*20
/            "HOLOGRAPHY"
/  EVER SINCE 1900 MAN HAS BEEN ABLE TO RECORD AND RETAIN
/AS A PERMANENT RECORD ALMOST ANY SCENE THAT HIS EYES
/PERCEIVED - THROUGH THE PROCESS OF PHOTOGRAPHY. THE
/OPTICAL LENS HAD BEEN INVENTED AND USED SEVERAL CENTURIES
/BEFORE, AND THE FORMATION OF OPTICAL IMAGES WITH LENSES
/WAS WELL UNDERSTOOD BY 1900. WITH THE INVENTION OF THE
/PHOTOGRAPHIC PROCESS THE IMPORTANCE OF THE LENS IN
/SCIENTIFIC INVESTIGATION WAS GREATLY ENHANCED. THE
/FORTUNATE COMBINATION OF LENS AND PHOTOGRAPHIC EMULSION
/MADE POSSIBLE THE CHARTING OF STARS, PLANETS, AND GAL-
/AXIES; THE RECORDING OF OPTICAL SPECTRA; THE PICTURING
/OF MINUTE MICROSCOPIC SPECIMENS; THE STORAGE OF LARGE
/AMOUNTS OF DATA IN THE FORM OF SMALL RECORDED IMAGES;
/AND MYRIAD OTHER USES. BECAUSE OF ITS SCIENTIFIC IMPORT-
/ANCE, THE SCIENCE OF PHOTOGRAPHY HAS ADVANCED STEADILY
/OVER THE PAST 70 OR MORE YEARS; EVEN TODAY NEW AND
/IMPORTANT USES ARE BEING FOUND.
/  NOW SCIENCE HAS AT ITS DISPOSAL A NEW METHOD OF
/FORMING OPTICAL IMAGES: HOLOGRAPHY.
/  HOLOGRAPHY IS A RELATIVELY NEW PROCESS WHICH IS SIMI-
/LAR TO PHOTOGRAPHY IN SOME RESPECTS BUT IS NONETHELESS
/FUNDAMENTALLY DIFFERENT. BECAUSE OF THIS FUNDAMENTAL
/DIFFERENCE, HOLOGRAPHY AND PHOTOGRAPHY WILL NOT BE
/COMPETING IN THE SAME AREAS. THERE ARE SEVERAL APPLIC-
/ATIONS FOR WHICH HOLOGRAPHY IS MORE SUITABLE THAN
/PHOTOGRAPHY, WHEREAS MOST OF THE MORE IMPORTANT USES OF
/PHOTOGRAPHY REMAIN UNCHALLENGED. FURTHER, THERE ARE
/SEVERAL TASKS WHICH CAN BE PERFORMED WITH HOLOGRAPHY BUT
/NOT AT ALL WITH CONVENTIONAL PHOTOGRAPHY.
/  IN ORDER TO POINT OUT THE FUNDAMENTAL DIFFERENCES
/BETWEEN HOLOGRAPHY AND PHOTOGRAPHY, WE SHOULD UNDERSTAND
/IN A GENERAL WAY HOW EACH WORKS.
/  PHOTOGRAPHY BASICALLY PROVIDES A METHOD OF RECORDING
/THE TWO-DIMENSIONAL IRRADIANCE DISTRIBUTION OF AN IMAGE.
/GENERALLY SPEAKING, EACH "SCENE" CONSISTS OF A LARGE
/NUMBER OF REFLECTING OR RADIATING POINTS OF LIGHT. THE
/WAVES FROM EACH OF THESE POINTS ALL CONTRIBUTE TO A
/COMPLETE WAVE, WHICH WE CALL THE "OBJECT" WAVE. THIS
/COMPLEX WAVE IS TRANSFORMED BY THE OPTICAL LENS IN SUCH
/A WAY THAT IT COLLAPSES INTO AN IMAGE OF THE RADIATING
/OBJECT. IT IS THIS IMAGE WHICH IS RECORDED ON THE
/PHOTOGRAPHIC EMULSION.
/  HOLOGRAPHY IS QUITE DIFFERENT. WITH HOLOGRAPHY, ONE
/RECORDS NOT THE OPTICALLY FORMED IMAGE OF THE OBJECT BUT
/THE OBJECT WAVE ITSELF. THIS WAVE IS RECORDED IN SUCH A
/WAY THAT A SUBSEQUENT ILLUMINATION OF THIS RECORD SERVES
/TO RECONSTRUCT THE ORIGINAL OBJECT WAVE, EVEN IN THE
/ABSENCE OF THE ORIGINAL OBJECT. A VISUAL OBSERVATION OF
/THIS RECONSTRUCTED WAVEFRONT YIELDS A VIEW OF THE OBJECT
/OR SCENE WHICH IS PRACTICALLY INDISCERNIBLE FROM THE
/ORIGINAL. IT IS THUS THE RECORDING OF THE OBJECT WAVE
/ITSELF, RATHER THAN AN IMAGE OF THE OBJECT, WHICH
/CONSTITUTES THE BASIC DIFFERENCE BETWEEN CONVENTIONAL
/PHOTOGRAPHY AND HOLOGRAPHY.
/  A BRIEF DESCRIPTION OF HOW THE OBJECT WAVE IS RECORDED
/WILL BE USEFUL BEFORE TRACING THE HISTORY OF HOLOGRAPHY.
/ONE STARTS WITH A SINGLE BEAM OF LIGHT WHICH HAS ORIGIN-
/ATED FROM A SMALL SOURCE. THE REQUIREMENTS THAT THIS BEAM
/OF LIGHT BE OF ONE COLOR AND THAT IT ORIGINATES FROM A
/SMALL SOURCE TOGETHER FORM THE CONDITION THAT THE LIGHT
/BE "COHERENT." THIS SINGLE BEAM OF LIGHT IS THEN SPLIT
/INTO TWO COMPONENTS, ONE OF WHICH IS DIRECTED TOWARDS THE
/OBJECT OR SCENE; THE OTHER IS DIRECTED TO A SUITABLE
/RECORDING MEDIUM, USUALLY A PHOTOGRAPHIC EMULSION. THE
/COMPONENT BEAM THAT IS DIRECTED TOWARDS THE OBJECT IS
/SCATTERED, OR DIFFRACTED, BY THAT OBJECT. THIS SCATTERED
/WAVE CONSTITUTES THE OBJECT WAVE, WHICH IS NOW ALLOWED TO
/FALL ON THE RECORDING MEDIUM. THE WAVE THAT PROCEEDS
/DIRECTLY TO THE RECORDING MEDIUM IS TERMED THE "REFERENCE
/WAVE." SINCE THE OBJECT AND REFERENCE WAVES ARE MUTUALLY
/COHERENT, THEY WILL WILL FORM A STABLE INTERFERENCE PAT-
/TERN WHEN THEY MEET AT THE RECORDING MEDIUM. THIS INTER-
/FERENCE PATTERN IS A COMPLEX SYSTEM OF FRINGES - SPATIAL
/VARIATIONS OF IRRADIANCE WHICH ARE RECORDED IN DETAIL ON
/THE PHOTOGRAPHIC EMULSION. THE MICROSCOPIC DETAILS OF THE
/INTERFERENCE PATTERN ARE UNIQUE TO THE OBJECT WAVE;
/DIFFERENT OBJECTS WILL PRODUCE DIFFERENT INTERFERENCE
/PATTERNS.
/  THE DETAILED PERMANENT RECORD OF THIS INTERFERENCE
/PATTERN ON THE PHOTOGRAPHIC EMULSION IS CALLED THE
/"HOLOGRAM," FROM WHICH THE WORD "HOLOGRAPHY" IS DERIVED.
/THIS "HOLOGRAM" NOW CONSISTS OF A COMPLEX DISTRIBUTION
/OF CLEAR AND OPAQUE AREAS CORRESPONDING TO THE RECORDED
/INTERFERENCE FRINGES. WHEN THE HOLOGRAM IS ILLUMINATED
/BY A LIGHT SIMILAR TO THE ORIGINAL REFERENCE WAVE USED TO
/RECORD THE HOLOGRAM, LIGHT WILL ONLY BE TRANSMITTED
/THROUGH THE CLEAR AREAS, RESULTING IN A COMPLEX WAVE.
/BECAUSE OF THE ACTION OF THE INTERFERENCE FRINGES,
/HOWEVER, THIS TRANSMITTED WAVE CONVENIENTLY DIVIDES INTO
/THREE SEPARATE COMPONENTS, ONE OF WHICH EXACTLY DUPLI-
/CATES THE ORIGINAL. BY VIEWING THIS RECONSTRUCTED WAVE,
/ONE SEES AN EXACT REPLICA OF THE ORIGINAL OBJECT, EVEN
/THOUGH THE OBJECT IS NOT PRESENT DURING THE RECONSTRUCT-
/ION PROCESS. THUS HOLOGRAPHY IS A TWO-STEP PROCESS BY
/WHICH IMAGES CAN BE FORMED. IN THE FIRST STEP A COMPLEX
/INTERFERENCE PATTERN IS RECORDED AND BECOMES THE HOLO-
/GRAM. IN THE SECOND STEP THE HOLOGRAM IS ILLUMINATED IN
/SUCH A WAY THAT PART OF THE TRANSMITTED LIGHT IS AN EXACT
/REPLICA OF THE ORIGINAL OBJECT WAVE. THE FUNDAMENTAL
/DIFFERENCE BETWEEN HOLOGRAHY AND CONVENTIONAL PHOTOGRAPHY
/IS NOW QUITE EVIDENT.
/  THE METHOD OF OPTICAL IMAGERY IS NOT REALLY NEW. NEARLY
/TWO DECADES AGO BRITISH RESEARCH SCIENTIST DENNIS GABOR
/FIRST CONCEIVED OF, AS HE CALLED IT, "A NEW TWO-STEP
/METHOD OF OPTICAL IMAGERY." IT IS ONLY IN THE PAST FEW
/YEARS, HOWEVER, THAT THE METHOD HAS BECOME WIDELY KNOWN
/AND USED. THE MODERN RENAISSANCE IN HOLOGRAPHY HAD TO
/AWAIT THE GENERAL AVAILABILITY OF THE LASER AS A SOURCE
/OF COHERENT LIGHT.