Patent Publication Number: US-2013235160-A1

Title: Optical pulse shaping

Description:
TECHNICAL FIELD 
     Embodiments of the invention relate to shaping optical pulses. 
     BACKGROUND 
     A time of flight (TOF) three dimensional (3D) camera acquires distances to features in a scene that the camera images by timing how long it takes temporally modulated light that it transmits to illuminate the scene to travel and make a “round trip” to the features and back to the camera. The known speed of light and the round trip time to a given feature in the scene is used to determine a distance of the given feature from the TOF 3D camera. 
     In a “gated” TOF 3D camera, a train of light pulses may be transmitted by a light source to illuminate a scene that the camera images. Upon lapse of a predetermined same delay interval, hereinafter an “exposure delay”, after each light pulse in the train of light pulses is transmitted, the camera is shuttered, or “gated” ON, for a short exposure period that ends when the camera is shuttered, or “gated”, OFF. The camera images light reflected from the transmitted light pulses by features in the scene that reaches the camera during each exposure period and is incident on pixels of the camera&#39;s photosensor. Distance to a feature in the scene imaged on a pixel of the photosensor is determined as a function of an amount of light that the feature reflects from the transmitted light pulses that is registered by the pixel during the exposure periods. 
     Light reflected by a feature in the scene from a transmitted light pulse in the train of light pulses reaches the TOF 3D camera as a reflected light pulse having pulse width and pulse shape substantially the same as the pulse width and pulse shape respectively of the transmitted light pulse from which it was reflected. Pulse shape of a light pulse refers to intensity of light in the light pulse as a function of location along the light pulse width, or to intensity of light in the light pulse on a surface on which the light pulse is incident as a function of time. 
     Sensitivity of pixels in the TOF 3D camera photosensor for registering light in the reflected light pulse during an “associated” exposure period following the transmitted light pulse is a function of time. The function is generally substantially equal to zero at the shutter ON and OFF times that define the exposure period and has a maximum at some time between the ON and OFF times. A shape of a curve representing the sensitivity function is referred to as a “shape” of the exposure period. 
     An amount of light in the reflected light pulse that is registered by the pixel imaging the feature during the associated exposure period is proportional to a convolution between the reflected light pulse and the exposure period. The convolution is a function of a round trip time for light to propagate to the feature and back to the gated TOF 3D camera. An amount of reflected light registered by the pixel for all the reflected light pulses incident on the pixel from the feature measures a sum of the convolutions between the shapes of the reflected light pulses and their respective associated exposure periods, and may be used to determine distance to the feature. Accuracy and resolution of distances provided by a TOF 3D camera generally improve as the transmitted light pulses and thereby the reflected light pulses are matched to the exposure periods to have similar or substantially same shapes. 
     Hereinafter, for convenience of presentation a convolution between the shape of a light pulse and an exposure period is referred to as a convolution between the light pulse and the exposure period. 
     SUMMARY 
     An aspect of an embodiment of the invention relates to providing a method of exposing a camera to light from a light pulse having a desired pulse shape by adjusting timing of light pulses that provide light to which the camera is exposed relative to exposure periods of the camera so that the light pulses emulate a light pulse having the desired pulse shape. An amount of light from the light pulses registered by the camera during the exposure periods is substantially the same as an amount of light that would be registered by the camera from a single light pulse having the desired pulse shape during a single exposure period of the camera. 
     In an embodiment of the invention, the camera is a TOF 3D camera and the light pulses are light pulses in a train of light pulses transmitted by a light source in the TOF 3D camera to illuminate a scene that the TOF 3D camera images. The exposure periods are the associated exposure periods of the TOF 3D camera, each of which follows a transmission time of a transmitted light pulse in the train of light pulses upon lapse of an exposure delay. 
     To provide a desired pulse shape, in accordance with an embodiment of the invention, exposure delays between transmission times of light pulses in the train of light pulses and ON times of their associated respective exposure periods of the TOF 3D camera are adjusted by different perturbation periods. The perturbation periods are chosen so that were the light pulses in the train of light pulses ordered in time relative to a common time origin by their perturbation periods and added together, they would provide a compound light pulse, hereinafter an “emulated light pulse”, having the desired pulse shape. Adding light pulses together refers to adding their pulse shapes or their intensities. 
     In an embodiment of the invention, the desired pulse shape of the emulated light pulse is similar to, or substantially the same as, the shape of the exposure periods. In an embodiment of the invention, the pulse shape of the emulated light pulse is advantageously higher at the leading edge than at the trailing edge to compensate, at least partly, for decrease in intensity of reflected light from features that are farther from the TOF 3D camera. 
     As a result of the perturbation periods, reflected light pulses from features in the scene reach the TOF 3D camera at arrival times relative to the ON time of the exposure periods that are functions not only of round trip times of light to and back from the features, but also of the perturbation periods. Light reflected from each transmitted light pulse by a given feature in the scene arrives at the TOF 3D camera following a delay from a transmission time of the transmitted light pulse that is equal to a sum of the perturbation delay associated with the transmitted light pulse as well as the round trip time of light to and back from the given feature. A sum of the convolutions of each reflected light pulse from the given feature and its associated exposure period is also a function of the perturbation periods. The “sum convolution” is equal to a convolution of the pulse shape of the emulated light pulse provided by the “time perturbed” transmitted light pulses and a single exposure period. 
     A distance to the given feature determined responsive to the sum convolution in accordance with an embodiment of the invention, may therefore be provided by the TOF 3D camera responsive to a convolution of the shape of the exposure periods of the TOF 3D camera with a light pulse having a desired, advantageous pulse shape. 
     In the discussion, unless otherwise stated, adjectives such as “substantially” and “about” modifying a condition or relationship characteristic of a feature or features of an embodiment of the invention, are understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended. 
     This Summary is provided to introduce a selection of 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 to limit the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Non-limiting examples of embodiments of the invention are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. 
         FIG. 1A  schematically shows a TOF 3D camera imaging a scene to determine distances to features in the scene; 
         FIG. 1B  shows a timeline graph that illustrates relative timing of light pulses in a train of light pulses transmitted by the TOF 3D camera shown in  FIG. 1A , light pulses reflected by features in the scene, and exposure periods of the TOF 3D camera; 
         FIG. 2  shows a timeline graph that illustrates relative timing of light pulses in a train of light pulses different from those illustrated in  FIG. 1B , light pulses reflected by features in the scene and exposure periods of the TOF 3D camera; 
         FIGS. 3A and 3B  shows timeline graphs that illustrate configuring and using an emulated light pulse to determine distances to features in the scene shown in  FIG. 1A , in accordance with an embodiment of the invention; and 
         FIG. 4  schematically shows an emulated light pulse having a pulse shape advantageous for compensating for decrease in intensity of reflected light pulses that are reflected by distant features to a TOF 3D camera, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following text of the detailed description, features of a TOF 3D camera are shown in  FIG. 1A  and discussed with reference to the figures. Operation of the TOF 3D camera shown in  FIG. 1A  is discussed with reference to a timeline graph shown in  FIG. 1B . The timeline graph illustrates timing of transmission times of transmitted light pulses used to illuminate a scene imaged by the TOF 3D camera shown in  FIG. 1A  and timing relationships between light reflected from the transmitted light pulses and exposure periods of the camera. In the timeline graph of  FIG. 1B  the transmitted light pulses have shape and duration that are substantially the same as the shape of the exposure periods.  FIG. 2  shows a timeline graph illustrating relative timing of exposure periods, transmitted light pulses, and reflected light pulses for transmitted light pulses that have pulse widths different from duration of the exposure periods.  FIGS. 3A and 3B  graphically illustrate configuring transmission times of light pulses in accordance with an embodiment of the invention to generate an emulated light pulse shaped similar to a shape of exposure periods of the TOF 3D camera.  FIG. 4  schematically illustrates an emulated light pulse in accordance with an embodiment of the invention that is configured to compensate, at least partly, for reduction in intensity of light received from features of a scene that a camera images that are relatively far from the camera. 
       FIG. 1A  schematically shows a gated TOF 3D camera  20  being used to determine distances to features in a scene  30  having objects  31  and  32 . TOF 3D camera  20 , which is represented very schematically, comprises an optical system, represented by a lens  21 , and a photosensor  22  having pixels  23  on which the lens system images scene  30 . TOF 3D camera  20  optionally comprises a shutter  25  for shuttering the camera ON and OFF, a light source  26 , and a controller  24  that controls shutter  25  and light source  26 . Whereas TOF 3D camera  20  is schematically shown having a shutter  25  separate from photosensor  22 , a TOF 3D camera may comprise a photosensor that includes circuitry operable to shutter ON and shutter OFF the photosensor and thereby the camera. A reference to shuttering ON or shuttering OFF a TOF 3D camera is understood to include shuttering ON and OFF the camera using any methods or devices known in the art, irrespective of whether or not specific reference is made to a “separate” shutter. 
     To determine distances to features in scene  30 , controller  24  controls light source  26  to transmit a train  40  of transmitted light pulses  41 , to illuminate scene  30 . Transmitted light pulses  41  are schematically represented by rectangular pulses associated with an overhead arrow  42  indicating direction of propagation of the light pulses. Features in scene  30  reflect light from each transmitted light pulse  41  towards TOF 3D camera  20  as a reflected light pulse. 
     In  FIG. 1A , exemplary features  131  and  132  comprised in objects  31  and  32  respectively are schematically shown reflecting light from transmitted light pulses  41  as trains  45  and  46  of reflected light pulses  47  and  48  respectively. Overhead arrows  67  and  68  schematically indicate direction of propagation of light pulses  47  and  48  respectively. Reflected light pulses, such as light pulses  47  and  48  generally have reduced intensity compared to transmitted light pulses  41  from which they were reflected but substantially a same pulse width and a same pulse shape as the transmitted light pulses. Light pulses used in a TOF 3D camera, such as transmitted light pulses  41  used by TOF 3D camera  20 , typically have a pulse width between about 5 and 10 ns (nanoseconds). 
     Upon lapse of a predetermined exposure delay, “T L ,” after a time at which each transmitted light pulse  41  is transmitted, controller  24  opens shutter  25  to shutter ON TOF 3D camera  20  for a short exposure period. Typically the short exposure period has a duration between about 10 ns and 20 ns and may have duration equal to the pulse width of transmitted light pulses  41 . The short exposure period is used to determine how long it takes light to propagate from TOF 3D camera  20  in a transmitted light pulse  41  and return to the camera in a reflected light pulse. Light in a reflected light pulse from a given feature in scene  30  that reaches TOF 3D camera  20  during the short exposure period following a transmitted light pulse  41  from which it was reflected is registered by a pixel  23  on which the camera images the given feature. An amount of light from a reflected light pulse that is registered during the short exposure period is substantially proportional to a convolution between the reflected light pulse and the exposure period. Reflected light registered by the pixel responsive to all transmitted light pulses  41  in light pulse train  40  provides a measure of the round trip transit time of light from TOF 3D camera  20  to the feature and back to the camera, and may be used to determine a distance to the feature imaged on the pixel. 
     For example, light in reflected light pulses  47  from feature  131  is imaged on, and registered by a pixel  23  designated  23 - 131  in  FIG. 1A , and light in reflected light pulses  48  from feature  132  is imaged on, and registered by a pixel designated  23 - 132  in the figure. The amounts of light registered by pixels  23 - 131  and  23 - 132  are substantially proportional to the convolutions of exposure periods of TOF 3D camera  20  with reflected light pulses  47  and  48 . The convolutions are a function of the round trip transit times of light from light source  26  to features  131  and  132  and back from the features to TOF 3D camera  20 . The amounts of light registered by pixels  23 - 131  and  23 - 132  during the exposure periods provide measures of the convolutions and are used, optionally by controller  24 , to determine distances from TOF 3D camera  20  to features  131  and  132  respectively. 
     Let the pulse width of a transmitted light pulse  41  and duration of a short exposure period following each transmitted light pulse  41  be the same and equal to “τ”. Let distance to a feature, “f”, such as feature  131  or  132 , in scene  30  be “D(f),” and an amount of reflected light registered by a pixel that images the feature be “Q(f)”. Then distance D(f) may be given by an expression, 
         D ( f )= cT   L /2±( c τ)(1 −Q ( f )/ Q   O ( f ))/2.  1)
 
     In equation 1 “c” is the speed of light, and “Q O (f)” is an amount of light that would be registered by the pixel were reflected light pulses from the feature to be temporally coincident with the short exposure periods. Various methods are known in the art to determine Q O (f) and when the plus or minus sign in the expression for D f  applies. Q O (f) is generally determined by controlling TOF 3D camera  20  to transmit a pulse train of light pulses having pulse width τ and registering light from features during long exposure period of the camera following transmission of each light pulse. 
     By way of example, equation 1) may be written for distance, “D( 131 )”, of feature  131  (schematically shown imaged on pixel  23 - 131  in  FIG. 1A ) from TOF 3D camera  20  as 
         D (131)= cT   L /2±( c τ)(1 −Q (23-131) Q   O (23-131)/2.  2)
 
     In general, a TOF 3D camera operating with transmitted light pulse width, “τP”, and an exposure period duration “τ E ” may provide distances to features in a scene located between a nearest distance, D N =c(T L −τ P )/2, and a farthest, D F =c(T L +τ E )/2 from the TOF 3D camera. A dynamic distance range “DDR” of the TOF 3D camera is therefore equal to about (τ P +τ E )/2. For TOF 3D camera  20  operating as described above with τ P =τ E =τ, DDR=cτ. 
       FIG. 1B  shows a timeline graph  200  that schematically illustrates relative timing of transmitted light pulses  41  in light pulse train  40 , exposure periods of TOF 3D camera  20 , and reflected light pulses  47  and  48 . The graph schematically illustrates convolutions between transmitted light pulse  47  and  48  and short exposure periods of TOF 3D camera  20 . Timeline graph  200  comprises timelines  202 ,  204 ,  206 , and  208 . 
     Transmitted light pulses  41  are schematically represented by rectangles along timeline  202  and are indicated as having a light pulse width τ. Short exposure periods are schematically represented by dashed rectangles  49  along timeline  204  and are indicated as having duration τ. A short exposure period  49  is associated with each transmitted light pulse  41 , and is indicated as starting following a exposure delay T L  after the light pulse  41  is transmitted. Reflected light pulses  47  and  48  reflected by features  131  and  132  respectively from transmitted light pulses  41  are shown along timelines  206  and  208 . Short exposure periods  49  shown along timeline  204  are reproduced along timelines  206  and  208  to show relative timing between the short exposure periods and reflected light pulses  47  and  48 . Height of reflected light pulses  47  and  48  in  FIG. 1B  is smaller than height of short exposure periods  49  for convenience of presentation and to distinguish the reflected light pulses from the exposure periods. Height of the reflected light pulses  47  and  48  is smaller than that of transmitted light pulses  41  to indicate that intensity of the reflected light pulses is less than that of the transmitted light pulses. 
     A shaded area A( 23 - 131 ) of a reflected light pulse  47  in a region of the light pulse that temporally overlaps a short exposure period  49 , indicates a magnitude of a convolution between reflected light pulse  47  and short exposure period  49 . An amount of light, “Q( 23 - 131 )”, in reflected light pulse  47  that is registered by pixel  23 - 131 , which images feature  131 , is proportional to the convolution and is represented by shaded area A( 47 - 49 ) in  FIG. 1B . A duration of the overlap is equal to τQ( 23 - 131 )/Q O ( 23 - 131 ), which is a term in the equation 2) for D( 131 ). As noted above, Q O ( 23 - 131 ) is an amount of light that would be registered by pixel  23 - 131  were light pulse  47  completely coincident with short exposure period  49 . 
     Similarly, a magnitude of the convolution between a reflected light pulse  48  from feature  132  and a short exposure period  49  is indicated by a shaded area A( 23 - 132 ) of reflected light pulse  48  in a region of reflected light pulse  48  that temporally overlaps the exposure period. An amount of light, Q( 23 - 132 ), in reflected light pulse  48  that is registered by pixel  23 - 132 , which images feature  132 , is proportional to the convolution and shaded area A( 48 - 49 ). A duration of the overlap is equal to τQ( 23 - 132 )/Q O ( 23 - 132 ) in the equation for D f . 
     In  FIG. 1A  feature  132  is shown closer to TOF 3D camera  20  than is feature  131  and for a given transmitted light pulse  41 , a reflected light pulse  48  arrives at TOF 3D camera  20  earlier than a reflected light pulse  47  from feature  131 . As a result, for exposure delay T L , reflected light pulse  48  overlaps its associated exposure period  49  less than reflected light pulse  47 , and an amount of reflected light registered by pixel  23 - 132  is less than an amount of reflected light registered by pixel  32 - 131 . Area A( 48 - 49 ), which provides a measure of reflected light registered by pixel  23 - 132  is therefore smaller than area A( 47 - 49 ), which provides a measure of reflected light registered by pixel  23 - 131 . 
     In  FIG. 1A  and  FIG. 1B  transmitted light pulses  41  and reflected light pulses  47  and  48  are shown as ideal square pulses with substantially zero rise times, zero fall times, and perfectly uniform intensities. Exposure periods  49  are also shown as ideal and having a perfectly rectangular shape with sensitivity of pixels  23  in TOF 3D camera  20  rising with zero rise time at an ON time of an exposure period to maintain a constant sensitivity for a duration of the exposure period until an OFF time of the exposure period. At the OFF time sensitivity for registering light falls abruptly to zero with zero fall time. However, practical light pulses and exposure periods have non-zero rise and fall times, and generally do not respectively provide ideally uniform intensities and sensitivities. 
     In general, it is advantageous for determining distances to features in a scene that light pulses transmitted by a TOF 3D camera, such as TOF 3D camera  20 , to illuminate the scene have a pulse shape that matches a shape of the short exposure periods during which light reflected from the transmitted light pulses is registered. In many situations, to provide improved accuracy and resolution of distance measurements provided by a TOF 3D camera, it is advantageous that the transmitted light pulse shape be similar to, or substantially the same as, the shape of the exposure periods. 
     However, light pulses transmitted by a TOF 3D camera are generally provided by light sources comprising lasers or light emitting diodes coupled to switching circuitry that is subject to inductances, capacitances, and resistances that are not readily adjusted. As a result, it may often be impractical to adjust transmitted light pulse shapes provided by the light sources so that they have a desired pulse shape that may be matched to exposure periods of a TOF 3D camera. 
       FIG. 2  shows a timeline graph  300  that schematically illustrates relative timing of transmitted and reflected light pulses, and exposure periods for TOF 3D camera  20  imaging scene  30  and objects  31  and  32  ( FIG. 1A ) with light pulses having pulse shapes substantially different from a shape of exposure periods of the TOF 3D camera. Timeline graph  300  comprises timelines  302 ,  304 ,  306 , and  308 . 
     Light source  26  ( FIG. 1A ) transmits light pulses that are schematically represented by small rectangles  341  shown along timeline  302  and are assumed by way of example to have a pulse width equal to about τ/3. Short exposure periods of TOF 3D camera  20  that are associated with transmitted light pulses  341  have non-zero rise and fall times and are schematically represented by dashed trapezoids  349  along timeline  304 . Short exposure periods  349  are assumed to have a pulse width τ, and each has an ON time that is delayed from a transmission time of its associated transmitted light pulse  341  by a same exposure delay T L . Light pulses reflected from transmitted light pulses  341  by features  131  and  132  ( FIG. 1A ) are represented by rectangles  347  and  348  along timelines  306  and  308  respectively. Dashed trapezoids  349  representing exposure periods of TOF 3D camera  20  that are associated with transmitted light pulses  341  are reproduced along timelines  306  and  308  to illustrate relative timing of the reflected light pulses and the exposure periods. 
     TOF 3D camera  20  operating with light pulses  341  having pulse width τ P =τ/3 and exposure period duration τ D =τ, has a dynamic range DDR, ignoring effects of rise and fall times, that may be given, as noted above, by an expression DDR=c(τ+τ/6)/2. Under the operating conditions that apply for  FIG. 2 , DDR of TOF 3D camera  20  is about 7/12 that of TOF 3D camera  20  operating under the operating conditions that apply for  FIG. 1B . 
     Light in reflected light pulses  347  and  348  arrive at TOF 3D camera  20  following a same round trip time as light in reflected light pulses  47  and  48  ( FIG. 1A ) respectively and exposure periods  49  ( FIG. 1B) and 349  occur following a same exposure delay T L  relative to a transmission time of their associated transmitted light pulses  41  and  341 . As a result of the reduced DDR noted above that characterizes operation of TOF 3D camera with transmitted light pulses  341  and exposure periods  349 , reflected light pulses  347  and  348  have no temporal overlap with their associated exposure periods  349 . Therefore no light is registered from features  131  and  132  by TOF 3D camera  20  operating under the conditions on which timeline graph is based and the TOF 3D camera does not provide distance measurements to features  131  and  132 . 
     A TOF 3D camera, such as TOF 3D camera  20 , may not be limited to using a single exposure delay. TOF 3D camera  20  may function to determine distances to features  131  and  132  using an exposure delay T L  shorter than that shown in  FIGS. 1A and 2 . For the shorter exposure delay sufficient temporal overlap may exist between exposure periods  349  and reflected light pulses  347  and  348  to provide distances to features  131  and  132 . However, because of the mismatch between pulse length of transmitted light pulses  341  and exposure periods  349 , and mismatch between their shapes, convolutions between light pulses reflected from transmitted light pulses  341  and exposure periods  349  are generally less sensitive to differences in distances of features in scene  30  than are convolutions for matched light pulses and exposure periods. For operation of TOF 3D camera  20  with transmitted light pulses  341  and exposure periods  349  therefore, resolution and accuracy for measurements it produces for distances to features  131  and  132  are generally impaired relative to resolution and accuracy obtained with transmitted light pulses  41  and exposure periods  49  shown in  FIG. 1B . 
       FIG. 3A  shows a timeline graph  400  that schematically illustrates operating TOF 3D camera  20  to image scene  30  ( FIG. 1A ) with an emulated transmitted light pulse having a pulse shape matched to the shape of the TOF 3D camera&#39;s exposure periods, in accordance with an embodiment of the invention. 
     In  FIG. 3A , TOF 3D camera  20  is assumed to illuminate scene  30  with a train of light pulses comprising transmitted light pulses  441 ,  442 , . . . ,  446 , and to image light reflected from the transmitted light pulses by features in scene  30  during short exposure periods  451 ,  452 , . . . ,  456  that are respectively associated with transmitted light pulses  441 ,  442 , . . . ,  446 . Transmitted light pulses  441 , . . . ,  446  are assumed by way of example, to have a same pulse shape as transmitted light pulses  341  shown in  FIG. 2 , and exposure periods  451 ,  452 , . . . ,  456  are assumed to have a same shape as that of exposure periods  349  shown in  FIG. 2 . 
     Light reflected from transmitted light pulses  441 , . . . ,  446  by feature  131  in scene  30  ( FIG. 1A ) propagates to TOF 3D camera  20  as reflected light pulses  541 ,  542 , . . . ,  546  respectively, which are schematically shown as rectangular pulses along timeline  406 . Similarly, light reflected from transmitted light pulses  441 , . . . ,  446  by feature  132  propagates to TOF 3D camera  20  as reflected light pulses  641 ,  642 , . . . ,  646  respectively that are schematically shown as rectangular pulses along timeline  408 . 
     In accordance with an embodiment of the invention, controller  24  controls light source  26  and/or shutter  25  ( FIG. 1A ) to adjust exposure delays between light pulses transmitted by light source  26  to illuminate scene  30  and ON times of their associated exposure periods by different perturbation periods. The perturbation periods are determined so that reflected light pulses reflected by a given feature in scene  30  from different transmitted light pulses arrive at different times relative to the ON times of their respective associated exposure periods and provide an emulated light pulse having a desired pulse shape. 
     By way of example, in  FIG. 3A  controller  24  optionally controls timing of exposure periods  451 , . . . ,  456  so that they repeatedly occur with a fixed period. Witness lines  410  shown along timeline  402  indicate “standard” transmission times for transmitted light pulses  441 ,  442 , . . . ,  446  transmitted by light source  26 . For a light pulse transmitted at a standard transmission time indicated by witness line  410  by light source  26 , an exposure delay to an associated exposure period is equal to T L . In accordance with an embodiment of the invention, to provide a desired emulated light pulse, controller  24  delays transmission of transmitted light pulses  441 ,  442 ,  443 ,  444 ,  445 , and  446  relative to the standard transmission times indicated by witness lines  410  by perturbation periods equal to 0, τ/2, τ, τ, 3τ/2, and 2τ respectively. Exposure delays between transmitted light pulses  441 , . . .  446 , and their respective associated exposure periods  451 , . . . ,  456 , are therefore, as indicated in timeline graph  400 , equal to T L , (T L −τ/2), (T L −τ), (T L −τ), (T L −3τ/2), and (T L −2τ). 
     Reflected light pulses  541 , . . . ,  546  reflected by feature  131  reach pixel  23 - 131  ( FIG. 1A ) of TOF 3D camera  20  relative to the ON times of their associated exposure periods at times that are perturbed by the perturbation periods of their associated transmitted light pulses. For example, assume that reflected light pulse  541 , for which the perturbation period of its associated transmitted light pulse  441  is 0, arrives at pixel  23 - 131  at a time “T a ” relative to the ON time of its associated exposure period  451 . Then reflected light pulses  542 ,  543 ,  544 ,  545 , and  546  arrive at pixel  23 - 131  at times, (T a −τ/2), (T a −τ), (T a −τ), (T a −3τ/2), and (T a −2τ) respectively. Whereas reflected light pulses  541  and  542  arrive prior to the ON times of their respective associated exposure periods  451  and  452 , and light they contain is therefore not registered by pixel  23 - 131 , light in portions of reflected light pulses  543 ,  544 ,  545 , and  546  arrives during exposure periods  453 ,  454 ,  455 ,  456 , and portions of the light they contain are registered by pixel  23 - 131 . 
     Transmitted light pulses  441 , . . . ,  446  provide an emulated light pulse in accordance with an embodiment of the invention. The emulated light pulse comprises a time ordered sum of the light in light pulses  441 , . . . ,  446  for which each light pulse  441 , . . . ,  446  contributes to the sum at a time delayed from a leading edge of the emulated light pulse that is equal to its perturbation period. The leading edge of the emulated light pulse is a leading edge of a transmitted light pulse, an “earliest” transmitted light pulse, that contributes to the emulated light pulse, which in  FIG. 3A  is light pulse  441 . The leading edge of transmitted light pulse  441  is its transmission time, which in  FIG. 3A  is coincident with its associated standard transmission time indicated by witness line  410 . An amount of light from reflected light pulses  541 ,  542 ,  543 ,  544 ,  545 , and  546  that reaches and is registered by pixel  23 - 131  is a same amount of light which pixel  23 - 131  would register from a reflection of a single light pulse that has a pulse shape identical to the emulated light pulse and is transmitted by light source  26  at a transmission time at which transmitted light pulse  441  is transmitted. 
     Similarly, An amount of light that pixel  23 - 132  registers from reflected light pulses  641 ,  642 ,  643 ,  644 ,  645 , and  646  that reaches and is registered by pixel  23 - 132  is a same amount of light which pixel  23 - 131  would register from a reflection of the emulated light pulse provided by transmitted light pulses  441 , . . . ,  446 . 
       FIG. 3B  shows a timeline graph  500  that reproduces timelines  402 ,  406  and  408  from timeline graph  400  in  FIG. 3A  and schematically shows an emulated transmitted light pulse  440  provided by transmitted light pulses  441 , . . . ,  446 . Emulated transmitted light pulse  440  comprises transmitted light pulses  441 , . . . ,  446  stacked in order of their respective perturbation delays relative to the transmission time of transmitted light pulse  441  indicated by witness line  410  associated with transmitted light pulse  441 . Transmitted light pulse  441  is shown in solid lines and light pulses  442 , . . . ,  446  “virtually” transposed to the transmission time of light pulse  441  to illustrate how they contribute to emulated transmitted light pulse  440  are shown in dashed lines. By choosing perturbation periods in accordance with an embodiment of the invention, as discussed above and as shown in  FIGS. 3A and 3B , emulated light pulse  440  has a pulse width τ equal to the duration of exposure periods  451 , . . . ,  456  and a trapezoidal shape similar to that of the exposure periods. 
     Reflection of light in emulated transmitted light pulse  440  by feature  131  is schematically shown as an “emulated reflected light pulse”  540 . Emulated reflected light pulse  540  is a compound pulse formed from reflected light pulses  541 , . . . ,  546  similarly to the manner in which emulated transmitted light pulse  440  is formed from transmitted light pulses  441 , . . . ,  446 . An amount of reflected light from reflected light pulses  541 , . . . ,  546  registered by pixel ( 23 - 131 ) that images feature  131  ( FIG. 1A ) is equal to a convolution of emulated reflected light pulse  540  with exposure period  451 . A shaded area A*( 23 - 131 ) of emulated reflected light pulse  540  represents that portion of emulated reflected light pulse  540  that contributes to the convolution. 
     Similarly, an amount of reflected light from reflected light pulses  641 , . . . ,  646  registered by pixel ( 23 - 132 ) that images feature  132  ( FIG. 1A ) is equal to a convolution of emulated reflected light pulse  640  with exposure period  451 . A shaded area A*( 23 - 132 ) of emulated reflected light pulse  640  represents that portion of emulated reflected light pulse  640  that contributes to the convolution. 
     It is noted that for the operating conditions of TOF 3D camera  20  that apply for  FIG. 2  and 
       FIGS. 3A and 3B  TOF 3D camera  20  illuminates scene  30  with transmitted light pulses having a same pulse width τ/3. However, by temporally configuring transmitted light pulses  441 , . . . ,  446  to provide an emulated reflected light pulse  540  having a pulse width T, in accordance with an embodiment of the invention, the dynamic distance range DDR, of TOF 3D camera is substantially increased. Whereas, as noted above, TOF 3D camera  20  has a DDR equal to about ( 7/12)cτ under the operating conditions that apply for  FIG. 2 , the TOF 3D camera has a DDR equal to about cτ under the operating conditions that apply for  FIGS. 3A and 3B , which provide emulated transmitted light pulse  540 . Under the operating conditions that apply for  FIG. 2  distance measurements to features  131  and  132  cannot be acquired but distance measurements may be acquired under the operating conditions, which provide an emulated transmitted light pulse in accordance with an embodiment of the invention that apply for  FIGS. 3A and 3B   
     Whereas in the description above, transmitted light pulses are timed to provide an emulated light pulse having a pulse shape similar to an exposure period, practice of embodiments of the invention are not limited to tailoring light pulses to match a shape of an exposure period. For example, an amount of light from a transmitted light pulse, such as transmitted light pulses  41  and  441  ( FIGS. 1A and 3A ) that illuminates a feature in scene  30 , such as features  131  and  132  in scene  30 , typically decreases by the square of a distance of the feature from TOF 3D camera  20 . As a result, an amount of light registered by a pixel  23  that images the feature, which is useable to provide a distance to the feature decreases in proportion to a square of the distance of the feature from TOF 3D camera  20 . 
     In an embodiment of the invention, to moderate a reduction in registered light with distance, an emulated transmitted light pulse is configured to have a greater amount of light in a trailing half of the emulated transmitted light pulse than in a leading half of the emulated light pulse. Optionally, the emulated transmitted light pulse has a parabolic shape, for which intensity of light in the emulated transmitted light pulse increases substantially quadratically with displacement from a trailing edge of the light pulse. 
     For example, light reflected from a transmitted light pulse by features relatively close to TOF 3D camera  20  that reaches and is registered by TOF 3D camera  30  during the camera&#39;s exposure periods is typically light reflected predominantly from portions of the transmitted light pulses closer to the trailing edges of the light pulses. On the other hand, light reflected from a transmitted light pulse by features relatively far from TOF 3D camera  20  that reaches and is registered by TOF 3D camera  20  during the camera&#39;s exposure periods is typically light reflected from portions of the transmitted light pulse closer to the leading edges of the light pulses. Therefore, an emulated transmitted light pulse having more light in its trailing half than in its leading half, in accordance with an embodiment of the invention, operates to moderate decrease in registered light with distance. An emulated light pulse having a parabolic pulse shape that increases substantially quadratically with displacement from a trailing edge of the light pulse provides illumination of features in scene  30  for TOF 3D camera  20  that operates to substantially match and cancel the inverse square falloff of illumination with distance, and provide illumination of scene  30  that may appear similar to ambient illumination. 
     By way of example,  FIG. 4  schematically shows an emulated light pulse  666  comprising light pulses  667  that has a shape  668  similar to a parabolic shape, for which intensity of light in the emulated light pulse increases substantially quadratically with displacement from a trailing edge  701  of the emulated light pulse to a leading edge  702  of the emulated light pulse. 
     It is noted that whereas emulated light pulse  666  is discussed in a context of a TOF 3D camera, an emulated light pulse similar to emulated light pulse  666  may be advantageous for use with a camera that provides contrast images, that is “pictures” of a scene. Light pulses similar to emulated light pulse  666  may provide advantageous illumination of features in a scene that are relatively far from the “contrast” camera. 
     In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb. 
     Descriptions of embodiments of the invention in the present application are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described, and embodiments of the invention comprising different combinations of features noted in the described embodiments, will occur to persons of the art. The scope of the invention is limited only by the claims.