Patent Publication Number: US-2022229206-A1

Title: Light curtain system and enhanced geometric configurations

Description:
The present application is related to the following applications naming the same inventors, and assigned to the same entity; each of said applications incorporated herein by reference: U.S. patent application Ser. No. 15/574,655, entitled MULTI-LAYERED SAFETY SYSTEM, filed Nov. 16, 2017; U.S. patent application Ser. No. 15/574,659, entitled DIFFUSION SAFETY SYSTEM, filed Nov. 16, 2017; U.S. patent application Ser. No. 15/574,663, entitled POWER BEAMING VCSEL ARRANGEMENT, filed Nov. 16, 2017; U.S. patent application Ser. No. 15/574,667, entitled LOCATING POWER RECEIVERS, filed Nov. 16, 2017; U.S. patent application Ser. No. 15/574,668, entitled WIRELESS POWER TRANSMITTER RECEIVER, filed Nov. 16, 2017, and U.S. patent application Ser. No. 16/079,073, entitled REMOTE POWER SAFETY SYSTEM, filed Aug. 22, 2018. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure generally relates to detection of objects in or around a hazardous area. More particularly, but not exclusively, the present disclosure relates to a light curtain safety system around a high-flux power beam or field. 
     Description of the Related Art 
     One form of remote power is a laser power beaming system. A laser power beaming system, also called an optical wireless power system, includes at least one transmitter and at least one corresponding receiver. The transmitter forms a high-flux beam of laser light, which is projected through the air over a distance toward the receiver. 
     In conventional laser power beaming systems, the transmitter has a laser assembly that converts electric power into optical power (i.e., light). One or more mutually coherent or incoherent lasers in the laser assembly are used to generate the optical power; however, the one or more lasers may also be replaced by one or more light emitting diodes (LEDs), super-radiant diodes, or some other high-intensity light source. The light output of the laser assembly passes through various optical elements (e.g., optical fibers, lenses, mirrors, etc.), which convert the raw laser light into a beam of desired size and shape, and which aim the light beam toward the receiver. After leaving the transmitter, the high-flux light beam travels through free space toward the receiver. 
     The receiver, which may be in a remote area having an absence of easily available power, has a light reception module to receive the high-flux beam of laser light. At the receiver, light from the high-flux optical beam is captured by the light reception module. Power from the high-flux optical beam is captured, either directly or via collecting optics such as lenses or mirrors, and converted at least partly back to another form of useful power. In some cases, the light reception module includes an array of photovoltaic (PV) cells which convert light to direct current (DC) electricity. In other cases, the light reception module converts light to electricity in other ways, for example by converting the optical power to heat, which drives a heat engine (e.g., Stirling engine, turbine), a thermoelectric device, or some other device. In cases where the laser light is converted to usable electric power, the electric power is stored or transported to one or more circuits where the power is consumed. 
     The dangers of high-flux laser light are addressed in various U.S. and international standards. For example, in ANSI Z136.1, a laser eye exposure hazard is defined as being a function of the laser wavelength, flux (in W/cm 2 ), pulse width, and the apparent angular size (AAS) of the source. Accordingly, it is known that in some power beaming systems, the optical high-flux power beam is above the safe limit for exposure to living tissue such as a human or animal eye. In addition to eye or other tissue damage, the high-flux power beam may also cause burns or other damage to non-living objects. For these reasons, known power beaming systems may try to detect when people, animals, or other objects are in or will imminently enter the high-flux beam path during the time the beam is activated. 
     In some cases, a “light curtain” is used to detect objects moving in proximity to a high-flux power beam. The light curtain includes one or more light sources (i.e., emitters) arranged around a high-flux power beam transmitter to direct light across a gap towards one or more detectors arranged around the power beam receiver. The light curtain also includes a control circuit that provides a binary (e.g., “go/no-go”) output. The control circuit provides a “go” output if each detector in the light curtain is illuminated by a light source. In some embodiments, the control circuit provides a “no-go” output if light is blocked from reaching one or more detectors, which will happen when an opaque object such as a human hand enters the area between the light source and the light detector. 
     In some known light curtain technology, each detector monitors a single line-of-sight light path from one light source, and the controller is configured to respond only if that path is blocked. In some cases, the controller provides additional outputs, for example indicating a large size or location of the object based on the number of light paths that are blocked, or based on which particular light path is blocked. 
     Emitters in the light curtain emit light at a specific wavelength or in a specific wavelength range. In some cases, the light from the emitters is also pulsed at a particular rate. In these light curtains, the detection system includes one or more filters that permit the detector to ignore light signals that are different from the selected wavelength or outside of the selected wavelength range. In this way, the known light curtain will not respond to steady light sources, such as sunlight, or light modulated with an incorrect frequency or pattern such as fluorescent light sources which flicker at 120 Hz. 
     U.S. Pat. No. 7,078,666 to Tuominen teaches a variant of a standard light curtain, which is described as a “virtual insulator.” In Tuominen, low power light sources are located around a high power transmitter, and low power light detectors are located around a high power receiver. In this way, the virtual insulator is configured to surround a high-power laser beam. The virtual insulator of Tuominen uses multiple light sources with individual detectors, and Tuominen pulses light sources and logically combines the detector outputs such that if any one detector differs in its light-detected/light-not-detected state from other detectors, the high power beam is deactivated. 
     All of the subject matter discussed in the Background section is not necessarily prior art and should not be assumed to be prior art merely as a result of its discussion in the Background section. Along these lines, any recognition of problems in the prior art discussed in the Background section or associated with such subject matter should not be treated as prior art unless expressly stated to be prior art. Instead, the discussion of any subject matter in the Background section should be treated as part of the inventor&#39;s approach to the particular problem, which in and of itself may also be inventive. 
     BRIEF SUMMARY 
     The present disclosure is directed toward a beam projection system, including a beam transmitter, a beam receiver, a plurality of emitters, and a plurality of detectors. The beam transmitter has one or more light sources to transmit a beam. The beam transmitter also has a centerline. The beam receiver is arranged to receive the beam from the beam transmitter. The beam receiver has an acceptance centerline. The plurality of emitters and the plurality of detectors form a light curtain that is patterned about the beam. The plurality of emitters are positioned to surround the beam. The plurality of emitters each have a centerline and each have a divergence angle centered on the centerline. The placement and orientation of the plurality of emitters about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters to the incoming beam. The detection of one or more of the plurality of emitters by one or more of the plurality of detectors depends on the divergence angles of each of the plurality of emitters and the tilt of the beam receiver relative to the beam centerline. 
     In some embodiments, the beam projection system further includes concentration optics with a maximum acceptance angle for incoming light. In others embodiments, the beam projection system further includes an optical element for shaping a beam profile. In still other embodiments, the beam projection system further comprises concentration optics that include an aperture to narrow the light cone and sharpen an intensity profile. In yet other embodiments, the beam projection system specifies an receiver acceptance angle for the plurality of emitters, and if the detectors lose a signal from the plurality of emitters, then the beam is automatically turned off before the beam receiver is tilted at an angle too large relative to the incoming beam. 
     In one or more embodiments of the beam projection system, the placement and orientation of the plurality of emitters about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters within one-half degree of parallel to the incoming beam. In other embodiments of the beam projection system, the placement and orientation of the plurality of emitters about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters within one-quarter degree of parallel to the incoming beam. In still other embodiments of the beam projection system, the placement and orientation of the plurality of emitters about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters within one-tenth degree of parallel to the incoming beam. 
     In some embodiments of the beam projection system, the plurality of emitters each have a divergence angle centered on the centerline, and at least some of the divergence angles are different than other divergence angles. In other embodiments of the beam projection system, the detection based on the divergence angles of each of the plurality of emitters and the tilt of the beam receiver relative to the beam centerline controls whether the beam is transmitted. In still other embodiments of the beam projection system, the detection based on the divergence angles of each of the plurality of emitters and the tilt of the beam receiver relative to the beam centerline controls modulation of the beam. 
     In one or more other embodiments, the system to detect obstacles, includes a beam transmitter, a beam receiver, and a plurality of emitters and a plurality of detectors. The beam transmitter has one or more light sources to transmit a beam. The beam receiver is arranged to receive the beam from the beam transmitter. The plurality of emitters and the plurality of detectors form a light curtain that is patterned about the beam. The plurality of emitters surrounding the beam and the plurality of detectors surrounding the beam being are configured to detect an obstacle in at least one of many line-of-sight transmission paths between the plurality of emitters and the plurality of detectors. An outer shell of these many line-of-sight transmission paths is configured as a shell cylinder of separate emitters. Additionally, at least some of the plurality of emitters, at least some of the plurality of detectors, or both, are positioned at multiple radial distances from a center of the shell cylinder. 
     In another aspect of some embodiments, the positioning of at least some of the plurality of emitters, at least some of the plurality of detectors, or both, at multiple radial distances from the center of the shell cylinder further includes having two or more concentric rings of emitters including the shell cylinder surrounding the center of the shell cylinder. In still another aspect of some embodiments, the positioning of the at least some of the plurality of emitters, at least some of the plurality of detectors, or both, at multiple radial distances from the center of the shell cylinder further includes having one or more emitters in an interior of the beam. In yet another aspect of some embodiments, the positioning of the at least some of the plurality of emitters, at least some of the plurality of detectors, or both, at multiple radial distances from the center of the shell cylinder further includes having a random or pseudo random array of emitters distributed in an annulus area within the shell cylinder. 
     In one or more other embodiments of the system to detect obstacles, the detection of an obstacle in at least one of many line-of-sight transmission paths between the plurality of emitters and the plurality of detectors controls whether the beam is transmitted. In still other embodiments of the system to detect obstacles, the detection of an obstacle in at least one of many line-of-sight transmission paths between the plurality of emitters and the plurality of detectors controls modulation of the beam. 
     In some implementations, the system to detect obstacles, includes a beam transmitter, a beam receiver, and a plurality of emitters and a plurality of detectors. The beam transmitter has one or more light sources to transmit a beam. The beam receiver is arranged to receive the beam from the beam transmitter. The plurality of emitters and a plurality of detectors form a light curtain patterned about the beam. The plurality of emitters surrounding the beam and the plurality of detectors surrounding the beam being configured to detect an obstacle in a line-of-sight transmission path between an emitter in the plurality of emitters and a detector in the plurality of detectors. At least some of the plurality of emitters, at least some of the plurality of detectors, or both, are positioned away from the beam on arms that extend outwardly away from the beam. This configuration increasing the separation between the power beam and the line-of-sight transmission paths between the plurality of emitters and the plurality of detectors, which in turn increases how much time a safety system has to detect an object in the line-of-sight transmission path and take remedial measures, such as turn off the beam or otherwise modulate the beam. 
     In one or more other embodiments of the system to detect obstacles, the arms are extendable and retractable. In another aspect of the one or more other embodiments, the arms are foldable and unfoldable. In still another aspect of the one or more other embodiments, the arms include flexible webbing material that spreads out when extended to catch scattered light from the system. In yet another aspect of the one or more other embodiments, the arms are thinner and lighter than standard beam housing enclosures. In another aspect of the one or more other embodiments, the arms are constructed from Printed Circuit Board. In still another aspect of some embodiments of the system, the arms are encased in sealing materials to protect electronics from the environment. In yet another aspect of some embodiments of the system, the detection of an obstacle in the line-of-sight transmission path controls whether the beam is transmitted. In another aspect of some embodiments of the system, the detection of an obstacle in the line-of-sight transmission path controls modulation of the beam. 
     Again, in other implementations, the system to detect obstacles includes a beam transmitter, a beam receiver, and a plurality of emitters and a plurality of detectors. The beam transmitter has one or more light sources to transmit a beam. The beam receiver is arranged to receive the beam from the beam transmitter. The plurality of emitters and a plurality of detectors form a light curtain patterned about the beam. The plurality of emitters surrounding the beam and the plurality of detectors surrounding the beam being configured to detect an obstacle in a line-of-sight transmission path between an emitter in the plurality of emitters and a detector in the plurality of detectors. The plurality of emitters are connected together in series without a central controller. The timing of the emissions from the plurality of emitters is controlled by each of the plurality of emitters that are connected together in series. In this manner, an emitter in the plurality of emitters signals an adjacent emitter to produce a beam in conjunction with the emitter in the plurality of emitters producing a beam. 
     In some embodiments, one or more additional emitters are added to the plurality of emitters in series without connecting to a central controller. The timing of the emissions of the additional emitters is controlled by an emitter of the plurality of emitters that is connected together in series with the additional emitters. In this manner, an emitter in the plurality of emitters signals an adjacent emitter in the additional emitters to produce a beam in conjunction with the emitter in the plurality of emitters producing a beam. 
     This Brief Summary has been provided to introduce certain concepts in a simplified form that are further described in detail below in the Detailed Description. Except where otherwise expressly stated, the summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments are described with reference to the following drawings, wherein like labels refer to like parts throughout the various views unless otherwise specified. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements are selected, enlarged, and positioned to improve drawing legibility. The particular shapes of the elements as drawn have been selected for ease of recognition in the drawings. One or more embodiments are described hereinafter with reference to the accompanying drawings in which: 
         FIG. 1  is a power beaming system having exemplary arrangements of guard beam emitters and guard beam detectors; 
         FIG. 2  is a plurality of emitters and detectors, the emitters having overlapping guard beam light fields; 
         FIGS. 3A-3C  are guard beam signal intensity plots that show different methods of distinguishing multiple guard beam emitter light signals from each other; 
         FIG. 4A  is a system diagram of a first power beam safety system embodiment; 
         FIG. 4B  is a system diagram of a second power beam safety system embodiment; 
         FIG. 5A  is a logical flow diagram generally showing one embodiment of a process for utilizing a plurality of safety sensors with a power beam to turn off the power beam; 
         FIG. 5B  is a logical flow diagram generally showing one embodiment of a process for utilizing a plurality of safety sensors that detect signals of multiple wavelengths to interrupt a power beam based on an intrusion by one or more detected objects; 
         FIG. 5C  is a logical flow diagram generally showing a process to distinguish a first guard beam light signal from a second guard beam light signal; 
         FIG. 6A  is a power beaming system embodiment having guard beam emitters operating with signals of different wavelength; 
         FIG. 6B  is the power beaming system embodiment of  FIG. 6A  with a foreign object intruding on a guard beam; 
         FIGS. 7A-7E  are guard beam emitter embodiments; 
         FIG. 8  is a power beaming system having a light-transparent foreign object intruding on a high flux power beam; 
         FIGS. 9A-9B  are a power beaming system embodiment arranged to detect light-transparent foreign objects; and 
         FIGS. 10A-10B  are a high-flux power beam receiver embodiment arranged to detect a reflective or transparent foreign object and corresponding intensity profiles for the embodiment at various points in the receiver. 
         FIG. 11A  displays an emitter for use in controlling the output cone from emitters for angle sensing, in accordance with a disclosed embodiment; 
         FIG. 11B  displays a beam intensity profile for use in controlling the output cone from emitters for angle sensing, in accordance with a disclosed embodiment; 
         FIG. 11C  displays an emitter and lens for use in controlling the output cone from emitters for angle sensing, in accordance with a disclosed embodiment; 
         FIG. 11D  displays an emitter and aperture apparatus for use in controlling the output cone from emitters for angle sensing, in accordance with a disclosed embodiment; 
         FIG. 12A  displays a ring of emitters located at a radial distance from the center of the shell cylinder, in accordance with a disclosed embodiment; 
         FIG. 12B  displays two concentric rings of emitters located at multiple radial distances from the center of the shell cylinder, in accordance with a disclosed embodiment; 
         FIG. 12C  displays a ring of emitters located at a radial distance from the center of the shell cylinder, and further having one or more emitters in the interior of the power beam area, in accordance with a disclosed embodiment; 
         FIG. 12D  displays three concentric rings of emitters located at multiple radial distances from the center of the shell cylinder, a random distribution of emitters in an annulus area, or a pseudo-random distribution of emitters in an annulus area, in accordance with a disclosed embodiment; 
         FIG. 13A  displays a first view of the geometry of a many-to-many paths embodiment, showing what beam paths look like for two concentric rings of emitters, in accordance with one disclosed embodiment; 
         FIG. 13B  displays a second view of the geometry of a many-to-many paths embodiment, showing beam paths for two concentric rings of emitters, in accordance with another disclosed embodiment; 
         FIG. 14A  displays a side view of an orthographic projection of a many-to-many paths embodiment, showing beam paths for two concentric rings of emitters, in accordance with another disclosed embodiment; 
         FIG. 14B  displays an end view of an orthographic projection of a many-to-many paths embodiment, showing beam paths for two concentric rings of emitters, in accordance with another disclosed embodiment; 
         FIG. 15  displays a perspective view of a beam projection system with emitters that are spaced away from the power beam on whiskers. 
         FIG. 16A  displays an exploded view of an assembly beam projection system with PV array, in accordance with another disclosed embodiment. 
         FIG. 16B  displays a beam projection system with parallel incident light paths impinging on a dome, in accordance with another disclosed embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various disclosed embodiments. However, one skilled in the relevant art will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with computing systems including client and server computing systems, as well as networks have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments. 
     Prior to setting forth the embodiments however, it may be helpful to an understanding thereof to first set forth definitions of certain terms that are used hereinafter. 
     The term “free space,” as it is used in the present disclosure, means any reasonably transparent medium such as air or vacuum, water, gas, and the like. Free space is distinguished from a mechanical medium such as an optical fiber or conduit that confines or encloses a high flux light beam or field. Within the present disclosure, a free space path may include one or more mirrors, lenses, prisms, or other discrete optical elements that redirect or alter particular characteristics of the high flux light. 
     The term “power beam” is used, in all its grammatical forms, throughout the present disclosure and claims to refer to a high-flux light transmission that may include a field of light, that may be generally directional, and that may be arranged for steering/aiming to a suitable receiver. The power beams discussed in the present disclosure include beams formed by high flux laser diodes or other like sources sufficient to deliver a desirable level of power to a remote receiver without passing the power over a conventional electrical conduit such as wire. 
     In the present disclosure, the term “light,” when used as part of a safety system such as a guard beam, refers to electromagnetic radiation including visible light, ultraviolet light, and mid- or short-wavelength infrared light. Shorter or longer wavelengths, including soft X-rays and thermal infrared, terahertz (THz) radiation, or millimeter waves, are also considered to be light within the present disclosure when such light can be reflected, blocked, attenuated, or otherwise used to detect obstacles of the sizes and compositions of interest. 
     The term “guard beam” refers to a light-based system deployed in proximity to a high-flux power beam and arranged as one or more portions of a safety system. The guard beam may be formed as a light beam or field at a power level that is comparatively low with respect to the high-flux power beam or some other hazardous region. The interruption of a guard beam may be used to indicate the presence of an unsafe object. The interruption of a guard beam may generate one or more control signals that are used to prevent, extinguish, disable, block, or otherwise control the high-flux power beam or other hazard. For example, interruption of a guard beam may generate a control signal used by a safety system to shut down a high-flux power beam transmitter. In some cases the guard beam may only be partially interrupted. For example, in some cases, one or more foreign objects may reflect light as a side reflection that is at the edges of the emitted sensor beam(s) or from sensor light that is scattered in the air. In other cases, the guard beam may be fully interrupted causing a direct reflection due to one or more foreign objects being completely between an emitter and a detector. 
     In the present disclosure, a source for a guard beam is referred to as an “emitter.” The term emitter is distinguished from the term, “transmitter,” which indicates a source of a high-flux power beam. An emitter may be any type of optical or infrared emitting device, including without limitation a laser or a light emitting diode (LED), or an emitter may be any other source of electromagnetic energy, such as a light, having a flux level lower than the high flux power beam and determined to be safe. Along these lines, the detection module for a guard beam includes a “detector.” The term “detector” is distinguished from the term, “receiver,” which indicates a reception module for a high-flux power beam. A detector may be any type of optical or infrared sensing device, including without limitation a photodiode, a phototransistor, an avalanche photodiode, a photomultiplier tube, a portion of a 1-D or 2-D array of sensors such as a CCD or CMOS image sensor, or any other circuit or device (e.g., an emitter of radar or mm-waves) arranged to cooperatively detect signals from a corresponding emitter (e.g., a rectifier or transceiver). Emitters and/or detectors may include various electronic, optical, mechanical, electromechanical, and other components in addition to a light source and a photodetector, respectively, and some of said components are not described herein for brevity. 
     The term “near,” when used in the present disclosure in the context of, for example, a foreign object being “near” or impeding a guard beam, a high-flux power beam, or the like is a definite term of distance that is determinable based on a context of use. The term, however, is not subject to a formulistic or mathematically precise definition. For example, when a small-size, small-mass object (e.g., a bumble bee) is stationary or slowly moving toward a high-flux power beam, the small-size, small-mass object may be determined to be “near” the high-flux power beam when the object is within one or two inches of the high flux power beam. When the small-size, small-mass object is quickly moving toward the high-flux power beam, the object may be determined to be “near” the high-flux power beam when the object is within several feet of the high-flux power beam. When a medium-size, medium-mass object (e.g., a human being or a body part of a human being), is stationary or slowly moving toward a high-flux power beam, the medium-size, medium-mass object may be determined to be “near” the high-flux power beam when the object is within one to 12 inches of the high-flux power beam. When the medium-size, medium-mass object is quickly moving toward the high-flux power beam (e.g., a human being walking, running, swinging an arm, or the like), the object may be determined to be “near” the high-flux power beam when the object is within several feet of the high-flux power beam. Along these lines, when a large-size, large-mass object (e.g., an automobile, an aircraft, a watercraft, a piece of construction equipment, or the like) is stationary or slowly moving toward a high-flux power beam, the large-size, large-mass object may be determined to be “near” the high-flux power beam when the object is within one or two feet of the high flux power beam. When the large-size, large-mass object is quickly moving toward the high-flux power beam, the object may be determined to be “near” the high-flux power beam when the object is within dozens or even several hundred feet of the high-flux power beam. Accordingly, the determination of whether or not an object is “near” a high flux power beam may be determined based on a predicted time of obstruction, size of an object, speed of an object, power intensity of the high flux power beam, or other such factors that contribute toward a determination of imminent obstruction by the object. 
     The present disclosure may be understood more readily by reference to the following detailed description of the preferred embodiments of the disclosure. It is to be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to be limiting. It is further to be understood that unless specifically defined herein, the terminology used herein is to be given its traditional meaning as known in the relevant art. 
     In many cases, the flux (W/m 2 ) in an optical high flux power beam is substantially above the safe limit for exposure to living tissue such as a human or animal eye. In some cases, the flux is high enough to cause eye damage and/or other non-eye damage such as burns or other changes to living tissue. It is thus important to detect when people, animals, or other objects are in or will imminently enter the high flux beam path during the time the beam is activated. In these and other cases, it may also be important to deter and/or prevent people, animals, and objects from entering the beam path while the beam is activated or will soon be activated. 
     In addition to direct exposure to a high flux power beam, hazardous amounts of light may be reflected specularly or diffusely by objects in, or passing through, the power beam. In some cases the high flux power beam may be intense enough to ignite flammable objects (e.g., paper, cardboard). Thus, unless the beam path is generally inaccessible to objects and living beings (e.g., in outer space), a laser power beaming system is improved by including a safety system arranged to detect hazards, including objects in or near the high flux beam path, and to shut off the high flux power beam or prevent the high flux power beam from being activated. 
     In general, embodiments of safety systems described herein may include multiple subsystems or mechanisms for detecting hazards, controlling the high flux power beam activation, and otherwise providing safety features. Some of the embodiments provide redundant or overlapping coverage. 
     In some safety system embodiments described herein, the high flux power beam is shut off at the source (e.g., by commanding a power supply to turn off). In some safety system embodiments described herein, the high flux power beam is blocked (e.g., with a mechanical or electronic shutter). In still other safety system embodiments described herein, the interaction between the high flux power beam and one or more objects is detected and/or prevented in other ways. 
     Some safety systems are employed in connection with other hazards that cannot easily be protected by physical barriers. Optical “fences” are used, for example, to detect when operators reach into hazardous areas around machinery. These other safety systems are very different from the safety systems described herein, however, because high flux power beams are very different from other types of dangerous machines. Notably, high flux power beams are invisible, which makes safety systems even more important. For example, laser power beaming systems are exceptional in the need for very rapid response to small objects due to the possibility of hazardous reflections over a long distance. Improved safety sensors are therefore of high value for power beaming, although they may be useful in other applications as well. 
     One problem with existing light curtains is that the emitters in the known systems are not individually distinguished by the detectors. It is therefore possible for a certain detector to receive (i.e., to “see”) a light signal from more than its corresponding emitter. This reception of light from a non-corresponding emitter can lead to the light curtain failing to detect a small obstacle which partially or completely blocks one emitter if the certain detector continues to respond to other emitters. This problem can be resolved at least in part by using optics or baffles to limit the field of view of the emitters and/or the detectors such that each detector responds only to its single, corresponding emitter, but this solution is inadequate for longer-range or small-beam-spacing applications where spatially resolving individual emitters and/or detectors is difficult, and the solution is also undesirable because it requires precise positioning and maintenance of emitter and detector alignment. 
     In some embodiments described herein, a wireless power safety system includes a light curtain comprising a plurality of emitters that are distinguishably modulated, filtered, or otherwise differentiated such that each emitter, or each group out of two or more groups of emitters, has one or more uniquely identifiable characteristics (e.g., a unique signature). This particular wireless power safety system embodiment also includes one or more detectors arranged to selectively detect or otherwise identify a corresponding one or more emitters based on the one or more uniquely identifiable characteristics. The light curtain in this wireless power safety system embodiment further comprises a controller coupled to the one or more detectors, which can determine if any of a set of emitter-detector optical paths has been blocked or attenuated by an obstacle, and provide a corresponding output. 
     In some cases, the emitters of the embodiments described herein are distinguished at least in part by the one or more wavelengths of light they emit. The wavelengths may be inherent in the individual emitters (e.g., red, green, and blue LEDs or laser diodes operating at different wavelengths). Alternatively, or in addition, the wavelengths may be determined by filters of one type or another. The wavelength or wavelength combination of a particular emitter may be fixed or variable (e.g., by a tunable filter, by changing drive currents to the individual colors of a multicolor LED, or by other means). In these embodiments, the detectors may be inherently sensitive to selected wavelengths. Alternatively, or in addition, the light signals that reach the one or more detectors may be filtered and thereby responsive to selected wavelengths. In some embodiments, for example, one or more detectors are configured to detect some specific wavelength combinations and reject others. In these cases, a particular detector may comprise multiple photosensitive elements filtered to detect different wavelengths, and an associated electronic circuit may be configured to determine the ratio of the detected intensities at those wavelengths. In some embodiments, the detector may determine whether the intensities are consistent with a specific emitter. In some embodiments, the detector provides multiple outputs to a controller, which determines which emitter was the source of a particularly received light signal. 
       FIG. 1  is a power beaming system  100  having exemplary arrangements of guard beam emitters and guard beam detectors. In the power beaming system  100 , a transmission unit  102  includes power-beaming transmitter  104 . In some cases, the power-beaming transmitter  104  converts electric power into optical power (i.e., light). The power-beaming transmitter  104  may operate in the near-infrared (NIR) portion of the optical spectrum wavelength, such as between 0.7 and 2.0 μm, though wavelengths from a different part of the electromagnetic spectrum may also be used. The power-beaming transmitter  104  can be formed from a single laser or multiple lasers, which may be mutually coherent or incoherent. In some cases, the one or more lasers may be replaced by one or more light emitting diodes (LEDs), super-radiant diodes, or some other high-intensity light source. In addition to the high flux generation source, the power-beaming transmitter  104  includes particular optical structures such as optical fibers, lenses, mirrors, and various other beam shaping and focusing elements that create a high-flux power beam  106  of a desired size, shape, power distribution, and divergence. Various other elements of the power-beaming transmitter  104  aim the high-flux power beam  106  in a desired direction toward a reception unit  108 . Certain electronic, mechanical, and/or electromechanical structures of the power-beaming transmitter  104  direct the operations of the transmission unit  102 . 
     The high-flux power beam  106  produced by the power-beaming transmitter  104  is transmitted through free space from a first location, where the transmission unit  102  is positioned, toward a second location, where the reception unit  108  is positioned. At the reception unit  108 , the high flux power beam  106  strikes power-beam receiver  110 . The power-beam receiver  110  may include certain collecting optics such as lenses or mirrors, certain protective coverings, a power conversion means such as a photovoltaic array, and one or more safety mechanisms. The power conversion means (e.g., photovoltaic array, Stirling engine, or some other opto-electric or thermoelectric device) converts at least some portion of the received high-flux power beam  106  into electricity under the control and direction of the power beam receiver  110 . 
     In  FIG. 1 , a first cross-section of the high-flux power beam  106  is shown as “A,” and a second cross-section of the high-flux power beam  106  is shown as “B.” The cross section at “A” shows a view looking toward the power beaming transmitter  104 , and the cross section at “B” shows a view looking toward the power beam receiver  110 . 
     Two different light curtain embodiments are also presented in  FIG. 1 . In a first embodiment, illustrated at the top of  FIG. 1  above the high-flux power beam  106 , a light curtain is formed having a plurality of guard beam emitters  112  patterned about the power-beaming receiver  110  and a plurality of corresponding guard beam detectors  114  patterned about the power beaming transmitter  104 . In a second embodiment, illustrated at the bottom of  FIG. 1  below the high-flux power beam  106 , a light curtain is formed having a plurality of guard beam emitters  112  and a corresponding plurality of guard beam detectors  114  patterned about the power beaming transmitter  104 . In this second light curtain embodiment, a plurality of retro-reflectors  116  are positioned about the power-beaming receiver  110 . 
     In some embodiments, the light curtain includes emitters  112  having different colors of light emitting diodes (LEDs). For example, in the upper embodiment of  FIG. 1 , a light curtain consists of a repeating row of LEDs: red (R), green (G), blue (B), red, green, blue, and so on. Only some of the colored LEDs are illustrated in  FIG. 1  for simplicity. The detectors  114  in the upper embodiment are correspondingly filtered based on the wavelength (i.e., color) of the expected emitter light signal. In the illustrated case, if each emitter  112  is spaced 3.3 centimeters (cm) apart, then each detector  114  can distinguish between same-colored emitters  112  that are spaced 10 cm apart. That is, each detector  114  will respond to a directly opposite emitter  112  emitting a same color, but the detector  114  will not respond to an adjacent emitter  112  of a different color, and the detector  114  will not receive a signal from a same color emitter  112  that is 10 cm away from directly opposite due to the ease of directionally aiming the emitters  112  toward corresponding detectors  114 . More or fewer colors could certainly be implemented, however, in the present example, by selecting three colors, the emitters  112  and detectors  114  can be spaced about 3.3 cm apart (10 cm/3 colors=3.3 cm/color), and therefore, the guard beam will reliably detect obstacles as small as about 4 cm. 
     In other embodiments, the light curtain includes emitters  112  that are different based on some other characteristic such as synchronized timing, modulation, encoded patterns, and the like. For example, in the lower embodiment of  FIG. 1 , a light curtain consists of a repeating row of LEDs: “a”, “b”, “c”, a, b, c, and so on. Here, the emitters  112  of the light curtain may sequentially emit signals “a”, then “b”, then “c”, and the pattern is repeated. Alternatively, the emitters  112  of the light curtain may emit signal “a” with a first pulsed, modulated, or otherwise encoded pattern while concurrently or otherwise emitting signals “b” and “c” with different pulsed, modulated, or otherwise encoded patterns. In these cases, corresponding detectors  114  are arranged to detect a pattern of their line-of-sight emitter  112 . In the exemplary embodiments, three wavelengths (e.g., three colors) have been described, and emitters having other distinguishing characteristics have also been discussed. It is recognized that the embodiment discussed in the present disclosure may include more or fewer wavelengths, more or fewer patterns, and more or fewer other distinguishing characteristics. Several other non-limiting embodiments of emitter and detector encodings are later described in the present disclosure. 
     Turning to another feature in the lower embodiment of  FIG. 1 , emitters  112  and detectors  114  are co-located in proximity to each other in a particular pattern around the power-beaming transmitter  104 . The co-located set of emitters  112  and detectors  114  may otherwise be referred to in the present disclosure as a single-end system or a single-ended system, which is different from conventional systems that locate light sources and light detectors at opposite ends of a power beam path (i.e., emitters at either a power beam transmitter or a power beam receiver, and detectors at either a power beam receiver or a power beam transmitter, respectively). 
     Although some types of retro-reflectors are used in conjunction with some types of known beam-break sensors such as garage door openers, these systems generally employ “bicycle reflector” type retro-reflectors, molded corner cube reflectors, or reflective tape, with each emitter-detector pair providing a single sensing beam path. These rudimentary systems are different from the embodiments discussed herein for several reasons including that the embodiments described herein encode emitted light signals, operate light curtains with signals of different wavelengths, employ retro-reflectors  116  having particular optical properties (discussed herein), and for other reasons. 
     In one aspect of the disclosed embodiments, retro-reflectors  116  are used so that both the emitters  112  and detectors  114  for any kind of guard beam system can be co-located at the same end of the beam path. One embodiment, for example, locates retro-reflectors  116  in proximity to the power-beam receiver  110  and locates the emitters  112  and detectors  114  in proximity to the power-beam transmitter. This type of single-ended arrangement allows the reception unit  108  to be passive, and all sensing related to safety is located at the transmission unit, where control of the high-flux power beam is preferably performed. 
     The retro-reflectors  116  in  FIG. 1  may be discrete retro-reflectors such as corner cubes or cat&#39;s eye reflectors, which have a lens with a reflector in its focal plane. Alternatively, or in addition, the retro-reflectors  116  in  FIG. 1  may be areas of retroreflective tape, such as that manufactured by 3M CORPORATION. In some embodiments, such tape generally consists either of sheets of small corner cubes or sheets of glass spheres that act as cat&#39;s eye reflectors. In some embodiments the retro-reflectors  116  may be continuous strips of retro-reflective RR material. In other embodiments, the retro-reflectors  116  are separate, non-contiguous “dots” of retro-reflective material. 
     As illustrated in the lower embodiment of  FIG. 1 , a first retro-reflective surface  116  is positioned remote to a first emitter  112   a  and first detector  114   a,  wherein the first emitter  112   a  is positioned in proximity to the first detector  114   a,  and wherein a first optical path includes two first sub-paths: one first sub-path between the first emitter  112   a  and the first reflective surface  116 , and another first sub-path between the first reflective surface  116  and the first-detector  114   a.  A second reflective surface  116  is positioned remote to a second emitter  112   b  and a second detector  114   b,  wherein the second emitter  112   b  is positioned in proximity to the second detector  114   b,  and wherein the second optical path also includes two second sub-paths: one second sub-path between the second emitter  112   b  and the second reflective surface  116 , and another second sub-path between the second reflective surface  116  and the second-detector  114   b.    
     In the single-ended embodiments discussed with respect to  FIG. 1 , emitters  112  and detectors  114  are co-located at one end of the high-flux power beam  106  or other region to be protected. One or more retro-reflectors  116  are discussed, however other embodiments are also contemplated. For example, the retro-reflectors  116  of  FIG. 1 , in some embodiments, are implemented as a continuous retro-reflective surface such as retro-reflective tape. In other embodiments, for example when the power-beaming transmitter  104  and a power-beam receiver  110  can be appropriately aligned, the retro-reflectors  116  of  FIG. 1  may be implemented by a suitable flat or curved specular reflector (e.g., a mirror), a diffusely reflective or scattering surface (e.g. a white-painted surface), or some other means to literally reflect or virtually (i.e., mimic) reflect the emitted guard beam signals. 
     In both the upper embodiment of  FIG. 1  and the lower embodiment of  FIG. 1 , there is a one-to-one correspondence between emitters  112  and detectors  114 . That is, for each emitter  112 , there is a corresponding detector  114 . In other embodiments, one-to-one correspondence is not necessary or present. For example, in some embodiments, a detector  114  may employ selectable or tunable filters or other characteristics such that a single detector  114  can respond to different wavelengths or wavelength combinations at different times. In addition, or in the alternative, an emitter  112  may be controllable to emit signals having different wavelengths, different light patterns, different encoding, or some other different characteristics such that a single emitter  112  can emit a plurality of signals, each of the plurality of signals directed to a different corresponding detector  114 . 
       FIG. 2  is a plurality of emitters  112  and detectors  114 , the emitters  112  having overlapping guard beam light fields. In some cases, the emitters  112  have a one-to-one correspondence with the detectors  114 . In other cases, an emitter  112  is arranged to generate two or more guard beam signals wherein each of the two or more guard beam signals has at least one distinguishable characteristic from the other guard beam signals. In these or in still other cases, a detector  114  is arranged to detect a plurality of guard beam signals having different characteristics. The overlapping guard beam signals produced by emitters  112  and detected by detectors  114  of  FIG. 2  illustrate the flexibility of the embodiments described herein. Furthermore, as evident in  FIG. 2 , a nearly impenetrable light curtain can be formed around a high-flux power beam  106  such that whenever a foreign object of nearly any size, shape, and speed approaches the high-flux power beam  106 , the foreign object can be detected and the high-flux power beam can be safely interrupted. 
       FIGS. 3A-3C  are guard beam signal intensity plots distinguishing multiple guard beam emitter light signals from each other. In  FIG. 3A , a signal intensity plot illustrates time domain multiplexing of a plurality of emitter signals; in  FIG. 3B , a signal intensity plot illustrates pulse frequency domain multiplexing of a plurality of emitter signals; and in  FIG. 3C , a signal intensity plot illustrates wavelength (e.g., “color”) domain multiplexing. 
     In some embodiments, emitters such as emitters  112  of  FIG. 1  are distinguished at least in part by time of illumination. For example, an emitter  112 , or a group of emitters  112 , may be assigned one or more time slots from a plurality of time slots. During a respective time slot, each emitter  112  or group of emitters  112  that is assigned to the time slot is turned on by an emitter driver. Outside of an assigned time slot, the particular emitter  112  or group of emitters  112  is attenuated, turned off, shielded, or otherwise prevented from emitting its guard beam signal. In these embodiments, any particular detector  114 , or a plurality of detectors  114 , may be activated or otherwise enabled to detect card beam light signals during any time slot, during any set of time slots, or during all time slots. A controller coupled to the one or more detectors  114  may be configured to act on the absence of a response from one or more detectors  114  during a time slot when each of the particular detectors  114  is expected to receive a guard beam signal from at least one emitter  112 . The controller may, for example, determine that the absence of a signal is indicative of a foreign object impinging on the power beam  106  (e.g., near or impeding a high-flux power beam  106 ), and the controller may further generate a responsive signal or otherwise cause or direct an interruption of the high-flux power beam  106 . 
       FIG. 3A  illustrates one embodiment of time domain multiplexing wherein a first emitter  112  or first set of emitters  112  is assigned time slot “A” and second and third emitters  112  or second and third sets of emitters  112  are respectively assigned time slots “B” and “C.” In the embodiment of  FIG. 2 , emitters  112  generate and emit guard beam signals during their assigned time slots, and a controller (not shown in  FIG. 2 ) is arranged to detect the emitted guard beam signals. 
     In some embodiments, including the embodiments represented in the signal intensity plots of  FIGS. 3A-3C , a controller that directs operations of one or more detectors may be synchronized in time with one or more emitter drivers. In these embodiments, the controller may be synchronized in time with the emitter drivers via a system-wide unique spatial and/or temporal pattern of emitted guard beam signals that are recognizable by the controller. For example, all emitters may be turned off for one time slot, or all emitters may be turned on for one time slot, or some set of emitters may be turned on for a determined number (e.g., two) of successive time slots as a synchronization signal. After the synchronization signal is emitted, subsequent emitter activations may be emitted lasting for only one time slot. Alternatively, in some embodiments, a separate synchronization signal, such as a light pulse from an emitter to a detector separate from the light curtain, may be used. In still other embodiments, synchronization between emitters and detectors may be accomplished by reference to a common signal, such as a GPS signal, a cellular radio signal, or some other distinguishable signal. 
     The signal intensity plot of  FIG. 3B  illustrating pulse or modulation frequency domain multiplexing may be implemented in some embodiments by an arrangement of emitters  112  pulsed in some cases in a repeating sequence (e.g., 1, 2, 3, 4, . . . N, 1, 2, . . . ). In other cases, the pulsing or modulation can occur concurrently amongst two or more emitters  112  or in some other pattern or arrangement. In these embodiments, a controller may look for an output from the corresponding detectors  114  according to the expected repeating sequence (e.g., 1, 2, 3, 4, . . . N, 1, 2, . . . ). In these embodiments, the controller will generate an alarm signal if any detector  114  does not detect an emitted pulse at the correct time (e.g., with the in a determined time window). In a related and alternative embodiment, the emitters  112  and detectors  114  may have wide fields of view (e.g.,  FIG. 2 ), such that each detector  114  responds to all emitters  112 . In these embodiments, the controller may be arranged to generate an alarm signal if any detector  114  fails to detect a pulse in any time slot. Due to the wide fields of view, the plurality of emitters  112  and detectors  114  creates a light curtain having a dense “weave” of paths, and when any of the paths in the dense weave is blocked, the object, which may be very small, will be detected. 
     In some embodiments, emitters  112  are distinguished at least in part by a modulation frequency. An emitter  112 , or a group of emitters  112 , may be modulated by varying a drive current of an LED, for example in a sinusoidal or square-wave fashion. In these cases, the drive current will have a frequency substantially higher than the inverse of the desired response time of the light curtain so as to permit fast detection of an object impinging on a high-flux power beam  106  (e.g., approaching or impeding a high-flux power beam  106 ). The modulation of any emitter  112  may be 100% (i.e., turning the LED fully on and turning the LED fully off), or the modulation may be at a lower percentage. Modulating an emitter  112  at less than 100% may improve response time. Alternatively, or in addition, modulating different emitters  112  at different frequencies or amplitudes may permit detection and distinguishing of different emitters  112  byte particular ones of the detectors  114 . 
     In some cases, different emitters  112  are modulated at different frequencies. In these cases, some or all of the detectors  114  may be coupled to one or more filter elements, each of which responds to the detection of light modulated at a particular frequency. For example, a row of 10 LED or laser diode emitters  112  may be modulated at, respectively, 1.0 MHz, 1.05 MHz, 1.1 MHz, and so on up to 1.45 MHz. In this way, individual detectors  114  may be arranged to identify the particular emitted guard beam signal from its assigned, corresponding emitter  112 . Alternatively, or in addition, one or more detectors  114  may be arranged to identify guard beam signals emitted by a plurality of emitters  112 , each having a different modulation rate. Accordingly, in some embodiments each detector  114  and its associated filter may respond to a single emitter  112 . In other embodiments, emitters  112  and detectors  114  may have wide fields of view, and each detector  114  may be coupled to several filters, again creating a “weave” of paths from each emitter  112  to multiple detectors  114  and vice versa. In these cases, blocking any one line-of-sight path between an emitter  112  and a detector  114  is detected and determined to be a foreign object impinging on a high-flux power beam  106  (e.g., near or impeding a high-flux power beam  106 ). 
     In some embodiments, a more complex coded modulation may be used in place of a simple single-frequency modulation. Such modulations may include multi-frequency modulations, orthogonal pseudo-random binary coded modulations, and the like. In these cases, the controller associated with one or more detectors  114  is arranged to capture signal data from one or more detectors  114 , transformationally analyze the captured signal data (e.g., using a fast Fourier Transform (FFT)), and detect dominant modulated signals within a large volume of signal detection data caused by a plurality of different emitters  112 , sunlight, other ambient light, other man-made light, and the like. Use of such complex modulations reduces the chance that any undesired or unexpected light source mimics a modulated emitter  112 . 
     In  FIG. 3C , a plurality of emitters  112  are arranged to emit guard beam light signals, each having a different wavelength. In the signal intensity plot of  FIG. 3C , three emitters  112 , or three sets each having a plurality of emitters  112 , are arranged such that each emitter  112  or set of emitters  112  generates and emits a guard beam light signal of a different wavelength (e.g., color). One or more detectors  114  may be arranged with one or more filters particular to one or more of the colors associated with the emitters  112 . A controller is arranged to direct the operations of the detectors  114 , and when an expected signal of a particular wavelength or color is not received, the controller determines that a foreign object is impinging on a high-flux power beam  106  (e.g., near or impeding the high-flux power beam  106 ). In these cases, the controller is arranged to generate a signal that causes or otherwise directs an interruption of the high-flux power beam  106 . 
     In some embodiments, emitters  112  surround a power-beam receiver  110 , while detectors  114  are located about a power-beaming transmitter  104 . In these embodiments, the active line-of-sight paths (i.e., paths responsive to obstacles) between the emitters  112  and receivers  114  form a protective light curtain around a high-flux power beam  106 . In other embodiments, the emitters  112  may be located about the power-beaming transmitter  104 , and the detectors  114  located about the power-beam receiver  110 . In these embodiments, a separate signaling pathway may be arranged to either transmit an alarm signal from the reception unit  108  toward the transmission unit  102  directing an interruption of the high-flux power beam  106  or a “safe” signal directing transmission of the high-flux power beam  106 ; the absence of such safe signal being interpreted by the reception unit as an alarm signal directing an interruption of the high-flux power beam  106 . 
     The embodiments described herein using distinguishable emitters  112  provide benefits that may not be realized with conventional beam-break systems. For example, the safety curtain technology described in the present disclosure can be used effectively at distances greater than would be feasible with a conventional beam-break system, because the detectors  114  do not need to optically resolve each individual emitter  112  or vice versa to detect the blocking of a single emitter  112  or detector  114  by an obstacle. In addition because individual emitters can be distinguished, a light curtain may be formed around a high-flux power beam  106  in a very tight weave, which is not possible in conventional beam-break systems. 
       FIG. 4A  is a system diagram of a first power beam safety system embodiment  400 A. The power beam safety system embodiment  400 A can be cooperatively implemented by the transmission unit  102  and reception unit  108  of  FIG. 1 . 
     The first power beam safety system embodiment  400 A includes a power-beaming transmitter  104  for generating and transmitting a high-flux power beam  106 . The power beam safety system embodiment  400 A also includes an executive controller  404 . 
     The executive controller  404  includes a processor  406 , a memory  408 , and an input/output interface  410 . The processor  406  includes one or more processing units (e.g., central processing units) that execute instructions to perform actions, including actions to perform embodiments described herein to utilize sensor data to interrupt (e.g., shut down, block, power down, or the like) the high-flux power beam  106 . The executive controller  404 , the processor  406 , or the executive controller  404  and the processor  406  together, may be interchangeably referred to in the present disclosure as a controller. 
     The memory  408  may be utilized to store information, such as the computer-readable instructions that are utilized by the processor  406 , and other information, such as, for example, thresholds to compare against sensor data to determine if an object is impinging on a high-flux power beam  106  (e.g., near, impeding, or about to impede the high-flux power beam  106 ). 
     The input/output interface  410  provides a communication interface between the executive controller  404  and a plurality of sensors. The executive controller  404  receives information from the sensors and, in some embodiments, provides commands to the sensors via the input/output interface  410 . 
     The sensors in the power beam safety system embodiment  400 A may include one or more cameras  420 , one or more rangefinders  422 , one or more single-ended sensing systems  424  (e.g., emitters  112  and detectors  114 ), one or more acoustic sensors  414 , and in addition or in the alternative, other sensors. The sensors may be configured to have different or overlapping (fully or partially) effective areas to detect the one or more same or different objects. 
     The acoustic sensor  414  may utilize directional listening technology to detect an object. The cameras  420   a - 420   c  may capture images from various directions. The rangefinders  422   a - 422   c  may be configured to detect an object impinging on the power beam (e.g., near, impeding, or about to impede the power beam) at some distance between the power-beaming transmitter  104  and the power-beam receiver  110 . 
     The single-ended sensing systems  424   a - 424   c  of the power beam safety system embodiment  400 A may include one or more pairs or other pluralities of emitters  112  and detectors  114 , which are illustrated in  FIG. 1  and described with respect to  FIG. 1  and elsewhere. For example, the single-ended sensing systems  424   a - 424   c  each include one or more emitters  112  that emit time-varying optical signals that are reflected back to one or more corresponding detectors  114 . The signals received at the one or more detectors  114  are used to determine if an object is impinging on the high-flux power beam  106  (e.g., near, impeding, or about to impede the high-flux power beam  106 ). In various embodiments, the single-ended sensing systems  424   a - 424   c  may utilize a real-time clock  412  to generate and emit time-varying optical signals. 
     The executive controller  404  monitors the sensors via the input/output interface  410  for sensor output information regarding detected objects. The executive controller  404  also monitors and controls the power level of the power-beaming transmitter  104 . In some embodiments, some sensors may provide an object-intrusion indication that is a “binary” signal (e.g., yes/no) that is activated when the sensor detects an object. For example, a single-ended sensing system  424  may determine that a line-of-sight between a first emitter  112  and a first detector  114  has been blocked by a foreign object. In these or other embodiments, different sensors may provide object-detection information associated with the detection of an object (e.g., images from a camera  420 , distance reading from a rangefinder  422 , and the like). The executive controller  404  is arranged to utilize the object-detection information and one or more thresholds to determine if an object is impinging on the high-flux power beam  106  (e.g., near, impeding, or about to impede the high-flux power beam  106 ). In addition, the executive controller  404  is also arranged to monitor and control the power level of the power-beaming transmitter  104 . 
     In at least one embodiment, the executive controller  404  interrupts the transmission of the high-flux power beam  106  by the power-beaming transmitter  104  via switch  416  when a single sensor detects an object impinging on the high-flux power beam  106  (e.g., near, impeding, or about to impede the high-flux power beam  106 ). In other embodiments, information from a plurality of sensors may be utilized to determine if an object is impinging on the high-flux power beam  106  (e.g., near, impeding, or about to impede the high-flux power beam  106 ). In this embodiment along with others, the executive controller  404  may also, for example, direct the power-beaming transmitter  104  into a lower intensity mode as objects change position in proximity to the high-flux power beam  106 . The executive controller  404  may, for example, direct the power-beaming transmitter  104  to remain in a low-flux mode, a power-off state, or some other condition based on one or more conditions detected or otherwise determined using the various sensors described herein. 
     In various embodiments, information from some sensors is weighted higher (e.g., if a sensor is more accurate) than information from other sensors (e.g., if a sensor is less accurate), such that a combination of the sensor information is compared against one or more thresholds to determine if an object is impinging on the high-flux power beam  106  (e.g., near, impeding, or about to impede the high-flux power beam  106 ). 
     Along these lines, a wide variety of sensors may be employed in the embodiments discussed herein. In some cases, the sensors provide raw data output signals that need to be processed in order to provide useful data for safety sensing. In other cases, individual sensors have circuitry that pre-process raw signal data before sending processed information to an attached processor or controller such as executive controller  404 . In cases where the sensors send raw data, a higher level processor such as processor  406  analyzes raw data and determines when the high-flux power beam  106  should be interrupted. In other cases, where a sensor pre-processes data, a higher level processor such as processor  406  may simply receive an indication of object detection or other such high-level information. In these cases, the safety system may process the high-level indication in real time to interrupt the high-flux power beam  106 . In still other cases, the executive controller (e.g., processor  406 ) may have access to both high-level object intrusion information and the raw data from the sensor. In this case, the executive controller can perform more elaborate processing such as combining information from several sensors to determine whether or not the high-flux power beam  106  should be interrupted. 
       FIG. 4B  is a system diagram of a second power beam safety system embodiment  400 B. In the second power beam safety system embodiment  400 B, a power-beaming transmitter  104  includes a laser power supply, a laser, and particular optics that cooperate to generate and transmit a high-flux power beam  106 . When a particular foreign object is impinging on the high-flux power beam  106  (e.g., detected near or impeding the high-flux power beam  106 ), a switch  416  is used to stop or otherwise interrupt transmission of the high-flux power beam  106 . 
     In  FIG. 4B , the switch  416  is configured as an interlock circuit. The switch  416  may include a plurality of electronic, mechanical, or electromechanical switches, shades, shutters, circuit breakers, or other means to interrupt the transmission of the high-flux power beam  106 . Operations of the switch  416  may be directed by an executive safety controller  406 . 
     The executive safety controller  406  may direct operations of a plurality of safety systems, or otherwise receive input and take action based on the operations of one or more safety systems. For example, the executive safety controller  406  is communicatively coupled to a first safety system (i.e., safety sensor A), which may include one or more cameras  420 , and the executive safety controller  406  is communicatively coupled to a second safety system (i.e., safety sensor B), which may include one or more rangefinders  422 . The executive safety controller  406  may be coupled to any number of additional safety sensors including one or more single-ended guard beams that form a light curtain. 
     In the second power beam safety system embodiment  400 B of  FIG. 4B , a plurality of emitters  112  and a plurality of detectors  114  are arranged as described in the present disclosure. In some cases, the plurality of emitters  112  and the plurality of detectors  114  are configured as one or more single-ended sensor systems cooperatively positioned or otherwise arranged with reflecting means such as retro-reflectors  116 . In other cases, the plurality of emitters  112  and the plurality of detectors  114  of  FIG. 4B  are arranged about a power-beaming receiver  110  ( FIG. 1 ) and a power-beam transmitter  104  ( FIG. 1 ), respectively. 
     In  FIG. 4B , a pre-processing unit  407  receives raw or processed data from the one or more detectors  114 . The pre-processing unit  407  is optionally coupled to switch  416 . In these cases, detection data from one or more detectors  114  is very quickly analyzed and, when an object is impinging on the high-flux power beam  106  (e.g., determined to be near or impeding the high-flux power beam  106 ), the transmission of the high-flux power beam  106  is very quickly interrupted. Alternatively, or in addition, data from the pre-processing unit  407  is passed to the executive safety controller  404   a  for additional analysis, consolidation, processing, or the like. 
     The executive safety controller  404   a  of  FIG. 4B  performs along the same lines as the executive controller  404  of  FIG. 4A . The executive safety controller  404   a  may be arranged to monitor and control a power level of the power-beaming transmitter  104 , monitor and control an interlock  416 , monitor and control a plurality of emitters  112  and detectors  114 , and perform other functions. For example, in some embodiments, the executive safety controller  404   a  may reduce power, power down, or direct another power level in the power-beaming transmitter based on singular, aggregated, or any other combination of raw data from any number of emitters  112  and detectors  114 . In other cases, the executive safety controller  404   a  may enable or disable operations of the power-beaming transmitter  104  based on a history of sensor data, a prediction of future interruptions, a schedule, or any other characteristics. 
     Non-limiting and exemplary operation of certain aspects of the disclosure will now be described with respect to  FIGS. 5A-5C . In at least one of various embodiments, processes  500 A- 500 C described in conjunction with  FIGS. 5A-5C , respectively, may be implemented by or executed on one or more computing devices, such as an executive controller  404 , single-ended sensing system  424 , or other object detection systems for power beams. 
       FIG. 5A  is a logical flow diagram generally showing one embodiment of a process  500 A for utilizing a plurality of safety sensors with a power beam  106  to interrupt the power beam  106 . 
     Process  500 A begins after a start block. At block  502  a high-flux power beam  106  is generated and transmitted from a transmission unit  102 . In various embodiments, the high-flux power beam  106  is transmitted to a reception unit  108 , which is coupled with other circuitry to convert power from the high-flux power beam  106  into electricity. As a result, electronics coupled to the reception unit  108  can be wirelessly charged or otherwise receive power derived from the power source of the transmission unit  102  and the high-flux power beam  106 . 
     Process  500 A proceeds to block  504 , where a plurality of sensors such as one or more cameras  420 , one or more range finders  422 , and one or more single-ended sensing systems  424 , which may include detectors  114 , are monitored for information indicating that an object is impinging on the high-flux power beam  106  (e.g., impeding, near, or about to impede the high-flux power beam  106 ). As described elsewhere herein, these sensors utilize a variety of different technologies to detect the presence of an object, and are positioned and configured to detect objects that are impinging on the high-flux power beam  106  (e.g., impeding, near, or about to impede the high-flux power beam  106 ). Similarly, sensors can be configured to detect objects that have different characteristics (e.g., size, shape, reflective properties, direction and rate of movement, location, distance from transmitter or sensor, etc.). 
     When a sensor detects an object or otherwise generates data associated with a detected object, the sensor outputs information regarding the detected object. In some embodiments, the output is an object-intrusion indication, which is a “binary” signal that indicates that the sensor has detected an object. In other embodiments, the output is object-detection information, which includes additional information about the detected object. This additional information may include, but is not limited to, size of the object, location of the object relative to the high-flux power beam  106 , distance from the transmission unit  102 , or sensor, speed and direction the object is moving, or other information that is used to determine if the object is in fact impinging on the high-flux power beam  106  (e.g., impeding, near, or about to impede the high-flux power beam  106 ). 
     Process  500 A continues at block  506 , where a determination is made whether an object-intrusion indication is received from one or more of the plurality of sensors. As mentioned herein, the object-intrusion indication may be a “binary” signal that indicates that an object was detected by the sensor. If an object-intrusion indication is recited from any one of the plurality of sensors, process  500 A flows to block  508 ; otherwise, process  500 A loops to block  502  to continue to transmit the high-flux power beam  106  and monitor the sensors for object intrusions. 
     At block  508 , the high-flux power beam  106  is stopped in response to receipt of the object-intrusion indication. Stopping the high-flux power beam  106  may include powering down or halting the high-flux power beam  106 , closing a shutter or cap on the transmitter to prohibit the high-flux power beam  106  from being projected towards the receiver, putting the high-flux power beam  106  into a safe mode of operation, or otherwise interrupting or terminating the transmission of the high-flux power beam  106  from the transmission unit  102 . 
     In various embodiments, a positive indication of a detected object for just one of a plurality of sensors results in the high-flux power beam  106  being immediately stopped. In this way, even a false positive detection will result in the high-flux power beam  106  being stopped. And since, in some embodiments, the sensor coverage overlaps, there is a higher likelihood that if one sensor misses an object, another sensor will detect it. 
     In other embodiments, a combination of object-intrusion indications from multiple sensors is used to determine whether the high-flux power beam  106  is stopped or not. For example, in at least one embodiment, the high-flux power beam  106  will not be stopped unless two or more sensors provide an object-intrusion indication. In this way, a malfunctioning sensor may not unnecessarily keep stopping the high-flux power beam  106 . A malfunctioning sensor may be identified when it routinely detects an object in an overlapped sensor area, but the other overlapping sensors do not detect the object. This lack of consistency between sensors indicates that one of the sensors is less reliable than the others, and may also indicate that one sensor is malfunctioning. In some embodiments, the system may be calibrated or re-calibrated to account for a sensor that is malfunctioning or is consistently providing object-intrusion indications when other sensors are not. 
     After block  508 , process  500 A loops to block  504  to continue to monitor the plurality of sensors to determine whether the object is still impinging on the high-flux power beam  106  (e.g., near or impeding the high-flux power beam  106 ) at decision block  506 . If the object is no longer impinging on the high-flux power beam  106  (e.g., near or impeding the high-flux power beam  106 ) the transmission unit  102  may begin retransmitting the high-flux power beam  106  (at block  502 ). In this way, process  500 A is perpetual. 
       FIG. 5B  is a logical flow diagram generally showing one embodiment of a process  500 B for utilizing a plurality of safety sensors that detect signals of multiple wavelengths to interrupt a high-flux power beam  106  based on an intrusion by one or more detected objects. 
     Process  500 B begins after a start block. At block  512  a high-flux power beam  106  is generated and transmitted from a transmission unit  102  toward a reception unit  108 , which is coupled with other circuitry to convert power from the high-flux power beam  106  into electricity. 
     At block  514 , a light curtain is formed using a plurality of emitters  112  and detectors  114  positioned about the high-flux power beam  106 . In some cases, there is a one-to-one correspondence between each emitter  112  and its corresponding detector  114 . In other embodiments, one emitter  112  projects guard beam signals that are detected by a plurality of detectors  114 . In these, or other embodiments, a plurality of emitters  112  project guard beam signals that are detected by a single detector  114 . 
     In some cases two or more guard beams from one or more emitters  112  are formed according to different wavelengths. As discussed herein, some foreign objects that may obstruct a high-flux power beam  106  are transparent, some are opaque, some are highly reflective, some scatter light, some diffuse light, and some other foreign objects have different optical properties. By selecting different wavelengths having certain desirable characteristics for some guard beams, foreign object having particular optical properties may be detected. In addition, or in the alternative, the guard beam signals from one in emitter  112  may be distinguished from the guard beam signals of another emitter  112 . 
     At block  516 , at least some returns from the guard beam signals emitted at block  514  are received by one or more detectors  114 . In some cases, signals that are expected are received; in some cases signals that are expected are not received; and in some cases, signals that are not expected are received. 
     Processing continues from block  516  to block  518 . In some cases, a light curtain is formed in a single-ended arrangement or a double-ended arrangement. Guard beams in the light curtain are emitted by emitters  112 , and guard beam signals are detected by detectors  114 . In cases where no foreign objects are obstructing a guard beam, an expected guard beam signal is received. In other cases where an expected guard beam signal is not received, a determination is made that a foreign object is obstructing the guard beam. Upon such determination, the high-flux power beam  106  is interrupted. In still other cases, a guard beam having a particular wavelength is emitted by a particular emitter  112 . Due to the selected wavelength of the particular guard beam, a corresponding detector does not anticipate receiving a return of the particular guard beam. For example, if the guard beam is unobstructed, the guard beam will be absorbed by a particular wavelength filter. Alternatively, if the guard beam is obstructed, the foreign object obstructing the guard beam may reflect the guard beam where it can be unexpectedly detected by a particular detector  114   
     At block  520 , signals that are expected or are otherwise missing are analyzed. Also at block  520  signals that are expected to be missing but are otherwise received are also analyzed. In cases where expected signals are missing, or in cases where other signals are expected to be missing but are instead unexpectedly detected, processing advances to block  522 , where the high-flux power beam  106  is interrupted. Conversely, if all of the expected guard beam signals are received, and no unexpected guard beam signals are received, processing returns to block  512 , where transmission of the high-flux power beam  106  continues. 
     After a high-flux power beam  106  is interrupted at block  522 , processing returns to block  514 . Unless otherwise diverted, the light curtain continues to operate through blocks  514  to  520 . If a previously determined foreign object is no longer detected, and if expected guard beam signals are received while unexpected guard beam signals are not received, the high-flux power beam  106  may again be transmitted. Process  500 B is perpetual. 
       FIG. 5C  is a logical flow diagram generally showing a process  500 C to distinguish a first guard beam light signal from a second guard beam light signal. The logical flow diagram of process  500 C is associated with a particular power beaming system such as power beaming system  100  of  FIG. 1 . The power beaming system includes a light curtain system having a plurality of M emitters and a plurality of N detectors. M and N are integers. In some cases, such as in process  500 C, there are an equal number of emitters and detectors (i.e., M equals N). In other cases, however, there may be different numbers of emitters and detectors (i.e., M does not equal N). 
     In the process  500 C of  FIG. 5C , the light curtain system performs a time domain multiplexing of the plurality of M emitters. 
     At block  532 , a synchronization signal is sent to each of the N detectors. The synchronization signal may include a particular pattern of guard beam signal, a time value associated with the emission of a guard beam signal, or some other synchronization signal. After the synchronization signal is sent, processing advances to block  534 . 
     At block  534 , a guard beam signal is sent from each emitter, and some or all of the detectors operate to receive an indication of the guard beam signal. In the embodiment, any detector in line of sight of the emitter will expect to receive the emitted guard beam, and if the expected guard beam is not received, the particular detector will form or begin to form a “detection” signal. 
     In the process  500 C, each emitter emits its guard beam signal according to its assigned timeslot. In cooperation, each detector attempts to receive the associated guard beam signal according to the assigned timeslot. Due to the time synchronization at block  532 , emitters and corresponding detectors follow a common determined pattern. In some cases, emitters are assigned sequential timeslots. In some cases, timeslots for any given emitter follow a particular pattern. In some cases, timing is dynamically adjusted. For example, the light curtain system may dynamically change assignments to particular timeslots, lengths of particular timeslots, ordering of particular timeslots, or other characteristics. In the processing of block  534 , along the lines of  FIG. 3A , emitters are assigned to sequential timeslots. 
     At block  536 , received signals are initially analyzed, parsed and stored in a particular data structure such as an array. In some cases, data collection is ongoing as a background task. In other cases, data is collected from each detector in the light curtain, and data is processed in sets. In still other cases, data collection and processing happens in other ways. Processing advances to block  538 . 
     At block  538 , one or more return signals, individually or collectively, are compared against particular thresholds, previous signals, predicted signals, or some other values. In this way, guard beam signals that are expected to have a particular amplitude, timing, or other characteristics may be analyzed, and anomalies may be noted in order to determine if a foreign object is impinging on a particular one or more guard beams (e.g., near or impeding a particular one or more guard beams). In some cases, individual guard beams are analyzed. In other cases, collections of guard beam data are analyzed. In these and still other cases, guard beam data may be analyzed historically, predictively, and a normalized fashion, in a weighted fashion, or according to other expectations and contexts of the particular light curtain system. 
     At block  540 , if the analysis and block  538  indicates the presence of a foreign object impinging on one or more guard beams (e.g., near or impeding one or more guard beams), the high-flux power beam  106  may be interrupted. Interrupting the high-flux power beam  106  may include a single switch, an interlock circuit, or some other means. 
     In the present disclosure, guard beams are passed between emitters and detectors in order to form a light curtain around a high-flux power beam  106 . In some embodiments, each emitter  112  sends a light signal (e.g., from an LED or a laser), which is then sensed by a detector  114  (e.g., by a photodiode) when the line-of-sight beam path between the emitter  112  and the detector  114  is clear. If the light signal is blocked, then the absence of the detected light signal is interpreted as an object impinging on the high-flux power beam  106  (e.g., near or impeding the high-flux power beam  106 ), which causes the generation of a signal that is used to interrupt transmission of the high-flux power beam  106 . Certain embodiments described herein include a beam path having a first sub-path between the emitter  112  and a retro-reflector  116 , and a second sub-path between the retro-reflector  116  in the detector  114 . 
     The present inventors have recognized that in some cases, a foreign object (e.g., an opaque object such as a hand, for an indoor system, or a vehicle, animal, or bird, for a large-scale outdoor system) may be a substantially reflective foreign object. In embodiments where emitters  112 , detectors  114 , and retro-reflectors form a single-ended system, it is possible that the substantially reflective foreign object, which has intruded into the line-of-sight beam path, will reflect the guard beam signal strongly enough that the responsive detector  114  will fail to detect the foreign object. This failure by the detector, which is also referred to as a false negative, indicates to the power beaming system that there is no intrusion when in fact there is one. Accordingly, in a case where a high-flux power beam  106  should be interrupted, the safety system falsely determines that no intrusion is present, and transmission of the high-flux power beam  106  dangerously continues. 
       FIG. 6A  is a power beaming system embodiment having guard beam emitters  112  operating with signals of different wavelength.  FIG. 6B  is the power beaming system embodiment of  FIG. 6A  with a foreign object  120 A intruding on a guard beam. The guard beam emitters  112  of  FIGS. 6A-6B  participate in a solution that can better distinguish a light signal returned by a retro-reflector  116  and other objects that could intrude in the beam path. 
     In some embodiments, one or more in emitters  112  of a light curtain safety system transmit at least two separate wavelengths of light in the guard beam light signal. These guard beam signals having the at least two separate wavelengths may be from separate emitters  112  or from the same emitter  112 . 
     The at least two separate wavelengths are described herein as a first wavelength (λ 1 ) and a second wavelength (λ 2 ) or some variation thereof. Such descriptions are broadly interpreted as two or more signals having different wavelengths unless expressly stated as not having different wavelengths. The embodiments described herein may have signals of multiple wavelengths. Accordingly, while the present disclosure references and describes a first wavelength (λ 1 ) and a second wavelength (λ 2 ), inventive embodiments along the lines described herein are not so limited and may implement three or more wavelengths. Accordingly, the emitters  112  described herein may generate guard beam signals of any number of wavelengths. 
     Guard beam signals having at least two separate wavelengths may be produced from a single emitter  112  that is arranged with a suitable spectral bandwidth, such as an incandescent source, a white LED (i.e., which is generally a blue LED with phosphor to convert some blue light to longer wavelengths), or some other signal source. In the alternative, or in addition, guard beam signals having at least two separate wavelengths may be produced from two or more emitters  112 , each arranged to produce a signal having a particular wavelength. 
     In some embodiments, an emitter  112  is arranged as a broadband emitter that may be combined or otherwise arranged to cooperate with one or more narrowband filters. The one or more narrowband filters may be either transmissive filters or blocking filters, and the emitter  112  and filter combination is arranged to produce a desired emitted spectrum. In some embodiments, a single source may have a variable wavelength, and in such cases, a single source (e.g., an emitter  112 ) may generate and transmit different wavelengths at different times. 
     The target reflector, which is shown in  FIGS. 6A-6B  as a discrete retro-reflector  116 , but which also may be retroreflective tape, a specular flat or curved mirror, a diffusing surface, or some other means, is configured to reflect at least one emitted wavelength or wavelength range differently from at least one other wavelength or wavelength range. The target reflector may include a wavelength filter  118 , such as an interference filter, dye filter, or colored-glass filter. 
     Preferentially, the two wavelengths of the emitted guard beam signals are separated by a small amount (e.g., (λ1−λ2)/λ1&lt;0.1)) so that most materials other than the target reflector will reflect the two wavelengths similarly. The target reflector on the other hand includes a narrow-band wavelength filter  118 , such as a multilayer interference filter, which selectively transmits or blocks guard beam signals having one of the two wavelengths. 
     The narrow-band wavelength filter functionality may be integrated directly with the target reflector as a material or coating with wavelength-dependent reflectivity or transmission. For example, the wavelength filter  118  may be implemented as a glass cover which significantly attenuates blue light compared to red light, preferably with a sharp transition in wavelength between transmission and attenuation. In some embodiments, the target reflector will reflect signals formed with a first wavelength (λ 1 ) without attenuation, and will either completely absorb signals formed with a second wavelength (λ 2 ), or reflect such signals formed with the second wavelength (λ 2 ) with substantial (e.g., 10×) attenuation. 
     In some embodiments, detectors  114  are directly sensitive to the wavelength of the detected light. For example, a detector  114  in some embodiments may comprise two photodiodes covered with filters for two different wavelengths. As another example, a detector  114  may comprise two photodiodes with one photodiode unfiltered (i.e., to allow detection of signals formed with either the first wavelength or the second wavelength), and a second photodiode filtered (i.e., to detect only one of the two wavelengths). 
     In still other embodiments, the two wavelengths are made distinguishable by another means. For example, in some cases, the two wavelengths may be emitted at different times (e.g., alternating between a longer and a shorter wavelength at, for example, 10 kHz). In some cases, signals can be pulsed with different wavelengths at different pulse rates. In some cases, signals can be modulated with different wavelengths at different frequencies (e.g., 100 kHz and 150 kHz, or 100 kHz and 1 MHz). In this way, the output of a detector  114  can be electronically filtered to produce at least two outputs (e.g., intensity for wavelength 1 [I 1 ] and intensity for wavelength 2 [I 2 ]), and the outputs can be processed to produce, for example, total intensity (I 1 +I 2 ) and one separate intensity I 1 , total intensity (I 1 +I 2 ) and intensity ratio (I 1 /I 2 ), or some other combination. 
     As shown in the embodiment of  FIG. 6A , a guard beam signal formed with the first wavelength (λ 1 ) is emitted by emitter  112  and passes through the wavelength filter  118 . The guard beam formed with the first wavelength (λ 1 ) is reflected by the retro-reflector  116 , passes back through the wavelength filter  118 , and is detected by one or more detectors  114 . In contrast to the guard beam signal formed with the first wavelength (λ 1 ), the guard beam signal formed with the second wavelength (λ 2 ) is blocked by the wavelength filter  118 . 
     In  FIG. 6B , a foreign object  120 A blocks the line-of-sight path between an emitter  112  and the detector  114 , which includes an expected first sub-path from emitter  112  to retro-reflector  116 , and an expected second sub-path from retro-reflector  116  to detector  114 . In this case, the foreign object  120 A breaks the two line-of-sight sub-paths between the emitter  112  and the retro-reflector  116  and between the retro-reflector  116  and the detector  114 . The foreign object  120 A reflects both guard beam signals (i.e., the guard beam signal formed with the first wavelength (λ 1 ) and the guard beam signal formed with the second wavelength (λ 2 )), and both guard beams are reflected directly to the detector  114 . 
     In the embodiment of  FIG. 6B , the detector  114  is arranged to recognize guard beam signals formed with both the first wavelength (λ 1 ) and the second wavelength (λ 2 ). Despite the ability to recognize signals with both wavelengths, the detector  114  is only expected to receive guard beam signals formed with the first wavelength (λ 1 ), because guard beam signals formed with the second wavelength (λ 2 ) are expected to be attenuated by wavelength filter  118  and not reflected back to the detector  114 . When, however, detector  114  does receive a guard beam signal formed with the second wavelength (λ 2 ), the detector  114  or an associated controller determines that a substantially reflective foreign object  120 A has impinged (e.g., impeded) the line-of-sight path to or from the retro-reflector  116 . Based on such determination, the transmission of a high-flux power beam  106  is interrupted. 
     The interpretation that a guard beam signal (e.g., guard beam signal formed with the second wavelength (λ 2 ) in  FIG. 6B ) unexpectedly received at a detector  114  is caused by a reflective foreign object  120 A impinging on a high-flux power beam  106  (e.g., near or impeding a high-flux power beam  106 ) may be made with a processor  406  ( FIGS. 4A, 4B ), which may be configured as an executive safety control device or similar control circuit. For example, a control circuit coupled to one or more detectors may be arranged to respond to a decrease in detected intensity of the detected light at the strongly-reflected wavelength. In these or other embodiments, the control circuit may be arranged to respond to an increase in intensity of detected light at the poorly-reflected wavelength. In these or still other embodiments, the control circuit may be arranged to respond to a change in a ratio between detected light signals at the two wavelengths. 
     In the embodiments described herein, a decrease in intensity at the strongly-reflected wavelength indicates that the guard beam has been blocked, such as by an opaque, light-absorbing object entering the beam. An increase in intensity or change in ratio indicates that the subject guard beam has been fully or at least partially blocked from reaching a corresponding target reflector, but the guard beam is being reflected or scattered back from an object, such as a hand or piece of clothing, which has a substantially different ratio of reflectivities at the two wavelengths than the target reflector. 
     In the present disclosure, light or sensitivity to light at a particular wavelength (e.g., guard beam signals formed with the first wavelength (λ 1 ) and guard beam signals formed with the second wavelength (λ 2 )) may be understood as light or sensitivity to light at a single wavelength or as a spectral distribution of light or sensitivity to light in a band of wavelengths around the particular wavelength. 
       FIGS. 7A-7E  are guard beam emitter  112  embodiments of the types described in the present disclosure. The emitters  112  of  FIGS. 7A-7E  may be flexibly arranged to provide guard beam signals formed with one or more particular wavelengths. For example, in  FIG. 7A , two emitters  112  each provide a particular guard beam signal. A first one of the emitters  112  provides a first guard beam signal  122 A formed with a first wavelength (λ 1 ), and a second one of the emitters  112  provides a second guard beam signal  122 B formed with a second wavelength (λ 2 ). In the embodiment of  FIG. 7A , the two emitters  112  are positioned in proximity to each other such that the guard beam signal of each emitter  112  forms a cone, and the cones of each of the two emitters  112  overlap. In this way, a foreign object that crosses a line-of-sight of the first guard beam will also very likely cross a line-of-sight of the second guard beam. A guard beam signal intensity plot in  FIG. 7A  illustrates a substantially equal amplitude (i.e., intensity) of the first and second guard beam signals, and the signal intensity plot also illustrates the relative closeness of the first wavelength (λ 1 ) to the second wavelength (λ 2 ). 
     In  FIG. 7B , along the lines of the embodiment of  FIG. 7A , two emitters  112  each provide a particular guard beam signal. A first one of the emitters  112  provides a first guard beam signal  122 A formed with the first wavelength (λ 1 ), and a second one of the emitters  112  provides a second guard beam signal  122 B formed with the second wavelength (λ 2 ). A dichroic means  124  such as a dichroic mirror, dichroic lens, or some other dichroic structure gathers, redirects, and otherwise combines the first and second guard beam signals. In this way, a foreign object impinging on a high-flux power beam  106  (e.g., moving near or impeding a high-flux power beam  106 ) by crossing into a light curtain formed in part by the first guard beam signal will also cross into the second guard beam signal. In such embodiments as those of  FIGS. 6A and 6B  and described in the present disclosure, the foreign object which impacts both the first and second guard beam will be detected. 
     In  FIG. 7C , an emitter  112  is arranged to produce a first guard beam signal  122 A formed with the first wavelength (λ 1 ) and a second guard beam signal  122 B formed with the second wavelength (λ 2 ). In the embodiment, the first and second guard beam signals are passed through a common projection lens  126 . Along the lines of  FIGS. 7A and 7B , the emitted signal cone of the first and second guard beam signals overlap. When the a foreign object crosses into the signal path of one guard beam, the foreign object will also cross into the signal path of the other guard beam in cases where a corresponding detector  114  is configured to receive a signal having a particular wavelength, but not expecting to receive the signal with a particular wavelength, the foreign object will be detected. 
       FIG. 7D  illustrates an emitter  112  arranged to produce a dual wavelength guard beam signal  122 C formed with both a first wavelength (λ 1 ) and a second wavelength (λ 2 ). In  FIG. 7E , the emitter  112  of  FIG. 7D  is again employed to produce the dual-wavelength guard beam signal  122 C. In  FIG. 7D , the dual-wavelength guard beam signal  122 C is emitted toward one or more corresponding detectors  114 . In  FIG. 7D , the dual-wavelength guard beam signal  122 C is passed through a particular wavelength filter  118  to form a filtered dual-wavelength guard beam signal  122 D, which is emitted toward one or more corresponding detectors  114 . Corresponding intensity plot diagrams are presented in  FIGS. 7D and 7E  to illustrate generated signals. 
     The light curtain generation guard beam emitter  112  embodiments of  FIGS. 7A-7E  may be provided individually or in any desirable combination to achieve the benefits described in the present disclosure. Signals emitted by the emitters  112  of the embodiments of  FIGS. 7A-7E  are distinguishable from each other, and detectors  114 , which may be arranged with cooperative controllers (e.g., processor  406  of  FIG. 4A ), are used to distinguish one or more particular emitted signals amongst other guard beam signals, ambient light, man-made light, and other electromagnetic signals which would otherwise affect conventional systems. The use of these embodiments prevents a detection of false negatives, which arise when a foreign object that is impinging on a high-flux power beam  106  (e.g., near or impeding a high-flux power beam  106 ) goes undetected, or is not detected as early as otherwise possible. 
     The present disclosure describes guard beams that are passed between emitters  112  and detectors  114  in order to form a light curtain around a high-flux power beam  106 . The present disclosure also describes embodiments wherein, when a guard beam signal is blocked, the absence of the detected light signal is interpreted as an object impinging on the high-flux power beam  106  (e.g., near or impeding the high-flux power beam  106 ), which causes an interruption to the transmission of the high-flux power beam  106 . Certain embodiments describe a guard beam path having first and second sub-paths between an emitter  112 , a detector  114 , and an intervening retro-reflector  116 . 
     The present inventors have recognized that in some cases, a foreign object may be a substantially light transparent foreign object. In embodiments where emitters  112 , detectors  114 , and retro-reflectors form a single-ended system, it is possible that the substantially light transparent foreign object, which has intruded into the line-of-sight beam path, will permit the guard beam signal to pass to its corresponding detector  114 . In this case, the responsive detector  114  will again fail to detect the foreign object (i.e., a false negative) and indicate to the power beaming system that there is no intrusion when in fact there is one. Accordingly, in a case where a high-flux power beam  106  should be interrupted, the safety system falsely determines that no intrusion is present, and transmission of the high-flux power beam  106  dangerously continues. 
     One danger in power beaming systems recognized by the inventors is that when a transparent object, such as a sheet of glass or plastic, enters the protected space behind a light curtain, reflections of a high-flux power beam  106  off of the transparent object will present a hazard to humans, animals, property, or other objects. In the case of laser power beaming, for example, a glass surface of a foreign object may reflect a significant portion of the power beam (e.g., up to 4% or more) out from the projected beam path, thereby presenting a severe eye hazard. Worse still, since a sheet of glass, for example, has two surfaces (i.e., a first surface where a power beam enters the sheet of glass and a second surface where the power beam exits the sheet), the amount of power beam flux dangerously reflected outward may be doubled (e.g., up to 8% or more). While the introduction of such a transparent object is less likely than introducing an opaque object, certain cases where such an event would occur, for example by accident or as a result of a deliberate attempt to defeat the protection system, have been contemplated. 
     In some embodiments of the present disclosure, a beam-break or light-curtain safety system operates with a guard beams formed with the plurality of wavelengths. In some cases, a first wavelength (λ 1 ) is in the visible or near-infrared range. In these cases, a second wavelength (λ 2 ) is selected to be one that is significantly absorbed by common transparent materials, such as silica-based glasses, common plastics (e.g., acrylics and polycarbonates), and other such materials. Preferred wavelengths for guard beams formed with a second wavelength (λ 2 ) include signals in the ultraviolet B (290-315 nm) range, signals in the mid-infrared (2800-5000 nm) range, and other such signals that are strongly absorbed by common types of glass and plastic. In some cases, signals are formed with a different wavelength so as to detect selected UV-transmitting glasses and plastics, which do transmit UV-B. 
     In some cases, emitters and detectors that are arranged to emit and detect guard beam signals formed with the second, transparent-detection wavelength are configured and positioned in the similar ways as the emitters and detectors that are arranged to emit and detect card beam signals formed with the first wavelength (λ 1 ). In other cases, the emitters and detectors arranged to emit and detect guard beam signals formed with the second wavelength (λ 2 ) are configured and positioned differently. Emitters, for example, that are associated with guard beam signals formed with the second wavelength (λ 2 ) may be physically located further apart from other such emitters. These emitters can be spaced more widely because small transparent objects, which may otherwise pass between widely spaced guard beams, are likely to refract light and thus disrupt the visible/near-IR beams. Alternatively, very hazardous objects (e.g., large, flat sheets of glass or plastic) will be large enough to still interrupt guard beams that are spaced more widely apart. Along these lines, detectors that are configured to detect guard beam signals formed with the second wavelength (λ 2 ) may be similarly spaced more widely then detectors responsive to light signals formed with other wavelengths. 
       FIG. 8  is a power beaming system  100  having a light-transparent foreign object  120 B intruding on a high-flux power beam  106 . In this embodiment, the light curtain is not illustrated for simplicity. In this embodiment, the transmission unit  102  generates and projects the high-flux power beam  106  toward a reception unit  108 . The transparent object  120 B impinges on the high-flux power beam  106  (e.g., impedes the high-flux power beam  106 ), causing hazardous reflections from both the first surface of the transparent object  120 B closest to the transmission unit  102 , and from the second surface of the transparent object  120 B, furthest from the transmission unit  102 . 
     In some light curtain embodiments, it may be desirable to place the emitters  112  and detectors  114  that are arranged to emit and detect guard beam signals formed with the second wavelength (λ 2 ) at opposite ends of the protected area. Such a double-ended arrangement may be implemented even if emitters  112  and detectors  114  that are arranged to emit and detect guard beam signals formed with the first wavelength (λ 1 ) are configured in a single-ended arrangement (i.e., where emitter/detector assemblies are located at one end and retro-reflective structures are located at the other end). These double-ended arrangements may be implemented due to a limited availability of UV or mid-IR retro-reflectors or for other reasons. 
     Conversely, in some embodiments, it may be desirable to place the emitters  112  and detectors  114  that are arranged to emit and detect guard beam signals formed with the second wavelength (λ 2 ) at one end and use a UV or mid-IR retro-reflector at the other end. Such a single-ended arrangement may be used in order to pass the guard beam formed at the second wavelength (λ 2 ) through a possible obstacle two times, thereby increasing the absorption length and attenuation of the guard beam signal due to a partially-absorbing obstacle. 
       FIGS. 9A-9B  are a power beaming system embodiment arranged to detect light-transparent foreign objects. In  FIG. 9A , a transmission unit  102  is arranged to generate and project a high-flux power beam  106  (not shown) toward a reception unit  108 . In the embodiment of  FIG. 9A , two guard beams of a light curtain are illustrated. A first emitter  112  and a first detector  114  are located at the transmission unit  102 . Together, the first emitter  112  and first detector  114  form a single-ended system that communicates a first guard beam signal  122 A formed at the first wavelength (λ 1 ) from the first emitter  112  along a first sub-path to a first retro-reflector  116 , and back from the first retro-reflector  116  along a second sub-path to the first detector  114 . Also in  FIG. 9A , a second emitter  112  is located at the reception unit  108 . The second emitter  112  is arranged as a double-ended guard beam system with the first detector  114  at the transmission unit  102 . The second emitter unit  112  communicates a second guard beam  122 B signal formed with the second wavelength (λ 2 ) directly to the first detector  114 , which is arranged to receive both the first guard beam signal  122 A and the second guard beam signal  122 B. 
       FIG. 9B  illustrates the same portion of a dual-wavelength light curtain as illustrated in  FIG. 9A . In  FIG. 9B , however, a substantially transparent foreign object  120 B (i.e., a transparent (glass) pane) is now obstructing the first and second guard beams. In  FIG. 9B , the substantially transparent foreign object  120 B partially attenuates the first guard beam signal  122 A and completely blocks the second guard beam signal  122 B. In a conventional guard beam system, a guard beam would substantially pass through the substantially transparent foreign object  120 B and provide a false negative indication that the high-flux power beam was safe to transmit. Conversely, in the embodiment of  FIG. 9B , the attenuation of the first guard beam  122 A and absence of the second guard beam  122 B at the detector  114  are used to recognize (i.e., determine) the presence of the substantially transparent foreign object  120 B, which indication is used to interrupt transmission of the high-flux power beam  106 . 
     A dual-wavelength light curtain system may be used in conjunction with a sensitive threshold detector  114  arranged to detect a guard beam signal formed at the first wavelength (λ 1 ). These embodiments may be implemented because many transparent objects (e.g., glass objects, plastic objects, and the like) will reflect at least several percent of the guard beam signal formed at the first wavelength (λ 1 ) per surface, and thus, unless the transparent object is exactly perpendicular to the light-curtain beams, the transparent object will attenuate the guard beam by several percent (e.g., 8% or more) on a single pass or twice as much (e.g., 16% or more) on two passes. Accordingly, using attenuation of the guard beam formed at the first wavelength (λ 1 ) alone to detect transparent obstacles may be unsatisfactory due to noise or slow variations in the detected light intensity, which will cause excessive false alarms if sensitive threshold detector  114  is set to reliably detect low levels of beam attenuation. However, strong attenuation of the guard beam formed with the second wavelength (λ 2 ) combined with a small attenuation of the guard beam formed with the first wavelength (λ 1 ) may provide an acceptably reliable indicator that the attenuation of the guard beam formed with the second wavelength (λ 2 ) was due to an obstacle and not due to emitter or detector problems, atmospheric scattering in the UV range, or other effects that would disrupt the guard beam formed with the second wavelength (λ 2 ). 
     In some cases, foreign objects may be partly-transmissive even in the mid-IR rage, or beams may be larger, or diffraction may have more tendency to “fill in” blocked beams. In these cases, even if a foreign object obstructs a guard beam, it may be desirable to select a change-in-signal threshold differently for, particularly, a mid-IR system than for a visible/near-IR system. 
       FIGS. 10A-10B  are a high-flux power beam receiver embodiment arranged to detect a reflective foreign object and corresponding intensity profiles for the embodiment at various points in the receiver. The embodiment of  FIGS. 10A-10B  is implemented as a dual-wavelength single-ended light curtain wherein one or more emitters  112  (not shown) generate and emit a first guard beam signal  122 A formed at the first wavelength (λ 1 ) and a second guard beam signal  122 B formed at the second wavelength (λ 2 ). One or more detectors  114  (not shown) are arranged to detect signals at the first wavelength (λ 1 ) and the second wavelength (λ 2 ). At the reception unit  108 , a power beam receiver  110  is arranged to receive a high-flux power beam  106  (not shown). A plurality of retro-reflectors  116  are arranged about the power beam receiver  110 . Each retro-reflector  116  is positioned behind a particular wavelength filter  118 . In the embodiment of  FIG. 10A , the first guard beam  122 A and second guard beam  122 B travel substantially the same path toward the reception unit  108 . In this case, when a foreign object impinges on a first guard beam  122 A (e.g., impedes a first guard beam  122 A), the foreign object will also impinge on a second guard beam  122 B (e.g., impede a second guard beam  122 B). In this way, the dual-wavelength single-ended light curtain embodiment of  FIG. 10A  is arranged to detect any foreign objects that impinge on the high-flux power beam  106  (e.g., come near or impede the high flux power beam  106 ), wherein the foreign objects may be any type of opaque objects, substantially reflective objects, light transparent objects, or the like. 
     In the present case of  FIGS. 10A-10B , the second wavelength (λ 2 ) and the wavelength filter  118  may be cooperatively selected such that the second guard beam signal  122 B will be attenuated when it reaches the reception unit  108 . In this case, if the second guard beam signal  122 B is obstructed by a reflective object, then the second guard beam signal  122 B will unexpectedly be returned to a detector, and the opaque (i.e., reflective) object will be detected. 
     The intensity plots of  FIG. 10B  illustrate particular signal intensities of the first guard beam signal  122 A formed at the first wavelength (λ 1 ) and the second guard beam signal  122 B formed at the second wavelength (λ 2 ). In the top-most signal intensity plot, if neither guard beam is obstructed, both the first guard beam signal  122 A and the second guard beam signal  122 B are incident upon filter  118 . Correspondingly, in the middle signal intensity plot, it is shown that the first guard beam signal  122 A passes through the wavelength filter  118  and strikes the retro-reflector  116 . In contrast, in the middle signal intensity plot, the second guard beam signal  122 B is attenuated by the wavelength filter  118 , and the amplitude of the second guard beam signal  122 B is very low. In the bottom-most signal intensity plot of  FIG. 10B , the first guard beam signal  122 A is reflected back to a corresponding detector  114  (not shown). Concurrently, the second guard beam signal  122 B is even further attenuated by the wavelength filter  118 , and the amplitude of the second guard beam signal  122 B is barely detectable. 
     Controlling the Output Cone from Emitters for Angle Sensing 
     Referring now to  FIGS. 11A-11D , a beam projection system  1100  is shown. In some embodiments, the beam projection system  1100  includes a beam transmitter, a beam receiver, a plurality of emitters, and a plurality of detectors. The beam transmitter has one or more light sources to transmit a beam.  FIG. 1  shows a single emitter  1100  from the plurality of emitters. The beam transmitter also has a centerline. The beam receiver is arranged to receive the beam from the beam transmitter. The beam receiver has an acceptance centerline. The plurality of emitters  1110  and the plurality of detectors form a light curtain that is patterned about the beam. The plurality of emitters  1110  are positioned to surround the beam. The plurality of emitters each have a centerline and each have a divergence angle centered on the centerline. 
     As shown in  FIG. 11A , in some embodiments the beam projection system  1100  includes light curtain emitters  1110  that emit light towards the detectors. The light curtain emitters  1110  may be used in controlling an output cone for angle sensing. 
     As shown in  FIG. 11B , in one or more embodiments, each individual emitter  1100  of the light curtain emitters emit light in a generally cone-shaped region, with a beam intensity profile that falls towards zero near the edges of the output cone. In some embodiments, the beam intensity profile may have other variations in between the peak and the edges of the output cone. 
     Referring now to  FIG. 11C , in one or more embodiments, the beam intensity profile is shaped with an optical element  1130  that is placed between the light curtain emitters  1110  and the light curtain detectors. By way of example only, and not by way of limitation, this optical element  1130  may be a lens. In some such embodiments, the optical element  1130  is used to narrow an angle for purposes of maximizing the light intensity at the detectors. For example, the light emitted from the light curtain emitters  1110  may have a divergence angle θ 1  before it reaches the optical element  1130 . The emitted light may then be narrowed to θ 2  by the optical element, where θ 2  is less than θ 1 . 
     As shown in  FIG. 11D , in one or more embodiments, an aperture apparatus  1140  is positioned between at least some of the light curtain emitters  1110  and the detectors to further narrow the light cone. In some embodiments, the aperture apparatus  1140  may be used to sharpen the beam intensity profile (as shown in  FIG. 11B  at  1150 ), either alternatively or additionally to the narrowing of the light cone. In this regard, the aperture apparatus  1140  will cause the beam intensity profile to have a sharper fall-off at the edges of the cone. The aperture apparatus  1140  may be used either in addition to the optical element  1130  or instead of the optical element. For example, the light emitted from the light curtain emitters  1110  may have a divergence angle θ 1  before it reaches the aperture apparatus  1140 . The emitted light may then be narrowed to θ 2  by the aperture apparatus  1140 , where θ 2  is less than θ 1 . 
     In another aspect of the beam projection system  1100 , these light curtain emitters  1110  (particularly when using this final divergence angle θ 2 ) can be used in conjunction with other design parameters of the power receiver to serve a function beyond their main function for active safety sensing. In some embodiments, the light curtain emitters  1110  are used by the system to indicate when the power beam receiver is tilted beyond a given angle relative to the incoming power beam. This is due to the fact that the placement and orientation of the light curtain emitters  1110  on the power receiver are usually designed such that the centerline of the outgoing emitter beams are parallel (or nearly parallel) to the nominal incoming power beam. 
     The expectation is that the optical axis of each light curtain emitter is approximately parallel to the incoming power beam centerline. Similarly, the light curtain detector axes are near-parallel to the outgoing power beam centerline, at least when it is in the nominal center of its steering range. Notably, fabrication and assembly errors tend to make these axes not exactly parallel, and there are a variety of factors that determine the allowable tolerance stack-up of these potential misalignment errors. Those factors include mechanical flatness of mating surfaces, thermal mismatch, as well as light curtain beam divergence angle and detector field of view angle. As one example, if the emitters were further off-parallel from the incoming power beam centerline than the half-angle of their emitted beam divergence, then the light cone from them would not be expected to reach the detectors. Therefore, any fabrication and assembly tolerances would be required to be smaller (with some amount of margin) than the emitter beam divergence half-angle. In some cases, this amount of misalignment from parallel is less than 10 degrees, in other cases less than 5 degrees, in other cases less than 3 degrees, in other cases less than 1 degree, in other cases less than 0.5 degrees, in other cases less than 0.25 degrees, or in other cases less than 0.10 degrees. 
     Otherwise stated, in one or more embodiments of the beam projection system  1100 , the placement and orientation of the plurality of emitters  1110  about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters  1110  within ten degrees of parallel to the incoming beam. In other embodiments of the beam projection system  1100 , the placement and orientation of the plurality of emitters  1110  about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters  1100  within five degrees of parallel to the incoming beam. In still other embodiments of the beam projection system  1100 , the placement and orientation of the plurality of emitters about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters  1110  within three degrees of parallel to the incoming beam. In yet other embodiments of the beam projection system  1100 , the placement and orientation of the plurality of emitters about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters  1110  within one degree of parallel to the incoming beam. 
     In one or more embodiments of the beam projection system  1100 , the placement and orientation of the plurality of emitters  1110  about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters  1110  within one-half degree of parallel to the incoming beam. In other embodiments of the beam projection system  1100 , the placement and orientation of the plurality of emitters  1110  about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters  1100  within one-quarter degree of parallel to the incoming beam. In still other embodiments of the beam projection system  1100 , the placement and orientation of the plurality of emitters about the acceptance centerline of the beam receiver are configured to align the centerlines of the plurality of emitters  1110  within one-tenth degree of parallel to the incoming beam. 
     In some embodiments, the beam projection system shuts off the beam when the power beam receiver is tilted beyond a given angle relative to the incoming power beam, due to the light curtain detecting drop-out of the emitter signal as determined using the angle of the light curtain emitters  1110 . In other embodiments, the beam projection system modulates the beam when the power beam receiver is tilted beyond a given angle relative to the incoming power beam, as determined using the angle of the light curtain emitters  1110 . 
     In another embodiment of the beam projection system  1100 , if the receiver uses concentrating optics, those concentrating optics generally have a maximum acceptance angle for incoming light. While adding other sensors to detect the relative angle of the incoming power beam and the receiver could be done, this would increase the expense and complexity of the receiver. As described above, using the angle of the light curtain emitters  1110  to determine when the power beam receiver is tilted beyond a given angle relative to the incoming power beam achieves a similar result without increasing the expense and complexity of the receiver. 
     In still other embodiments of the beam projection system  1100 , a cone angle for some light curtain emitters  1100  is specified such that the light curtain detectors lose the signal from this subset of emitters at the specified angle. In one or more embodiments, the loss of the signal by the detectors is used to automatically turn off the power beam before the receiver is tilted at an angle too great relative to the incoming power beam. This embodiment may be particularly useful when the beam intensity profile of the light curtain emitters  1100  have relatively sharp edges. 
     Placement of Emitters and Detectors 
     Referring now to  FIGS. 12A-12D , in one or more embodiments of an object detection system  1200 , an efficient geometry for placing light curtain emitters  1210  and detectors around a power beam  1220  is a circle. In other embodiments of the object detection system  1200 , efficient geometries for placing light curtain emitters  1210  and detectors around a power beam  1220  include rectangle, square, rounded rectangle, and rounded square. In one or more other embodiments, the object detection system  1200  includes a beam transmitter, a beam receiver, and a plurality of emitters  1210  and a plurality of detectors (as shown in  FIG. 1 ). The beam transmitter has one or more light sources to transmit a beam. The beam receiver is arranged to receive the beam from the beam transmitter. The plurality of emitters  1210  and the plurality of detectors form a light curtain that is patterned about the beam  1220 . The plurality of emitters  1210  surrounding the beam  1220  and the plurality of detectors surrounding the beam being are configured to detect an obstacle in at least one of many line-of-sight transmission paths between the plurality of emitters  1210  and the plurality of detectors. 
     As shown in  FIG. 12A , a ring of light curtain emitters  1210  is positioned around the power beam  1220 . Each of the many light curtain emitters  1210  emits light that is received by each of many detectors. Otherwise stated, each light curtain emitter  1210  emits light that is received by many detectors, and each detector  1210  receives light that is emitted by many light curtain emitter  1210  (i.e., many-to-many beam paths). The “many-to-many” beam paths, which connect each of many emitters to each of many detectors, create a dense weave of paths that can directly detect a foreign object if it crosses part or all of one or more of those beams. 
     The outer “shell” of these beam paths, which is created from the circle of light curtain emitters  1210  and detectors, is shaped like a cylinder. The diameters of the ends of that cylinder corresponds to the diameters of the rings on which the light curtain emitters  1210  and detectors are positioned, respectively. Notably, in some embodiment of the object detection system  1200 , the light curtain emitters  1210  and detectors are positioned such that some of those beam paths cross through the interior of the light curtain shell cylinder (as shown in  FIGS. 14A and 14B ). 
     Referring now to  FIG. 12B  (in comparison to  FIG. 12A ), a technological improvement to the circular geometry of light curtain emitters  1210  and detectors around a power beam  1220  is positioning light curtain emitters  1210  at multiple radial distances from the center of the shell cylinder, thereby creating multiple concentric rings of light curtain emitters  1210 . In other embodiments, the detectors are positioned at multiple radial distances from the center of the shell cylinder, thereby creating multiple concentric rings of detectors. In still other embodiments, both light curtain emitters  1210  are positioned at multiple radial distances from the center of the shell cylinder, and detectors are positioned at multiple radial distances from the center of the shell cylinder, thereby creating multiple concentric rings of light curtain emitters  1210  and multiple concentric rings of detectors. In one or more embodiments with two concentric rings of light curtain emitters  1210  and/or detectors, the inner concentric ring would likely have fewer emitters and/or detectors due to the shorter perimeter, and limitations on physical spacing between the emitters and/or detectors. 
     As shown in  FIG. 12C , in some embodiments of the object detection system  1200  a ring of emitters are positioned at a radial distance from the center of the shell cylinder, and additionally one or more light curtain emitters  1210  are positioned in the interior of the power beam area. In other embodiments of the object detection system  1200  a ring of emitters are positioned at a radial distance from the center of the shell cylinder, and additionally one or more detectors are positioned in the interior of the power beam area. In some such embodiments, the one or more light curtain emitters  1210  positioned in the interior of the power beam area and take space within the area defined by photovoltaic array. Other embodiments may use a dichroic to have one or more detectors combined with the power beam in the transmitter optics. Other embodiments may use a dichroic to have one or more emitters combined with the power beam in the receiver optics. 
     Referring now to  FIG. 12D , in other embodiments of the object detection system  1200 , still additional configurations may be used that employ multiple radial distances from the center of the shell cylinder. In one or more embodiments, three concentric rings of emitters are positioned at multiple radial distances from the center of the shell cylinder. In other embodiments, a light curtain emitters  1210  are positioned in a random distribution in an annulus area at multiple radial distances from the center of the shell cylinder, as shown in  FIG. 12D . In still other embodiments, a light curtain emitters  1210  are positioned in a pseudo-random distribution in an annulus area at multiple radial distances from the center of the shell cylinder. In yet other embodiments, a light curtain emitters  1210  are positioned in other structured patterns of distribution in an annulus area at multiple radial distances from the center of the shell cylinder. 
     In one or more other embodiments of the object detection system  1200 , the detection of an obstacle in at least one of many line-of-sight transmission paths between the plurality of emitters  1210  and the plurality of detectors controls whether the beam is transmitted. In still other embodiments of the system to detect obstacles, the detection of an obstacle in at least one of many line-of-sight transmission paths between the plurality of emitters  1210  and the plurality of detectors controls modulation of the beam  1220 . 
       FIG. 13A  shows a first view of a first embodiment of an object detection system  1300  that utilizes many-to-many beam paths with cross-sections, i.e., each of many (but not necessarily all) light curtain emitters  1310  emitting light that is received by each of many (but not necessarily all) detectors  1320 . Specifically,  FIG. 13A  shows beam paths  1340  from two concentric rings of emitters  1310 , and emissions from two light curtain emitters  1310  transmitting to five different detectors  1320 . For ease of viewing, not all of the emissions for each of the light curtain emitters  1310  are shown. Notably, this side view is showing the some of the light curtain emitters being combined. 
       FIG. 13B  shows a second view of a second embodiment of an object detection system  1300  that utilizes many-to-many beam paths  1340 , i.e., each of many (but not necessarily all) light curtain emitters  1310  emitting light that is received by each of many (but not necessarily all) detectors  1320 . Specifically,  FIG. 13B  shows beam paths  1340  from two concentric rings of emitters  1310 , and emissions from four light curtain emitters  1310  transmitting to two different detectors  1320 . For ease of viewing, not all of the emissions for each of the light curtain emitters  1310  are shown. In this second view, it is easier to visually distinguish the inner and outer rings of light curtain emitters  1310 . This configuration of multiple rings and many-to-many beam paths  1340  creates overlaps in the paths that are denser than for embodiments that use only a single ring. Thus, this configuration reduces the possibility that a foreign object could traverse the light curtain without being detected. 
     Referring now to  FIG. 14A , an orthographic projection side view of a first embodiment of an object detection system  1400  is shown that utilizes many-to-many beam paths  1410 . This embodiment shows beam paths  1410  for two concentric rings of emitters  1420 . Again, each of many (but not necessarily all) of the light curtain emitters  1420  are emitting light that is received by each of many (but not necessarily all) of the detectors  1430 . Significantly, in this orthographic projection side view of this embodiment of the object detection system  1400 , some of the light curtain emitters  1420  emit light that crosses into the interior of the power beam area  1440 . 
     Referring now to  FIG. 14B , an orthographic projection end view of a second embodiment of an object detection system  1400  is shown that utilizes many-to-many beam paths  1410 . Again, the density of the many-to-many beam paths  1340  with these layouts is higher than for a single ring, thereby reducing the possibility that a foreign object could traverse the light curtain without being detected. This embodiment shows beam paths for two concentric rings of emitters  1420 . Again, each of many (but not necessarily all) of the light curtain emitters  1420  are emitting light that is received by each of many (but not necessarily all) of the detectors  1430 . Significantly, in this orthographic projection end view of this embodiment of the object detection system  1400 , some of the light curtain emitters  1420  emit light that crosses into the interior of the power beam area  1440 . 
     Maximizing Separation Between Light Curtain Paths and Power Beam 
     As shown in  FIG. 15 , in some embodiments of an object detection system  1500 , the light curtain paths between the light curtain emitters  1510  and the detector are spaced away from the power beam. In some implementations, the object detection system  1500  includes a beam transmitter (not shown), a beam receiver  1530 , and a plurality of emitters  1510  and a plurality of detectors (not shown). The beam transmitter has one or more light sources to transmit a beam. The beam receiver is arranged to receive the beam from the beam transmitter. The plurality of emitters  1510  and a plurality of detectors form a light curtain patterned about the beam. The plurality of emitters surrounding the beam and the plurality of detectors surrounding the beam being configured to detect an obstacle in a line-of-sight transmission path between an emitter in the plurality of emitters and a detector in the plurality of detectors. 
     Specifically, at least some of the plurality of emitters  1510 , at least some of the plurality of detectors, or both, are positioned away from the power beam on arms  1520  (e.g., “whiskers”) that extend outwardly away from the power beam. This outward extension of the emitters  1510  on whisker arms  1520  increases separation between the power beam and the line-of-sight transmission path between each emitter in the plurality of emitters  1510  and each detector in the plurality of detectors. The separation in turn increase time that a safety system has to detect an object in the line-of-sight transmission path and turn off the power beam (or take other remedial action). 
     Otherwise stated, the distance between a light curtain beam path (particularly the beam paths that define the outer shell cylinder) and the power beam affects how much time the safety system has to detect a foreign object and turn off the power beam before the foreign object is exposed to high intensity, and thus, hazardous light. Therefore, these embodiments of the beam projection system  1500  that maximize the physical separation between the power beam and the light curtain beam paths that define the outer shell cylinder while minimizing the additional mass, surface area, or both, that are required for the maximized separation. Thus, these systems provide a technological improvement over systems without such physical separation. 
     In one or more other embodiments of the object detection system  1500 , the arms  1520  are extendable and retractable. In other embodiments of the object detection system  1500 , the arms  1520  are foldable and unfoldable. In still other embodiments of the object detection system  1500 , the arms  1520  include flexible webbing material that spreads out when extended to catch scattered light from the system. 
     Referring still to  FIG. 15 , in some embodiments of the beam projection system  1500  that maximize the physical separation between the power beam and the light curtain beam paths, the housing enclosure for both the transmitter and the receiver will have various limitations on their physical dimensions. These physical dimensions may be influenced by a variety of factors, including by way of example only, and not by way of limitation: where the components are being placed, wind loading, aerodynamic factors (e.g., if mounted on a flying vehicle), weight, and other factors. Accordingly, these factors may limit the maximum separation between light curtain beam paths and the power beam. In some embodiments, all of the light curtain emitters  1510  (or the detectors) on “whisker” extension arms  1520 , which extend away from the beam and are thinner and lighter than thicker arms or larger radial housing structure. Therefore, these “whisker” extension arms  1520  do not have the same limitations as the main housing structure or other enclosure. 
     In one or more embodiments of the beam projection system  1500  that maximize the physical separation between the power beam and the light curtain beam paths, these “whisker” extension arms  1520  are made out of printed circuit board (PCB), and encased in standard sealing materials to protect the electronics from the environment. The type of embodiment also enables any separate physical structure that would normally provide enclosure of the light curtain emitters  1510  and detector to be eliminated, thereby reducing costs and complexity. 
     Daisy Chaining of Light Curtain Emitters 
     In most power beam projection systems and objection detection systems, as described above, there is a central controller that controls the emissions of the light curtain emitters. Thus, each of the many light curtain emitters in the system is individually connected to the central controller, which is generally located in the central section of the beam projection system or objection detection system. Accordingly, adding additional light curtain emitters to the system can be problematic since the light curtain emitters must be connected into the central controller, which adds significantly more wiring since each separate emitter requires its own wire routed back to the central controller. 
     However, some embodiments of the disclosed beam projection systems and objection detection systems provide the technological improvement of having each of the light curtain emitters include its own simpler controller, and having each of the light curtain emitters connected together in series without a central controller. The controllers in each of the light curtain emitters execute a timing of emissions such that the each emitter in the plurality of light curtain emitters are configured to emit light and then signal an adjacent emitter in the plurality of light curtain, in series throughout the circle of daisy chained light curtain emitters. A daisy chain is a wiring scheme in which multiple devices are wired together in sequence or in a “ring.” This embodiment of light curtain emitters that are connected in series and that each have individual controllers, provides technical improvements to the scalability of the beam projection system or objection detection system. In some embodiments, one wire may be run to provide a clock signal that all of the daisy-chained emitters can use to synchronize the proper timing for their emission. 
     Protective Receiver Dome 
     Referring now to  FIGS. 16A and 16B , in some implementations, a PV array  1602  may be covered by a dome  1604  that is transparent at least to the frequency of light of the power beam.  FIG. 16A  shows an exploded view of an assembly of PV array  1602  and dome  1604 , while  FIG. 16B  shows a cross-sectional view. These views may be referred to herein collectively as  FIG. 16 . It will be understood that while, to avoid cluttering the figure, dome  1604  is shown in  FIG. 16  as sealing directly to the array surface  1602 , in some implementations the dome may seal not directly to the surface of the array, but to other components surrounding the array, such as the carrier, the heat sink, or other structures. Dome  1604  serves to protect PV cells  1606  (and any other interior components) from dust, rain, or other particles that may accumulate and interfere with power beaming over time. In implementations where dome  1604  is present, it may also serve as an optical component of the system by dispersing light reflected from the dome. 
       FIG. 16B  shows two parallel incident light paths  1608  impinging on dome  1604 . Dome  1604  is sufficiently thin that its top and bottom surfaces may be considered to be substantially parallel, so that the portion of each light beam  1608  that is transmitted through the dome may be shifted slightly toward the center, but the two beams remain substantially parallel. However, any reflected portions of the impinging beams  1608  (shown in the figure as thinner lines reflecting away) will no longer be parallel, because of the curvature of dome  1604 . Since reflections of the incident beams are dispersed in this fashion, the reflected light is quickly dissipated, and does not constitute a safety hazard at a distance away from the receiver in the same way that it might if it reflected from a flat surface. While some light is usually reflected when light moves from one transmission medium to another, it may still be beneficial in many situations for dome  1604  to be coated with an antireflective coating to minimize the amount of light reflected. Since no antireflective coating is perfect, the beam-dispersing effect of dome  1604  may nevertheless be desirable even when such efforts are made to minimize reflections. 
     Certain words and phrases used in the present disclosure are set forth as follows. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or,” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof in all grammatical forms, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. The term “controller” means any device, system, or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware, or software, or some combination of at least two of the same. The functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Other definitions of certain words and phrases may be provided within this patent document. Those of ordinary skill in the art will understand that in many, if not most instances, such definitions apply to prior as well as future uses of such defined words and phrases. 
     Where one or more figures included in the present disclosure illustrates a data flow diagram, the illustrated process is a non-limiting process that may be used by embodiments of high flux power beam or field systems. In this regard, each described process may represent a module, segment, or portion of software code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that in some implementations, the functions noted in the process may occur in a different order, may include additional functions, may occur concurrently, and/or may be omitted. 
     In some cases, the figures in the present disclosure illustrate portions of one or more non-limiting computing device embodiments such as the transmission unit  102  and the reception unit  108 . The computing devices may include operative hardware found in conventional computing device apparatuses such as one or more processors, volatile and non-volatile memory, serial and parallel input/output (I/O) circuitry compliant with various standards and protocols, wired and/or wireless networking circuitry (e.g., a communications transceiver), one or more user interface (UI) modules, logic, and other electronic circuitry. In addition, or in the alternative, the computing device embodiments may be electronic circuits formed to carry out operations of a finite state machine. 
     Processors, such as those that may be employed in the transmission unit  102  (e.g., executive controller  404 ) and the reception unit  108 , may include central processing units (CPU&#39;s), microcontrollers (MCU), digital signal processors (DSP), application specific integrated circuits (ASIC), and the like. The processors interchangeably refer to any type of electronic control circuitry configured to execute programmed software instructions. The programmed instructions may be high-level software instructions, compiled software instructions, assembly-language software instructions, object code, binary code, micro-code, or the like. The programmed instructions may reside in internal or external memory or may be hard-coded as a state machine or set of control signals. According to methods and devices referenced herein, embodiments describe software executable by the processor and operable to execute certain ones of the method acts. 
     As known by one skilled in the art, a computing device has one or more memories such as memory  408 , and each memory comprises any combination of volatile and non-volatile computer-readable media for reading and writing. Volatile computer-readable media includes, for example, random access memory (RAM). Non-volatile computer-readable media includes, for example, read only memory (ROM), magnetic media such as a hard-disk, an optical disk drive, a floppy diskette, a flash memory device, a CD-ROM, and/or the like. In some cases, a particular memory is separated virtually or physically into separate areas, such as a first memory, a second memory, a third memory, etc. In these cases, it is understood that the different divisions of memory may be in different devices or embodied in a single memory. The memory in some cases is a non-transitory computer medium configured to store software instructions arranged to be executed by a processor. 
     The computing devices illustrated herein may further include operative software found in a conventional computing device such as an operating system or task loop, software drivers to direct operations through I/O circuitry, networking circuitry, and other peripheral component circuitry. In addition, the computing devices may include operative application software such as network software for communicating with other computing devices, database software for building and maintaining databases, and task management software where appropriate for distributing the communication and/or operational workload amongst various processors. In some cases, the computing device is a single hardware machine having at least some of the hardware and software listed herein, and in other cases, the computing device is a networked collection of hardware and software machines working together in a server farm to execute the functions of one or more embodiments described herein. Some aspects of the conventional hardware and software of the computing device are not shown in the figures for simplicity. 
     When so arranged as described herein, each computing device may be transformed from a generic and unspecific computing device to a combination device comprising hardware and software configured for a specific and particular purpose. 
     Database structures, if any are present in the power beaming system embodiment  100  or in other embodiments, may be formed in a single database or multiple databases. In some cases hardware or software storage repositories are shared amongst various functions of the particular system or systems to which they are associated. A database may be formed as part of a local system or local area network. Alternatively, or in addition, a database may be formed remotely, such as within a “cloud” computing system, which would be accessible via a wide area network or some other network. 
     Input/output (I/O) circuitry and user interface (UI) modules include serial ports, parallel ports, universal serial bus (USB) ports, IEEE 802.11 transceivers and other transceivers compliant with protocols administered by one or more standard-setting bodies, displays, projectors, printers, keyboards, computer mice, microphones, micro-electro-mechanical (MEMS) devices such as accelerometers, and the like. 
     Buttons, keypads, computer mice, memory cards, serial ports, bio-sensor readers, touch screens, and the like may individually or in cooperation be useful to an operator of the power beaming system embodiment  100 . The devices may, for example, input control information into the system. Displays, printers, memory cards, LED indicators, temperature sensors, audio devices (e.g., speakers, piezo device, etc.), vibrators, and the like are all useful to present output information to the operator of the power beaming system embodiment  100 . In some cases, the input and output devices are directly coupled or otherwise electronically coupled to a processor or other operative circuitry. In other cases, the input and output devices pass information via one or more communication ports (e.g., RS-232, RS-485, infrared, USB, etc.) 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the disclosed embodiments. That the upper and lower limits of these smaller ranges may be independently included in the smaller ranges is also encompassed within the disclosed embodiments, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosed embodiments. 
     Unless defined otherwise, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosed embodiments belong. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, a limited number of the exemplary methods and materials are described herein. 
     As used in the present disclosure, the term “module” refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor and a memory operative to execute one or more software or firmware programs, combinational logic circuitry, or other suitable components (i.e., hardware, software, or hardware and software) that provide the functionality described with respect to the module. 
     A processor (i.e., a processing unit), as used in the present disclosure, refers to one or more processing units individually, shared, or in a group, having one or more processing cores (e.g., execution units), including central processing units (CPUs), digital signal processors (DSPs), microprocessors, micro controllers, state machines, and the like that execute instructions. In the present disclosure, the terms processor in any of its grammatical forms is synonymous with the term controller. 
     In the present disclosure, memory may be used in one configuration or another. The memory may be configured to store data. In the alternative or in addition, the memory may be a non-transitory computer readable medium (CRM) wherein the CRM is configured to store instructions executable by a processor. The instructions may be stored individually or as groups of instructions in files. The files may include functions, services, libraries, and the like. The files may include one or more computer programs or may be part of a larger computer program. Alternatively or in addition, each file may include data or other computational support material useful to carry out the computing functions of the systems, methods, and apparatus described in the present disclosure. 
     The terms, “real-time” or “real time,” as used interchangeably herein and in the claims that follow, are not intended to imply instantaneous processing, transmission, reception, or otherwise as the case may be. Instead, the terms, “real-time” and “real time” imply that the activity occurs over an acceptably short period of time (e.g., over a period of microseconds, milliseconds, seconds, minutes or some other time frame as the context of the term&#39;s use implies), and that the activity may be performed on an ongoing basis (e.g., stopping the transmission of a high flux power beam or field). An example of an activity that is not real-time is one that occurs over an extended period of time (e.g., hours, days, weeks, months, years, or some other time frame as the context of the term&#39;s use implies) or that occurs based on intervention or direction by a person or other activity. 
     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, e.g., “including, but not limited to.” 
     Reference throughout this specification to “one embodiment” or “an embodiment” and variations thereof means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content and context clearly dictates otherwise. It should also be noted that the conjunctive terms, “and” and “or” are generally employed in the broadest sense to include “and/or” unless the content and context clearly dictates inclusivity or exclusivity as the case may be. In addition, the composition of “and” and “or” when recited herein as “and/or” is intended to encompass an embodiment that includes all of the associated items or ideas and one or more other alternative embodiments that include fewer than all of the associated items or ideas. 
     The headings and Abstract of the Disclosure provided herein are for convenience only and do not limit or interpret the scope or meaning of the embodiments. 
     The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, application and publications to provide yet further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.