PATENT DOCUMENT

Publication Number: US-10097771-B2
Application Number: US-201615233997-A
Country: US
Kind Code: B2

Title: Wideband ambient light rejection

Abstract:
Optical apparatus includes an image sensor and objective optics, which are configured to collect and focus optical radiation over a range of wavelengths along a common optical axis toward a plane of the image sensor. A dispersive element is positioned to spread the optical radiation collected by the objective optics so that different wavelengths in the range are focused along different, respective optical axes toward the plane.

Claims:
The invention claimed is: 
     
       1. Optical apparatus, comprising:
 an image sensor; and 
 objective optics, which are configured to collect and focus optical radiation over a range of wavelengths within at least one of an infrared and a visible part of the optical spectrum toward a plane of the image sensor, and which comprise a dispersive element configured so that the optics have a chromatic aberration within the range in excess of one micrometer per nanometer of wavelength at the plane. 
 
     
     
       2. The apparatus according to  claim 1 , wherein the optics are configured so that the chromatic aberration causes different wavelengths in the range originating from a given object point to focus at different, respective image points along an axis perpendicular to the plane. 
     
     
       3. The apparatus according to  claim 1 , wherein the optics are configured so that the chromatic aberration causes different wavelengths in the range originating from a given object point to focus along different, respective axes. 
     
     
       4. The apparatus according to  claim 1 , wherein the dispersive element is configured to cause the image sensor to capture a sharp image of the optical radiation emitted from an object at a target wavelength within the range, while spreading broadband background radiation within the range across the plane. 
     
     
       5. The apparatus according to  claim 4 , and comprising a projection assembly, which is configured to project the optical radiation at the target wavelength onto the object, so that the emitted optical radiation collected by the objective comprises the projected optical radiation that is reflected from the object. 
     
     
       6. The apparatus according to  claim 5 , wherein the projected optical radiation comprises a predetermined radiation pattern, and wherein the sharp image captured by the image sensor comprises an image of the pattern projected onto the object. 
     
     
       7. The apparatus according to  claim 4 , and comprising a processor, which is configured to process the sharp image captured by the image sensor while rejecting interference caused by the background radiation. 
     
     
       8. The apparatus according to  claim 7 , wherein the processor is configured to construct a 3D map of the object by processing the sharp image. 
     
     
       9. The apparatus according to  claim 1 , wherein the dispersive element comprises a diffractive element. 
     
     
       10. The apparatus according to  claim 9 , wherein the diffractive element comprises a grating. 
     
     
       11. The apparatus according to  claim 10 , wherein the grating is configured to direct a selected diffraction order onto the image sensor, and wherein the apparatus comprises an interference filter positioned between the grating and the image sensor so as to block stray diffraction orders, other than the selected diffraction order, from reaching the image sensor. 
     
     
       12. The apparatus according to  claim 1 , wherein the dispersive element comprises a refractive element. 
     
     
       13. Optical apparatus, comprising:
 an image sensor; and 
 objective optics, which are configured to collect and focus optical radiation over a range of wavelengths toward a plane of the image sensor, and which comprise a dispersive element configured so that the optics have a chromatic aberration in excess of one micrometer per nanometer of wavelength at the plane, 
 wherein the dispersive element has a first tilt relative to an optical axis of the objective optics, and wherein the apparatus comprises a wedged lens, which is configured to focus the optical radiation with a second tilt, opposite to the first tilt. 
 
     
     
       14. The apparatus according to  claim 13 , wherein the wedged lens comprises a section of an axially-symmetrical lens. 
     
     
       15. A method for imaging, comprising:
 collecting and focusing optical radiation over a range of wavelengths within at least one of an infrared and visible part of the optical spectrum along an optical path toward a plane of an image sensor; and 
 positioning a dispersive element in the optical path so that the radiation is focused with a chromatic aberration within the range in excess of one micrometer per nanometer of wavelength at the plane. 
 
     
     
       16. The method according to  claim 15 , wherein the chromatic aberration causes different wavelengths in the range originating from a given object point to focus at different, respective image points along an axis perpendicular to the plane. 
     
     
       17. The method according to  claim 16 , wherein the chromatic aberration causes different wavelengths in the range originating from a given object point to focus along different, respective axes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/296,472, filed Jun. 5, 2014, which is a division of U.S. patent application Ser. No. 13/031,627, filed Feb. 22, 2011 (now U.S. Pat. No. 8,786,757), which claims the benefit of U.S. Provisional Patent Application 61/306,980, filed Feb. 23, 2010, and of U.S. Provisional Patent Application 61/374,373, filed Aug. 17, 2010, both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optical systems and devices, and specifically to techniques for capturing images of narrowband features against a background of ambient light. 
     BACKGROUND OF THE INVENTION 
     Computer vision systems in a variety of applications operate by projecting light onto an object and then detecting the light reflected back from the object. For example, three-dimensional (3D) mapping systems commonly project a pattern of structured light or short pulses of light onto an object and then extract 3D contours from an image of the patterned or pulsed light. 
     One system of this sort is described, for example, in U.S. Patent Application Publication 2008/0240502, whose disclosure is incorporated herein by reference. This publication describes apparatus for mapping an object using an illumination assembly, which includes a transparency containing a fixed pattern of spots. A light source transilluminates the transparency with optical radiation so as to project the pattern onto the object. An image capture assembly captures an image of the pattern that is projected onto the object. A processor processes the image captured by the image capture assembly so as to reconstruct a 3D map of the object. The image capture assembly may comprise a bandpass filter, which is chosen and positioned so that the image sensor receives light in the emission band of the light source, while filtering out ambient light that might otherwise reduce the contrast of the image of the projected pattern that is captured by the sensor. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention that are described hereinbelow provide methods and apparatus that may be used to improve the quality of images formed with narrowband optical radiation. 
     There is therefore provided, in accordance with an embodiment of the present invention, optical apparatus, including an image sensor and objective optics, which are configured to collect and focus optical radiation over a range of wavelengths along a common optical axis toward a plane of the image sensor. A dispersive element is positioned to spread the optical radiation collected by the objective optics so that different wavelengths in the range are focused along different, respective optical axes toward the plane. 
     In disclosed embodiments, the dispersive element is configured to cause the image sensor to capture a sharp image of the optical radiation emitted from an object at a target wavelength within the range, while spreading broadband background radiation within the range across the plane. 
     In one embodiment, the apparatus includes a projection assembly, which is configured to project the optical radiation at the target wavelength onto the object, so that the emitted optical radiation collected by the objective includes the projected optical radiation that is reflected from the object. The projected optical radiation may include a predetermined radiation pattern, and the sharp image captured by the image sensor may include an image of the pattern projected onto the object. 
     In some embodiments, the apparatus includes a processor, which is configured to process the sharp image captured by the image sensor while rejecting interference caused by the background radiation. The processor may be configured to construct a 3D map of the object by processing the sharp image. 
     In some embodiments, the dispersive element includes a diffractive element, such as a grating. In one embodiment, the grating is configured to direct a selected diffraction order onto the image sensor, and the apparatus includes an interference filter positioned between the grating and the image sensor so as to block stray diffraction orders, other than the selected diffraction order, from reaching the image sensor. Alternatively or additionally, the dispersive element includes a refractive element. 
     Typically, the dispersive element is configured so that the optical radiation is focused with a chromatic aberration in excess of one micrometer per nanometer of wavelength at the plane of the image sensor. 
     In a disclosed embodiment, the dispersive element has a first tilt relative to the optical axis, and the apparatus includes a wedged lens, which is configured to focus the optical radiation with a second tilt, opposite to the first tilt. The wedged lens may include a section of an axially-symmetrical lens. 
     There is also provided, in accordance with an embodiment of the present invention, optical apparatus, including a projection assembly, which is configured to project optical radiation at a target wavelength onto an object. An imaging assembly includes an image sensor and optics, which are configured to collect and focus the optical radiation reflected from the object, along with background radiation over a range of wavelengths, toward a plane of the image sensor. The optics include a dispersive element, which is configured to direct the collected optical radiation so as to form a sharp image of the optical radiation at the target wavelength in the plane of the image sensor, while spreading the background radiation across the plane. A processor is configured to process the sharp image captured by the image sensor while rejecting interference caused by the background radiation. 
     In one embodiment, the projected optical radiation includes a predetermined radiation pattern, the sharp image captured by the image sensor includes an image of the pattern projected onto the object, and the processor is configured to construct a 3D map of the object by processing the sharp image. 
     Typically, the dispersive element is configured to spread the optical radiation collected by the optics so that different wavelengths in the range are focused along different, respective optical axes toward the plane of the image sensor. 
     In one embodiment, the dispersive element includes a dispersive focusing element, such as a diffractive lens, which is configured to focus the radiation at the target wavelength onto the plane of the image sensor, while other wavelengths in the range are focused ahead of or behind the plane. 
     The apparatus may include a motion mechanism, which is configured to shift a position of one or more elements of the optics relative to the plane so as to select the target wavelength that is to be imaged sharply in the plane. 
     There is additionally provided, in accordance with an embodiment of the present invention, optical apparatus, including an image sensor and objective optics, which are configured to collect and focus optical radiation over a range of wavelengths toward a plane of the image sensor. The objective optics include a dispersive element configured so that the optics have a chromatic aberration in excess of one micrometer per nanometer of wavelength at the plane. 
     The optics may be configured so that the chromatic aberration causes different wavelengths in the range originating from a given object point to focus at different, respective image points along an axis perpendicular to the plane. Alternatively or additionally, the optics may be configured so that the chromatic aberration causes different wavelengths in the range originating from a given object point to focus along different, respective axes. 
     There is further provided, in accordance with an embodiment of the present invention, a method for imaging, which includes collecting and focusing optical radiation over a range of wavelengths along a common optical axis toward a plane of an image sensor. A dispersive element is positioned to spread the collected optical radiation so that different wavelengths in the range are focused along different, respective optical axes toward the plane. 
     There is moreover provided, in accordance with an embodiment of the present invention, a method for imaging, which includes projecting optical radiation at a target wavelength onto an object. The optical radiation reflected from the object is collected and focused, along with background radiation over a range of wavelengths, toward a plane of an image sensor. A dispersive element is positioned to direct the collected optical radiation so as to form a sharp image of the optical radiation at the target wavelength in the plane of the image sensor, while spreading the background radiation across the plane. The sharp image captured by the image sensor is processed while rejecting interference caused by the background radiation. 
     There is furthermore provided, in accordance with an embodiment of the present invention, a method for imaging, which includes collecting and focusing optical radiation over a range of wavelengths along an optical path toward a plane of an image sensor. A dispersive element is positioned in the optical path so that the radiation is focused with a chromatic aberration in excess of one micrometer per nanometer of wavelength at the plane. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top view of an optical imaging device, in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic side view of an imaging assembly with a dispersive element, in accordance with an embodiment of the present invention; 
         FIG. 3A  is a schematic representation of a projected pattern of optical radiation; 
         FIG. 3B  is a schematic representation of a pattern of background radiation; 
         FIG. 3C  is a schematic representation of an image of the pattern of  FIG. 3A  that is captured in the presence of the background radiation of  FIG. 3B ; 
         FIG. 3D  is a schematic representation of an image of the pattern of  FIG. 3A  that is captured in the presence of the background radiation of  FIG. 3B  using dispersive filtering of the background radiation, in accordance with an embodiment of the present invention; 
         FIG. 4  is a schematic side view of an imaging assembly with a dispersive element, in accordance with another embodiment of the present invention; and 
         FIG. 5  is a schematic side view of an imaging assembly with a dispersive focusing element, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Ambient radiation interference is a problem in many computer vision systems. Such systems commonly operate by processing an image of interest. The image is formed on an image sensor by objective optics, which collect and focus optical radiation in a given wavelength range onto the image sensor plane. In many applications, the image of a scene is formed at a particular wavelength. In such applications, ambient background radiation collected from the scene at other wavelengths within the range of the objective optics tends to introduce spurious features and reduce the contrast (and hence reduce the quality) of the image captured by the image sensor. Systems that use projected radiation in a narrow wavelength band, such as 3D mapping and various sorts of laser scanning systems, tend to be particularly vulnerable to this sort of background interference. 
     Some systems use optical filters, such as interference filters, to block at least a portion of the incoming ambient radiation (as described in the above-mentioned US 2008/0240502, for example). The passband of such filters, however, is strongly dependent on the incident angle of the radiation on the filter. In other words, a narrowband interference filter that passes a target wavelength at normal incidence will typically block a ray of the same wavelength that is incident at a 10° angle, for example. When the filter is to be interposed in the path of focused radiation, the passband is generally made broad enough to pass the rays at the target wavelength over the full range of angles that is collected by the objective optics. Manufacturing tolerances on the filter passband (width and center wavelength) may result in further widening of the filter. The resultingly broad passband permits a substantial amount of ambient radiation to reach the image sensor, as well. 
     Embodiments of the present invention that are described hereinbelow address this problem by positioning a dispersive element in the optical path, so as to spread the optical radiation collected by the objective optics as a function of wavelength. For effective spreading, the inventors have found it advantageous to use a dispersive element that causes the optics to have a chromatic aberration in excess of one micrometer per nanometer of wavelength at the plane of the image sensor. In some designs, the resulting chromatic aberration may be still greater—in the range of 4-5 micrometers per nanometer, for example. As a result, each wavelength emanating from a given point in the object is focused to a different point with respect to the image sensor plane. 
     In some embodiments, the dispersive element causes different wavelengths in the range received by the optics to be focused along different, respective optical axes toward the plane of the image sensor (even though the objective optics by themselves focus all the wavelengths along a common optical axis). As a result, each individual wavelength remains sharply focused, but image features that appear over all or a large part of the range of incoming wavelengths will be smeared in the image sensor plane, since these wavelengths all have different focal axes. 
     Therefore, if the radiation emitted from an object in a scene imaged by the optics includes a narrowband feature at some target wavelength (such as a pattern of radiation projected onto and reflected from the object at the target wavelength), the image sensor will capture a sharp image of this feature. Broadband features, however, such as parts of the scene illuminated by ambient background radiation, will be spread over multiple pixels in the image, and therefore smeared. The effect of the dispersive element is thus equivalent to applying a low-pass filter to broadband (ambient) radiation, without significantly affecting the narrowband radiation at the target wavelength. 
     Other embodiments of the present invention use a strongly-dispersive focusing element to spread the focal points of the different wavelengths in the range. The focusing element is typically designed and positioned so that the target wavelength is focused sharply in the plane of the image sensor, while other wavelengths are focused to points ahead of or behind the plane. A diffractive optical element, such as a Fresnel lens, may be used effectively for this purpose, although refractive elements may also be used, provided they have sufficient chromatic dispersion. Thus, as in the embodiments described above, the image at the target wavelength is sharp, while broadband features are effectively low-pass filtered. 
     Although the embodiments described above and shown in the figures use either a dispersive spreading element or a dispersive focusing element, in alternative embodiments both types of dispersive elements may be used together in order to enhance the filtering effect. 
     System Description 
       FIG. 1  is a schematic side view of an imaging system  20 , in accordance with an embodiment of the present invention. A set of X-Y-Z axes is used in this figure and in parts of the description that follows to aid in understanding the orientation of the figures, wherein the X-Y plane is the frontal plane of system  20 , and the Z-axis extends perpendicularly from this plane toward the scene. The choice of axes, however, is arbitrary and is made solely for the sake of convenience in describing embodiments of the invention. 
     An illumination assembly  22  projects a patterned radiation field  24  onto an object  26  (in this case a hand of a user of the system) in a scene. An imaging assembly  28  captures an image of the scene within a field of view  30 . A controller  31  or other electronic processor processes the image in order to generate a 3D map of object  26  (also referred to as a “depth map”). Further details of this sort of mapping process are described, for example, in the above-mentioned US 2008/0240502 and in PCT International Publication WO 2007/105205, whose disclosure is also incorporated herein by reference. The 3D map of the user&#39;s hand (and/or other parts of the user&#39;s body) may be used in a gesture-based computer interface, but this sort of functionality is beyond the scope of the present patent application. Furthermore, the principles and benefits of the embodiments described below are not limited to this sort of system, but may rather be applied in substantially any context in which narrowband features are to be imaged against a broadband background. 
     Illumination assembly  22  comprises a projection module  32 , which generates a beam of patterned radiation, and projection optics  34 , which project the beam onto field  24 . Module  32  comprises a narrowband radiation source, such as a laser diode, which emits optical radiation to generate the beam at a target wavelength. In a typical embodiment, the radiation source in module  32  emits near-infrared radiation, but the principles of the present invention are equally applicable to target wavelengths in any part of the optical spectrum. 
     The term “optical radiation,” as used in the context of the present patent application and in the claims, refers to any or all of visible, infrared and ultraviolet radiation, and is used interchangeably with the term “light.” The bandwidth of the radiation source in module  32  may be in the range of 1 nm or less, typically up to about 10 nm, depending on application requirements; and the term “target wavelength” is used in the present description and in the claims to refer to the center wavelength of the emitted radiation band. The term “narrowband” is used in a relative sense, to refer to the narrow bandwidth of the object features of interest (such as the bandwidth of the radiation source in module  32 ) relative to the ambient background radiation, which typically extends over a band of hundreds of nanometers. The terms “ambient” and “background” are used to refer to radiation in the scene that is outside the narrow band of interest. 
     Imaging assembly  28  comprises objective optics  36 , which form an optical image of the scene containing object  26  onto an image sensor  38 , such as a CMOS integrated circuit image sensor. The image sensor comprises an array of sensor elements  40 , as is known in the art. The sensor elements generate respective signals in response to the radiation focused onto them by optics  36 , wherein the pixel value of each pixel in the electronic images output by image sensor  38  corresponds to the signal from a respective sensor element  40 . A dispersive element  42  is positioned to spread the optical radiation collected by objective optics  36 , as described in detail with reference to the figures that follow. 
     Embodiment I—Dispersive Beam Spreading 
       FIG. 2  is a schematic side view of imaging assembly  28  with dispersive element  42 , in accordance with an embodiment of the present invention. Element  42  may comprise any suitable type of dispersive optical component and dispersive material. For example, element  42  may comprise a refractive element, such as a slab of a dispersive type of vitreous material, such as flint glass, or crystalline material, inclined at an angle as shown in the figure so that rays of different wavelengths are refracted in the material at different angles. As another example, element  42  may comprise a dispersive prism. As yet another example (described below in greater detail with reference to  FIG. 4 ), element  42  may comprise a diffractive element, such as a grating. Alternatively, other types of dispersive elements may be used for the purposes of this embodiment and are considered to be within the scope of the present invention. In any case, as noted earlier, it is desirable for good wavelength separation that the dispersive element introduce chromatic aberration of at least one micrometer of focal shift per nanometer of wavelength. Although element  42  is shown in the figures as a transmissive element, assembly  28  may alternatively comprise a reflective dispersive element, such as a diffraction grating operating in a reflective mode. 
     Optics  36  in assembly  28  focus incoming radiation over a broad band along a common optical axis, as shown in  FIG. 2 . Dispersion in element  42 , however, causes different wavelengths in the range collected by optics  36  to be shifted and thus focused along different, respective optical axes toward the plane of image sensor  38 . As a result, each wavelength in a given broadband ray is focused to a different, respective focal point  44 ,  46 ,  48  in the sensor plane. The optics shown in this figure are greatly simplified in order to demonstrate the principles of the embodiment with conceptual clarity. 
     In particular, placement of dispersive element  42  in a location at which the rays from optics  36  are converging, as shown in  FIG. 2 , introduces focal aberrations that should desirably be corrected by another optical element or elements (not shown). Such aberrations may be alleviated by placing the dispersive element at a location in the optical path in which the rays are approximately collimated. One such configuration is shown in  FIG. 4 . As another alternative, the dispersive element may be placed ahead of the objective optics, i.e., between the object and the first focusing element of the optics. The dispersive element in this sort of embodiment may be fabricated, for example, as a dome (with suitable refractive and/or diffractive properties) or may comprise one or more slabs and/or prisms of dispersive material. 
     As a result of the dispersion introduced by element  42  (or by other types of dispersive elements, as noted above), a fine feature in a broadband image formed by optics  36  will be focused to point  44  for one wavelength and to point  48  for another wavelength, for example. Since image sensor  38  is sensitive to the entire wavelength range, this feature will be smeared over multiple pixels in the electronic image that is output by the image sensor. Dispersive element  42  thus acts as a low-pass filter with respect to broadband image features. On the other hand, a fine feature in a narrowband image will be focused to a single focal point  46  and will therefore remain sharply-focused in the electronic image notwithstanding the shift caused by dispersive element  42 . 
       FIGS. 3A-D  are schematic representations of images that illustrate the effect of dispersive element  42 , in accordance with an embodiment of the present invention.  FIG. 3A  shows a projected pattern  50  of narrowband optical radiation, comprising bright spots  52  against a dark background. Projection assembly  22  ( FIG. 1 ) may project a pattern of this sort onto object  26 . At the same time, broadband ambient radiation in the area of object  26  creates a background image  55 , shown in  FIG. 3B , which is captured by optics  36  along with the narrowband pattern. For the sake of illustration, the background image is taken to be an arbitrary pattern of bright features against a dark background, but the principles of the embodiments disclosed herein are equally applicable to any sort of sharp features in a broadband image. 
       FIG. 3C  shows an image  60  that would be captured by image sensor  38  in the absence of dispersive element  42 . Bright spots  52  from pattern  50  are mixed with features of background image  55 . Although spots  52  are distinguished in  FIG. 3C  from the parts of pattern  55 , in practice the background image on image sensor  38  may be as bright as or brighter than the pattern image. As a result, controller  31  may have difficulty in processing the image to distinguish between the pattern and the background. 
       FIG. 3D  shows an image  65  of the pattern that is captured by image sensor when dispersive element  42  is used to filter background image  55 . Now the features of the background image are spread over many pixels and are therefore blurred in image  65 . Narrowband spots  52 , however, remain in sharp focus. Although the background radiation from image  55  causes some reduction in the contrast of spots  52  in image  65 , controller  31  can readily distinguish these spots from the background features and thus reject the interference caused by the background radiation. 
       FIG. 4  is a schematic side view of an imaging assembly  70  with a dispersive filtering element  82 , in accordance with another embodiment of the present invention. This assembly may be used in place of imaging assembly  28  in system  20  ( FIG. 1 ). Detailed design parameters of assembly  70  are presented below in an Appendix. 
     Objective optics  72  in assembly  70  comprise plastic, axially-symmetrical aspheric lenses  74 ,  76 ,  78 ,  80 . These elements together act as a field reducer, as well as a telecentric objective. The chief rays of all fields are nearly collimated at the exit from objective optics  72 , while the rays inside each field are slightly tilted around the chief rays by angles that depend on the F-number of the design. 
     Element  82  is a linear transmissive diffraction grating, tilted at the Littrow angle relative to the optical axis of objective optics  72 . The diffraction grating causes different wavelength to be diffracted at different angles according to the grating equation sin α+sin β=λ/d, wherein α is the incident angle, β is the diffraction angle, λ is the light wavelength, and d is the grating period. The Littrow incidence angle is defined as the angle for which 2 sin α=λ/d. When used at the Littrow angle, the diffraction grating generally has maximal efficiency, meaning that most of the incoming light is directed into a single diffraction order (typically the −1 order). The grating may be blazed, as is known in the art, to optimize this efficiency. 
     The rays exiting from the diffraction grating are tilted and focused by an aspheric wedged lens  84  onto image sensor  38 . As shown in  FIG. 4 , the wedged lens focuses the optical radiation with a tilt that is opposite to the tilt of element  82 . For ease of fabrication, the wedged lens may comprise, for example, an off-axis section of an axially-symmetrical lens, as illustrated in the figure. An interference filter  86  may be used to block radiation that is far from the target wavelength, as well as blocking stray orders of the diffraction grating (orders other than −1 in the present example). Because of the sensitivity of filter  86  to angle of incidence, as explained above, the filter will block stray diffraction orders at the target wavelength (and other nearby wavelengths) while passing the desired order. Thus, interference filter  86  serves in this embodiment as an angular filter, in addition to its role in rejecting light outside its passband. 
     In an alternative embodiment, the position of filter  86  may be interchanged with wedged lens  84 , and the filter will still able to perform its dual filtering function. In this case, grating  82  may be formed on the same substrate as filter  86 , and the filter will then be able to perform both the dispersive and angular filtering functions. In such an embodiment, it is desirable that the grating be formed on the front surface of the filter, i.e., the surface that is more distant from image sensor  38 . 
     As explained above, assembly  70  is designed to focus different wavelengths at a relative offset onto the plane of image sensor  38 . Each wavelength is still imaged with good quality, and all wavelengths within the passband of the filter are in good focus at the same time on the plane of the image sensor, but they are shifted relative to one another. This effect causes broadband image features to smear in the direction of the shift (the direction of the grating operation, as shown in the figure), while the narrowband projected pattern is imaged with high contrast. 
     Element  82  will cause the location of the pattern in the image formed on image sensor  38  to shift according to the actual wavelength of projection module  32 . This wavelength may vary due to manufacturing tolerances and temperature changes, for example. Controller  31  may be programmed to compensate for this shift. 
     Lenses  74 ,  76 ,  78 ,  80  may be produced as a single, rotationally-symmetrical assembly, which may be used to focus assembly  70  by rotational movement. The remaining elements of assembly  70  are not axially symmetrical, and are therefore typically assembled statically in a suitable holder. 
     Embodiment II—Dispersive Focusing 
       FIG. 5  is a schematic side view of an imaging assembly  90  with a dispersive focusing element  94 , in accordance with an embodiment of the present invention. Assembly  90  may be used in place of assembly  28  in system  20 , for example. Objective optics  92  in assembly  90  collect and focus optical radiation over a range of wavelengths along a common optical axis toward image sensor  38 . Dispersive focusing element  94 , however, focuses different wavelengths at different focal lengths, so that only the target wavelength has its focal point  96  in the plane of the image sensor. Other wavelengths are focused ahead of or behind the plane. Therefore, narrowband features at the target wavelength will be in sharp focus in the electronic image output by the image sensor, while broadband features, due to background radiation for example, will be smeared. 
     Dispersive focusing element  94  may, for example, comprise a diffractive lens, such as a Fresnel lens. The focal length of a diffractive lens is given as a function of wavelength by the formula 
               =         λ   0     λ     ⁢     f   0         ,         
wherein λ 0  is the “design” wavelength, i.e., the wavelength at which the focal length of the lens is f 0 . For diffractive lenses with aspheric behavior, some corrections may be introduced into the above formula, but the strongly dispersive behavior of the lens with wavelength is still present. To create an imaging optical system that focuses different wavelengths at different focal distance in this manner, it may be sufficient to replace one or more refractive surfaces of the optics (such as the last lens, as in assembly  90 ) with its diffractive counterpart. In any case, the optical system as a whole should be designed with a strong dependence of the overall focal length on element  95  in order to provide the desired wavelength separation. As in the preceding embodiments, it is desirable that the optics (including element  94 ) have a chromatic aberration in excess of one micrometer per nanometer of wavelength at the plane of image sensor  38 .
 
     The combined focal length of the hybrid refractive-diffractive optics shown in  FIG. 5  changes with wavelength, thus achieving selective focusing of the target wavelength of the projected pattern onto the image sensor plane, while de-focusing broadband background features. The amount of focal length variation of element  94  is chosen so as to provide sufficient focal sensitivity with wavelength for background filtering on the one hand, while leaving the entire bandwidth of the projected radiation in good focus on the other. The amount of focal variation is proportional to the focal length of the diffractive lens in the system, as seen in the above formula. The overall assembly  90  may be designed with the defocusing trade-off in mind, so as to correct for possible diffractive aberrations and introduce the desired amount of wavelength sensitivity. 
     Because dispersive focusing element  94  is the last element in the optics before image sensor  38 , element  94  can be refocused for any given target wavelength (within the design range). The focus may thus be adjusted for changes in the wavelength of projection module  32 , as noted above. Wavelengths outside a narrow band around the target wavelength will be defocused and smeared. Alternatively, such refocusing may also be accomplished by suitable movement of a dispersive focusing element at another position within the optical train. 
     To facilitate the wavelength-dependent focusing, assembly  90  may comprise a motion mechanism  98 , which adjusts the position of dispersive focusing element  94 . Mechanism  98  may comprise, for example, a fine mechanical drive or a piezoelectric drive, or any other suitable type of mechanism that is known in the art. In this embodiment, mechanism  98  shifts the relative positions of element  94  and image sensor  38  in order to select the focal wavelength and to maintain focal stability in response to wavelength changes and other factors that may change during operation of assembly  90 . Alternatively or additionally, mechanism  98  may be configured to shift the positions of one or more other elements of optics  92 . 
     In other embodiments (not shown specifically in the figures) a similar sort of mechanism may be used to adjust and maintain wavelength stability in the sorts of axis-shifting embodiments that are shown in  FIGS. 2 and 4 . 
     Although the description of  FIG. 5  above relates to a diffractive optical element, other types of dispersive focusing elements, such as a highly-dispersive refractive lens, may alternatively be used in assembly  90 . Furthermore, dispersive focusing elements may be used in combination with dispersive filtering elements (such as element  42  or  82 ) for enhanced effect. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. 
     Appendix—Optical Design Parameters 
     The parameters listed below provide details of the optical design shown in  FIG. 4 . The parameters were extracted from a design file generated by the ZEMAX® optical design software (ZEMAX Development Corporation, Bellevue, Wash.) that was used in creating and evaluating the design. 
     General Lens Data 
     
         
         Surfaces:  24   
         Stop:  6   
         System Aperture: Float By Stop Size=0.797 
         Glass Catalogs: PLASTIC B-SYSTEM 
         Ray Aiming: Real Reference, Cache On 
         X Pupil shift: 0 
         Y Pupil shift: 0 
         Z Pupil shift: 0 
         X Pupil compress: 0 
         Y Pupil compress: 0 
         Apodization: Uniform, factor=0.00000E+000 
         Temperature (C): 2.00000E+001 
         Pressure (ATM): 1.00000E+000 
         Adjust Index Data To Environment: Off 
         Effective Focal Length: 2.319484 (in air at system temperature and pressure) 
         Effective Focal Length: 2.319484 (in image space) 
         Back Focal Length: 1.842413 
         Total Track: 32.02137 
         Image Space F/#: 2.915449 
         Paraxial Working F/#: 2.915448 
         Working F/#: 2.701257 
         Image Space NA: 0.1690324 
         Object Space NA: 2.673263e-007 
         Stop Radius: 0.797 
         Paraxial Image Height: 1.932521 
         Paraxial Magnification: −1.558752e-006 
         Entrance Pupil Diameter: 0.7955839 
         Entrance Pupil Position: 8.985321 
         Exit Pupil Diameter: 3.55047 
         Exit Pupil Position: 9.209573 
         Field Type: Angle in degrees 
         Maximum Radial Field: 60.07529 
         Primary Wavelength: 0.83 μm 
         Lens Units: Millimeters 
         Angular Magnification: −0.2828611 
         Fields: 12 
         Field Type: Angle in degrees 
       
    
                                                     #   X-Value   Y-Value   Weight                                                            1   0.000000   0.000000   1.000000           2   12.000000   0.000000   3.000000           3   0.000000   14.500000   5.000000           4   0.000000   −14.500000   3.000000           5   24.000000   0.000000   3.000000           6   0.000000   39.800000   4.000000           7   0.000000   −39.800000   2.000000           8   45.000000   0.000000   8.000000           9   45.000000   −39.800000   3.000000           10   45.000000   39.800000   3.000000           11   22.500000   −19.900000   1.000000           12   22.500000   −19.900000   1.000000                        
Vignetting Factors
 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                 # 
                 VDX 
                 VDY 
                 VCX 
                 VCY 
                 VAN 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 2 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 3 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 4 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 5 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 6 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 7 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 8 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 9 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 10 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 11 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 12 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                   
               
            
           
         
       
         
         Wavelengths: 1 
         Units: μm 
       
    
     
       
         
           
               
               
               
             
               
                   
               
               
                 # 
                 Value 
                 Weight 
               
               
                   
               
             
            
               
                 1 
                 0.830000 
                 1.000000 
               
               
                   
               
            
           
         
       
     
     SURFACE DATA SUMMARY 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
             
            
               
                 Surf 
                 Type 
                 Radius 
                 Thickness 
                 Glass 
                 Diameter 
               
               
                   
               
               
                 OBJ 
                 STANDARD 
                 Infinity 
                 1488030 
                 3873671 
                 0 
               
               
                 1 
                 STANDARD 
                 Infinity 
                 5 
                 27.30599 
                 0 
               
               
                 2 
                 EVENASPH 
                 48.87732 
                 0.7494415 
                 E48R-BS 
                 11.88 
               
               
                 3 
                 EVENASPH 
                 4.623508 
                 2.246475 
                 9.48 
                 −1.353716 
               
               
                 4 
                 EVENASPH 
                 −33.61659 
                 0.7992448 
                 E48R-BS 
                 7.2 
               
               
                 5 
                 EVENASPH 
                 7.473258 
                 3.387089 
                 4.96 
                 0 
               
               
                 STO 
                 STANDARD 
                 Infinity 
                 3.205891 
                 1.594 
                 0 
               
               
                 7 
                 EVENASPH 
                 −17.68054 
                 2.168681 
                 E48R-BS 
                 5.88 
               
               
                 8 
                 EVENASPH 
                 −6.876255 
                 0.8340925 
                 7.86 
                 −2.255111 
               
               
                 9 
                 EVENASPH 
                 99.57123 
                 3.328151 
                 E48R-BS 
                 10.1 
               
               
                 10 
                 EVENASPH 
                 −5.335159 
                 0.9305261 
                 10.1 
                 −4.524261 
               
               
                 11 
                 COORDBRK 
                 — 
                 0 
                 — 
                 — 
               
               
                 12 
                 STANDARD 
                 Infinity 
                 0.67 
                 POLYCARB 
                 12 
               
               
                 13 
                 DGRATING 
                 Infinity 
                 0 
                 12 
                 0 
               
               
                 14 
                 COORDBRK 
                 — 
                 1.753535 
                 — 
                 — 
               
               
                 15 
                 COORDBRK 
                 — 
                 0 
                 — 
                 — 
               
               
                 16 
                 EVENASPH 
                 10.02237 
                 6.186684 
                 E48R-BS 
                 17.4 
               
               
                 17 
                 EVENASPH 
                 −12.99827 
                 0 
                 17.4 
                 0 
               
               
                 18 
                 COORDBRK 
                 — 
                 −0.5455667 
                 — 
                 — 
               
               
                 19 
                 COORDBRK 
                 — 
                 0 
                 — 
                 — 
               
               
                 20 
                 STANDARD 
                 Infinity 
                 0.7 
                 1.800000 
                 35.000000 
               
               
                 21 
                 STANDARD 
                 Infinity 
                 −0.7 
                 7.989695 
                 0 
               
               
                 22 
                 COORDBRK 
                 — 
                 0.7 
                 — 
                 — 
               
               
                 23 
                 STANDARD 
                 Infinity 
                 0.6071292 
                 8.284734 
                 0 
               
               
                 IMA 
                 TILTSURF 
                 — 
                 6.447033 
                 — 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Surf 
                 Conic 
                 Comment 
               
               
                   
               
               
                 OBJ 
               
               
                 1 
               
               
                 2 
                 0 
               
               
                 3 
               
               
                 4 
                 0 
               
               
                 5 
               
               
                 STO 
               
               
                 7 
                 0 
               
               
                 8 
               
               
                 9 
                 0 
               
               
                 10 
               
               
                 11 
                 Element 
                 Tilt 
               
               
                 12 
                 0 
               
               
                 13 
               
               
                 14 
                 Element 
                 Tilt 
               
               
                 15 
                 Element 
                 Tilt 
               
               
                 16 
                 0 
               
               
                 17 
               
               
                 18 
                 Element 
                 Tilt 
               
               
                 19 
                 Element 
                 Tilt 
               
               
                 20 
                     8.308056 
                 0 
               
               
                 21 
               
               
                 22 
                 Element 
                 Tilt 
               
               
                 23 
                 Dummy 
               
               
                 IMA

Metadata:
Filing Date: 20160811
Publication Date: 20181009
Grant Date: 20181009
Priority Date: 20100223
Inventors: SHPUNT, ALEXANDER
GILBOA, NIV
BEZDIN, HAIM
Assignee: APPLE INC
CPC Classifications: [{"code": "H04N23/45", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N23/45", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N23/75", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04N23/11", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N5/2258", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N9/646", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N13/257", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N13/271", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0955", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/4205", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N5/332", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N5/2254", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/1066", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N13/106", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0944", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N5/238", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/1066", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/4205", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N13/106", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N13/271", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0944", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N13/271", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N13/257", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0955", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N13/106", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N13/257", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N9/646", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/1066", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N23/11", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 44476210