LASER PHOTOGRAPHY Barry Miller

Aviation Week and Space Technology Los A ngeles, Calif.

Lasers offer science and technology an exciting new tool for examining challenging questions by fresh approaches. Literally hundreds of ideas for putting lasers to work in important experiments, in medicine, in high-speed computation, in measurement, in communications, and in military science have been proposed. In a number of instances these have been pursued with promising results and in rare cases startling ones.

The use of a laser line-scanning camera system for nighttime reconnais- sance by the military provides an important example of the remarkable utility of these devices. Their successful use for this tactical military applica- tion within the terrestrial atmosphere suggests a possible extension into the extraterrestrial arena. Obtaining photographs of darkened areas of the moon or possibly of planetary surfaces from an orbiting spacecraft or a flyby is one possibility. The relatively meager output power available from continuous wave lasers at present may preclude getting pictures of meaningful resolution from reasonable space vehicle altitudes. But the output powers of lasers are being raised daily, and with the emergence of higher power and higher efficiency devices, planetary photography using laser line-scanners may appear feasible. Certainly for those planetary mapping applications where only gross resolution is tolerable their use may be significant.

Only limited details of what the military and a handful of industrial organizations have achieved with laser camera line-scanning systems are known.' Results with relatively primitive systems indicate that the resolution of pictures obtained under darkened evening conditions a t least rival those taken with comparable photographic systems in bright daylight environment.

The accompanying photographs taken by a rudimentary laser line- scanning system from aircraft flying at altitudes of less than a mile under typical noctural conditions offer qualitative evidence of the power of the technique. Automobiles, homes, footpaths, and shrubbery on a typical street of a residential community are clearly visible. A multitude of cracks in an airport runway surface that ordinarily would not be visible to the naked eye appear in a picture taken with the laser camera system due to the absorption of laser light.

These pictures were taken with a system having resolution of about a milliradian and using a helium neon laser with a continuous wave output of about 20 milliwatts. The aircraft carrying the equipment were traveling at speeds in the 100 to 300 knot range and a t altitudes up to several thousands of feet.

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FIGURE 2. Laser line-scanning camera records differential light absorption caused by multitude of cracks in airport runway.

For the Air Force, the laser line-scanning system has great value. I t can be substituted for a normal nighttime aerial reconnaissance camera dependent on magnetic flares or powerful strobe lights for its illumination of targets. Aside from requiring heavy, cumbersome power supplies, these illumination aids give an enemy sure evidence of the presence of the reconnaissance air- craft and can hasten its destruction.

The laser serves as its own illuminator and is a quasi-secretive one a t that. Because the laser beam is so narrow (on the order of millimeters in diameter in

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FIGURE 3. Ship at sea photographed at night from low altitude. Light on top of mast appears as line across picture due to scanning action of laser camera system.

a typical possible system) and because i t is virtually non-divergent, its detection by an observer on the ground would be unlikely unless he were to gaze directly at it.

The laser camera system makes use of the extremely narrow, collimated laser beam as a light source to illuminate terrain areas. The principle laser properties of interest are the narrowness of the beam, which defines the resolution of the scanning system, and the beam’s radiance, which deter- mines how much energy is projected into the far field.

Simplified diagrams illustrating one basic concept for a nighttime aerial photographic laser camera system are shown in FIGURES 4. A continuous- wave laser beam is split into two beams. One beam is passed through a Pockels cell modulator, striking a six-sided rotating prism scanner that reflects it onto film, which records it. The camera functions as a recording device to take advantage of bandwidth and the resolution storage capability of film, not as a direct imaging device.

The remaining portion of, the beam is passed through the prism scanner down toward the earth. Prism scanning motion is a t right angles to the direc- tion of flight of the aircraft with scanning action occurring through selectable angles of perhaps 30 degrees or more. In this manner, the rotating prism scans the beam in a lateral direction, the movement of the aircraft in a forward direction.

Reflected energy is then picked up by a Schmidt lens, which images the light onto a photomultiplier detector. I ts video output, which corresponds to the reflectivity of the scanned terrain, drives the modulator. Hence the returned energy is modulating the original beam, or impressing an a.c. signal onto a d.c. carrier beam, which is recorded.

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FIGURE 4. Diagrams indicating early method of proving feasibility of nighttime

There are more sophisticated techniques for performing line-scanning, probably using a scanner capable of achieving scan rates well in excess of the several hundred lines per sec. in this approach.

At altitudes in excess of a mile, the system performance degraded sub- stantially due to inadequate power available from the commercial helium neon laser. No doubt the altitude capability for terrestrial applications is ex- pandable several-fold by using more powerful, continuous wave lasers, such as high-power ionized gas lasers, which have generated outputs in tens of

aerial photo reconnaissance using laser line scanning system.

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watts rather than milliwatts. CW powers of greater magnitudes can be ex- pected in the normal evolution of this technology.

A 10-watt argon laser might yield an improvement in performance of as much as 50 db. figured on the basis of four orders of magnitude increase in power output over the helium neon device and an extra factor of ten gain in photo cathode quantum efficiency achieved by switching to the 4880 angstrom wavelength of the argon laser. This type of improvement could be traded for higher altitude operation or appreciable gains in resolution. Even more improvements may be expected in the months ahead, if the rapid progress in lasers over the past six years serves as a valid guideline.

As an example of typical parameters of a low-altitude terrestrial system, consider one laser line-scanner proposed for this application some time ago. It provided a V/H range of 0.05 to 2 radianslsec. and would be capable of operating a t 600 kt. aircraft speeds a t 500 ft . 1,200 kt. a t 1,000 ft . and proportionately higher speeds at higher altitudes. Typical angular resolu- tion was 0.3 to 0.5 milliradians, yielding a photographic resolution of 60135 lines per millimeter. It was intended for scanning a 53 degree lateral line. Anticipated weight was 150 lb. It required a kilowatt of power and generated 0.2 watt output. System effective focal length would be 2-114 in., and ground resolution would be 6 /4 lines/ft. at 500 ft., with 1 in. limiting ground resolution.

For extraterrestrial photographic missions, particularly those where lighting will be inadequate for ordinary photographic imaging, the laser line-scanner might offer a promising alternative. Certainly applications in which pictures with even the grossest resolution by terrestrial standards are acceptable could elicit interest in this technique.

High-power pulsed lasers might offer another photographic possibility for higher altitude operation. In this case, single broad pictures would be syn- chronized to laser pulses, although the number of pictures that could be taken in any interval would be restricted to the relatively slow repetition rates of pulsed devices.

REFERENCES 1. MILLER, B. 1965. Air reconnaissance aided by line-scanning laser camera. Avia. Wk.

& Space Tech. Apr. 26,1965: 80.