Rotating shutter system for hostile laser source location technique

A hostile laser source location technique employs a television camera in conjunction with a rotating shutter to pinpoint the location of a hostile laser source without placing the pilot/operator at risk for eye damage. For an attack scenario the pilot/operator could then make changes in his tactics to negate the source or to continue his attack run. The shutter made of high optical density material (e.g., OD-4), except for an open aperture, provides optics protection for the camera system from both damage and dazzle. A detector is used to detect the laser pulse (flash) which immediately signals the proper frame to be frozen on a monitor. The laser source appears as a bright spot and is easily pinpointed.

DEDICATORY CLAUSE
 The invention described herein may be manufactured, used, and licensed by
 or for the Government for governmental purposes without the payment to us
 of any royalties thereon.
 BACKGROUND OF THE INVENTION
 Optical filters have been designed that attenuate sufficiently at a
 particular wavelength thus leaving the rest of the visible spectrum open.
 Since the use of filters greater than optical density-2 (OD-2) limits the
 performance of pilot/operator in bright sunlight conditions, it follows
 that the use of OD-2 filters in a low ambient light scenario would
 likewise limit the performance for non-instrumental surveillance at the
 latter conditions. Bright light in the visible region, (e.g., wavelength
 from about 397 micrometers or microns to about 723 micrometers or microns)
 and to the infrared region to the limit of perception of human eye (which
 is about 100 microns or 100 micrometers), emitting from a point source
 relates to radiation of a different nature than that which is emitting
 from a wide spectrum electromagnetic radiation source extending in many
 directions. Radiation from a point source such as radiation from a
 frequency agile laser is the subject matter which is covered in greater
 detail hereinbelow.
 Laser radiation particularly in the visible region has always presented a
 potential hazard to the human eye. More recently this potential has been
 under exploitation by military organizations both foreign and domestic.
 The human eye focuses light in the visible region which greatly increases
 the intensity of light on the retina from that striking the cornea and the
 lens.
 Certain laser beams are 10,000 times brighter so that the sun's rays, and
 much more hazardous. When a beam of laser radiation is absorbed by living
 tissue, the extent of damage caused is dependent on several things: the
 energy level of the radiation, the type of tissue irradiated, and the
 wavelength of the laser radiation, and the time of exposure to the
 radiation.
 Intense laser-energy when absorbed by the body is converted into heat. This
 heat coagulates the protein in the body's tissues (in a similar manner
 when boiling water coagulates egg albumin) and destroys the cells.
 The human eye is the most vulnerable tissue to all types of laser
 radiation. The tissue in the retina (that portion of the eye upon which
 the light or image is focused, and specifically the fovea of the retina)
 is particularly susceptible to damage because the lens of the eyeball
 concentrates and focuses the laser beam on the fovea of the retina.
 The interaction of a laser beam with eyeball tissue as received by the
 cornea and focused by the lens on the fovea of the retina is described as
 follows:
 1. As the laser beam impinges on the eyeball, part of the beam is prevented
 from entering the eye by the iris, a colored disc behind the cornea (the
 outwardly convex transparent membrane forming part of the anterior outer
 coat of the eye); the iris acts like an automatic photographic shutter and
 constricts when high-intensity light impinges on the eye;
 2. The shutter action of the iris prevents part of the light from reaching
 the retina;
 3. The part of light which reaches the retina also effects a thin,
 dark-brown choroid membrane containing arteries, veins, and pigment cells
 which surrounds the retina, and since this membrane is dark colored and
 can easily absorb radiation, it must therefore be protected;
 4. The laser beam is converged and focused on the fovea of the retina by
 the lens; and,
 5. As projected from laser-welding principles we know that the focus is the
 hottest point, therefore, the laser-energy density at the fovea is about
 10.sup.4 to 10.sup.6 times more concentrated than that received by the
 cornea and the lens. Hence, it is recognized why eye damage can occur due
 to this magnitude of light concentration.
 As concluded from the above described interactions, it is recognized why
 eye damage can occur due to this magnitude.
 Since the laser used as a weapon against the human eye, eyes behind optics,
 and optics themselves is considered a threat to U.S. forces, scientists
 have looked hard at contermeasure techniques. Protective goggles or
 glasses have been the only real product of research efforts. The bulk of
 the work has been in the types of lenses/filters to go in this eyewear.
 Filters have been designed that attenuate sufficiently at a particular
 wavelength thus leaving the rest of the visible spectrum open. This is a
 real advantage to the pilots or operators in performing their tasks. With
 the advent of frequency agile lasers however, it has become necessary to
 filter over the entire visible region. This presents a real handicap to
 the operators in performing their tasks particularly in a low ambient
 light scenario.
 Designing a filter in the form of goggles which filter over the entire
 visible region is not a practical approach. Another name for goggles which
 filter the complete visible spectrum is blindfolds. Thus, the limitation
 to an operator's performance is recognized, and it follows that instrument
 flight conditions would then be required; however, the ability to locate
 and pinpoint a source of light would be lost. Since OD-2 goggles pass
 1/100th of the light that strikes them which means that the agile laser
 threat could increase laser output by 100 times and thus negate the
 filters. Finally, the goggles allow for exposure of both eyes
 simultaneously. It follows that another approach is desired since complete
 protection of both eyes from a first laser pulse does not appear possible.
 Therefore, scientists had at this point in time overlooked an approach for
 protection from the first laser pulse but concentrated efforts towards
 providing protection from the second and subsequent laser pulses. The
 probability however, is in the operators favor of not being damaged by the
 first laser pulse in that he will not be looking directly at the laser
 source when the first pulse arrives or that the first pulse will be off
 target but still detectable. This scheme also protects the operator from
 instinctively looking towards the source when the next pulse arrives.
 Applicants' cross-referenced, concurrently filed application is designed
 with the emphasis to operator's eye protection. Protection of optics from
 damaging laser flashes is a desirable feature which is described below and
 which also prevents eye damage in location technique.
 The invention described hereinbelow provides optics protection while
 providing a technique to pinpoint the location of a hostile laser source.
 A system which provides protection from laser pulses including the first
 one while also avoiding the risk of damage to optics is a desirable
 enovation. The operator will also suffer no degradation of his performance
 due to insufficient light required if the optics (camera) employed to
 pinpoint the hostile laser source location is provided protection.
 An object of this invention is to provide a technique to pinpoint the
 location of a hostile laser source without placing the operator at risk
 for eye damage.
 Another object of this invention is to provide a technique to pinpoint the
 location of a hostile laser source which offers protection to the optics
 employed in the technique.
 Still a further object of this invention is to additionally provide a
 detector for a laser pulse (flash) which immediately causes the detected
 laser pulse to be frozen on a monitor whereby the laser source appears as
 a bright spot which is easily pinpointed.
 SUMMARY OF THE INVENTION
 A method of locating a hostile laser source which employs a television
 camera system in combination with a rotating shutter which provides optics
 protection for the camera system from both damage and dazzle. The rotating
 shutter is made of high optical density material (e.g., OD-4 material),
 except for an open aperature which permits the light sensitive portion of
 the optics of the camera system to receive a view of the vicinity without
 being exposed to a hostle laser beam. A multi-directional detector is used
 to detect a first laser pulse (flash) which immediately signals the proper
 frame to be frozen on a TV monitor, and the exact location is easily
 pinpointed.
 The rotating shutter system comprises a rotatable shutter having an open
 sector which is rotated at a predetermined rate in front of a TV camera to
 prevent flickering or dazzling of the field of view. The shutter is
 mounted to variable speed motor which controls the rate of rotation of the
 shutter and the open sector. A TV camera is mounted in a fixed
 relationship to the open sector of the rotating shutter. In operation, the
 TV camera and rotatable shutter system is in combination with a detector
 system for laser pulses over the visible range from the far infrared
 through to the near infrared. The detector system is in electrical
 communication with a control circuit, a frame grabber, TV camera, and TV
 monitor as illustrated in the Drawing and as described hereinabove to
 complete the desired functions.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 The method of locating a hostile laser source without placing the operator
 at risk for eye damage employs a detector system in electrical
 communication with a TV camera, control circuitry, frame grabber, and TV
 monitor. A rotating shutter system provides the TV camera system with a
 view of the vicinity while lessening the chances of exposing is the TV
 camera system to a hostile laser beam. The view obtained by the TV camera
 in this manner enables the first laser pulse (flash) to be recorded in the
 form of a proper frame which is frozen on a TV monitor. The laser source
 appears in the proper frame as a bright spot and is easily pinpointed in
 the frozen frame.
 In further reference to the FIGURE of the Drawing, the method of locating a
 hostile laser source 10 is depicted in diagrammatic form. A hostile beam
 11 is depicted as radiating toward a rotatable shutter 12 which is
 rotatable at a predetermined speed by motor 13. The shutter is constructed
 of a major portion of an opaque or high density material (e.g., OD-4) with
 an open sector 14 or aperture which permits TV camera 15 to receive a view
 of the vicinity of hostile beam while being provided protection from
 damage or dazzle. The rate of rotation of the illustrated disk of 48 times
 per second or 2880 RPM for a single open sector enables the camera to
 transmit a picture without flicker while sufficient to obtain picture and
 protecting the camera system from laser damage. By adding a second open
 sector, the speed of the motor can be reduced by one-half.
 In further describing the invention while it is in service, while referring
 to the Drawing, the hostile laser pulse 11 irradiates both a detector 16
 and the shutter 12 (with 10 open sector 14) which is rotated by a motor
 13. The output of the detector 16 is sent to the control circuitry unit
 17. The function of control circuit 17 may be accomplished by a simple
 threshold detector. If the signal exceeds a preset threshold the control
 circuitry unit 17 triggers a sample and hold circuit (frame grabber) 18
 which freezes the frame of incidence and displays it on the TV monitor 19.
 Prior to this, the camera 15 had been continuously displaying the scene
 (in its field of view) on the TV monitor 19. By the laws of probability
 the camera 15 is looking through OD-4 material at the time of the hostile
 laser pulse 11; but, subsequently without damage to the light sensitive
 portion of the camera, the camera displays the laser pulse which is then
 easily pinpointed as a bright spot in the frozen scene. If the operator
 chooses to negate the source he can switch back to live video and utilize
 it in making his approach to this target. The pilot would probably not be
 able to utilize this technique for the entire approach. He would however,
 eliminate the risk to his eyes (inherent in protective eyewear) while
 utilizing this camera system to approach the target rather than by direct
 use of his eyes.