Abstract:
Aspects of the subject disclosure are directed towards safely projecting a diffracted light pattern, such as in an infrared laser-based projection/illumination system. Non-diffracted (zero-order) light is refracted once to diffuse (defocus) the non-diffracted light to an eye safe level. Diffracted (non-zero-order) light is aberrated twice, e.g., once as part of diffraction by a diffracting optical element encoded with a Fresnel lens (which does not aberrate the non-diffracted light), and another time to cancel out the other aberration; the two aberrations may occur in either order. Various alternatives include upstream and downstream positioning of the diffracting optical element relative to a refractive optical element, and/or refraction via positive and negative lenses.

Description:
BACKGROUND 
       [0001]    A known technique for active stereo camera sensing uses an infrared (IR) projected pattern in conjunction with one or more cameras capable of detecting IR. As higher resolution three-dimensional (3D) sensors are devised to operate at larger and larger field of views and distances, and with higher and higher resolutions, the total IR illumination power needs to increase. 
         [0002]    IR lasers in conjunction with one or more diffractive optical elements (DOEs) have been used as power sources to produce such patterns. However, at such power levels, the intensity of the DOE&#39;s non-diffracted light (or zero-order light, usually a fixed fraction of total energy) is an eye-safety hazard. 
         [0003]    One common way to reduce the intensity of the non-diffracted light is to have two spot-generating DOEs in a series. The first DOE creates the complex dot pattern, and the second one replicates it by a small number N×N, e.g., 3×3. As the non-diffracted light is a fixed ratio of the total power, (e.g., represented by α), the final power of the non-diffracted light is α/N 2 . Thus, the second DOE effectively decreases the intensity power by the factor N 2 . This resulting decrease in power is counter to the need for increased IR illumination power. 
       SUMMARY 
       [0004]    This Summary is provided to introduce a selection of representative concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used in any way that would limit the scope of the claimed subject matter. 
         [0005]    Briefly, various aspects of the subject matter described herein are directed towards an eye safe diffraction system generally based upon reducing the power of the non-diffracted light by introducing defocusing or other aberrations. In one aspect, a DOE set, compromising of one or more DOEs passes the undiffracted light (i.e. the zero order) unaffected while the diffracted light (i.e. non-zero order light) is diffracted to the desired light pattern. The DOE set is also configured to aberrate (e.g. focus, defocus, add a Zernike wavefront or any other arbitrary aberration) the diffracted light. A refractive optical element (ROE) optically coupled to the DOE set, aberrates refractivelly (e.g. focus, defocus, add a Zernike wavefront or any other arbitrary aberration) the undiffracted light and the diffracted light from the DOE set. The aberration introduced by the DOE set and the ROE are such that their total effect to the diffracted light is cancelled or substantially cancelled while the undiffracted light is only affected by the ROE. 
         [0006]    In one aspect, a projection mechanism is configured to output a pattern of diffracted light spots, and to defocus non-diffracted light output therefrom. The projection mechanism comprises optically coupled components including a laser light source, a collimating lens, a refractive lens, and a diffractive optical element set comprising at least one diffractive element able to introduce aberrations to the beam (e.g. defocus by means of encoding a diffractive lens element, such as a diffractive Fresnel lens or Fresnel zone plate in the DOE) The refractive lens is configured to aberrate the non-diffracted light, and to cancel or substantially cancel abberations of the diffracted light spots by the diffractive element. 
         [0007]    In one aspect, a diffractive optical element set is optically coupled with a refractive lens. The refractive lens defocuses and thus expands the non-diffracted light that is not diffracted by the diffractive optical element set, and cancels or substantially cancels focusing introduced to the diffracted light introduced by the diffractive optical element set. 
         [0008]    Other aspects and advantages may become apparent from the following detailed description when taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present invention is illustrated by way of example and not limited in the accompanying figures in which like reference numerals indicate similar elements and in which: 
           [0010]      FIG. 1  is a block diagram representing example components of a device that projects diffracted light in a diffraction pattern while diffusing non-diffracted (zero-order) light, according to one example implementation. 
           [0011]      FIG. 2  is a representation of an arrangement of optical-related components, including a diffractive optical element and a downstream negative lens, in which the arrangement outputs a diffracted light pattern while expanding non-diffracted light based upon focusing, according to one example implementation. 
           [0012]      FIG. 3  is a representation of an arrangement of optical-related components, including upstream negative lens relative to a diffractive optical element, in which the arrangement outputs a diffracted light pattern while expanding non-diffracted light based upon focusing, according to one example implementation. 
           [0013]      FIG. 4  is a representation of an arrangement of optical-related components, including a diffractive optical element and a downstream positive lens, in which the arrangement outputs a diffracted light pattern while expanding non-diffracted light based upon focusing, according to one example implementation 
           [0014]      FIG. 5  is a representation of an arrangement of optical-related components, including an upstream positive lens relative to a diffractive optical element, in which the arrangement outputs a diffracted light pattern while expanding non-diffracted light based upon focusing, according to one example implementation. 
           [0015]      FIG. 6  is a representation of an arrangement of optical-related components, including a plurality of diffractive optical elements in series and a downstream negative lens, in which the arrangement outputs a diffracted light pattern while expanding non-diffracted light based upon focusing, according to one example implementation. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Various aspects of the technology described herein are generally directed towards a light pattern projection system that is eye safe. As is known with laser illumination, zero-order light is light not diffracted into the light pattern, referred to herein as non-diffracted light. Thus, if left as is, the non-diffracted light is dangerous to the eye. The technology described herein makes the non-diffracted light eye safe by expanding (e.g., defocusing) or blocking it, while leaving the projected diffracted light pattern substantially unchanged. 
         [0017]    To this end, diffracted light is both aberrated diffractively and aberrated refractively in one stage, while non-diffracted light is not. Note that as used herein, “aberration” and the like are used in an optical sense, e.g., as a lower frequency change in the wavefront compared with the high frequencies introduced by the DOE, that can be encoded both on a diffractive element (DOE) and/or a refractive element (ROE), (and not to imply any unwanted/uncontrolled/negative effect on the system). In another (prior or subsequent) stage, both the diffracted light and the non-diffracted light are aberrated. In this other stage, the diffracted light is basically aberrated to cancel the prior or subsequent aberrations, such that overall after both stages the diffracted light is not aberrated overall. Because the non-diffracted light is only aberrated in this other stage, the non-diffracted light is aberrated and thus defocused to an eye safe level. 
         [0018]    It should be understood that any of the examples herein are non-limiting. For instance, various arrangements of lenses are used as examples, however the technology described herein is not limited to these arrangements. As such, the present invention is not limited to any particular embodiments, aspects, concepts, structures, functionalities or examples described herein. Rather, any of the embodiments, aspects, concepts, structures, functionalities or examples described herein are non-limiting, and the present invention may be used various ways that provide benefits and advantages in optics, laser technology and light pattern projection in general. 
         [0019]      FIG. 1  shows a generalized block diagram in which a device  102  such as a built into or part of a home entertainment device is configured with projection mechanism including a laser  104  to project light (e.g., IR) in an illumination pattern. The illumination pattern need not be evenly distributed, and indeed may change over time. Stereo cameras  106 A and  106 B capture the reflection from an illuminated person  108  (and/or other object) and use the captured images as desired; note that a single camera may be used in a given implementation. 
         [0020]    For example, a structured illumination depth mapping system may use a light pattern to introduce geometric parallax that is then recorded by a number of cameras. One of the most efficient ways to illuminate a scene is by using a laser diode together with a diffractive optical element (DOE). Laser diodes are one of the most efficient ways to create light, and DOEs can create an arbitrary array of spots in the scene. However, as the distance, resolution and Field of View (FOV) of such systems increase, more light and thus a higher energy laser is needed to illuminate the scene. 
         [0021]    As described herein, a diffractive optical element mechanism  110  disperses the laser light into a large number of spots (also referred to as dots regardless of any shape or size) in a desired pattern, such as on the order of 100,000 dots. Some of the pattern (e.g., four dots of the typically many thousand) is represented in  FIG. 1  by the solid lines coming from the mechanism  110  and the dots on the image plane  112 . Note that neither  FIG. 1  nor its components, nor any of the figures or their components shown herein are intended to be to scale or convey any particular distance or relative sizes; however a typical distance from the image plane to the light source in one implementation is on the order of 0.5 to 4.0 meters ( FIG. 2 ). 
         [0022]    However, primarily due to fabrication inaccuracies in diffractive optical elements, some of the light referred to as non-diffracted light is not affected by a system&#39;s diffractive optical element. Although careful fabrication can reduce the power of the non-diffracted light to below one percent of the laser power, the non-diffracted light still may be above the eye safe limit/level. Given that the zero-order is a fixed ratio of the laser power, heretofore this put an upper limit on the allowed illumination power. 
         [0023]    As described herein, the non-diffracted light is made eye safe. More particularly, the non-diffracted light is that which is not affected by the diffractive optical element, but it is affected by a normal refractive optical element (ROE), e.g., lens. Thus, described herein is combining a diffractive (e.g., positive) lens encoded in the diffractive optical element and a ROE (e.g., a negative lens) that cancels the diffractive lens, whereby the spots illuminating a scene remain unaffected, with the exception of the non-diffracted light. The non-diffracted light, because it is not affected by the diffractive lens, is only affected by the ROE; a negative lens defocuses the non-diffracted light to make it eye safe. Note that as used herein, the term “cancel” refers to canceling the effect of a previous “downstream” aberration, or proactively canceling the effect of a subsequent “upstream” aberration. 
         [0024]    To this end, in addition to providing the dot pattern, as described herein the diffractive optical element with the mechanism  110  is configured to spread out the non-diffracted light. In three dimensions, the non-diffracted light is basically spread out into a wider cone, so that if the non-diffracted light enters the eye of the person  108 , the energy of the non-diffracted light is distributed over a much larger area so that only a portion of the energy can enter the eye, thus providing for safety. 
         [0025]    The spreading of the non-diffracted light is a function of distance, and thus some consideration is given to prevent someone from getting too close to the light output before the non-diffracted light can be sufficiently spread. For example, as shown in  FIG. 1 , a distance D from the light output to the nearest point a person&#39;s eye can be positioned may be provided, using any type of physical barrier, so that a person cannot get sufficiently close to the light output to damage the eye. The physical barrier may be provided by recessing the optics in the device, and/or via a tube or the like that physically increases the closest distance to the light output. Internal mirrors also may be used to increase the distance. 
         [0026]      FIG. 2  is a side-view, two-dimensional representation that shows an implementation of components of an example diffractive optical element with a non-diffracted light expansion mechanism  210 . The optical arrangement in this example embodiment comprises a laser  204 , a positive collimating lens  222 , a diffractive optical element  224  (encoded within a diffractive lens) and a ROE  226 . Note that such a diffractive optical element  224  with a diffractive lens may be referred to as a hologram, and may be designed based upon the known algorithms such as Gerchberg Saxton algorithm, and fabricated using known techniques such as etching glass, embossing/molding plastic, and so forth. 
         [0027]    The laser  204  and the collimating lens  222  makes the received light rays parallel; the focal length is represented by f. The light from the collimating lens  222  illuminates the diffractive optical element  224 , which has a diffractive lens (such as a Fresnel zone plate) encoded thereon. The diffractive optical element  224  performs two functions. One function splits the beam into many weaker beams, and another function (via the encoded diffractive lens) introduces a (e.g., relatively slight) focusing power. 
         [0028]    The negative ROE  226  cancels the focusing effect of the diffractive lens such that the net effect on the majority of the illumination spots is cancelled. Thus, for example, the spot corresponding to the point  228  is substantially positioned on the image plane  212  in  FIG. 2  as if the diffractive optical element  224  did not include focusing power and there was no negative ROE. 
         [0029]    However, the negative lens  226  does affect the non-diffracted light which was not affected by the diffractive optical element  224 , namely, the negative lens  226  defocuses the non-diffracted light as represented in  FIG. 2  by the dashed diverging lines. In other words, the non-diffracted light (not affected by the diffractive optical element  224 ) is also not affected by the diffractive lens, whereby the net effect is to remain unfocused after it leaves the optical arrangement. 
         [0030]    The defocusing (and thus its width) of the non-diffracted light may be kept to the minimum permissible diameter by the eye safety limitations to minimize the strength of the diffractive lens and negative refractive lens. Notwithstanding, the strength of the diffractive lens and negative refractive lens can be varied as desired to control how quickly (in terms of distance) the non-diffracted light spreads out, so that, for example, the strength may be used to compensate for different implementation scenarios. Similarly, the size (diameters) of the lenses may be varied for safety, e.g., larger lenses spread the light energy over a larger area, e.g., such as larger than the eye pupil. 
         [0031]    As can be seen, the lens arrangement of  FIG. 2  overcomes one of the primary challenges in designing a DOE-laser projector, namely ensuring that the spots in the scene are eye safe. The majority of spots are made eye safe because of diffraction, while the non-diffracted light spot is made safe by the defocusing/diffusion as described herein. 
         [0032]      FIG. 3  shows an alternative embodiment similar to  FIG. 2  except that in this arrangement  310 , a negative lens  336  is placed in front of the diffractive optical element  334 . The negative lens  336  spreads all light, including the non-diffracted light, however the diffractive optical element  334  refocuses the light (e.g., the dot  338  is refocused on the image plane  312 ) except for the non-diffracted light (represented by the diverging dashed lines) which continues to spread. 
         [0033]      FIG. 4  shows another alternative embodiment, in which rather than defocusing the non-diffracted light via a negative lens, an arrangement  410  comprising a positive lens  447  focuses the non-diffracted light to a point  449  from which the non-diffracted light again begins to diverge/defocus. Note that to cancel the effect of the positive lens  447  on the diffracted dots, the diffractive optical element  445  includes (e.g., is encoded with) a negative lens that cancels the positive lens  447  for all light other than the non-diffracted light. In this way, the dots are refocused on the image plane  412 , as represented by the converging solid lines/dot  448  in  FIG. 4 . A stop may be located at the point  449  to physically block the non-diffracted light from being projected further. For example, an opaque dot on an otherwise transparent surface may be positioned relative to point  449  to block the focused non-diffracted light. 
         [0034]      FIG. 5  shows another alternative embodiment, comprising an arrangement  510  in which an upstream positive lens  557  focuses light such that the diffracted light&#39;s focusing is canceled by a downstream diffractive optical element  555 , but the non-diffracted light&#39;s focusing is not canceled. Similar to  FIG. 4 , at a point  559 , the zero-order light is able to be blocked. Via the canceling, the diffracted light dots each converge at the image plane  512 , as represented by the dot  558 . 
         [0035]    The technology described herein is compatible with existing diffractive optical element technology. For example, as represented in  FIG. 6 , two (or more, up to any practical number of) diffractive optical elements  664  and  665  may be arranged in series. Unlike the existing technology, the last in the series (relative to the laser  604 ) may be encoded with a diffractive lens to defocus diffracted light for refocusing with a ROE, that is, for canceling out the effect on diffracted light dots. As before, the diffracted light is unaffected after passing through the ROE, whereby the non-diffracted light is defocused. Note that the last DOE in the series contains the diffractive lens so as to not focus the light before the last diffractive optical element in the series is reached. 
         [0036]    Note that  FIG. 6  shows an arrangement  610  similar to  FIG. 2  but for the two diffractive optical elements  666  and  666  in series. Thus, a collimating lens  662  provides the parallel rays to the first diffractive optical element  664 , which diffracts the diffracted light with its pattern into the next diffractive optical element  666  that further diffracts and converges the diffracted light for diverging by the downstream negative lens  666 . Notwithstanding, it is understood that two or more diffractive optical elements may be arranged in a given configuration, and that the various diffractive optical elements and ROE may be positioned in other ways. For example, an arrangement may have two or more diffractive optical elements arranged similar to (the single element in)  FIG. 3  with an upstream negative lens, or similar to (the single elements in)  FIGS. 4 and 5  with respective downstream and upstream positive lenses. An arrangement may have a ROE positioned between two diffractive optical elements, and so forth; as used herein diffractive optical elements “in series” include optically coupled elements, whether directly in series or indirectly in series with one or more intermediary components in between. 
         [0037]    Further, it should be noted that any of the diffractive optical elements/ROEs shown herein may be further divided into more diffractive optical elements/ROEs than those illustrated, and/or combined into a lesser number. For example, two or more ROEs may be used in a given arrangement. A single diffractive optical element (e.g., a unitary construction/assembly) with canceling positive and negative focusing of diffracted light may be used, with only negative or positive focusing (but not both that cancel) of the non-diffracted light. Thus, as used herein, “optically coupled” refers to passing light between separate optical components, and/or between different optical sub-components of a single component, regardless of the order of any components or any intermediary elements. 
         [0038]    Further, it should be noted that reflective optical elements may be used in addition to, or instead of, refractive optical elements in the manner described above in order to introduce aberrations that cancel, or substantially cancel the aberrations intentionally introduced by the diffractive optical element. 
       CONCLUSION 
       [0039]    While the invention is susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention. 
         [0040]    In addition to the various embodiments described herein, it is to be understood that other similar embodiments can be used or modifications and additions can be made to the described embodiment(s) for performing the same or equivalent function of the corresponding embodiment(s) without deviating therefrom. Still further, multiple components can share the performance of one or more functions described herein, and similarly, such functionality may be effected across a plurality of devices. Accordingly, the invention is not to be limited to any single embodiment, but rather is to be construed in breadth, spirit and scope in accordance with the appended claims.