Abstract:
A light emitting diode (LED) illumination device for generating modulated light for a scannerless range imaging system includes circuitry for generating a drive signal having a given phase and frequency characteristic; a plurality of light emitting diodes arranged in a plurality of LED banks, wherein each LED bank comprises a plurality of light emitting diodes connected in series to the circuitry to receive the drive signal; and a switching stage for simultaneously activating the plurality of LED banks such that the LED banks generate the modulated light according to the phase and frequency characteristics of the drive signal.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of three-dimensional image capture and in particular to a modulated illumination source used in the capture of image depth information with a scannerless range imaging system. 
     BACKGROUND OF THE INVENTION 
     Distance (or depth) information from a camera to objects in a scene can be obtained by using a scannerless range imaging system having a modulated illumination source and a modulated image receiver. In a method and apparatus described in U.S. Pat. No. 4,935,616, a scannerless range imaging system uses an amplitude-modulated high-power laser diode to simultaneously illuminate a target area. Conventional optics confine the target beam and image the target onto a receiver, which includes an integrating detector array sensor having hundreds of elements in each dimension. The range to a target is determined by measuring the phase shift of the reflected light from the target relative to the amplitude-modulated carrier phase of the transmitted light. To make this measurement, the gain of an image intensifier (in particular, a micro-channel plate) within the receiver is modulated at the same frequency as the transmitter, so the amount of light reaching the sensor (a charge-coupled device) is a function of the range-dependent phase difference. A second image is then taken without receiver or transmitter modulation and is used to eliminate non-range-carrying intensity information. Both captured images are registered spatially, and a digital processor is used to extract range data from these two frames. Consequently, the range associated with each pixel is essentially measured simultaneously across the whole scene. 
     A scannerless ranging system, such as the system described above, typically uses a laser for field illumination in order to capture depth information. Lasers are capable of high power and very high frequency modulation. However, a primary concern is eye safety, which requires proper safety glasses or alternatively requires the laser design to provide added protective measures, e.g., output radiant power limits, safe viewing distance, etc. To achieve sufficient powers of illumination for image capture, scannerless ranging systems use multimode lasers. Due to their nature, multimode laser diodes typically have significant spatial structure along with astigmatism, speckle and large width-to-height ratios (ellipticity) in the beam. As a result, large non-uniform regions can appear when using the beam to illuminate an object field. The spatial structure can also shift with varying temperatures and drive current. To overcome these effects, optical components, e.g., an anamorphic prism, are placed in the path of the illumination source to shape the beam and reduce ellipticity. Diffuser plates are also placed into the beam path to reduce spatial structure in the illumination, to provide beam spreading, and to comply with requirements for eye safety. Other optical components could also be used in lieu of a diffuser to produce beam uniformity and spreading. For long distances, the coherency property of a laser provides the advantage of maintaining low dispersion. 
     What is needed is an alternative to the use of laser diodes for depth capture to overcome some of the disadvantages presented by the use of a laser for field illumination, namely: the concern for eye safety, beam structure, speckle, additional optics and cost. Moreover, in the case of field illumination for closer range image capture (e.g., 40 feet or less), the coherency property of a laser is a disadvantage since beam spreading and good uniformity are desired, thus necessitating further optics. In consideration of power density, a single laser component does provide higher output power density (compared, e.g., to a single LED), however high current thresholds must be overcome to drive the laser. 
     As described in the Sandia Lab News (vol. 46, No. 19, Sep. 16, 1994), the scannerless range imaging system described in the &#39;616 patent may alternatively use an array of amplitude-modulated light emitting diodes (LEDs) to completely illuminate a target scene. Eye exposure to an LED source illumination can be tolerated, as one might expect in a picture-taking scenario where eye sensitivity is present. However, the design for the modulated LED source poses a challenge, particularly as to the scalability and reliability of the design, as well as to operation at the required high modulating frequencies. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a scannerless range imaging system that is capable of providing reliable illumination for closer range image capture. 
     A further objective of the invention is to provide improved eye safety in a scannerless range imaging system, particularly for closer range image capture. 
     A further object of the invention is to provide easily scalable, high frequency drive circuitry for an LED illumination source for a scannerless range imaging system. 
     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a light emitting diode (LED) illumination device for generating modulated light for a scannerless range imaging system includes circuitry for generating a drive signal having a given phase and frequency characteristic; a plurality of light emitting diodes arranged in a plurality of LED banks, wherein each LED bank comprises a plurality of light emitting diodes connected in series to the circuitry to receive the drive signal; and a switching stage for simultaneously activating the plurality of LED banks such that the LED banks generate the modulated light according to the phase and frequency characteristics of the drive signal. 
     This invention provides an alternative to the use of laser diodes for depth capture, thereby overcoming some of the disadvantages presented by the use of a laser for field illumination, namely: eye safety, beam structure, speckle, high threshold currents, additional optics and cost. 
     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the main components of a scannerless range imaging system in accordance with the invention. 
     FIG. 2 shows the primary assemblies of the LED illumination device shown in FIG.  1 . 
     FIG. 3 is a diagram of the illumination section of the LED illumination device shown in FIG.  2 . 
     FIG. 4 is a diagram of the LED driver circuitry of the LED illumination device shown in FIG.  2 . 
     FIG. 5 is a diagram of the interface circuitry of the LED illumination device shown in FIG.  2 . 
     FIGS. 6A and 6B illustrate timing diagrams for two consecutive frames with shifted phases, as generated by the interface circuitry shown in FIG.  5 . 
     FIG. 7 is a diagram of the power conditioning circuitry of the LED illumination device shown in FIG.  2 . 
     FIG. 8 is a block diagram of a known range imaging system which can be used to capture a bundle of images. 
     FIG. 9 is a diagram of a distributed layout of LEDs, as provided in one embodiment of the illumination section shown in FIG.  3 . 
     FIG. 10 is a diagram of a non-distributed layout of LEDs, as provided in another embodiment of the illumination section shown in FIG.  3 . 
     FIGS. 11A and 11B show diagrams of two arrangements of RGB and IR LEDs that provide both wide band illumination for capturing an intensity image and modulated illumination for capturing a range image. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Because range imaging devices employing laser illuminators and capture devices including image intensifiers and electronic sensors are well known, the present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. Elements not specifically shown or described herein may be selected from those known in the art. Certain aspects of the embodiments to be described may be provided in software. Given the system as shown and described according to the invention in the following materials, software not specifically shown, described or suggested herein that is useful for implementation of the invention is conventional and within the ordinary skill in such arts. 
     It is helpful to first review the principles and techniques involved in scannerless range imaging. Accordingly, referring first to FIG. 8, a range imaging system  10  is shown as a laser radar that is used to illuminate a scene  12  and then to capture an image bundle comprising a minimum of three images of the scene  12 . An illuminator  14  emits a beam of electromagnetic radiation whose frequency is controlled by a modulator  16 . Typically, in the prior art, the illuminator  14  is a laser device which includes an optical diffuser in order to effect a wide-field illumination. The modulator  16  provides an amplitude varying sinusoidal modulation. The modulated illumination source is modeled by: 
     
       
           L ( t )=μ L +η sin(2 πλt )  (Eq. 1)  
       
     
     where μ L  is the mean illumination, η is the modulus of the illumination source, and λ is the modulation frequency applied to the illuminator  14 . The modulation frequency is sufficiently high (e.g., 12.5 MHz) to attain sufficiently accurate range estimates. The output beam  18  is directed toward the scene  12  and a reflected beam  20  is directed back toward a receiving section  22 . As is well known, the reflected beam  20  is a delayed version of the transmitted output beam  18 , with the amount of phase delay being a function of the distance of the scene  12  from the range imaging system. The reflected beam  20  strikes a photocathode  24  within an image intensifier  26 , thereby producing a modulated electron stream proportional to the input amplitude variations. The output of the image intensifier  26  is modeled by: 
     
       
           M ( t )=μ M +γ sin(2 πλt )  (Eq. 2)  
       
     
     where μ M  is the mean intensification, γ is the modulus of the intensification and λ is the modulation frequency applied to the intensifier  26 . The purpose of the image intensifier is not only to intensify the image, but also to act as a frequency mixer and shutter. Accordingly, the image intensifier  26  is connected to the modulator  16 , causing the gain of a microchannel plate  30  to modulate. The electron stream from the photocathode  24  strikes the microchannel plate  30  and is mixed with a modulating signal from the modulator  16 . The modulated electron stream is amplified through secondary emission by the microchannel plate  30 . The intensified electron stream bombards a phosphor screen  32 , which converts the energy into a visible light image. The intensified light image signal is captured by a capture mechanism  34 , such as a charge-coupled device (CCD). The captured image signal is applied to a range processor  36  to determine the phase delay at each point in the scene. The phase delay term ω of an object at a range ρ meters is given by:              ω   =         2      ρλ     c        mod                 2      π             (Eq. 3)                                
     where c is the velocity of light in a vacuum. Consequently, the reflected light at this point is modeled by: 
     
       
           R ( t )=μ L +κ sin(2 πλt +ω)  (Eq. 4)  
       
     
     where κ is the modulus of illumination reflected from the object. The pixel response P at this point is an integration of the reflected light and the effect of the intensification:              P   =         ∫   0     2      π              R        (   t   )            M        (   t   )               t         =       2        μ   L          μ   M       +     κ                 π                 γ                   cos        (   ω   )                     (Eq.  5)                                
     In the range imaging system disclosed in the aforementioned &#39;616 patent, a reference image is captured during which time the micro-channel plate is not modulated, but rather kept at a mean response. The range is estimated for each pixel by recovering the phase term as a function of the value of the pixel in the reference image and the phase image. 
     A preferred, more robust approach for recovering the phase term is described in U.S. Pat. No. 6,118,946, entitled “Method and Apparatus for Scannerless Range Image Capture Using Photographic Film”, which is incorporated herein by reference. Instead of collecting a phase image and a reference image, this approach collects at least three phase images (referred to as an image bundle). This approach shifts the phase of the intensifier  26  relative to the phase of the illuminator  14 , and each of the phase images has a distinct phase offset. For this purpose, the range processor  36  is suitably connected to control the phase offset of the modulator  16 , as well as the average illumination level and such other capture functions as may be necessary. If the image intensifier  26  (or laser illuminator  14 ) is phase shifted by θ i , the pixel response from equation (5) becomes: 
       P   i =2μ L μ M π+κπγcos(ω+θ i )  (Eq. 6) 
     It is desired to extract the phase term ω from the expression. However, this term is not directly accessible from a single image. In equation (6) there are three unknown values and the form of the equation is quite simple. As a result, mathematically only three samples (from three images) are required to retrieve an estimate of the phase term, which is proportional to the distance of an object in the scene from the imaging system. Therefore, a set of three images captured with unique phase shifts is sufficient to determine ω. For simplicity, the phase shifts are given by θ k =2πk/3; k=0, 1, 2. In the following description, an image bundle shall be understood to constitute a collection of images which are of the same scene, but with each image having a distinct phase offset obtained from the modulation applied to the intensifier  26 . It should also be understood that an analogous analysis can be performed by phase shifting the illuminator  14  instead of the intensifier  26 . If an image bundle comprising more than three images is captured, then the estimates of range can be enhanced by a least squares analysis using a singular value decomposition (see, e.g., W. H. Press, B. P. Flannery, S. A. Teukolsky and W. T. Vetterling,  Numerical Recipes  ( the Art of Scientific Computing ), Cambridge University Press, Cambridge, 1986). 
     If images are captured with n≧3 distinct phase offsets of the intensifier (or laser or a combination of both) these images form an image bundle. Applying Equation (6) to each image in the image bundle and expanding the cosine term (i.e., P i =2μ L μ M π+κπγ (cos(ω)cos(θ i )−sin(ω)sin(θ i ))) results in the following system of linear equations in n unknowns at each point:                (           P   1               P   2             ⋮             P   n           )     =       (         1         cos                   θ   1               -   sin                     θ   1               1         cos                   θ   2               -   sin                     θ   2               ⋮       ⋮       ⋮           1         cos                   θ   n               -   sin                     θ   n             )                     (           Λ   1               Λ   2               Λ   3           )               (Eq.  7)                                
     where Λ=2μ L μ M π, Λ 2 =κπγ cos ω, and Λ 3 =κπγ sin ω. This system of equations is solved by a singular value decomposition to yield the vector Λ=[Λ 1 , Λ 2 , Λ 3 ] τ . Since this calculation is carried out at every (x, y) location in the image bundle, Λ is really a vector image containing a three element vector at every point. The phase term ω is computed at each point using a four-quadrant arctangent calculation: 
     
       
         ω=tan −1 (Λ 3 , Λ 2 )  (Eq. 8)  
       
     
     The resulting collection of phase values at each point forms the phase image. Once phase has been determined, range r can be calculated by:              r   =     ω                   c     4      πλ                 (Eq.  9)                                
     Equations (1)-(9) thus describe a method of estimating range using an image bundle with at least three images (i.e., n=3) corresponding to distinct phase offsets of the intensifier and/or illuminator. 
     Referring now to FIG. 1, the overall scannerless range imaging system is shown to comprise four main components in accordance with the present invention. The first component is an image capture device  40 , preferably a single lens reflex (SLR) camera body  42  having an image capture element  44 ; a typical capture element could be a photosensitive film (which would be used in a range imaging system as described in the aforementioned U.S. Pat. No. 6,118,946) or an electronic sensor, such as a charged-coupled-device (CCD). The image capture device  40  is able to capture a plurality of range images, i.e., the aforementioned bundle of images. Typically, for an electronic image capture element, the image capture device  40  would include image storage means (not shown) to store all the range images in the image bundle, as well as, e.g., a color texture image in addition to the range images. This storage can be accomplished by an on-camera storage means, such as a an internal memory together with output connections to, e.g., a PCMCIA card or a floppy disk (not shown) for receiving images from the internal memory. Also, a means for advancing, or driving, the image capture device  40  to prepare for a successive image capture must be available. Such capabilities are well known in relation to such image capture devices, and will not be described in detail. 
     The second component is an illumination device  46  for producing high-frequency amplitude modulated illumination of a desired average amplitude, amplitude modulus and frequency. Preferably, the illumination device  46  is configured as a removable unit  48  that is attached to the camera body  42  via a conventional hot shoe connection  50 . It is also desirable that the illumination device  46  includes the capability of shifting the phase of the amplitude modulation between a set of prescribed phase offsets (alternatively, this function may be performed by modulation of the reflected illumination in the optical portion of the scannerless range imaging system). It is also useful for the illumination device  46  to have a preferred operating wavelength, such as in the infrared region of the spectrum. 
     The illumination device  46  has the primary purpose of producing amplitude-modulated illumination with its phase controllable for generating a shift in the transmitted wave pattern for each range image in the image bundle (although, as mentioned before, this function may be performed by modulation of the reflected illumination). As will be explained in detail later, the illumination device  46  includes a light emitting diode (LED) light source and a modulation circuit for generating the requisite modulation signals of predetermined frequency with a set of predetermined phase offsets. The LED light source is preferably modulated at a modulation frequency of about 12.5 megahertz and the preferred phase offsets, as mentioned earlier are phase shifts θ in each range image given by θ k =2πk/3; k=0, 1, 2. Although the LED light need not necessarily be uniformly distributed, a controlled angle diffusion sheet, such as the Light Shaping Diffuser® from Physical Optics Corporation, is positioned in front of the light source in order to spread the modulated light across the desired field of view as uniformly as possible. 
     The illumination device  46  may also include a standard wide-band illumination source that is not modulated. This illumination source is used for intensity (non-ranging) photographic images, and may be based on a commonly known and understood flash of a standard camera system, e.g., a commonly available electronic flash of the type useful with photographic cameras. Alternatively, the intensity illumination could be provided by separate LEDs, e.g., red, green and blue LEDs, that are chosen so as to provide a white illumination similar to an electronic flash. One approach would be to cluster the RGB LEDs in and around each modulated (IR) LED used for ranging. There are many possibilities for such “clustering”; for example, as shown in FIG. 11A, an LED cluster could be constituted by one each of RGB and IR LEDs. Or, as shown in FIG. 11B, RGB LEDs could be clustered around each IR LED in a hexagonal layout. In either case, the layout shown would be replicated as necessary to provide the needed illumination. Source color temperature could be controlled by the current through each color group. And, whatever the layout, it would be superimposed upon the layouts that will be described for the modulated (IR) LEDs in connection with FIGS. 9 and 10. In any of these embodiments, the illumination device  46  would operate in either of the following modes: a) a first mode in which the light source is operated to illuminate the scene with a plurality (bundle) of exposures, each with a unique phase offset applied to its modulated frequency; and b) a second mode in which the standard wide-band illumination device is operated during capture of an intensity image, e.g., a color texture image. If ambient light is sufficient, of course, it may be unnecessary for the illumination device to operate in the second mode in order to capture an intensity image; in that case, the image capture device would be instructed to operate without flash. Moreover, the sequence of image capture may be reversed, that is, the second mode may be engaged before the first mode or, indeed, the second mode might in specific situations be engaged between the several exposures of the first mode. 
     The third component is an optical assembly  52 , which includes a lens  54  and a depth capture optical tube  56 . The optical tube  56  incorporates a modulating element  58 , such as the intensifier  26  (see FIG.  8 ), for modulating the incoming reflected scene light that is directed toward the capture element  44  in the camera body  42 . The optical tube  56  also includes an IR bandpass filter  59  in order to eliminate background illumination, and may incorporate other opto-mechanical elements, such as a shutter  57 . (If the range imaging system is further capable of capturing an intensity image, e.g., a color texture image, then the IR bandpass filter  59  would be removed or bypassed for that capture.) The optical assembly  52  is shown as a removable optical element that is connected to the image capture device  40  by means of a lens mount  60  on the camera body  42 . This is to further emphasize that the image capture device  40  may be based on a conventional camera body that could otherwise be fitted with a standard family of lenses for conventional photography. 
     The fourth component is a controller  62 , which manages the overall image capture process of the range imaging system, including the operation of the illumination device  46 , the image capture device  40 , and the optical assembly  52 . For instance, the controller  62  includes the necessary drivers and other circuitry (not shown) for operating the modulating element  58  in proper synchronism with an image capture event. (It should also be apparent that the controller  62  could be part of, or attached to, the camera body  42 , and the necessary electrical linkages to the optical assembly could be made directly through the camera body  42 , e.g., through contacts in the lens mount  60 . 
     As shown in FIG. 2, the illumination device  46  comprises three primary assemblies: an illumination section  70 , an LED driver stage  72  and a power and control interface  74 . The illumination device  46  also includes electrical connectors  76  and  78  for an exposure control signal input and a power input, a display and user control section  80  for displaying status indicators and providing one or more switches for on/off and other functions, and a camera mount  82  for attaching the device  46  to the hot shoe connector  50  on the camera body  42 . In addition, the illumination device  46  may be configured in two connected parts  46   a  and  46   b,  where the upper part  46   a  with the illumination section  70  can articulate about a pivot  46   c  for better aiming, etc. The entire illumination device  46  is intended to be a self contained unit that attaches to the hot shoe connector  50  of the camera body  42  via the camera mount  82 . This is to further emphasize that the illumination device  46  may be based on a conventional flash body that could be fitted to a standard camera body. 
     Referring to FIGS. 2 and 3, the illumination section  70  contains a plurality of light emitting diodes (LEDs)  84  mounted onto a heat sink (copper bar)  86 , a diffuser plate  88  (preferably a Holographic Light Shaping Diffuser® from Physical Optics Corporation) at the illumination exit port of the illumination section  70 , and a thin layer of insulation material  90  (preferably a material identified as Sil-PAD™ available from Bergquist Co., Chanhassen, Minn.) placed between the diodes  84  and the copper heat sink  86  to provide electrical isolation and good thermal conductivity. The circuit traces connecting the LEDs are completed on a printed circuit board  91  on the backside of the heat sink  86 . At the present time, infrared LEDs are available (e.g. part number CLE330E from Clairex Technologies Inc., which have a peak emission at 850 nm.) which can be modulated at frequencies sufficient for depth capture (e.g. 12.5 MHz.). High current threshold is not an issue with LEDs, which can provide illumination from a very low current (10&#39;s of milliamperes) to a high continuous current (100 milliamperes) to a very high current (amperes) in a pulsed mode. Moreover, the non-coherency property of the LED provides inherent dispersion that can be controlled using simple lenses mounted directly onto the LED at the time of manufacturing. 
     Ample illumination is obtained by using multiple LEDs. While only one LED  84  is shown in FIG. 3 for clarity of illustration, it should be understood that a plurality of LEDs are mounted through four rows  85  and  87  of hole perforations in the insulation material  90  and the heat sink  86 , respectively. In the preferred embodiment, five LED diodes, which are termed an “LED bank”, are connected together on the printed circuit board  91  to operate in series; eight banks (i.e., forty LEDs) are contained in the illumination device  46 . Grouping the LEDs  84  in this fashion provides a good compromise between the number of driver circuits required and the driver capability. Nonetheless, these numbers of LEDs and banks are not to be seen as limitations, and a greater or fewer number of diodes, or diodes per bank, may be used in other embodiments according to the invention. 
     In a distributed LED layout as shown in FIG. 9 for eight banks A-H, the LED diodes  84  are mounted on the circuit board  90  in a dispersed fashion so that a bank failure would have minimal impact on the distribution of the illumination. On the other hand, in a non-distributed LED layout as shown in FIG. 10 for eight banks A-H, the LED diodes  84  are mounted in a regular pattern, which is helpful in more easily maintaining uniform trace lengths and thereby avoiding any phase discrepancy between the LED banks. In either case, as shown in both FIGS. 9 and 10, rows of LED diodes  84  are arranged using an offset placement pattern to provide more uniform distribution of the output illumination. The radiating full angle provides wide and reasonably uniform field illumination, especially at a short distance. The diffuser  88  is used to provide an additional but small amount of diffusion (10°) of the LED output; this further enhances uniformity for close field illumination. The diffuser material also provides protection for the interior of the illumination compartment, which encases the LED driver circuitry. 
     Referring to FIG. 4, an LED driver circuit  100  provides controlled current drive for a bank  92  of LEDs  84 . Eight of these driver circuits  100  are contained within the LED driver section  72  of the illumination device  46  in order to drive eight banks of LEDs. Current through each of the LEDs  84  in the bank  92 , which is the same due to the series arrangement, is established via the input voltage Vmod x  to a first stage amplifier  102 . The input voltage Vmod x  controls the level of illumination generated by the bank of LEDs, i.e., both as to the mean level and the peak to peak modulation. A second stage amplifier  104 , using the reference voltage Vref, overcomes the offset forward bias needed by the LEDs, which is approximately +8.5 Vdc depending upon the diodes chosen and the operating current through the diodes. The effective gain on the first stage  102  is approximately 16 depending upon the diodes and trim circuit  112 . By using a second amplifier stage  104 , the dynamic range of the first stage is preserved for the range of the amplified output, that is, ˜16×Vmod x . An R-C network  106  provides pole-zero compensation for loop stability and a unity gain buffer amplifier  110  delivers the high current needed to drive the LED bank  92 . A trim potentiometer P in a current trim circuit  112  provides a limited adjustment in the load resistor R L , such that the LED banks  92  have matching diode radiance for a given input voltage Vmod x . A FET switch  108  controls the on/off state of the LED bank  92  in response to an LED On signal derived within the power and control interface circuitry  74 . The bank  92  is turned on during the exposure period of the camera and gated off otherwise; in this embodiment, the bank  92  is also gated on during camera focus. 
     Referring to FIGS. 5 and 6, the interface portion of the circuitry  74  establishes the on state of all LED banks  92  in response to a received exposure control signal input to the illumination device  46  through the exposure control signal input connection  76  (see FIG.  2 ). This exposure control signal is based on a clock signal generated by the controller  62  (see FIG. 1) when the controller references an actual camera exposure signal (the “camera exposure signal” shown in FIGS. 6A and 6B) generated by the camera body  42  at the beginning of an exposure period. The exposure control signal may either be optically coupled through a fiber optic receiver  116  or wire coupled through a coax connector  118 . The optical coupling enables control of the light illumination device  46  either locally or remotely while providing good common mode immunity to system noise. The exposure control signal is coupled to a signal conditioner and amplifier  120 , which functions as a differential receiver, particularly for the differential fiber optic signal, and produces a clock signal  122  (the “received clock signal” shown in FIGS.  6 A and  6 B). The duration of the exposure control signal is equal to the exposure period with the addition of a 12 ms precursor time interval. The precursor time interval is established by firmware within the camera itself. The precursor interval accommodates the initiation of electrical-mechanical operations, such as shutter opening in the depth capture optical tube  56 , prior to image exposure. Following the initial 12 ms, the received clock signal  122  is active for the duration of the exposure time established by the camera, as indicated by the camera exposure signal seen in FIGS. 6A and 6B. 
     The received clock signal  122  appears as a 12.5 MHz. clock occurring for the duration of the input exposure control signal. From this clock signal, the interface circuit generates the input modulation signal Vmod x  used to drive the eight LED banks  92 . The input clock signal also performs the following functions: a) it establishes the modulation frequency (e.g. 12.5 MHz.) of Vmod x ; b) it activates the output illumination of the illumination section  70  for the duration of the exposure; and c) it controls the change in phase shift (e.g., the earlier explained phase shifts given by θ k =2πk/3; k=0, 1, 2.) of the output illumination modulation with respect to the phase of the intensifier modulation (the “intensifier modulation” waveform shown in FIGS.  6 A and  6 B). These phase shifts are provided by the controller  62  in response to the received exposure control signal. (The intensifier modulation signal shown in FIGS. 6A and 6B is the modulation signal applied to the intensifier  26  shown in FIG. 8.) 
     The clock signal  122  is filtered through a resonant LC filter  124  and produces a sinusoidal output voltage  126 . The sinusoidal signal  126 , when added in a summing amplifier  128  to a selected dc offset voltage V op  becomes the controlling voltage for the LED bank  92 , i.e. the voltage signal Vmod x . The level of LED illumination follows Vmod x , both in amplitude and phase. As shown in the timing diagrams of FIGS. 6A and 6B, the input clock phase of Vmod x  is shifted with respect to the phase of the intensifier modulation in accordance with preset phase increment values, as explained in connection with FIG.  8 . (An additional inherent phase delay t 1  could exist due to the electronics.) An example of the controlled phase increment is 10 nanoseconds, which relates to a π/4 (45°) phase shift given a 12.5 MHz. clock (80 nanoseconds period). More specifically, FIG. 6B shows the phase of input voltage signal Vmod x  for the second frame shifted by t 2  relative to the first frame shown in FIG.  6 A. 
     The interface circuitry  74  activates the LED bank  92  only when the transmitted modulation signal is sufficiently present. As shown in FIG. 5, a detection circuit  132  monitors the received clock signal  122 . If the received clock signal  122  is present for more than a predetermined time period (e.g., 1.28 μs or 16 clock cycles), the detection circuit  132  generates the LED On signal and the LED drive current is activated (see FIG.  4 ); if the received clock signal  122  is absent for a predetermined time (e.g., 0.16 μs or 2 clock cycles), the detection circuit  132  disables the LED On signal and the LED drive current is deactivated. The detection circuit can take a variety of conventional forms, such as a logical counter, an R-C network with a comparator, or one-shot circuitry, depending upon the precision needed. 
     Referring to FIG. 7, the power and control interface circuitry  74  includes power conditioning circuitry  140 , which receives an external supply (e.g., +18 Vdc) through the power input connector  78 . Then, through the use of internal regulators  142   a,    142   b  and  142   c,  the power conditioning circuitry  140  establishes the +/− dc supplies (e.g., +V s , −V s , +V c ) needed for the other circuitry in the illumination circuit  46 . Levels for the three supplies are typically +15 Vdc, −15 Vdc, +5 Vdc, respectively. Since the supply regulation is conventional, the particular designs for conditioning and regulation of the supplies are readily understood by one of ordinary skill in this art and will not be further described. 
     The invention has been described with reference to one or more preferred embodiments. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  10 
                 range imaging system 
               
               
                   
                  12 
                 scene 
               
               
                   
                  14 
                 illuminator 
               
               
                   
                  16 
                 modulator 
               
               
                   
                  18 
                 output beam 
               
               
                   
                  20 
                 reflected beam 
               
               
                   
                  22 
                 receiving section 
               
               
                   
                  24 
                 photocathode 
               
               
                   
                  26 
                 image intensifier 
               
               
                   
                  30 
                 microchannel plate 
               
               
                   
                  32 
                 phosphor screen 
               
               
                   
                  34 
                 capture mechanism 
               
               
                   
                  36 
                 range processor 
               
               
                   
                  40 
                 image capture device 
               
               
                   
                  42 
                 camera body 
               
               
                   
                  44 
                 image capture element 
               
               
                   
                  46 
                 illumination device 
               
               
                   
                  46a 
                 upper part 
               
               
                   
                  46b 
                 lower part 
               
               
                   
                  46c 
                 pivot 
               
               
                   
                  48 
                 removable unit 
               
               
                   
                  50 
                 hot shoe connection 
               
               
                   
                  52 
                 optical assembly 
               
               
                   
                  54 
                 lens 
               
               
                   
                  56 
                 optical tube 
               
               
                   
                  57 
                 shutter 
               
               
                   
                  58 
                 modulating element 
               
               
                   
                  59 
                 IR bandpass filter 
               
               
                   
                  60 
                 lens mount 
               
               
                   
                  62 
                 controller 
               
               
                   
                  70 
                 illumination section 
               
               
                   
                  72 
                 LED driver stage 
               
               
                   
                  74 
                 power and control interface 
               
               
                   
                  76 
                 connector 
               
               
                   
                  78 
                 connector 
               
               
                   
                  80 
                 display and user control section 
               
               
                   
                  82 
                 camera mount 
               
               
                   
                  84 
                 light emitting diode(s) 
               
               
                   
                  85 
                 rows 
               
               
                   
                  86 
                 heat sink 
               
               
                   
                  87 
                 rows 
               
               
                   
                  88 
                 diffuser plate 
               
               
                   
                  90 
                 insulation material 
               
               
                   
                  91 
                 printed circuit board 
               
               
                   
                  92 
                 LED bank 
               
               
                   
                 100 
                 LED driver circuit 
               
               
                   
                 102 
                 first stage amplifier 
               
               
                   
                 104 
                 second stage amplifier 
               
               
                   
                 106 
                 R-C network 
               
               
                   
                 108 
                 FET switch 
               
               
                   
                 110 
                 unity gain amplifier 
               
               
                   
                 112 
                 current trim circuit 
               
               
                   
                 116 
                 fiber optic receiver 
               
               
                   
                 118 
                 coax connector 
               
               
                   
                 120 
                 signal conditioner and amplifier 
               
               
                   
                 122 
                 received clock signal 
               
               
                   
                 124 
                 resonant LC filter 
               
               
                   
                 126 
                 sinusoidal output signal 
               
               
                   
                 128 
                 summing amplifier 
               
               
                   
                 130 
                 unity gain buffer amplifier 
               
               
                   
                 132 
                 detection circuit 
               
               
                   
                 140 
                 power conditioning circuitry 
               
               
                   
                 142a 
                 regulator circuit 
               
               
                   
                 142b 
                 regulator circuit 
               
               
                   
                 142c 
                 regulator circuit