Abstract:
Systems and methods for preventing high-radiant-flux light, such as laser light or a nuclear flash, from causing harm to imaging devices, such as a camera or telescope. In response to detection of high-radiant-flux light, the proposed systems have the feature in common that a shutter is closed sufficiently fast that light from the source will be blocked from reaching the image sensor of the imaging device. Some of the proposed systems include a folded optical path to increase the allowable reaction time to close the shutter.

Description:
BACKGROUND 
       [0001]    This disclosure generally relates to systems and methods for the protection of imaging devices against light having a high radiant flux. As used herein, the term “high-radiant-flux light” means light having any one or more of the following measurable properties: high radial intensity (W/sr), high irradiance (W/m 2 ) and high radiance (W·sr −1 ·m −2 ). Such high-radiant-flux light may be coherent (e.g., laser light) or incoherent. 
         [0002]    A general problem is to achieve high probability of mission success, at acceptably low cost, despite hazards. A hazard of increasing concern is high-radiant-flux light, which can damage optical sensors (hereinafter “image sensors”) needed to (a) operate a vehicle safely or (b) collect data, such as agricultural data or military surveillance data. This hazard usually arises from lasers aimed at a vehicle. However, it may also arise from arc welding equipment, exceptionally large or hot fires, a lightning bolt, or a nuclear blast. 
         [0003]    Imaging devices, such as cameras and telescopes, are especially vulnerable to high-radiant-flux light. By design, such imaging devices use a lens or mirror to focus light onto an image sensor (such as a focal plane array) comprising a multiplicity of pixels. This greatly increases the light intensity on pixels corresponding to the location of the high-radiant-flux light source in the image. Thus, light that is harmless to a structural surface may have damaging radiant flux at the image sensor. The high-radiant-flux light can damage the image sensor by thermal shock, melting, or other mechanisms. 
         [0004]    One approach to solving this problem is using a laser sensor to detect the presence of high-radiant-flux light in the imaging device&#39;s field of view. As used herein, the term “laser sensor” means a sensor that detects high-radiant flux light (defined above). (For avoidance of doubt, it should be noted that the term “laser sensor” as used herein does not mean a sensor that detects laser light only or a sensor that detects all laser light. Instead the laser sensor detects any light having a radiant flux in excess of a specified threshold, including but not limited to high-radiant-flux laser light.) The laser sensor transmits a signal via a signal line to a shutter inside the imaging device. The shutter closes, blocking the light from reaching the image sensor of the imaging device. This approach suffices for the weakest threats, such as accidental exposure to lasers used in a light show, but it is insufficient for the more intense light commonly encountered in military situations due to reaction time delays in such a system. 
         [0005]    It would be advantageous to equip imaging devices with protection systems that can block even the highest-intensity light before it damages the image sensor. 
       SUMMARY 
       [0006]    The subject matter disclosed in detail below is directed to systems and methods for preventing high-radiant-flux light, such as laser light or a nuclear flash, from causing harm to imaging devices, such as a camera or telescope. In response to detection of high-radiant-flux light, the proposed systems share the common feature that a shutter is closed sufficiently fast that light from the source will be blocked from reaching the focal plane of the imaging device. Most of the proposed systems include a folded optical path to increase the allowable reaction time for closing the shutter. 
         [0007]    One aspect of the subject matter disclosed in detail below is an imaging device comprising: a laser sensor configured to output an activation signal in response to impingement thereon of light having a radiant flux greater than a specified threshold; an image sensor comprising a multiplicity of elements that convert impinging light to electrical signals; a first path-bending optical component disposed along an optical path that extends from a point in a vicinity of the laser sensor to the image sensor; a first shutter disposed along a portion of the optical path that extends from the first path-bending optical component to the image sensor; and a signal line connected to carry the activation signal from the laser sensor to the first shutter. The laser sensor, the signal line and the first shutter can be configured so that in response to some light and other light, both having a radiant flux greater than the specified threshold, concurrently arriving at the laser sensor and a starting point of the optical path respectively, the first shutter will become opaque prior to the other light impinging thereon in response to receipt of the activation signal from the laser sensor via the signal line. The optical path is configured to produce a time-of-flight delay for light traveling from the vicinity of the laser sensor to the first shutter, and the laser sensor, the signal line, and the first shutter are configured to produce a shutter delay from the time a high-radiant-flux arrives at the laser sensor to the time the first shutter becomes opaque, wherein the time-of-flight delay is greater than the shutter delay. The imaging device may further comprise a second shutter disposed along a portion of the optical path that extends from the first path-bending optical component to the first shutter. In accordance with one embodiment, the first shutter comprises an electro-optical shutter and the second shutter comprises a mechanical shutter. 
         [0008]    Another aspect of the subject matter disclosed in detail below is an instrument comprising: a laser sensor configured to output an activation signal in response to impingement thereon of light having a radiant flux greater than a specified threshold; a first path-bending optical component disposed along an optical path that extends from a point in a vicinity of the laser sensor to a focal plane of the instrument; a shutter disposed along a portion of the optical path that extends from the first path-bending optical component to the focal plane; and a signal line connected to carry the activation signal from the laser sensor to the shutter. The instrument may further comprise second, third and fourth path-bending optical components, wherein the second path-bending optical component is disposed along a portion of the optical path that extends from the first path-bending optical component to the focal plane, the third path-bending optical component is disposed along a portion of the optical path that extends from the second path-bending optical component to the focal plane, and the fourth path-bending optical component is disposed along a portion of the optical path that extends from the third path-bending optical component to the focal plane. 
         [0009]    A further aspect is an imaging device comprising: a laser sensor configured to output an activation signal in response to impingement thereon of light having a radiant flux greater than a specified threshold; an image sensor comprising a multiplicity of elements that convert impinging light to electrical signals; means for increasing a time-of-flight of light along an optical path that extends from a point in a vicinity of the laser sensor to the image sensor; a shutter disposed along a portion of the optical path that extends from the volume of substance having a high index of refraction to the image sensor; and a signal line connected to carry the activation signal from the laser sensor to the first shutter. In some embodiments, the structure that performs the function of increasing a time-of-flight of light along an optical path comprises a volume of substance having a high index of refraction. In other embodiments, the structure that performs the function of increasing a time-of-flight of light along an optical path comprise one or more reflective surfaces. 
         [0010]    Yet another aspect of the subject matter disclosed below is a method comprising: (a) detecting a radiant flux entering an optical instrument that has above a specified threshold; (b) when the specified threshold is reached or exceeded, sending an activation signal via a signal line to a shutter disposed inside the optical instrument; (c) delaying the arrival of the entering light at a shutter inside the optical instrument by an amount of time equal to a time-of-flight delay; and (d) in response to sending of the activation signal, the shutter becomes opaque at a time which is subsequent to a time when step (a) occurs by a total shutter delay, wherein the time-of-flight delay is greater than the total shutter delay. 
         [0011]    Other aspects of systems and methods for the protection of imaging devices against high-radiant-flux light are disclosed below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a diagram representing an imaging device that is focusing incoming light onto a focal plane, thereby increasing the light&#39;s intensity. 
           [0013]      FIG. 2  is a diagram representing an imaging device equipped with a shutter that can be activated to close when high-radiant-flux light is detected by a laser sensor. 
           [0014]      FIG. 3A  is a graph of light intensity at a focal plane of an imaging device versus time for two laser attacks of differing intensities I 1  and I 2 . 
           [0015]      FIG. 3B  is a graph of temperature at a focal plane of an imaging device versus time for two laser attacks that would produce the respective intensity profiles depicted in  FIG. 3A . 
           [0016]      FIG. 4  is a diagram depicting structural and functional aspects of an imaging device equipped with a protection system having one shutter in accordance with a first embodiment. 
           [0017]      FIG. 5  is a diagram depicting structural and functional aspects of an imaging device equipped with a protection system having a slower mechanical shutter “upstream” from a faster electro-optical shutter in accordance with a second embodiment. 
           [0018]      FIG. 6  is a graph of temperature at the focal plane versus time for the imaging device depicted in  FIG. 5 . 
           [0019]      FIG. 7  is a diagram depicting structural and functional aspects of a telescope having a Cassegrain reflector, which telescope is further equipped with a protection system having a shutter near the secondary mirror in accordance with a third embodiment. 
           [0020]      FIG. 8  is a diagram depicting structural and functional aspects of a telescope having a Cassegrain reflector, which telescope is further equipped with a protection system having a shutter behind the primary mirror in accordance with a fourth embodiment. 
           [0021]      FIG. 9  is a diagram depicting structural and functional aspects of a Newtonian telescope equipped with a protection system having a shutter near the laser sensor in accordance with a fifth embodiment. 
           [0022]      FIG. 10  is a diagram depicting structural and functional aspects of an imaging device equipped with a protection system having a shutter in accordance with a sixth embodiment in which multiple path-bending optics create a very long optical path. 
           [0023]      FIG. 11  is a diagram depicting structural and functional aspects of an imaging device equipped with a protection system having a shutter in accordance with a seventh embodiment in which the optical path is effectively lengthened by a substance with high index of refraction. 
           [0024]      FIG. 12  is a diagram depicting structural and functional aspects of a periscope equipped with a protection system having a shutter and an extended optical path in accordance with an eighth embodiment. 
       
    
    
       [0025]    Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals. 
       DETAILED DESCRIPTION 
       [0026]    Illustrative embodiments of a protection system are described in some detail below. However, not all features of an actual implementation are described in this specification. A person skilled in the art will appreciate that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0027]    By design, an imaging device uses a lens or mirror to focus light onto an image sensor.  FIG. 1  depicts an imaging device  10   a  that has a lens  12 , an image sensor  14 , and a housing  16 . The housing  16  has an aperture through which incident light propagates on its way to the focal plane of the imaging device  10   a.  The arrows in  FIG. 1  indicate optical paths  18  of respective incoming light rays  18  that are being focused by the lens  12  onto a portion of the image sensor  14 , thereby increasing the light&#39;s intensity. (Other light rays are not shown to avoid clutter in the drawing.) This greatly increases the intensity on pixels corresponding to the location of the high-radiant-flux light source in the image. Thus, light that is harmless to a structural surface may have damaging intensity at the image sensor  14 . 
         [0028]    One approach to solving this problem is depicted in  FIG. 2 , which shows an imaging device  10   b  that has a lens  12 , an image sensor  14 , a laser sensor  20 , and a shutter  22 , all of which components may be attached directly or indirectly (by support means not shown) to a housing  16 . The laser sensor  20  detects the presence of high-radiant-flux light in the imaging device&#39;s field of view. The laser sensor  20  transmits an activation signal via a signal line  24  to the shutter  22  when high-radiant-flux light is detected. In response to receipt of that activation signal, the shutter  22  becomes opaque, thereby blocking at least some of the incoming light from reaching the image sensor  14  disposed at the focal plane of the imaging device  10   b.    
         [0029]    The approach depicted in  FIG. 2  may be insufficient for blocking the intense light commonly encountered in military situations. The reason for this insufficiency lies in the following three delays: (1) the laser sensor response time Δt sensor  (any laser sensor requires a non-zero time to detect the arrival of high-radiant-flux light and send a signal); (2) the signal transit time Δt transit  (the signal must travel from the laser sensor  20  to the shutter  22 ; it cannot travel faster than the speed of light, 0.3 meter per nanosecond); and (3) the shutter response time Δt response  (no shutter can close instantly; it requires a nonzero reaction time to become opaque after a signal arrives; the shorter the reaction time, the more costly the shutter). These delays can be summed to produce a single value Δt shutter , which is the total shutter delay from when high-radiant-flux light arrives at the laser sensor  20  to when the shutter  22  becomes opaque. 
         [0030]      FIG. 3A  is a graph of light intensity at an image sensor versus time for two laser attacks of differing intensities I 1  and I 2 .  FIG. 3B  is a graph of temperature at the same image sensor versus time for the same laser attacks. 
         [0031]    The plot labeled I 1  in  FIG. 3A  represents a relatively low-intensity attack beginning at time t 0 . The plot labeled T 1 (t) in  FIG. 3B  represents the corresponding temperature at the focal plane. Starting at t 0 , the temperature rises from an initial value T 0  toward the damage threshold T damage . Before it reaches that threshold, the shutter  22  closes at time t close . The temperature stops rising, and the image sensor  14  survives. 
         [0032]    The plot labeled I 2  in  FIG. 3A  represents a higher-intensity attack. The plot labeled T 2 (t) in  FIG. 3B  represents the corresponding temperature at the focal plane. As above, the temperature rises from the initial value T 0 , but given the higher intensity I 2 , the temperature rises faster than in the weak attack. The temperature exceeds the damage threshold T damage  before the shutter  22  closes at t close . In this event, the image sensor  14  may be damaged or destroyed. In contrast, the protection systems described in detail below can block even the highest-intensity light before it damages the focal plane. 
         [0033]      FIG. 4  depicts structural and functional aspects of an imaging device  10   c  equipped with a protection system in accordance with a first embodiment. The imaging device  10   c  comprises the following elements: a lens  12  (or other image-forming optics), an image sensor  14  (or other image sensor); a mirror  28  (or other path-bending optic(s); a laser sensor  20 ; a shutter  22  (electro-optical, mechanical, etc.); a signal line  24  (a wire, coaxial cable, or optical fiber); a housing  16   a  having an aperture; and a baffle  26 . As shown in  FIG. 4 , the mirror  28  reflects light from the lens  12  onto the image sensor  14 . The shutter  22 , when closed, blocks the light&#39;s path to protect the image sensor  14 . The optical path length from a point near the laser sensor  20  to the shutter  22  via the mirror  28  is much longer than the length of the signal line  24  from the laser sensor  20  to the shutter  22 . The elongated housing  16   a  surrounds and protects the components and the optical path. Baffle  26  blocks any light scattered by diffraction, dust or other blemishes on lens  12  from reaching shutter  22  via any path with an optical path length less than the optical path length from a point near the laser sensor  20  to the shutter  22  via the mirror  28   
         [0034]    In accordance with the embodiment depicted in  FIG. 4 , the process for protecting the imaging device  10   c  from incoming high-radiant-flux light includes the following major steps: 
         [0035]    (1) The laser sensor  20  (placed near the aperture) detects light having a radiant flux above a specified threshold. 
         [0036]    (2) When the specified threshold is reached or exceeded, the laser sensor  20  sends an activation signal (electrical or optical) via the signal line  24  to the shutter  22 . 
         [0037]    (3) Meanwhile, the high-radiant-flux light travels by a long optical path through the housing  16   a.  A first portion  18   a  of that optical path (indicated by a first arrow in  FIG. 4  and having a length Δs 1 ) extends from a point in the vicinity of the laser sensor  20  to the mirror  28 ; a second portion  18   b  of that optical path (indicated by a second arrow in  FIG. 4  and having a length Δs 2 ) extends from the mirror  28  to the shutter  22 . 
         [0038]    (4) In response to receipt of the activation signal from the laser sensor  20 , the shutter  22  becomes opaque. 
         [0039]    (5) When the high-radiant-flux light reaches the shutter  22  (now in an opaque state), enough of the incident light can be blocked to prevent damage to the image sensor  14 . 
         [0040]    (6) The laser sensor  20  detects a return to light levels below the specified threshold and sends a de-activation signal which causes the shutter  22  to open (i.e., become transparent). 
         [0041]    In operation, the optical path length Δs is long enough to impose a time-of-flight delay Δs/c, where c is the speed of light, and this time-of-flight delay is greater than the total shutter delay Δt shutter . That is, 
         [0000]      Δ s/c&gt;Δt   shutter    (1)
 
         [0000]    For a two-legged optical path as shown in  FIG. 4 , and showing all elements of Δt shutter , this equation becomes: 
         [0000]      (Δ s   1   +Δs   2 )/ c&gt;Δt   sensor   +Δt   transit   +Δt   response    (2)
 
         [0000]    where Δs 1  is the optical path length from a point in the vicinity of the laser sensor  20  to the mirror  28 , and Δs 2  is the optical path length from the mirror  28  to the shutter  22 . 
         [0042]    The common elements of the various imaging devices disclosed herein will be described more generally in the following individually entitled sub-sections. 
       Image Sensor 
       [0043]    The image sensor  14  may comprise a staring focal plane array, such as a charge coupled device (CCD) sensitive to visible or infrared wavelengths. In the alternative, it can be a single-pixel camera (compressive imaging system), an imaging photomultiplier, a vidicon tube, a photochemical film, or others. 
       Image-Forming Optics 
       [0044]    The image-forming optics may comprise a lens, a mirror, or a combination thereof that focuses light on the image sensor to create an image. It may be a single optical element such as the lens  12  shown in  FIG. 4 , or a multi-element system such as an achromatic lens, a Newtonian mirror system, or a Schmidt-Cassegrain lens-mirror combination. 
       Path-Bending Optics 
       [0045]    The path-bending optics may comprise an optical element, such as a mirror  28 , that changes the direction of light. Preferably, the direction is changed by at least 90°. This includes light that forms the image and light that can damage the image sensor. Changing the light&#39;s direction allows the path length of the light to be much greater than the path length for the shutter signal. Besides a single flat mirror  28  as shown in  FIG. 4 , this optical element may be a reflecting prism, multiple mirrors, or combinations thereof. In some embodiments (described n more detail later), the mirror is curved and is part of the image-forming optics. 
       Laser Sensor 
       [0046]    The laser sensor is a photosensitive electronic device that has roughly the same field of view as the imaging device. When a sufficiently high-radiant-flux light appears in its field of view, the laser sensor transmits a signal quickly—typically in less than a nanosecond. As seen in  FIG. 4 , preferably the laser sensor  20  is positioned near the shutter  22  so the signal line  24  from the laser sensor  20  to the shutter  22  will be short. The laser sensor  20  is typically much less sensitive than the image sensor  14 , as it only needs to respond to high-radiant-flux light. It can also survive exposure to light having a higher radiant energy than what the image sensor  14  can be exposed to. A typical laser sensor comprises a processor having a thresholding function, a photodetector, and a lens or other focusing element to provide directionality. 
       Shutter 
       [0047]    The shutter  22  is a device that has two states. In one state, the shutter  22  at least partially blocks the passage of light. In the other state, the shutter  22  allows the passage of light. The shutter  22  may comprise crossed polarizers surrounding a fast-acting magneto-optical or electro-optical device such as a Pockels cell, a Kerr cell, a Faraday modulator, or an active-matrix liquid-crystal grid (similar to the technology used in liquid crystal displays). To give the shutter  22  as much time as possible to receive the activation signal and to respond, the shutter  22  is typically adjacent to the image sensor  14  and as close as possible to the laser sensor  20 . 
       Signal Line 
       [0048]    As depicted in  FIG. 4 , the signal line  24  carries a signal from the laser sensor  20  to the shutter  22 . The signal line  24  is configured to take as direct a route as possible from laser sensor  20  to shutter  22 . For an optical signal, the signal line  24  may be optical fiber (signal speed ˜2×10 8  m/s), or it may be a free-space path (perhaps shielded by a hollow tube) along which the signal moves at 3×10 8  m/s. For an electrical signal, the signal line  24  is configured to have minimal inductance and capacitance per unit length to achieve the highest possible signal speed. 
       Housing 
       [0049]    Time must be given for the shutter  22  to receive the activation signal from the laser sensor  20  and to change state. Elongating the path that the light takes to reach the image sensor  14  allows this to occur. There are multiple housing designs which can be used. The key element is the distance the light must travel versus the distance the signal must travel. At 0.3 meter per nanosecond, a two-meter path gives six nanoseconds of delay. The housing  16   a  shown in  FIG. 4  could be designed to achieve a two-meter path in one meter of length by positioning the mirror  28  one meter from each of the laser sensor  20  and the shutter  22 . 
         [0050]    Alternative embodiments of imaging devices equipped with protection systems will now be described in detail for the purpose of illustrating manifold ways in which the concept disclosed herein may be reduced to practice. 
         [0051]    In some alternative embodiments, there is a second shutter which is mechanical. A mechanical shutter is too slow to act before the high-radiant-flux light reaches the image sensor, but once closed, it blocks 100% of the light. Affordable electro-optical shutters typically do not block 100% of the light, so some alternative embodiments use both types of shutters in tandem: the electro-optical shutter acts quickly to block most of the light, and the mechanical shutter subsequently blocks the rest of light. 
         [0052]      FIG. 5  depicts structural and functional aspects of an imaging device  10   d  equipped with a protection system in accordance with a second embodiment. In this embodiment, the protection system comprises a slower mechanical shutter  22   a  “upstream” from a faster electro-optical shutter  22   b.  The mechanical shutter  22   a  is tougher (i.e., more rugged) than the electro-optical shutter  22   b,  so as shown in  FIG. 5 , the mechanical shutter  22   a  is positioned to protect the weaker, more costly electro-optical shutter  22   b  from prolonged exposure to high-radiant-flux light. 
         [0053]      FIG. 6  shows how adding a mechanical shutter  22   a  helps protect the image sensor  14 . (The signal line from the laser sensor  20  to the mechanical shutter  22   a  is not shown to avoid clutter in the drawing.) Given that the total shutter delay for the electro-optical shutter  22   b  is Δt shutter   _   1 , the electro-optical shutter  22   b  closes at time t close   _   1 , but since it does not block 100% of the light, the temperature of the image sensor  14  (or other image sensor) continues to rise slowly. Given that the total shutter delay Δt shutter   _   2  for the mechanical shutter  22   a  is longer than the total shutter delay Δt shutter   _   1  for the electro-optical shutter  22   b,  the mechanical shutter  22   a  closes at time t close   _   2 , which is later than the time t close   _   1  when the electro-optical shutter  22   b  closed. The closed mechanical shutter blocks 100% of light, so that the temperature at the image sensor  14  (or other image sensor) rises no further (i.e., does not reach the temperature T damage  at which damage might occur). 
         [0054]    Many imaging systems use a Cassegrain optical configuration. A Cassegrain reflector is a combination of a concave primary mirror and a convex secondary mirror, often used in optical telescopes. In a symmetrical Cassegrain reflector, both mirrors are aligned about the optical axis, and the primary mirror usually contains a hole in the centre, thus permitting the light to reach an eyepiece, a camera, or a light detector.  FIG. 7  depicts structural and functional aspects of a Cassegrain imaging device  10   e  (e.g., a telescope having a Cassegrain reflector) equipped with a protection system having a shutter  22  near the secondary mirror  32 .  FIG. 7  shows use of the Cassegrain primary mirror  30  as both a path-bending optical element and an image-forming optical element. The incoming light rays travel by respective long optical paths through the housing  16   b.  A first portion  18   a  of respective optical paths for two light rays extends from a point in the vicinity of the laser sensor  20  to the primary mirror  30 ; a second portion  18   b  of the respective optical paths for the two light rays extends from the primary mirror  30  to the secondary mirror  32 ; and a third portion  18   c  of the respective optical paths for the two light rays extends from the secondary mirror  32  to the image sensor  14 . 
         [0055]    A benefit of the embodiment depicted in  FIG. 7  is that light passes through the shutter  22  twice on its way to the image sensor  14 . This increases the effective opacity of the shutter  22 : a shutter  22  that blocks 90% of the light in a single pass blocks 99% of the light in a double pass. This allows an inexpensive shutter to work as well as a more expensive one. 
         [0056]    A disadvantage of the embodiment depicted in  FIG. 7  is that hardware around the shutter  22  may block some light, reducing the performance of the telescope.  FIG. 8  depicts an alternative that avoids this disadvantage. In this embodiment, a Cassegrain imaging device  10   f  is equipped with a protection system having a shutter  22  disposed behind the primary mirror  30  and in front of the image sensor  14 . Placing the shutter  22  behind the primary mirror  30  puts the extra shutter hardware out of the optical path. In some applications, this may be preferable despite losing the double-pass advantage provided by the embodiment depicted in  FIG. 7 . 
         [0057]      FIG. 9  depicts structural and functional aspects of a Newtonian imaging device  10   g  (e.g., a Newtonian telescope) equipped with a protection system having a shutter  22  near the laser sensor  20  in accordance with a further embodiment. Incoming light is reflected and focused by a concave primary mirror  30  onto a flat diagonal secondary mirror  32   a  the latter in turn reflects the focused beam onto the image sensor  14 . This optical configuration places the shutter  22  behind an aperture in the housing  16   c  and very close to the laser sensor  20  and keeps the extra shutter hardware out of the optical path. 
         [0058]      FIG. 10  depicts structural and functional aspects of an imaging device  10   h  equipped with a protection system having a shutter  22  in accordance with an alternative embodiment in which multiple path-bending optics create a very long optical path. The imaging device  10   h  comprises the following elements: a lens  12 , an image sensor  14 , mirrors  28   a  and  28   b,  a laser sensor  20 , a shutter  22 , a signal line  24 , a housing  16   d  having an aperture, and a pair of baffles  26   a  and  26   b.  As indicated by arrows in  FIG. 4 , the first mirror  28   a  reflects light from the lens  12  onto the mirror  28   b;  the mirror  28   b  reflects light from mirror  28   a  back onto mirror  28   a;  and the mirror  28   a  reflects light from mirror  28   b  toward the shutter  22 . The shutter  22 , when opaque, at least partially blocks the light&#39;s path to protect the image sensor  14 . The optical path length from a point near the laser sensor  20  to the shutter  22  is much longer than the length of the signal line  24  from the laser sensor  20  to the shutter  22 . More specifically, the incoming laser light travels by a long optical path through the housing  16   d.  A first portion  18   a  of that optical path (indicated by a first arrow in  FIG. 10 ) extends from a point in the vicinity of the laser sensor  20  to the mirror  28   a;  a second portion  18   b  of that optical path (indicated by a second arrow) extends from the mirror  28   a  to the mirror  28   b;  a third portion  18   c  of that optical path (indicated by a third arrow) extends from the mirror  28   b  to the mirror  28   a;  and finally a fourth portion  18   d  of that optical path (indicated by a fourth arrow) extends from the mirror  28   a  to the shutter  22 . 
         [0059]    In the embodiments depicted in  FIGS. 4, 5 and 7-9 , the imaging device has one or two major bends in the optical path. The embodiment depicted in  FIG. 10  use multiple path-bending optics to create very long optical paths in limited physical space. For simplicity of illustration, the version shown here keeps the optical path roughly in a single plane and has non-crossing legs in the optical paths. More complicated embodiments minimize the device&#39;s volume by having optical path legs that cross, e.g., in a star pattern or a three-dimensional mesh. Multiple reflecting surfaces can introduce substantial optical errors, so some embodiments use adaptive optics between the shutter and the image sensor to correct any errors. 
         [0060]    In the previously described embodiments, the apparatus included a folded optical path. Another way to delay the arrival of high-radiant-flux light at an image sensor is to use a substance having a high index of refraction to delay the time-of-flight. 
         [0061]      FIG. 11  depicts structural and functional aspects of an imaging device  10   i  in accordance with a further alternative embodiment. In this configuration, a portion of the optical path  18  is filled with a transparent substance  34  (solid, liquid, or gas) that has a high index of refraction n. That is, the speed of light in the material is slowed by a factor of, say, 1.33 (water) to as much as 4.0 (germanium, used in long-wave infrared imagers) or higher (exotic substances such as Bose-Einstein condensate). Thus the optical path is effectively lengthened by the high-index substance  34  disposed inside the housing  16   e.  Signals propagating along the signal line  24  are not slowed, so they can travel as fast as the speed of light in vacuum. Given an optical path of sufficient length, the activation signal reaches the shutter  22  well in advance of the high-radiant-flux light. The shutter  22  becomes opaque (i.e., closes) before the light reaches it whenever the following relation is true: 
         [0000]        nΔs/c&gt;Δt   shutter    (3)
 
         [0062]    More generally, the optical path may comprise multiple legs, each leg i having length Δs i  and index of refraction n i . In this case, the appropriate relation is: 
         [0000]      (Σ n   i   Δs   i )/ c&gt;Δt   shutter    (4)
 
         [0000]    where Σ denotes a sum over all legs. 
         [0063]    Some embodiments can use both a folded optical path and a path that is at least partially filled with a high-index substance. In addition, when a path-bending element is a prism, the refractive index of the prism is at least 1.3, so light traveling through it incurs a substantial delay. In some embodiments, the prism is designed to have a large internal path length and to incorporate material with unusually high index of refraction. 
         [0064]    The embodiments described above each have a shutter between the image-forming optics and the image sensor. In some embodiments, typically those where the image-forming optics have a short focal length, the image-forming optics may be between the shutter and the image sensor (i.e., “downstream” of the shutter). The path-bending optics and the laser sensor would remain “upstream” of the shutter. 
         [0065]    A special case of putting the image-forming optics “downstream” of the shutter is an embodiment that protects a human observer, i.e., the image-forming optics and the image sensor are both part of a human eye. Prior art includes many forms of periscope: those used in submarines, in armored turrets on tanks, fortresses, and naval vessels, in trench warfare, and in covert surveillance by police.  FIG. 12  is a diagram depicting structural and functional aspects of a periscope  8  equipped with a protection system having a shutter  22  placed in front of the eye of a human observer  6  and further having an extended optical path. The extended optical path through the periscope  8  allows shutter  22  to close before high-radiant-flux light reaches the eye of the human observer  6 . 
         [0066]    In the embodiment depicted in  FIG. 12 , the regular periscope structure is extended below the viewer&#39;s eye to increase the optical path. (Other parts of the periscope optics are omitted for clarity.) This longer path allows the signal from the laser sensor  20  to reach the shutter  22  before the high-radiant-flux light reaches the shutter  22 . A first portion  18   a  of that optical path (indicated by a first arrow in  FIG. 12 ) extends from a point in the vicinity of the laser sensor  20  to the mirror  36 ; a second portion  18   b  of that optical path (indicated by a second arrow) extends from the mirror  36  to a first facet of a prism  38 ; a third portion  18   c  of that optical path (indicated by a third arrow) extends from the first facet of prism  38  to a second facet of prism  38 ; and finally a fourth portion  18   d  of that optical path (indicated by a fourth arrow) extends from the mirror  28   a  to the shutter  22 . 
         [0067]    In other embodiments (not shown), the forward opening of the periscope can be at the same height as the human observer. In these embodiments, the point is not to see over an obstacle, but rather simply to protect the observer&#39;s eyes from high-radiant-flux laser light or other high-radiant-flux light. 
         [0068]    Alternative criteria for shutter closure may be employed. The activation threshold may be based on radiant intensity (e.g., watts per steradian), irradiance (e.g., watts per square meter), or radiance(e.g., watts per steradian per square meter). Slightly different laser sensors can be employed depending on whether radiance or irradiance is being detected. To detect irradiance, the laser sensor may comprise a single photodetector. To detect radiance, the laser sensor should comprise a focusing element, a photosensitive chip with multiple pixels, and a processor that issues the activation signal when the amount of light on one pixel exceeds the threshold. 
         [0069]    While imaging devices and instruments having means for laser protection have been described with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the teachings herein. In addition, many modifications may be made to adapt the concepts and reductions to practice disclosed herein to a particular situation. Accordingly, it is intended that the subject matter covered by the claims not be limited to the disclosed embodiments. 
         [0070]    The process claims set forth hereinafter should not be construed to require that the steps recited therein be performed in alphabetical order (any alphabetical ordering in the claims is used solely for the purpose of referencing previously recited steps) or in the order in which they are recited. Nor should they be construed to exclude any portions of two or more steps being performed concurrently or alternatingly.