwiched between the two transparent sheets of material. The attachment may be via adhesive, spacers, or other methods.

At block <NUM>, the wire grid polarizer is applied to the second surface of the second sheet. This is the exit surface, in one embodiment.

At block <NUM>, the resulting material is cut into appropriately sized longitudinal OPLEs. The process then ends.

Although this is illustrated as a flowchart, one of skill in the art would understand that the steps need not be taken in the order shown. For example, the wire grid polarizer may be applied to the optically transparent material at any time, before or after the second sheet is integrated into the OPLE structure. Similarly, the partially reflective coating may be applied at any time.

The process shown produces consistent longitudinal OPLEs. These longitudinal OPLEs can be used to lengthen the light path, which may be controlled by digitally modulating the polarization of the light impacting the OPLE. The OPLE and the digital light path length modulator is easily and consistently manufactured, and takes up very little space.

In one embodiment, an OPLE is made up of one or more polarization sensitive reflective elements, which cause light of one polarization state to travel a longer path than light of the other polarization state. In one embodiment, the OPLE comprises a cuboid with one or two diagonal polarization sensitive reflective elements, and quarter wave plate and a mirror on both sides. Light of a second polarization state is reflected by the polarization sensitive reflective element, passes through the quarter wave plate, is reflected by the mirror and passes through the quarter wave plate for the second time. This reverses the polarization of the light, which is reflected at least once more prior to exiting the orthogonal OPLE. In one embodiment, the structure supporting the polarization sensitive reflective elements are four triangular prisms arranged in a cuboid, which support two differently oriented angled polarization sensitive reflective elements and with a light path extender on one side. In one embodiment, the angled polarization sensitive reflective element comprises a wire grid polarizer or a thin-film polarizer coating. In one embodiment, the OPLE may be made up of one or more plates with a plurality of polarization sensitive reflective elements. A plurality of digital light path length modulators create a modulation stack.

In one embodiment, by using a modulation stack, the number of focal planes can be increased. This provides the capacity to build a system that can meet the physiological requirements of human vision, by creating a display in which the 3D cues of overlap, focus, and vergence match. This produces a better quality 3D display and can prevent the headaches associated with 3D displays.

This mechanism in one embodiment can also be used for image capture, and various other uses in which light waves or other waves in a similar spectrum are either projected or captured, including but not limited to cameras, binoculars, 3D printing, lithography, medical imaging, etc. Creating a simple, easy to manufacture digital light path length modulator is like the step from vacuum tubes to transistors; it enables more complex, cheaper, and much more dense digitally controlled elements, which can become building blocks for a wide range of uses.

<FIG> is diagram of one embodiment of an orthogonal optical light path length extender (OPLE) not according to the claimed invention. The orthogonal OPLE <NUM> includes four prisms 2010A, 2010B, 2010C, and 2010D, arranged to form a cuboid. In one embodiment, the cuboid is a square cuboid. The prisms 2010A, 2010B, 2010C, 2010D define an entry surface (base of prism 2010A), an exit surface (base of prism 2010C), and two sides (bases of prisms 2010B and 2010D). In one embodiment, the height (h1) of the face defined by prisms 2010A, 2010B, 2010C, and 2010D is between <NUM> and <NUM>. In one embodiment, the height is based on an aperture of the system.

Between the contact areas of the prisms 2010A, 2010B, 2010C, 2010D are angled polarization sensitive reflective elements (in one embodiment wire grid polarizers) 2030A, 2030B, 2040A, 2040B. The first diagonal formed by the prisms, formed by the shared edges prisms 2010A and 2010B and the shared edges of prisms 2010C and 2010D, has a wire grid polarizer in a first orientation 2030A, 2030B, and the perpendicular diagonal, formed by the shared edges of prisms 2010B and 2010C and the shared edges of prisms 2010A and 2010D, has a wire grid polarizer in a second orientation 2040A, 2040B.

A path length extender <NUM> is positioned at one side of the OPLE <NUM>, here on the base of prism 2010D. In one embodiment, the height of the path length extender <NUM> (h2) is between <NUM>/<NUM> to <NUM>.

On both sides of the OPLE <NUM> there is a quarter wave plate 2050A, 2050B and mirror 2060A, 2060B. In one embodiment, the quarter wave plate 2050A, 2050B is a birefringent material such as mica. In one embodiment, the quarter wave plate 2050A, 2050B is a polycarbonate film, which may be applied to the base of the side prism 2010B and the base of the path length extender <NUM>. In another embodiment, the quarter wave plate 2050B may be applied to the top of the path length extender <NUM> or the bottom of prism 2010D.

In one embodiment, the prisms 2010A, 2010B, 2010C, 2010D and path length extender <NUM> are made of material transparent to the wavelengths being used, e.g. optically transparent for light in optical wavelengths. The prisms 2010A, 2010B, 2010C, 2010D and path length extender <NUM> are glued together, in one embodiment.

<FIG> illustrates an alternative embodiment in which the prism on one side is a pentagonal prism <NUM>. In this embodiment, the path length extender <NUM> may be manufactured as part of one of the prisms, here prism <NUM>. Prism <NUM> replaces prism 2010D and path length extender <NUM> shown in <FIG>.

Additionally, in one embodiment there is a small non-reflective area <NUM> on the tip of the prisms forming the intersection of the prisms 2010A, 2010B, 2010C, <NUM>. In one embodiment, the non-reflective area <NUM> may be a black spot in the cross section. The non-reflective area <NUM> ensures that light hitting the intersection point does not cause scattering of the light.

The embodiments of <FIG> and <FIG> are not designed to be exclusive, and the elements may be mixed and matched.

<FIG> illustrate the light paths followed by light in the orthogonal OPLE of <FIG>. To enable seeing the light paths, the light bounced from the mirror is offset slightly. One of skill in the art would understand that this offset is for illustration purposes only. <FIG> illustrates the light path for light having a first polarization <NUM>. The light with the first polarization exits polarization rotator <NUM>, and enters orthogonal OPLE <NUM>. It passes through the wire grid with the first orientation, and is reflected by the wire grid with the second polarization. It passes through quarter wave plate, then is reflected by the mirror. Due to passing through the quarter wave plate twice, the light now has the second polarization. Therefore, it passes through the wire grid with the second orientation, and is reflected out of the orthogonal OPLE <NUM> by the wire grid with the first orientation. The length of the light path through the OPLE <NUM> is <NUM>*h1, twice the length of the sides of the square formed by the prisms. Note that although it is illustrated as having a thickness, the quarter wave plate does not add significantly to the light path length.

<FIG> illustrates the light path for light having a second polarization <NUM>. The light with the second polarization exits polarization rotator <NUM>, and enters orthogonal OPLE <NUM>. It is reflected by the wire grid with the first orientation, and passes through the light path extender. It passes through the quarter wave plate, and is reflected by the mirror back through the quarter wave plate and the path length extender. Due to passing through the quarter wave plate twice, the light now has the first polarization. Therefore, it is reflected out of the orthogonal OPLE <NUM> by the wire grid with the second orientation. The length of the light path through the OPLE <NUM> is <NUM>*h1 + <NUM>*h2, twice the length of the sides of the square formed by the prisms plus twice the length of the path length extender. In a typical configuration of a <NUM>×<NUM> prism, and a <NUM> light path extender, the difference in the light paths therefore is <NUM>%, <NUM> to <NUM>. In one embodiment, a polarization modulator is placed before the OPLE, so that light of one polarization is sent through the OPLE <NUM>, resulting in all of the light exiting at the same time, having traveled the same path length. In another embodiment, the light sent through the OPLE may include light of both polarizations, and the polarization selection may occur after the light goes through the OPLE <NUM>.

<FIG> illustrates the light path for light having the second polarization entering the OPLE <NUM> at a different location. As can be seen, in this instance the light passes through the wire grid with the first orientation, before being reflected by the wire grid with the second orientation through the light path extender. Thus, the distance traveled by the light is again <NUM>*h1 + <NUM>*h2.

<FIG> illustrates an alternative embodiment of the orthogonal OPLE. In this embodiment, there is only a single polarization sensitive reflective element (in one embodiment a wire grid polarizer) <NUM>, framed by the two quarter wave plates 2220A, 2220B and mirrors 2215A, 2215B. In one embodiment, this configuration may be built with two triangular prisms. In another embodiment, this configuration may be built with a single triangular prism.

In one embodiment, this configuration does not utilize a light path extender, in one embodiment, because there is a single polarization sensitive reflective element only the light with the second polarization is reflected. Light with the first polarization <NUM> passes straight through the OPLE <NUM>. Light with the second polarization <NUM> is reflected by the polarization sensitive reflective element <NUM>, passes through the quarter wave plate 2220B, is reflected by the mirror, and passes through the quarter wave plate 2220B again. It now has the first polarization and thus passes through the polarization sensitive reflective element <NUM> before encountering the second quarter wave plate 2220A, and being bounced back once more, with the polarization rotated back to the second polarization. It then impacts the polarization sensitive reflective element <NUM> for the third and last time, and is reflected out of the OPLE <NUM>. Thus, for a square cross-section of orthogonal OPLE <NUM>, the path length for the light with the first polarization is W, the width of the polarizer <NUM>, while the path length for the light with the second polarization is <NUM> + W (or <NUM>), since it bounces twice between the sides of the OPLE <NUM>.

<FIG> illustrates an alternative embodiment of the OPLE with a single polarization sensitive reflective element not according to the claimed invention. In this configuration, the OPLE <NUM> includes the same elements of polarization sensitive reflective element <NUM>, two quarter wave plates 2220A, 2220B, and two mirrors 2215A, 2215B. However, instead of utilizing prisms to position the polarization sensitive reflective element <NUM>, the polarization sensitive reflective element 2210is supported by a different support structure. In one embodiment, the support structure may be a thin sheet of glass, plastic, film, or other material that can provide support for a polarization sensitive reflective element such as a wire grid polarizer or a thin film polarizer coating and can maintain its structure. In one embodiment, the prisms may be replaced by air, and the polarization sensitive reflective element <NUM> may be supported on one or more edges of the support structure by being attached to a frame or other structure. In another embodiment, the polarization sensitive reflective element may be supported by a support structure such as a diagonal piece of glass, plastic, or other optically transparent material. This configuration may be useful if weight is a concern.

<FIG> illustrates an embodiment of the OPLE <NUM> shown in <FIG> not according to the claimed invention, without the prism supporting structure. This configuration includes a support framework for the polarization sensitive reflective elements 2240A, 2240B, but does not include the prisms shown in <FIG>. In one embodiment, the polarization sensitive reflective elements 2240A, 2240B are fastened to the top of the path length extender portion <NUM> of the OPLE <NUM>. In one embodiment, the path length extender may be formed by a framework which provides a spacing between the bottom of the polarization sensitive reflective elements 2240A, 2240B and the quarter wave plate 2244B.

<FIG> illustrates an alternative embodiment of an OPLE not according to the claimed invention. In this configuration, both sides of the OPLE <NUM> have a path length extender 2270A, 2270B. In this configuration, the difference in path length is the difference in the height of the path length extender 2270A, 2270B. In one embodiment, the system may provide an adjustable height, enabling changes in the light path length. In one embodiment, the height may be adjusted by moving the mirror relative to the rest of the OPLE to create a longer or shorter path length extender on either or both sides of the OPLE.

<FIG> illustrates another embodiment of an OPLE not according to the claimed invention. In this configuration, the light path length extension on one or both sides is provided by a curved mirror <NUM>. In one embodiment, no light path length extender is needed. In another embodiment, an optional light path extender is used. By using mirrors which have an optical power the virtual object distance is modulated. This may be combined with a light path extender (not shown).

<FIG> is a diagram of one embodiment of assembling the pieces of an orthogonal OPLE. The triangular prisms <NUM>, <NUM>, <NUM>, <NUM> are matched in size. In one embodiment, each prism <NUM>, <NUM>, <NUM>, <NUM> has isosceles triangle ends. The triangles in one embodiment are <NUM>-<NUM>-<NUM> triangles. In one embodiment, the prisms are made of glass or plastic that is transparent to the wavelengths used by the system. For visual object representation or capture, the prism is transparent to light in the visual frequency range.

The wire grid or other polarization sensitive reflective element (not shown) is placed on the prisms. In one embodiment, wire grids may be placed on the prisms, glued onto the prisms, or nano-imprinted on the prisms. In one embodiment, one side of each prism <NUM>, <NUM>, <NUM>, <NUM> has a wire grid placed on it, such that there are two prisms with polarization sensitive reflective elements of each orientation. In another embodiment, two prisms may have polarization sensitive reflective elements of opposite orientations placed on the two sides of the prism.

Once the polarization sensitive reflective elements are applied, the prisms <NUM>, <NUM>, <NUM>, <NUM> may be attached to each other. In one embodiment, the prisms are glued together with index matched glue, which does not have an optical effect.

The path length extender <NUM> is then attached to a base of a prism, here prism <NUM> In one embodiment, the path length extender <NUM> is also made of glass or plastic transparent to the wavelengths used by the system, and it is glued using index matched glue. In another embodiment, as shown above in <FIG>, one of the prisms may include an integral light path extender. In that configuration, the light path extender does not need to be attached to the prism.

The quarter wave plates <NUM>, <NUM> are then coupled to the sides of the cuboid formed by the prisms <NUM>, <NUM>, <NUM>, <NUM> and the light path extender <NUM>. In one embodiment, the quarter wave plates <NUM>, <NUM> may be a film applied to the base of the prism <NUM> and path length extender <NUM>. Mirrors <NUM>, <NUM> are coupled to the quarter wave plates <NUM>, <NUM>. In one embodiment, the mirrors are glued on, using index matched glue.

Although the prisms <NUM>, <NUM>, <NUM>, <NUM> here are shown as relatively short pieces, in one embodiment the system may be assembled as a large rectangle, and then cut to an appropriate size. The size, in one embodiment, depends on the aperture of the system. In one exemplary embodiment, the face formed by the prisms is 5mmx5mm (H), and the length of the OPLE (L) is <NUM>. The length may be between <NUM> and <NUM>.

<FIG> illustrates one embodiment of the assembly of an OPLE without the prisms not according to the claimed invention. In one embodiment, polarization sensitive reflective elements of the first orientation <NUM> and second orientation <NUM> are intersected. The polarization sensitive reflective elements <NUM>, <NUM> are placed on a support structure, in one embodiment. The support structure may be plastic, glass, film, or another optically clear material which can provide structure for the polarization sensitive reflective elements s <NUM>, <NUM>. In one embodiment, the polarization sensitive reflective elements <NUM>, <NUM> and their support structure have half slits, so the two polarization sensitive reflective elements <NUM>, <NUM> slide into each other forming an X shape. In one embodiment, the polarization sensitive reflective elements <NUM>, <NUM> are perpendicular to each other, and the wire grid polarizer with the first orientation is at a -<NUM> degree angle from the entrance surface of the OPLE.

In one embodiment, in this configuration the center of the OPLE has a non-reflective area to ensure that no negative optical effects are introduced into the system. In one embodiment, the OPLE includes the polarization sensitive reflective elements <NUM>, <NUM>, quarter wave plates <NUM>, <NUM>, and mirrors <NUM>, <NUM>.

The structure is supported by a framework <NUM>, illustrated for simplicity by framing elements. The framework in one embodiment may be plastic, glass, or another material, and need not be transparent as long as it is capable of supporting the mirror and polarization sensitive reflective elements. In one embodiment, the quarter wave plates <NUM>, <NUM> may be attached to the mirror <NUM>, <NUM>. In one embodiment, there may be a path length extender (not shown). In another embodiment, the bottom of polarization sensitive reflective elements <NUM>, <NUM> is positioned a height h2 above the quarter wave plate <NUM> and mirror <NUM> to create the spacing of the path length extender without requiring a physical object.

From <FIG> and <FIG> it should be clear how to assemble the various OPLE configurations shown in <FIG>, <FIG>, <FIG>, 2C, 2D, and 2E. Although these embodiments are separately shown, one of skill in the art would understand that elements from the configurations may be utilized in other configurations as well.

<FIG> is a diagram of one embodiment of a modulation stack including a plurality of OPLEs. This exemplary modulation stack includes four digital light path length modulators, each of the modulators <NUM>, <NUM>, <NUM>, and <NUM> includes a polarization modulator and an OPLE. In this example, the first OPLE <NUM> is a longitudinal OPLE <NUM>, the second OPLE is an orthogonal OPLE <NUM>, the third and fourth OPLEs are transverse OPLEs <NUM>, <NUM>. With the shown set of four different OPLEs, the system can create up to sixteen focal lengths by selectively modulating the polarization.

In one embodiment, because the light exits from both sides of a longitudinal OPLE, the longitudinal OPLE <NUM> is preferentially a first OPLE in a modulation stack <NUM> that includes longitudinal OPLEs.

Claim 1:
A system comprising:
an optical path length extender [OPLE] (<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,
<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>) including a plurality of polarization sensitive reflective elements (<NUM>, <NUM>), wherein all of the plurality of polarization reflective elements are parallel and at an angle between <NUM> and <NUM> degrees to an entrance surface of the OPLE, the OPLE comprising two light paths having different path lengths, such that light having a first polarization is directed through a first light path, and the light having a second polarization is directed through a second light path, such that a portion of an image is at a first focal length defined by the first light path, and a second portion of the image is at a second focal length defined by the second light path; and
an imager to capture or display the image.