Patent Description:
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.

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.

One approach to solving this problem is using a laser sensor to detect the presence of high-radiant-flux light in the imaging device'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.

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.

In accordance with its abstract, <CIT> states that in an optical viewing device means are provided to protect the observer against the blinding effects of intense light produced, e.g. by laser beams or atomic explosions, the means comprising an inertia free shutter in the form of a Kerr cell interposed in the optical path. A periscope comprises an objective lens through which light rays enter and travel along an optical path to an eyepiece lens, the optical path including, in succession, a prism, a bundle of optical fibers to produce a time delay of about twenty nanoseconds, a prism, a lens, a polarizer, a Kerr cell and an analyzer. Light rays from the same source as the mentioned light rays pass through lens, the prism, and the further lens to a photo-cell, the output of the photo-cell passing through a wideband amplifier and a rectifier to an output amplifier, the output amplifier being biased by a voltage at a terminal to establish a threshold operating level. When the threshold is exceeded the output amplifier applies a voltage through wires across the capacitor plates of the Kerr cell, causing rotation of the plane of polarizaion of the polarized light and thereby varying the amount of light transmitted by the analyzer. Above a certain light intensity the light transmission is zero. The optical delay means provide the necessary time for the optical shutter to be operated before the light reaches the observer's eyes. In a modification the optical delay means comprise multiple reflections from a pair of opposed mirrors. In another modification intended for protection against laser beams exhibiting a series of pulses, a pulse stretcher and an OR gate are interposed between the amplifiers to ensure that the shutter is kept closed for a period of time considerably longer than the duration of a single pulse.

In accordance with its abstract <CIT> states that a direct vision periscope has a periscope base unit with a rotatable observation head, an eyepiece box and an optical transmission section which is arranged in a tubular housing and forms an eyepiece branch. The periscope base unit and the eyepiece box are connected to one another via the eyepiece branch for direct optical transmission of a scene image to the eyepiece box. The eyepiece branch is articulated and variable in length by means of at least one biaxial articulation device and axial length compensation device. The rotation point of the articulation device is at the focal point of an intermediate image, and the axial length compensation is of a parallel beam path in the optical transmission section. The periscope may be used in a land vehicle, in particular a tank.

In accordance with its abstract <CIT> states an anti-flash shutter system for optical instruments of the type having means for receiving and reflecting light into an optical path terminating in a viewing eyepiece. Included serially in the optical path is a Kerr cell or like electro optical shutter, an objective lens, a high-speed electrically-operated mechanical shutter and a lens system.

The claimed invention is directed to an imaging device according to claim <NUM> and a method according to claim <NUM>.

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.

According to the claimed invention, an imaging device is provided, 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 comprises a second shutter disposed along a portion of the optical path that extends from the first path-bending optical component to the first shutter. The first shutter comprises an electro-optical shutter and the second shutter comprises a mechanical shutter.

The invention also related to an instrument comprising the claimed imaging device, including inter alia: 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.

The invention also relates to an imaging device as claimed comprising inter alia: 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.

The claimed invention also relates to a method comprising: (a) detecting light entering an optical instrument comprising the claimed imaging device according to the above that has a radiant flux above a specified threshold; (b) when the specified threshold is reached or exceeded, sending an activation signal via a signal line to a first shutter disposed inside the optical instrument; (c) delaying the arrival of the entering light at the first 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 first 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. The time-of-flight delay is greater than the total shutter delay. A second shutter is used in tandem with the first shutter. The first shutter comprises an electro-optical shutter and the second shutter comprises a mechanical shutter.

Other aspects of systems and methods for the protection of imaging devices against high-radiant-flux light according to the present invention are disclosed below.

Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals.

By design, an imaging device uses a lens or mirror to focus light onto an image sensor. <FIG> depicts an imaging device 10a that has a lens <NUM>, an image sensor <NUM>, and a housing <NUM>. The housing <NUM> has an aperture through which incident light propagates on its way to the focal plane of the imaging device 10a. The arrows in <FIG> indicate optical paths <NUM> of respective incoming light rays <NUM> that are being focused by the lens <NUM> onto a portion of the image sensor <NUM>, thereby increasing the light'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 <NUM>.

One approach to solving this problem is depicted in <FIG>, which shows an imaging device 10b that has a lens <NUM>, an image sensor <NUM>, a laser sensor <NUM>, and a shutter <NUM>, all of which components may be attached directly or indirectly (by support means not shown) to a housing <NUM>. The laser sensor <NUM> detects the presence of high-radiant-flux light in the imaging device's field of view. The laser sensor <NUM> transmits an activation signal via a signal line <NUM> to the shutter <NUM> when high-radiant-flux light is detected. In response to receipt of that activation signal, the shutter <NUM> becomes opaque, thereby blocking at least some of the incoming light from reaching the image sensor <NUM> disposed at the focal plane of the imaging device 10b.

The approach depicted in <FIG> may be insufficient for blocking the intense light commonly encountered in military situations. The reason for this insufficiency lies in the following three delays: (<NUM>) the laser sensor response time Δtsensor (any laser sensor requires a non-zero time to detect the arrival of high-radiant-flux light and send a signal); (<NUM>) the signal transit time Δttransit (the signal must travel from the laser sensor <NUM> to the shutter <NUM>; it cannot travel faster than the speed of light, <NUM> meter per nanosecond); and (<NUM>) the shutter response time Δtresponse (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 Δtshutter, which is the total shutter delay from when high-radiant-flux light arrives at the laser sensor <NUM> to when the shutter <NUM> becomes opaque.

<FIG> is a graph of light intensity at an image sensor versus time for two laser attacks of differing intensities I<NUM> and I<NUM>. <FIG> is a graph of temperature at the same image sensor versus time for the same laser attacks.

The plot labeled I<NUM> in <FIG> represents a relatively low-intensity attack beginning at time t<NUM>. The plot labeled T<NUM>(t) in <FIG> represents the corresponding temperature at the focal plane. Starting at t<NUM>, the temperature rises from an initial value T<NUM> toward the damage threshold Tdamage. Before it reaches that threshold, the shutter <NUM> closes at time tclose. The temperature stops rising, and the image sensor <NUM> survives.

The plot labeled I<NUM> in <FIG> represents a higher-intensity attack. The plot labeled T<NUM>(t) in <FIG> represents the corresponding temperature at the focal plane. As above, the temperature rises from the initial value T<NUM>, but given the higher intensity I<NUM>, the temperature rises faster than in the weak attack. The temperature exceeds the damage threshold Tdamage before the shutter <NUM> closes at tclose. In this event, the image sensor <NUM> 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.

<FIG> depicts structural and functional aspects of an imaging device 10c equipped with an illustrative protection system useful for understanding the claimed invention. The imaging device 10c comprises the following elements: a lens <NUM> (or other image-forming optics), an image sensor <NUM> (or other image sensor); a mirror <NUM> (or other path-bending optic(s); a laser sensor <NUM>; a shutter <NUM> (electro-optical, mechanical, etc.); a signal line <NUM> (a wire, coaxial cable, or optical fiber); a housing 16a having an aperture; and a baffle <NUM>. As shown in <FIG>, the mirror <NUM> reflects light from the lens <NUM> onto the image sensor <NUM>. The shutter <NUM>, when closed, blocks the light's path to protect the image sensor <NUM>. The optical path length from a point near the laser sensor <NUM> to the shutter <NUM> via the mirror <NUM> is much longer than the length of the signal line <NUM> from the laser sensor <NUM> to the shutter <NUM>. The elongated housing 16a surrounds and protects the components and the optical path. Baffle <NUM> blocks any light scattered by diffraction, dust or other blemishes on lens <NUM> from reaching shutter <NUM> via any path with an optical path length less than the optical path length from a point near the laser sensor <NUM> to the shutter <NUM> via the mirror <NUM>.

<FIG>, depicts the process for protecting the imaging device 10c from incoming high-radiant-flux light includes the following major steps:.

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 Δtshutter. That is, <MAT>.

For a two-legged optical path as shown in <FIG>, and showing all elements of Δtshutter, this equation becomes: <MAT> where Δs<NUM> is the optical path length from a point in the vicinity of the laser sensor <NUM> to the mirror <NUM>, and Δs<NUM> is the optical path length from the mirror <NUM> to the shutter <NUM>.

The image sensor <NUM> may comprise a staring focal plane array that includes a multiplicity of elements that convert impinging light to electrical signals, such as a charge coupled device (CCD) sensitive to visible or infrared wavelengths. In an alternative, it can be a single-pixel camera (compressive imaging system), an imaging photomultiplier, a vidicon tube, a photochemical film, or others.

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 <NUM> shown in <FIG>, or a multi-element system such as an achromatic lens, a Newtonian mirror system, or a Schmidt-Cassegrain lens-mirror combination.

The path-bending optics may comprise an optical element, such as a mirror <NUM>, that changes the direction of light. Preferably, the direction is changed by at least <NUM>°. This includes light that forms the image and light that can damage the image sensor. Changing the light'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 <NUM> as shown in <FIG>, this optical element may be a reflecting prism, multiple mirrors, or combinations thereof. The mirror can be curved and is part of the image-forming optics.

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>, preferably the laser sensor <NUM> is positioned near the shutter <NUM> so the signal line <NUM> from the laser sensor <NUM> to the shutter <NUM> will be short. The laser sensor <NUM> is typically much less sensitive than the image sensor <NUM>, 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 <NUM> 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.

The shutter <NUM> is a device that has two states. In one state, the shutter <NUM> at least partially blocks the passage of light. In the other state, the shutter <NUM> allows the passage of light. The shutter <NUM> 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 <NUM> as much time as possible to receive the activation signal and to respond, the shutter <NUM> is typically adjacent to the image sensor <NUM> and as close as possible to the laser sensor <NUM>.

As depicted in <FIG>, the signal line <NUM> carries a signal from the laser sensor <NUM> to the shutter <NUM>. The signal line <NUM> is configured to take as direct a route as possible from laser sensor <NUM> to shutter <NUM>. For an optical signal, the signal line <NUM> may be optical fiber (signal speed ~<NUM> x <NUM><NUM> m/s), or it may be a free-space path (perhaps shielded by a hollow tube) along which the signal moves at <NUM> x <NUM><NUM> m/s. For an electrical signal, the signal line <NUM> is configured to have minimal inductance and capacitance per unit length to achieve the highest possible signal speed.

Time must be given for the shutter <NUM> to receive the activation signal from the laser sensor <NUM> and to change state. Elongating the path that the light takes to reach the image sensor <NUM> 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 <NUM> meter per nanosecond, a two-meter path gives six nanoseconds of delay. The housing 16a shown in <FIG> could be designed to achieve a two-meter path in one meter of length by positioning the mirror <NUM> one meter from each of the laser sensor <NUM> and the shutter <NUM>.

According to the claimed imaging device 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 <NUM>% of the light. Affordable electro-optical shutters typically do not block <NUM>% of the light, so the claimed device uses 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.

<FIG> depicts structural and functional aspects of an imaging device 10d equipped with a protection system in accordance with the claimed invention. The protection system comprises a slower mechanical shutter 22a "upstream" from a faster electro-optical shutter 22b. The mechanical shutter 22a is tougher (i.e., more rugged) than the electro-optical shutter 22b, so as shown in <FIG>, the mechanical shutter 22a is positioned to protect the weaker, more costly electro-optical shutter 22b from prolonged exposure to high-radiant-flux light.

<FIG> shows how adding a mechanical shutter 22a helps protect the image sensor <NUM>. (The signal line from the laser sensor <NUM> to the mechanical shutter 22a is not shown to avoid clutter in the drawing. ) Given that the total shutter delay for the electro-optical shutter 22b is Δtshutter_1, the electro-optical shutter 22b closes at time tclose_1, but since it does not block <NUM>% of the light, the temperature of the image sensor <NUM> (or other image sensor) continues to rise slowly. Given that the total shutter delay Δtshutter_2 for the mechanical shutter 22a is longer than the total shutter delay Δtshutter_1 for the electro-optical shutter 22b, the mechanical shutter 22a closes at time tclose_2, which is later than the time tclose_1 when the electro-optical shutter 22b closed. The closed mechanical shutter blocks <NUM>% of light, so that the temperature at the image sensor <NUM> (or other image sensor) rises no further (i.e., does not reach the temperature Tdamage at which damage might occur).

Many imaging systems use a Cassegrain optical configuration. A Cassegrain reflector is a combination of a concave primary mirror <NUM> and a convex secondary mirror <NUM>, often used in optical telescopes. In a symmetrical Cassegrain reflector, both mirrors are aligned about the optical axis, and the primary mirror <NUM> usually contains a hole in the centre, thus permitting the light to reach an eyepiece, a camera, or a light detector. <FIG> depicts, for illustrative purposes, structural and functional aspects of a Cassegrain imaging device 10e (e.g., a telescope having a Cassegrain reflector) equipped with a protection system having a shutter <NUM> near the secondary mirror <NUM>. It is noted that the device as claimed utilizes a first shutter and a second shutter disposed along a portion of the optical path that extends from the first path-bending optical component to the first shutter, wherein the first shutter comprises an electro-optical shutter and the second shutter comprises a mechanical shutter. <FIG> shows use of the Cassegrain primary mirror <NUM> 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 16b. A first portion 18a of respective optical paths for two light rays extends from a point in the vicinity of the laser sensor <NUM> to the primary mirror <NUM>; a second portion 18b of the respective optical paths for the two light rays extends from the primary mirror <NUM> to the secondary mirror <NUM>; and a third portion 18c of the respective optical paths for the two light rays extends from the secondary mirror <NUM> to the image sensor <NUM>.

A benefit of the system as depicted in <FIG> is that light passes through the shutter <NUM> twice on its way to the image sensor <NUM>. This increases the effective opacity of the shutter <NUM>: a shutter <NUM> that blocks <NUM>% of the light in a single pass blocks <NUM>% of the light in a double pass. This allows an inexpensive shutter to work as well as a more expensive one.

Hardware around the shutter <NUM> may block some light, reducing the performance of the telescope. <FIG> depicts an alternative that avoids this.

As depicted in <FIG>, for illustrative purposes, a Cassegrain imaging device 10f is equipped with a protection system having a shutter <NUM> disposed behind the primary mirror <NUM> and in front of the image sensor <NUM>. It is noted that the device as claimed utilizes a first shutter and a second shutter disposed along a portion of the optical path that extends from the first path-bending optical component to the first shutter, wherein the first shutter comprises an electro-optical shutter and the second shutter comprises a mechanical shutter. Placing the shutter <NUM> behind the primary mirror <NUM> 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>.

<FIG> depicts, for illustrative purposes, structural and functional aspects of a Newtonian imaging device <NUM> (e.g., a Newtonian telescope) equipped with a protection system having a shutter <NUM> near the laser sensor <NUM>. It is noted that the device as claimed utilizes a first shutter and a second shutter disposed along a portion of the optical path that extends from the first path-bending optical component to the first shutter, wherein the first shutter comprises an electro-optical shutter and the second shutter comprises a mechanical shutter. Incoming light is reflected and focused by a concave primary mirror <NUM> onto a flat diagonal secondary mirror 32a the latter in turn reflects the focused beam onto the image sensor <NUM>. This optical configuration places the shutter <NUM> behind an aperture in the housing 16c and very close to the laser sensor <NUM> and keeps the extra shutter hardware out of the optical path.

<FIG> depicts, for illustrative purposes, structural and functional aspects of an imaging device <NUM> equipped with a protection system having a shutter <NUM> in which multiple path-bending optics create a very long optical path. It is noted that the device as claimed utilizes a first shutter and a second shutter disposed along a portion of the optical path that extends from the first path-bending optical component to the first shutter, wherein the first shutter comprises an electro-optical shutter and the second shutter comprises a mechanical shutter. The imaging device <NUM> comprises the following elements: a lens <NUM>, an image sensor <NUM>, mirrors 28a and 28b, a laser sensor <NUM>, a shutter <NUM>, a signal line <NUM>, a housing 16d having an aperture, and a pair of baffles 26a and 26b. As indicated by arrows in <FIG>, the first mirror 28a reflects light from the lens <NUM> onto the mirror 28b; the mirror 28b reflects light from mirror 28a back onto mirror 28a; and the mirror 28a reflects light from mirror 28b toward the shutter <NUM>. The shutter <NUM>, when opaque, at least partially blocks the light's path to protect the image sensor <NUM>. The optical path length from a point near the laser sensor <NUM> to the shutter <NUM> is much longer than the length of the signal line <NUM> from the laser sensor <NUM> to the shutter <NUM>. More specifically, the incoming laser light travels by a long optical path through the housing 16d. A first portion 18a of that optical path (indicated by a first arrow in <FIG>) extends from a point in the vicinity of the laser sensor <NUM> to the mirror 28a; a second portion 18b of that optical path (indicated by a second arrow) extends from the mirror 28a to the mirror 28b; a third portion 18c of that optical path (indicated by a third arrow) extends from the mirror 28b to the mirror 28a; and finally a fourth portion 18d of that optical path (indicated by a fourth arrow) extends from the mirror 28a to the shutter <NUM>.

In <FIG>, <FIG> and <FIG>, the imaging device has one or two major bends in the optical path. In <FIG> the invention uses 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. The device's volume can also be minimized 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.

As described here before, 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.

<FIG> also depicts structural and functional aspects of an illustrative imaging device 10i useful for understanding the claimed invention. In this configuration, a portion of the optical path <NUM> can be filled with a transparent substance <NUM> (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, <NUM> (water) to as much as <NUM> (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 <NUM> disposed inside the housing 16e. Signals propagating along the signal line <NUM> 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 <NUM> well in advance of the high-radiant-flux light. The shutter <NUM> becomes opaque (i.e., closes) before the light reaches it whenever the following relation is true: <MAT>.

More generally, the optical path may comprise multiple legs, each leg i having length Δsi and index of refraction ni. In this case, the appropriate relation is: <MAT> where Σ denotes a sum over all legs.

The invention uses a folded optical path and can optionally include 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 <NUM>, so light traveling through it incurs a substantial delay. The prism can bea prism designed to have a large internal path length and to incorporate material with unusually high index of refraction.

The invention can have a shutter between the image-forming optics and the image sensor. Alternatively, typically 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.

A special case of putting the image-forming optics "downstream" of the shutter is to protect 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> is a diagram depicting structural and functional aspects of a periscope <NUM> equipped with a protection system having a shutter <NUM> placed in front of the eye of a human observer <NUM> and further having an extended optical path. The extended optical path through the periscope <NUM> allows shutter <NUM> to close before high-radiant-flux light reaches the eye of the human observer <NUM>.

As depicted in <FIG>, for illustrative purposes, the regular periscope structure is extended below the viewer'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 <NUM> to reach the shutter <NUM> before the high-radiant-flux light reaches the shutter <NUM>. It is noted that the device as claimed utilizes a first shutter and a second shutter disposed along a portion of the optical path that extends from the first path-bending optical component to the first shutter, wherein the first shutter comprises an electro-optical shutter and the second shutter comprises a mechanical shutter. A first portion 18a of that optical path (indicated by a first arrow in <FIG>) extends from a point in the vicinity of the laser sensor <NUM> to the mirror <NUM>; a second portion 18b of that optical path (indicated by a second arrow) extends from the mirror <NUM> to a first facet of a prism <NUM>; a third portion 18c of that optical path (indicated by a third arrow) extends from the first facet of prism <NUM> to a second facet of prism <NUM>; and finally a fourth portion 18d of that optical path (indicated by a fourth arrow) extends from the mirror 28a to the shutter <NUM>.

The forward opening of the periscope can be at the same height as the human observer, whereby the point is not necessarily to see over an obstacle, but rather simply to protect the observer's eyes from high-radiant-flux laser light or other high-radiant-flux light.

Claim 1:
An imaging device (<NUM>) comprising:
a laser sensor (<NUM>) 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 (<NUM>) comprising a multiplicity of elements that convert impinging light to electrical signals;
a first path-bending optical component (<NUM>) disposed along an optical path (<NUM>) that extends from a point in a vicinity of said laser sensor (<NUM>) to said image sensor (<NUM>);
a first shutter (<NUM>) disposed along a portion of said optical path (<NUM>) that extends from said first path-bending optical component (<NUM>) to said image sensor (<NUM>); and
a signal line (<NUM>) connected to carry said activation signal from said laser sensor (<NUM>) to said first shutter (<NUM>), wherein
the device further comprising a second shutter (<NUM>) disposed along a portion of said optical path (<NUM>) that extends from said first path-bending optical component (<NUM>/<NUM>) to said first shutter (<NUM>), wherein said first shutter (<NUM>) comprises an electro-optical shutter (22b) and said second shutter comprises a mechanical shutter (22a).