Patent Publication Number: US-2023143755-A1

Title: Hybrid LADAR with Co-Planar Scanning and Imaging Field-of-View

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/432,105, which claims priority on U.S. Provisional Application Ser. No. 62/295,210, filed on Feb. 15, 2016, all disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of optical Time-of-Flight (ToF) measurement, and more specifically, to directionally-resolved ToF measurement technology, known as Laser Radar (LADAR). 
     BACKGROUND OF THE INVENTION 
     The need for early warning of an invading force dates back to the ancient world, which could involve scouts traveling on foot or by horse back, and eventually included use of telescopes and binoculars for an enhanced view of the enemy. In the modern era, beginning in World War II, radio detection and ranging (RADAR) systems were deployed, and which utilize reflected radio waves to identify the position of enemy aircraft. Sonar similarly uses sound waves to locate vessels within the oceans. Soon after the development of laser technologies, light/laser detection and ranging (LIDAR/LADAR) systems underwent development. 
     LADAR is generally based on emitting short pulses of light within certain Field-Of-View (FOV) at precisely-controlled moments, collecting the reflected light and determining its Time-of-Arrival, possibly, separately from different directions. Subtraction of the pulse emission time from ToA yields ToF, and that, in turns, allows one to determine the distance to the target the light was reflected from. LADAR is the most promising vision technology for autonomous vehicles of different kinds, as well as surveillance, security, industrial automation, and many other areas, where detailed information about the immediate surroundings is required. While lacking the range of radar, LADAR has a much higher resolution due to much shorter wavelengths that are used for sensing, and hence, comparatively relaxed diffractive limitations. It may be especially useful for moving vehicles, both manned, self-driving, and unmanned, if it could provide detailed 3D information in real-time, with the potential to revolutionize vehicles&#39; sensing abilities and enable a variety of missions and applications. 
     However, until recently, LADARs have been prohibitively large and expensive for vehicular use. They were also lacking in desirable performance: to become a true real-time vision technology, LADAR must provide high-resolution imagery, at high frame rates, comparable with video cameras, in the range of 15-60 fps, and cover a substantial solid angle. Ideally, a LADAR with omnidirectional coverage of 360° azimuth and 180° elevation would be very beneficial. Collectively, these requirements may be called “real-time 3D vision”. 
     A variety of approaches has been suggested and tried, including mechanical scanning, non-mechanical scanning, and imaging time-of-flight (ToF) focal-plane arrays (FPA). There is also a variety of laser types, detectors, signal processing techniques, etc. that have been used to date, as shown by the following. 
     U.S. Patent Application Pub. No. 2012/0170029 by Azzazy teaches 2D focal plane array (FPA) in the form of a micro-channel plate, illuminated in its entirety by a short power pulse of light. This arrangement is generally known as flash LADAR. 
     U.S. Patent Application Pub. No. 2012/0261516 by Gilliland teaches another embodiment of flash LADAR, with a two dimensional array of avalanche photodiodes illuminated in its entirety as well. 
     U.S. Patent Application Pub. No. 2007/0035624 by Lubard teaches a similar arrangement with a 1D array of detectors, still illuminated together, while U.S. Pat. No. 6,882,409 to Evans further adds sensitivity to different wavelengths to flash LADAR. Another approach is the use a 2D scanner and only one detector receiving reflected light sequentially from every point in the FOV, as taught by US Patent Application Pub. No. 2012/0236379 by da Silva. 
     Additional improvements to this approach are offered by U.S. Pat. No. 9,383,753 to Templeton, teaching a synchronous scan of the FOV of a single receiver via an array of synchronized MEMS mirrors. This arrangement is known as retro-reflective. 
     Yet another approach is to combine multiple lasers and multiple detectors in a single scanned FOV, as taught by U.S. Pat. No. 8,767,190 to Hall. 
     However, presently, such a real-time 3D LADAR remains elusive. 
     OBJECTS OF THE INVENTION 
     The present invention is aimed at overcoming the limitations of both scanning and imaging approaches to LADAR by combining their advantages and alleviating their problems, namely to:
         1. Improve Signal-to-Noise Ratio (SNR);   2. Lower the peak power of illumination sources, reduce their cost and improve efficiency;   3. Reduce overall cost and power consumption; and   4. Increase the resolution.       

     Further objects and advantages of the invention will become apparent from the following description and claims, and from the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     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. 
     In one embodiment, the above-noted objects of this invention may be achieved by combining a relatively large 1D array of time-sensitive detectors (preferably, hundreds of pixels), with a relatively fast 1D laser scanner (preferably, tens of kHz in frequency), in a co-planar configuration, that allows collection of one line of ToF data during each scan. Optionally, the planar FOV of both the scanner and the detector is also scanned in an orthogonal direction at a slower rate, thus providing a 2D FOV. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a concept for a 1D scanning and imaging hybrid system. 
         FIG.  2 A  illustrates a 1D hybrid LADAR installed on a Unmanned Aerial Vehicle (UAV) to obtain 2D ToF data. 
         FIG.  2 B  illustrates a 1D hybrid LADAR coupled to an additional scanning mirror to obtain 2D ToF data. 
         FIG.  2 C  illustrates a 1D hybrid LADAR installed on a rotating platform to obtain 2D ToF data. 
         FIG.  3 A  illustrates tight focusing of the laser beam on the scanning mirror of a LADAR and subsequent re-collimation with a cylindrical lens. 
         FIG.  3 B  is a bottom view of the LADAR of  FIG.  3 A . 
         FIG.  3 C  is a side view of the LADAR of  FIG.  3 A . 
         FIG.  4    illustrates forming a scan line on the surface of a diffractive element and imaging the scan line onto the target. 
         FIG.  5 A  illustrates a laser spot scanning across virtual detector pixels on the target. 
         FIG.  5 B  illustrates permissible misalignment while the laser spot is scanning across tall virtual detector pixels on the target. 
         FIG.  5 C  illustrates splitting each of the pixels shown in  FIG.  5 B  into two sub-pixels. 
         FIG.  5 D  illustrates positioning an array of micro-lenses in front of the detectors, and further illustrates how a relatively small pixel can be collecting light from a relatively large virtual pixel. 
         FIG.  6 A  illustrates the laser of the LADAR system being continuously ON. 
         FIG.  6 B  illustrates the laser being modulated with one short pulse per pixel. 
         FIG.  6 C  illustrates the laser being modulated with multiple short pulses per pixel. 
         FIG.  7    illustrates synchronizing the clock of an electronic control system with the scanning process, so that there is a constant integer number of clock pulses per scanning cycle. 
         FIG.  8    illustrates the electronic control system of  FIG.  7   . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used throughout this specification, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include”, “including”, and “includes” mean including but not limited to. 
     The phrases “at least one”, “one or more”, and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C”, “one or more of A, B, and C”, and “A, B, and/or C” mean all of the following possible combinations: A alone; or B alone; or C alone; or A and B together; or A and C together; or B and C together; or A, B and C together. 
     The word “pixel” is used throughout this specification to denote an individual detector of a 1D detector array. 
     Also, all references (e.g., patents, published patent applications, and non-patent literature) that are cited within this document are incorporated herein in their entirety by reference. 
     Furthermore, the described features, advantages, and characteristics of any particular embodiment disclosed herein, may be combined in any suitable manner with any of the other embodiments disclosed herein. 
     In accordance with at least one embodiment of the hybrid LADAR system disclosed herein, a fast 1D scanner/imager hybrid can collect one line of high-resolution ToF data within tens of microseconds. If needed, that hybrid can also be coupled to a secondary, slow scanning stage, producing raster ToF frames of thousands of lines at high frame rates, with total pixel throughput of tens or even hundreds of megapixels per second. 
     The following description lists several embodiments of the present invention, which are merely exemplary of many variations and permutations of the subject matter disclosed. 
     Mention of one or more representative features of a given embodiment is likewise exemplary: an embodiment can exist with or without a given feature, and likewise, a given feature can be part of other embodiments. 
     A preferable embodiment of a 1D hybrid scanner/imager is illustrated in  FIG.  1   . It comprises a 1D array of photo-detectors  1 , placed in a focal point of a receiving optical system  2 , and a scanning mirror  3 , its axis of rotation perpendicular to the direction of the detector array. A laser beam  5  generated by the laser  4  is directed onto the scanning mirror  3  through a collimating lens  6 . The positions of the optical system  2  and the scanner  3  are adjusted in such way that the fan of imaginary rays  7  emanating from individual pixels of the array, and the fan of real laser rays  8  emanating from the scanning mirror  3  lay in the same plane. Respectively, the laser scan line on the target overlaps with the FOV of the detector array. 
     Preferably, the scanning mirror  3  is a resonant MEMS mirror. Such mirrors, having dimensions of the reflective area of the order of 1 sq. mm, the resonant frequencies of tens of kHz, and the scan amplitude of tens of degrees, are becoming commercially available. While generally this invention would benefit from the fastest rate of scanning, a general tendency in scanning mirrors is that the faster scanning rate typically leads to narrower scan angle and smaller mirror size, which in turn increases the divergence of the scanned laser beam, thus limiting the number and size of the detectors in the array, and the amount of light that can be collected onto the detectors, especially, at longer ranges. 
     This invention might further benefit from non-mechanical beam scanners (NMBS), that are undergoing development, although their specifications and commercial availability remain unclear. For example, U.S. Pat. No. 8,829,417 to Krill teaches a phase array scanner, and U.S. Pat. No. 9,366,938 to Anderson teaches an electro-optic beam deflector device. NMBS may allow scan rates, or beam cross-sections, exceeding those of mechanical scanners. 
     A photo-detector array should preferably consist of high-sensitivity detectors, as the number of photons arriving to each detector, especially from longer ranges, might be exceedingly small. Significant progress has been recently achieved in design and manufacturing of Avalanche Photo Diodes (APD), working in both a linear, and a Geiger mode, which is also known as a Single-Photon Avalanche Detector (SPAD). The former are reported to be able to detect a light pulse consisting of just a few photons, while the latter can actually be triggered by a single photon, and both types would be suitable for embodiments of the present invention. It should be noted, however, that the most advanced detectors are expensive to fabricate, therefore, the present invention that needs only one row of detectors would be very cost-efficient in comparison with flash LADAR, that would typically require a 2D array of the same resolution. 
     One of the problems typically encountered in ground-based and vehicular LADARs intended to be used in a populated area is the eye safety hazard presented by its powerful lasers. Therefore, a preferable embodiment of this invention would use a laser with longer infra-red (IR) wavelength, exceeding approximately 1400 nm. Such longer wavelengths are not well-absorbed by human eyes and therefore don&#39;t pose much danger. Respectively, much more powerful lasers might be used in this spectral range. 
     Depending on a particular task the hybrid LADAR is optimized for, one of the following configurations of the slow stage may be used for ground surveillance, vehicular navigation and collision avoidance, control of industrial robots, other applications, or alternatively there may not be any slow stage at all. As depicted on  FIG.  2 A , the hybrid 1D LADAR of  FIG.  1    is attached to an aerial platform  15 , with the scan direction perpendicular to direction of platform motion. 
       FIG.  2 B  illustrates using a scanning mirror  16 , with its scanning direction being perpendicular to the scanning direction of the hybrid. Combined, they constitute a 2D LADAR preferably having comparable scan angles in both directions. Finally,  FIG.  2 C  illustrates positioning the hybrid of  FIG.  1    on a rotational stage  17 , which gives the combined LADAR a 360 degree FOV in a horizontal plane. 
     If a scanning mirror is used for the slow stage, its area should be sufficiently large to reflect the entire fan of beams emanating from the fast scanner, as well as the entire fan of rays coming to the optical reception system. However, scanning mirrors with active area of square centimeters, frequencies of tens of Hz, and scan angles of tens of degrees are commercially available. Thus, an exemplary combination of a detector array with 300 pixels, a fast scanner of 30 kHz, and a slow scanner of 30 Hz would provide a point cloud of ˜18M points a second, assuming that both scanners utilize both scan directions, thus enabling real-time 3D vision with the resolution of approximately 1000×300 pixels at 60 frames a second. Rotation at 60 revolutions per second, or linear motion on board a vehicle would also provide comparable point acquisition rates, without the limitations of the reflective area of the scanning mirror. 
     Preferably, the optical system  2  is configured to form the FOV of each individual pixel  11  of the detector array to match the divergence angle of the laser beam, which is generally possible and usually not difficult to achieve. In an exemplary embodiment, a laser beam of near-infrared (NIR) light of approximately 1 mm in diameter will have a far-field divergence of approximately 1 milliradian (mrad). A photo-detector pixel 20 um in size will have similar FOV when placed in a focal plane of a lens with 20 mm focal length. 
     It might also be advantageous to use additional elements in an optical transmission system to shape the laser beam, such as, for example, a cylindrical lens  23 , as depicted in  FIG.  3 A . In this case, the laser may still be nearly-collimated in the scanning direction, but may be tightly focused in the orthogonal direction, as illustrated by  FIG.  3 B , which shows the view from direction B in  FIG.  3 A . Cylindrical lens  23  subsequently re-collimates the beam in the orthogonal direction as illustrated on  FIG.  3 C , which shows the view from direction C in  FIG.  3 A . The advantage of such an arrangement lies in the possibility to reduce the dimension of the scanning mirror in the tightly-focused laser beam direction, as well as in reduced eye hazard presented by the laser beam expanded in at least one direction. 
     In another embodiment of the present invention, the laser beam can be tightly focused in both directions and be scanned across a diffractive element  21 , as illustrated on  FIG.  4   , forming a thin scan line on its surface. That scan line is subsequently imaged onto the target by the lens  20 . The lens  20  may be identical to the lens  2  in the optical reception system in front of the detector array, in which case the dimensions of the scan line on the surface of a diffractive element  21  should be similar to that of the detector array, thus insuring the equal divergence of both FOVs of the illuminating laser and the detector. Alternatively, the lenses  2  and  20  may have different magnification, and respectively, the scan line may have different dimensions from the detector array, however, the design should provide for a good overlap of both FOVs on the target, as further illustrated on  FIG.  5 A , where virtual pixels  31  denote the projections of the detector pixels  11  onto the target through the optical system  2 , while the laser spot  25  scans the same target along the scan line  22 , thus sequentially illuminating pixels  11  with light reflected from the target. As shown on the figure for clarity, at the moment when the laser spot assumes position  25   a , it illuminates the detector pixel  11   a . It should be noted that the desired shape of laser spot  25  may be achieved by a variety of optical methods, as illustrated on  FIG.  3    and  FIG.  4   , or other methods, without limiting the scope of this invention. 
     Pixels  11  may have different shapes: for example,  FIG.  5 B  illustrates pixels with their height exceeding the pitch of the array. For example, pixels of the array may be 20 micrometers wide and 60 micrometers toll. The advantage of such arrangement is that it may accommodate slight misalignment between the laser scan line  22  and the line of virtual pixels  31 . Likewise, the laser spot may also be oblong, or have some other desirable shape. It might also be advantageous to split each of the pixels  11  into two or more sub-pixels  11   b ,  11   c , as depicted on  FIG.  5 C . These sub-pixels would generate redundant information, as the scan spot  25  may cross either of virtual pixels  31   b , or  31   c , or both of them, at the same time, and the ToA measured by either sub-pixels or both would be treated as one data point. The advantage, however, may come from the fact that smaller sub-pixels would generally have lower noise level, therefore a laser spot of the same power is more likely to generate a response when illuminating a small sub-pixel, than a larger full pixel. 
     Some types of the high-sensitivity detectors, notably Geiger-mode APDs, are known to need considerable gaps between active pixels to eliminate cross-talk, thus limiting the fill factor of the arrays consisting of such detectors. To alleviate this problem, an array of micro-lenses  12  may be used in front of the detectors  11 , as depicted on  FIG.  5 D , with the micro-lens  12   a  specifically illustrating how a relatively small pixel  11   a  can still be collecting light from a relatively large virtual pixel  31   a.    
     It is also preferable to match the total extent of the array&#39;s FOV with the total scan angle, for example: 512 pixels, 20 um each, placed behind a 20-mm lens will subtend the angle of ˜29°. Respectively, the total scan angle should be the same or slightly greater to utilize every pixel of the array. 
     In a further embodiment of this invention, as the laser spot moves along the scan line, it is continuously energized, thus producing a time-domain response in the pixels it crosses, as illustrated on  FIG.  6 A , where the graph  10 A represents the intensity (I) vs. time (t) response in the pixel  11   a . The response starts when the laser beam in position  25   b  just touches the virtual pixel  31   a , and ends when the laser beam moves to position  25   c , just outside of the virtual pixel  31   a . In such an embodiment, the laser power, and hence, the sensitivity is maximized, however, the precision of ToF measurement may suffer, due to the ambiguity of the Time of Arrival (ToA) of a relatively long light pulse. 
     To alleviate this problem, the laser can be electrically-modulated with shorter pulses, as illustrated by  FIG.  6 B . For example, the laser is turned on when the laser spot is in the position  26   d , producing the response  10   d  in the pixel  11   a . Since the pulse is narrower, its ToA can be determined with greater precision. 
     One problem with this modulation method might arise if the laser is energized while its spot falls in between two virtual pixels, therefore each of the real pixels is getting only a fraction of the reflected light. Due to some parallax between the laser scanner and the detector array, the precise overlap between the laser spot and virtual pixels is somewhat dependent on the distance to the target, and hence not entirely predictable. To alleviate this problem, the laser may be modulated with more than one pulse per pixel, as illustrated on  FIG.  6 B . In this case, at least one of the consecutive modulation pulses emitted with the laser spot in positions  25   f ,  25   g ,  25   h , would fully overlap with the virtual pixel  31   a , thus producing full response  10   g  in the pixel  11   a . Responses from two other pulses  10   f  and  10   h  might be shared with adjacent pixels and hence be lower. 
     In any LIDAR system dependent on ToF measurements, it is important to accurately measure the ToA of a light pulse on the detector, as well as the time when the light pulse was emitted. In the present invention, additionally, it is important to precisely know the position of the scanned spot, as it determines which detector pixel will be illuminated. To achieve this, it is preferable to synchronize the clock of the electronic control system, denoted by pulses  50  on  FIG.  7   , with the scanning process, in such a way that there is a constant integer number of clock pulses per scanning cycle  51 . The vertical axis (S) on  FIG.  7    illustrates a scan angle, while the horizontal axis (t) illustrates time. Since many resonant scanners have unique, non-tunable resonant frequency, the system clock frequency may be changed instead to keep a integer number of clocks per period, preferably, by means of a Phase-Lock Loop (PLL) circuit. As long as this fixed relationship between the system clock and the scan angle is maintained, the system clock may be easily used for precise detector read-out timing, as well as for timing of the laser modulation pulses, if such modulation is used. Additionally, some types of detectors require quenching after they received a light pulse, and arming, to be able to receive the next pulse. Keeping pixels armed when they are not expected to be illuminated is undesirable, as it carries the risk of false positive due to internal noise. In most flash LADARs with such detectors, they are armed all at once, right before the illuminating laser pulse is emitted. In the present invention, it is preferable to arm detectors one-by-one, in the order they are illuminated by the scanning laser beam. As illustrated on  FIG.  7   , while the scanning beam proceeds from bottom to top, the pixel  11   d  would be armed by the system clock pulse  50   d , while pixel  11   e  will be armed considerably later by the system clock pulse  50   e.    
     A preferred embodiment of the electronic control system is illustrated by  FIG.  8   . The fast scanning mirror  3  is driven by the driver  63  at the frequency defined by the voltage-controlled oscillator (VCO)  67  through clock divider  65 . Being resonant, mirror  3  tends to oscillate at maximum amplitude while driven at its own resonant frequency and in a specific phase with respect to the drive signal. The mirror phase feedback  71  carries the information about the mirror&#39;s phase to the phase-locked loop (PLL)  62 , that compares it to the phase of the clock divider and adjusts the VCO to eliminate any phase error, thus maintaining mirror oscillations close to its resonant frequency. 
     System clock  68  is derived from the same VCO, thus insuring that all other timing blocks function in strict synchronization with the mirror motion. Specifically, the arming circuit  61  provides sequential pixel arming and the laser driver  64  generate laser pulses in specific relationship to the scanned laser spot, as discussed above. Likewise, the same system clock synchronizes the motion of the slow mirror  16  through the driver  66  in a fixed relationship with the motion of the fast mirror, for instance, one cycle of the slow mirror per 1024 cycles of the fast mirror. Read-out circuit  60  and signal processor  69  may use the same clock as well, although they, unlike other elements discussed above, don&#39;t have to be strictly synchronized with the mirror motion. They must, however, be fast enough to be able to read and process data from all pixels within one scan cycle of the fast mirror. 3D vision data  70  is generated based on the ToF measurement coming from the detector array  1  and then supplied to users, such as navigation system of autonomous vehicles or security surveillance system. 
     It should be noted that the present invention offers a considerable advantage in efficiency over other types of LADARs, especially flash LADARs, where short laser pulses are used to illuminate the entire scene at relatively long intervals. The advantage comes from the fact that in the present invention the laser is energized either continuously, or with a fairly high duty cycle: for example, a LADAR of the present invention using 10 ns pulses per pixel and generating 18M data points per second would have a duty cycle of approximately 35%. A flash LADAR using the same 10 ns pulses, and having the same frame rate of 60 fps, would have the laser duty cycle of only 0.00006%. Consequently, to generate the same average power and attain comparable range, the laser of that hypothetical flash LADAR would need instantaneous power almost 6 orders of magnitude greater. While pulsed lasers capable of producing very short powerful pulses do exist, they are known to have lower efficiency, larger size and higher cost than continuous lasers of the same average power. Aside from inability to deliver high average power, pulsed sources are typically less efficient, more complex, bulkier and costlier, than continuous or high-duty sources. The general physical explanation of lower efficiency is in the fact that the emitted power of photonic sources—lasers or LEDs—is typically proportional to the current, while parasitic losses on various ohmic resistances inside those sources are proportional to the square of the current. 
     Additionally, high-power pulsed laser sources are typically more dangerous in terms of eye safety. 
     It should be noted, that it is difficult (although not impossible) to place the scanner at the center of the optical system. At any other position, there will be some parallax between the FOVs of the optical transmission and the reception systems, hence the detailed mapping of the pixels onto a scan line will depend on the distance to the target. However, in a practical system such parallax can be kept to a minimum by placing the scanner in close proximity to the optical system. 
     The present invention as illustrated by the above-discussed embodiments, would provide serious advantages over other types of LADAR. 
     In a typical imaging LADAR, the light source emits a short pulse which illuminates all pixels at once. Respectively, each pixel receives only a small fraction of the total back-scattered signal. In the proposed hybrid, only one pixel receives all the back-scattered light emitted at a given moment. If we assumed that the illumination power is the same, then the signal strength on each pixel would be up by a factor comparable to the number of pixels, i.e. hundreds, if not thousands. In practice, pulsed sources generally have higher instantaneous power than continuous ones, but their average power is still considerably lower. 
     Conversely, in a typical scanning LADAR, at any given moment only a small portion of the photo-detector is receiving any signal, while the rest only generates noise and contributes to unwanted capacitance. The exception is so-called retro-reflective scanners, where a small detector FOV is directed through the same scanning system. However, this approach only works with large, slow scanners, where the mirrors are large enough to provide sufficient optical collection area for back-scattered light. Contrarily, high-speed scanners are usually tiny, just sufficient to fit the beam of the laser, and are usually of the order of 1 mm. 
     In either case, as illustrated above, a hybrid appears would have a considerable SNR advantage: higher signal than imaging-only, or lower noise than scanning-only device. 
     It is anticipated that a hybrid LADAR of the present invention will be able to use a regular laser diode as its illumination source—which is by far the cheapest and most efficient source among those suitable for LADARs. 
     Also, both cost and power consumption are roughly proportional to the total number of pixels fabricated by a given technology. So, substituting a 2D array by a 1D array is supposed to considerably reduce both cost and power consumption for the detector array, while the cost and power consumption of both fast and slow scan stages may be considerably lower, than that of the array of pixels or the illuminating laser.