re. a re . . * - 7 . . ’ . ° . * «ee 7a . . om ee ° ~ ee oe y oe 2h we ewe ee a ee ee rey — —— _— - . . . . “oe t are aa te ek hee. ete ere Ns ee MSY ete genes White + sos, 7 Coa - - . ¢ . ‘ . L “Ae . ° . . . - 26 February 1968 * : Somme weet —- . . -~* cot tm: -——— oOmm emee ce “oe y K . ~- t - ; . . = ° . 7 . woe - é . ° ° PROBLEMS OF PHOTOGRAPHIC IMAGERY ANALYSIS - - o- ere ee _~ oy . . RANRNAT OR RT mrs tt ~ ° , ° » —ecvee: © — were ow - we - . - . . - ~- ry *
INTRODUCT ION -3 SECTION r we fae 8 ae Oe te PR ~EMS OF PHOTOGRAPHIC IMAGERY Ay VSts ., . , . PRACTICAL PROBLEMS - 5” 2M Ae Imagery interpretation Small Objacts Near the Limits of Imagery - 7 Fine Oetall Beyond the Range of Human Vision-/2 Imagery with Weak Photo Contrasts -/¢ _ Imagery of Highly Reflective Surfaces-1f - Problems of Orientation and Viewing Angle -24 " B. Performance SkIII “Af oy tote By Communication -29.. a ey. Dd. Summary -3/  ° i SECTION 11 __ A. The 2. "Be BAS Te IMAGERY RESEARCH - 32 2 Film -33 : . Composition and Characteristics 733 | Processing Techniques - 34 Resolving Power and Ground Resolution - 3° B. Field Conditions ~45 "2. C. Vision -52 3. I. 2. The Nature of Light -75~ Light In the Atmosphere -77 Reflection and Imagery -.79 ee te cee Its Relation to Light - 52) a Visual Acuity -s#' Stereoscopic Viston -5¢ D. Characteristics of the Imagery - 573° E Object Recognition and identification “72 F. Summary - 72 -Tone - 5S — ; ; tear Brightness - 2 ¢ ee Sharpness cf Definition -64 © Contrast - &¢ Surround - 62 . | . 4 Lighting Defects -47 . fo. saree ee pee we
3 ( |. FIGURES ra toe Imagery of Roads and Wireline Poles and Shadows 2 - Mlcrodensity Trace Across Assault Guns and Tanks” ~ Isodensity Tracing of Assault Guns are - Imagery Revea! Ing Power Cables” - fo a” Pt 58 - = Absorption, ‘Transmisston, and Reflection of t Ligne Rays striking a ’ Transparent Surface i feo5 . SF a? ae r) : ° “oe raed  Sb- _ Diffuse Reflection from Surface Which ts Irregular Compared with Dimensions of Light Waves . oe . . 5c = Specular or Regular Reflection oe Ae mn eee a ee a, ec sere mee ones CM emery "6 - Love Fleld, Dallas, Texas, ee deg e pheenas a aetna ae 7] _~ Misshaped images of ulgst lest”: oy ‘ 6 = Typteal Characteristic Curve . 70 9 - Ganme-Dens ty Relationship ey, 10a - Diffraction Pattern Produced by Perfect Lens top - Resolution Between Two Close Images tl - Wavel ength- frequency Reiationship for the Visible and hajacent Spectrums for Light Traveling In Free Space 12 - Spectral Sensitivity of the Normal Human Eye 3 = Relative VisibI lity of the Normal Human Eye at Normal and Greatly Reduced Light Levels, Showlng the Purkinje Shift .. ‘14 - Relationship Between a Ray Incident on a Plane Surface and the Reflected and Refracted Rays Which Result \ “15 - Imagery ‘of Ralls and Tles on Railroad Bd 16 - Missile Site and Support Area [.°. Coc dae We Diagram IMlustrating the Difference Between Irradtatton and Halation ig - Amagery of the Edge of a Razor Blade from Fine angtaarse Gralned Emulstons ee re ee oe Lt cette me ms me r,
Ce ee PROL 45 OF PHOTOGRAPHIC IMAGERY ANAINSIS — / ad -INTRODUCT ION .The purpose of aertal photography Is to bring Into the laboratory for study end evaluation the most revealing and Informative coverage of specific focations on the earth's surface upon which nature or man has placed some- > thing of Interest to the analyst. Just how revealing and informative the coverage may be, and how satisfactory the study and evaluation, depend on the capabliiity of the series of media which comprise the photographic process ito transmit the appearance of the object on the ground to the brain of the - -2-Jaboratory analyst. The series of transmitting services consist of light ~- or radiant energy, the optical properties of the atmosphere, the optical system of the camera, the emulsion, and the reproductive quality of the ‘photographic film, as well as the reproducing processing, the optical instruments used In laboratory study, and the capability of the human eye to recognize detatis of the object's Image on the resulting photography. . Each of the transmitting services has discrete characteristics. Each, In - turn; degrades the Image, detracting from Its visuai clarity, modifying its “tone, - masking Its composition, or otherwise making the Image it-transmits ~~ ~~ .. something less true and less Intelligible than the Image It received. The -. - Job of the photo analyst Is to read as completely as possible the nature . and significance of the object. His ability to do so is largely a measure of hls tratning and experience In his trade. The capability with which he explains to his audience what he percelves and also the reasons for limitation "of his perception depend In no small fraction on the analyst's understandIng - _*. of the basic phystcal phenomena that determine the nature of the photo Image. . Only by judiciously evaluating tone variations on the photography does the . ...°- :.. . analyst detect an image In the first place. Only by expertly employing ~~ effective laboratory equipment can te distinguish as fully as possible the complex ‘of these tone variations, and only by his abliity to distingulsh the causes and values of these variations does he create the mental image and the _ object ‘description. - - i. Photographicat ly the object of Interest Is an Inert and passive thing. in order to be Identifiable It must exhibit material aspects such as size, | . shape, scalar values, and composition. An object is recognized because tt. 2T. “""" has a different tone or brightness from Its surroundings and because of the -. - “-~* variations of brightness and contrast over Its surface. {t may be associated with other objects. related to its function; Its, surroundings.may ‘be relevant; and the tight on It may or may not be advantageous to Its identification. “ ‘Whatever the Inheren* characteristics of the object and Its locale, the . ..... ._. Important aspect of the photography Is the quality of the Image which light, “SAM 1ZE 0 Cc ory". ‘ de ee es 3 yee sete cee . oo * ere
s - oe . oer - . . - ° failing upon the object, carries skyward by means of individual reflected light- rays. These in turn are transported through the atmosphere to the sertal camera, which Is sole distance overhead. If a desirable amount of light ts reflected from the object through a clear atmosphere, it can be expected that a high quality Image will reach the camera. The camera lens and body may adversely affect the Image falling upon the film, which Is able to capture the Image only within the limits of its composition and purpose. In the process of laboratory reproduction to positive photography, certain features of the Image may be Ideally reproduced at the expense of other ‘portions of the total Image. -In the human eye the visual process may be “Insufficient for detecting all the details carried thus far on the image. The final product desends ca the arelyst's 25ility to comenicste shat he Bes foots ot “Fa fresery. + is the surscse of fhls resort to consicer, in aS pervs, the _* photegrazhic process, from the object on the earth's surface to its cescrip- . tion In the analyst's report, with the Intention of Identifying the physical ‘and optical properties of each transmitting service and the functions of each which Iimit the final quality of the Image and Its identification and -evaluation. Consideration will be restricted to black-and-white, or _monochromatic, duplicate positive transparency, which Is the common media subject to direct laboratory study arid analysis. | wee ee . - . Pad . 4 ene cy et “e . 2 . Late eae we cate. eeeree teem ge: tgergerenm bees . ‘ . Dae DOr CARES Men Fe TR ERA NY yg AMER RPM Ong MEIER TI TANS, tie tw ame s > ove an e. Peg th tin to Ct 7 ~
© AS , SECTION | te. PRACTICAL PROBLEMS pF 7, A), te ry IMAGERY INT ERPRETATION There Is an endless variety of examples to which one can turn to iilustrate the business of reading, analyzing, presenting and communicating .— - “the substance that photographic imagery has to offer in support of man's .. ------ world-wide endeavor to know what Is there, what is happening, and what may “be the next situation to be considered in any locality. . The task Involved Is simply ability to assess the site and prepare whatever communication is -— - expected of the analyst. Difficulties-arise when coverage is poor or-of = - - smatt scale, for that ts when almost anything becomes a small object. In------7-- other words, the mere term "smal I. object" Is a comparative concept, .and. -.7 ~..—. . the practical problems Involved depend on what is expected in the. way of Lee analysis and reportage. re er) a a et er Se ome 2 The Imagery of sma objects, in the total concept In which the term _~— _Was. originally expected to be handied in this project, canbe separated Into “five classes, each of which requires a background knowledge different from — . that needed for the others. -First, there are those which are really smail--2.: -2- . “.on. the coverage, which are near the limits of human vision..: There.are “those ~~ “™ __whose finest detail is beyond the range of human vision but Is detectible .-.——:- _.... In brightness differences that can be mechanically or electronically. .-.2::r:-.-.) __. distingul shed. -There-are- those which -are-targe-enough to-see but which are --- _. too weak In con*rasts for visibility of pertinent detatis. There are images os _with highly reflective surfaces which are lightstruck and therefore distorted. And iastly, there are the endless variations of camera angle, sun or lighting angle, and target orientation, providing an image for which the analyst must Imaginatively fill in the invisible detail and reconcile its concept, some- _ ...times In order to prove or disprove one or another-of alternate choices in-- Identity. Each of these classes of image variations wiil be discussed In ~~ . . tura to elaborate background, essential factors, and possible points of ~:~ - departure in acquiring solutions, or, lacking them, a limited but. reasonable . _ understanding of the insoiution. Aitogether they require a thorough under-- ~ ==: _ . Standing of the mechanical aspects of vision and optics,.of photography and- —.- - flim, of imagery and analysis, and of geometrics and complexity of-photography - am ee eae wba koe _. Thus far we have followed the analyst in deriving the information from the Imagery, In the acquisition of knowledge. At this point his job is only ’ haif done. His communication of that know ledge to others is still ahead.
ae, ‘ q{) The effectiveness with which the commun?’cation fask as a whole is visualized often determines whether the new knowledge catches on, In common practice communication Is a team effort, and the analyst may find that, for several reasons, his work as an analyst is communicated by others who have made the . effort to develop communication as a skill. There is no objection to this type of teamwork, except that no one else can transmit the analyst's Ideas as well as he might with a ltittie more awareness and effort. Teamwork with graphic and mensuration specialists Is often desirable. The real problem here {fs one of kncwing the tools of communication, reasoned analysis, succinctly stated ideas, an apt vocabulary, and.a perspective of the.greater. -- - concept Into which his contribution is to fit. The opportunity to Improve in the role of communicator Is a real Iive Issue for the Imagery analyst. Exploitation of each of these limitations In analysis and communications - must concern both practical and theoretical approaches. We will first-address ourselves to the practical problems. These are briefiy summarized in the ~ first section of this report, and the broader background presented in the subsequent section. The Individual who Is interested only In the reasons for the difficulties need not go beyond the first section. For those who ere Interested tn fully understanding photoanalysis It is suggested that the | . latter section will be the more interesting and Informative. ~& tt = ‘ .
Small Objects Near the Limits of Imagery Lo Pe r . The human eye is very sensitive to light. It has been said that the mechanical energy produced by a pea falling from the height of an Inch would, If transformed Into luminous energy, be sufficient to give a faint Impression of light. Elther too Iittie or too much IIight can destroy an Image_falling . upon the eye. It is only within certain visual limits, and within the visible. -. wavelengths of light, that an Image ts satisfactorily registered In-the retina §- , of the eye and that real seeing occurs. But there are severajs. other involvements In reading photography. 7 , - ° re e J oo. sw eof -, eee - -- ---.-— There Is a limitation of object size, or visual acuity... Standard visual--- - aculty ts defined.as the ability to see an object which subtends only one —-~- —-- minute of arc.at_the eye, such objects being white on black, or black on white. ‘packground.. As.thIs has been.applied to- photography.to_represent_Its_resolving power, It has come ‘to Imply that the ground resoiution figure would be a™s 7° .2.—"* criterion of what can or cannot be seen on a photograph. But this is not—. =: -. -the-case. Imagery.on photography: deviates-from -this -standard.-ia -two distinct. - - directions. First, very few images on photography come from sharply black: ._ ___. " and-white contrasted objects ast" . \resolution-targets are’. “Theoretically, -. ~as such contrast is decreased the unit. Image loses definition,-making It fess. clear and requiring that the size of. the object. be--increased inorder that — ...- _It be visible. Secondly, the camera often registers Images of objects that » sre nowhere near as large as the ground resolution figure indicates they must _ - be ‘In order to be definable ‘unit Images. | “for. éxample',. At \bes_been possible, = over a period of years, to distinguish on some t “whether or not there are rails on railroad track beds. -.It--ts often found possibie-to see . - — “perimeter ‘fence rowsf "°° ~~ UF and median Strips on highways, rand: -sseitro “these dimensions\, re have no comparison to 2 “minute of arc at the eye. ‘In imagery analysis visual ‘ ‘detection reaches beyond the mechanically verif{abie.-- in-order—to understand—— - _ this we need to know how an Image becomes visible in the first. place, and 7-7... :. how we actually see it. - ‘ -—The photographic system ts made up of ‘two ‘Imagery. transmitting: components; .--- ‘the jens or camera system and the film. it-is evident that-for-whatever image Is to be captured by the film the representative light rays must first pass” | through the fens and then affect the film, In each case with sufficient clarity to register distinguishable differences In contrast and resolution. If an. Inferior lens is used the Imagery will be poor iregardless of the quality of the film, and vice versa. If both are of high quality we get-high quality imagery, and under certain: circumstances we get- imagery that-far surpasses. the expected ground resolution. Here we should say not only_why, but also. _ °¢ why not. - Why not get such Imagery as an expectable thing?. If we can see ay oe . , owe ¢ . 1 eetee be ae cag clneeenpe ce te hws
a ; e o telephone poles on the desert, cables at a power facility, and the shadows of fence posts [| | “at none of ‘these a foot in diameter, why do we not see them everywhere? — A second Important question needing clarification here is the manner In which we see an image, which In essence means how the mind handies the’. Image transmitted through the eye. Regardiess of the appearance of similarity, the eye-mind system Is only partly analogous to the lens-film system. The latter ts purely objective. it registers whatever comes through to the film, the clear line and the blurred, making no rejection and no analysis. Not so with the eye-mind system. The mind tries to make sense of whatever It: attempts to see, often rejecting and requiring a second look, and possibly misinterpreting the signal. Furthermore, in Imagery analysis the mind is 7 not dealing with a primary Image, but rather the image of an Image, for which tt has to fill in the G2Ps_ ‘and Supply reason In order to substantiate or Identify an object. . . With smail objects the problem is one of seeing and identifying rather than measuring. The sizes of objects on a single photography are not “necessarily comparable. ‘We may take as*an example a view of wire Ines - beside roads in a desert area (Figure |). The roads are for tracked vehicles, ‘the poles the Simple vertical Implanted pole with cross arms and Insulators. = \it cannot be expected that elther can- ‘be used” ‘as F ‘control for ‘deriving the width of the other. The width of the wider fine, the road, If It is clearly Imaged, may be fairly well derived from the photographic scaie, but not the narrower IIne, the pole, because it Is beyond the limit of resolution. If subjective evaluation is required, . Feasoned dimension for such an object must suffice. - a -_— se sere ew wee ewe were wm mt me oem ome mee 6 ee a re we ee es oo. . e emt es ere © Oe ee ee em ee - - me mee ee ee. a ote erm es me see © oe emer cemees + > omens -* . es . AED oo PD See we em cen ems se me. mem tes Oar ees cae. emer ane ff me meme. ws Gees cue Semmtcs EOS © EUs Ew +6 6 Oem 6 te ES me ote oem es : . "8
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om N Fine Detail Beyond the Range of Human Vision -A critical phase of photoanalysis comes where we took for things beyond the limits of human vision, as in cases where the > Proroena NSE seeks to (Gistinguish de detail he “nOws Is Theres ; _ Sia ed ata RE ee MP 2°83 Es i: ee wea et ep To" ‘this “end there - "ts assistance possidie from mechanical and electronic analysis. ; tome Te ee Phen oa Microdensity traces (Figure 2) are derived by mechanical curvilinear “"mensuration, and edge gradient analysis Is performed wherein, In tracing ._.. density variations, attempt is made to distinguish most preciseiy the con- figuration of an object and location of the edge or contact between the - -- object. Jmage_and its surround. Continued research by photogrammetrists a with better imagery and better controls ts expected-to—produce the, mathod=.... .." ology necessary for satisfactory determination of Gif ferences that human vision cannot resolve. . Se lee isodensity traces are offered as photogrammetric support to photo “~--@nalysis for-the purpose of distinguishing. differences in density too weak for detection by the human eyef . to be apparent. They are electronically derived plots of differences in density and represent - brightness differences of the Imagery (Figure 3). The patterns they form, generally highly magnified, commonly show the Invisible details of the film ~-...Breas_under Investigation. Only the trained Isodensity analyst can Interpret ‘a trace, which may be. sald’ to have the characteristics of a topographic map, ... . with Ilke densities Instead of IIke elevations being represented by the patterns derived from electronic scanning of photographic. area. Becduse their differences vary with actual contrasts In the photography they have a coincident relationship to the Imagery which the photoanalyst seeks to Interpret. The two analysts work as a team, each seeking to derive Informa- tion not available to the other, each depending on the other to recognize in ~ his own medium its peculfar variations, and each offering for consideration potnts which may add up to significant conclusions. The Isodensity study Is not a substitute or a verification of the photoanalyst's subjective inter- pretation, but rather serves as an independent analysis of objective variations - beyond ‘the visual! realm of the photoanalyst. atthe Isodensity analysis, as a support function, Is not requested for the majority of photoanalysis problems. It Is specifically desirable in the - detection of minutiae which are beyond the IImits of resolution of the © _ Imagery of slight differences In scale or detail -that make possible the more . accurate descriptions vital to distinguishing critical features or capabilities beyond visual recognition. As one result of this role the isodensity @2) _- we tote Seer sat °, 2 e ae ated : CARRE Sat oes ha onl Cones ee Cty mw ppeen noe sept egress wee mw eegee ee meresgetongecat cong on ent
- 5 “at 7. . ‘ee “ - oe . . S fe . . ‘ . . . . +e . - e ~~ : - . ts most often called upon to aid tn analysts of poor Imagery. ._He rarely sees the best coverage because there the photoanalyst has become accustomed to . depending on his own ‘visual acuity to reveal all the necessary imagery ode certain reserve that Is visually indetectable but nevertheless there in details. Oftentimes he Is unaware that In the Imagery there is still a Intinttesimal changes: in character pattern of film.. It has probably been. too Iittle understood that Isodensity support In the study of good coverage - could probably add to many requirements where derivation of- small ‘detail a might provide useful Information. ... 7 . oe, a3 ti - - «- wom se « tee pe cee ° ry . . - oe 4 woe -~ Another use of density snaiysts In preference fo high magnification “__. Amagery studies. is related to grain noise In the Imagery. in high optical__...— magnification the: grain-notse ts magnified along with Imagery" detail, finally destroying the Jatter. in tsodensity derivations itis possible: “a to detect density differences first and then enlarge the pattern with ‘the eT vs result that grain’ notse fails somewhat behind. ore ‘Ines between | two buildings. at an. electric. nstal lation n Eigure. 4). oS ren, For every step in improvement of Information derivation there will come, as, - oe eee oe Oe ad seem se os Pad in the Increased” quality of fiim that has come because of, improvement. Spe E SR EAS BIER ROME j Rican Pre: ER ate E ere has ccme opportunity to settle some of the recent problens In inter. ° “pretation of small objects. cpt rene g pene ete te bi sos leah ae ampere ples ee . ; o F Deis bs Loh “Oca oo wanes. a fant “But this does not’ mean: “Shae ae” small spieiee: ‘probiem wiil"é $6 “away better imagery is produced, an increased demand for more fine detail. ©.” o< ~ ofa nek Stade ald fvew oy mes FRB ETE A LE OS co a eae Bae 72" tow et EAE ph ENR oot OT RS EN LAN nn NICO wens TNE © getdate ee ae San Re ee A ab inet ee CL isle aed Sain “an. Occasional fy unexpectedly smal! objects are readily d discerned on imagery for which ground resolution does not approach anywhere near so “small an 7 -. object. One of the Interesting examples here is the set of cables or sas FIs anes mee cata P adits ‘ om ee 3 Ze, ‘ - ‘. caw at feanav cena? Tepe se! Pa teheve” copes ._..gathey have been aiternately seen ‘and unseen" saint eiasecsh cpdabites RSS, Lats rice Mead * Sat - e
a without the Irradiation effect, both as light lines on dark background and as dark Ines on lighter ground. The exact reason for their visibility Hes _ in the quality of the emulsion, but the exact physical reason for their discernibttity does not reconcile Itself with Inability to see such #Tne detal! elsewhere. re . woe SR eg Tene Sate eto comme eens gare gotten en ee oo \ oe me _ . No device has Been produced which” can Sither” vartiy” or refute the ‘Interpretation given by @ photoanalyst or by analysts who may not agree on a specific Interpreta- tion. In visual mensuration cooperation Is achieved between the two analytical skills. Where the machine operator Is unsure of the limits of the object being measured by such Instruments |” : ithe analyst Is often called upon to verify the points ‘for. point-to-point mensuration. This he is happy to do In order to forego the time consuming operation of the machine Itself. - There Is convincing evidence that the two skills are mutually beneficial. Perhaps at this point it is also quite obvious that the two should be xept separate rather than that the photoanalyst should be expected to do tfie most exacting types of mensuration. . The field of knowledge required Is too extensive for analysts in general to be content with attempting to perform with maximum quality In both fields. elated _ ; ; “SS Oe tomes oe mw cee
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imagery with Weak Photo Contrasts . yt wo. oe . y Much of the problem of object identification or visuallzation Is not a matter of size but of image clarity. If all photography were distinctly black and white, such as ground resolution targets are, this would be a standard problem with standard expectabilities, but most Imagery consists of gradations of gray tones that seem to merge Imperceptibly. To some degree this Is inherent in the use of granular emulsion. .Fleld conditions ‘are a major cause of such poor contrasts. Light reaching the camera lens Is attenuated by atmospheric humidity, haze, or overcast; the image may be blurred” because of lack of .. perfect coordination between #1 1m and vehicle motion; f” " nen ee ems Stee TRS Te ; OF ‘visibiftty may be’ ‘poor ‘pachise et: poor Vighting 2 ‘and | ‘Tighting’ ‘aigies at inappropriate times of day or season, f- eee Snow cover often masks ground surface detaiis, “bur it may “be. revealing because It sharpens contrast and can date activity. Obliquity of coverage may be a problem because of poor viewing angle but it too-is advantageous in many Instances. tn cases where one or more of these conditions is dominant, an object, even “one that Is large, may be Imperceptible, or If identifiable may still be too poorly imaged for full and satisfactory description. These are the usual conditions under which imagery Is acquired. The real problem of Identifying the fine detall of photographed objects is generally one of distinguishing between closely related tones produced in the relatively, slear Images of objects whose overall appearance Is not black and white but the In-between shades or colors that come out gray on so-called black and white photography. The large difficult objects fall into two . categories: (1) those which occur In shadow? _ of a nearby object; and (2) those whose gray tones closely match the surround. if the gray tones were effectively contrasted resolution could be good -pver fe “se --- wand objects of low scalar size distinguished but if the ‘tones are closely alike or merge imperceptibly object definition is apt to be poor. There has not been reported any.coherent body of research Into |. the expectable relationships between vision, visual acuity, and identification and description of objects only faintiy contrasted tn their included detail or In association with surrounds, at least no research that is of direct application to the photoanalyst. in this phase of his work he is operating In an area for which no physical parameters have been developed as guidelines or goals, with the result that whatever he accomplishes Is open to. criticism and greater expectations. ; . g &) . et ae oe ven eer eerste SECT CD CN CMT SL Ss dd a wares we one * oe oy Py er oye a te gee -* woe sa Tig On May IIT Oe ” ar Oe Ss Dee MEPRNIAT 2. oe stam et
‘The photoanalyst, personally, elther sees an Image well enough to recognize end describe It, or he does not, in which case he may be able to get help from available technical services. The image contrast can sometimes be Increased by developing’ techniques that enhance flim density differences. It Is worth while for the analyst to.discuss such contrast problems with the skilled staff of the photo reproduction laboratory, or | ‘+ with the photogrammetrist In case an exact edge presents the problem. ~'*: These techniques are most valuable In object mensuration, and they sometimes also lead to corroboration of Identity. gt hast . -mee . - -ee? “* hr a oe + Be. . < ewe ° ad . + oe z e * %.." ‘ ’ ‘= . - ~ Le “a 3 ae? eo bd e . . - . y ‘ . . . ° ms wo . s oe . o - ad . y . - a ae ween eo e : ie : ° : ot *, ” . .” 2 e . | . oe - ae - a - oo t - ~~e - ~ oy ee ro . . . . wes 2 = s . : ” * ‘* . . . 2 . . eeest . Ps aot ae . : ~ ~ ' * * ’ oe “ese . ° ~ - _—. “en - ae : . - ~ - +0 - bad ° - , oe e « e Py > -. — oe . ° oe ave . a » ° - r *e. - -_ ad oe “ . Par * Rose ae . ~ “6 . . . .. . s . e =e ad - eo e- . : a - _-— = . ao : - » . ea - . . . ~ ° .. - = - ¢ . ~ . a = . , . . - Sy .- ° ° oa \ - " | .° - - ¢- - “7 . . ° - +e rm awe. . . j . oe oot . . @ 7 ra Se eae get +, nt . Sateen tl cae le eer ee " . - ae" No . be Rae COR ee a et Bag ee a SP a ee ge A ne ars * PING ae THESE ED Oy Tb aM SD ONT ON rata
es Imagery of’ Highly Reflective Surfaces Metal objects, such as alrcraft and unpainted olf or chemical tanks, are generally as readily seen and measured as other types of construction, but occastonally, and often then In critical situations, the amount of light reflected from such objects Into the aerial camera is so bright as to partly efface object outline and detail. The analyst can most often distinguish the nature of such large objects by distinct outlines of portions of the Image, or by conclusions derived from the surround. Exact measurement or description of small *detail, especially that required to distinguish between similar appearing aircraft, for Instance, may be Impossible, although this bit of Information may be one of the most Important goals of an aerial mission. The majority of statements on such overbright light effects attribute the cause to glare, which In conversation Is frequently, though mistakenly, referred to as halation. The real cause of overbrightness therefore needs discussion and clarification. Three questions Invaribly express the problems of those concerned with such Identification assignments. They are: (1) under what circumstances does such overbrightness affect imagery?; (2) what is the . amount, percentage, or factor of errdér in mensuration of such image distortion?; and (3) what can be done to avoid its occurrence? - The first of these questions Is fairly easy to answer although It requires detailed objective explanation. Overbrightness Is almost wholly caused by natural and normal concentration of light reflection from the metallic surface Itself. To understand this we need to review basic explanations of the behavior of light, its distribution, scatter, or concentration, depending upon | the media through which [t passes, the surfaces through which it cannet go, and the manner In which It Is directed away from those surfaces. it is theorized that light travelling from the sun toward the earth Is propagated in direct rays until It-hits the earth's atmosphere, whereupon _ | " some of It Is diffused or scattered by minute particles contained therein. - Much of the sunlight falling through haze, overcast, and smog strikes the surface of the earth or the objects thereon, whence It can proceed no farther. Some of the light is absorbed and turns Into heat energy. ‘If the surface Is translucent some of the light will be i:onsmitted through It (Figure 5a), as_ through a window, but that which Is retiected (turned back by the object) carries an image to whatever optical system Is within viewing position. If the surface Is rough, either coarse or fine (Figure 5b), the reflected light Is diffused or scattered in all directions, and the object Is generally easy to see and easy to look at, although excessive diffuse light produces 2 - washed-out appearance which is readily apparent f°" a Oe we oct. 3 SO TENT ms NERO OS ete
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- ry ° (, e) e “* “The. smoother the surface may be the greater the proportion of reflected _Sight that is oriented in a particular direction (Figure 5c). That direction Is determined by the direction from which the light comes and the orientation of the bit of surface from which each light ray Is reflected. This relation- ship ts clearly explained In the physical law which states that the angle of Incidence of the IIght Is equal to the angle of reflection, these angles — belng measured from a line norma!, or vertically projected, to the reflecting surface. The law Js well Illustrated In the brightness of suniight reflected Into the eyes of a westbound driver in the evening when the sun Is low, or In the brightness of a smooth water surface as one looks into or out over it while facing the sun on a bright day. If one faces away from the sun the water appears dark, not because the sun Is not shining upon it also but ‘because the viewer Is tn the same direction as the sun, in the diagrammatic direction of the angle of Incidence, while the concentrated reflection .Is directed Into the alr out beyond the viewer in the opposite direction. For smooth surfaces this light concentration is called specular reflection. . A mirror ts such a surface. Any smooth metal surface, such as the body of a car or the curve of an aircraft wing has such a reflective capability. The brightness of the specularly reflected light, and the image it may carry, Is sald to be equal to the brightness of the source of light, viewed on a IIne through the point of incidence, times the reflecting power of the surface, which may be less than 0.1 for water or glass, but as much as 0.98 for some metals. Thus it is that polished metals are used for mirror surfaces, and that the brightness of reflection from metal equipment, such as tanks and - ‘aircraft bodies, sometimes appears too bright for comfortable visual observation. The brightness from such an object may be too great for the camera film to . capture a good clear Image; this is the reason why these items often appear as overbright blobs of hight or only partially discernible images on aerial film. The second question, concerning the amount; percentage, or factor of error In the mensuration of light distorted images, has no single or specific answer. Esch Item is a case by itself. The amount of distortion registered on the film, for example, will depend not only upon the nature, curvature, and angle of the reflecting surface, but also upon redirection and contro! of the light. For each object there is not necessariiy a unit reflection. This is well Illustrated in the photography of an airfield at Dallas wheré several aircraft were parked (Figure 6). Of these only one was subject to oyerbrightness, but for It the forward part of the fuselage between wing .and nose was distorted beyond definition, while the rear of the same aircraft was clearly outlined. [a star rn stmt cece * ~probtems, and some means have been achieved In avolding undes
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effects. First among these may be mentioned the use of polarizing filters to help reduce excessive diffuse light and to minimize reflectivity while , otherwise producing good quality film. in the photogrammetric and reproduce | .tion laboratories, too, microdensIty and film reproduction research ts. underway to help solve such problems. Sut this laboratory work cannot. erase from the negative the overbrightness already registered there nor + restore an Image that never reached the film. . ; ree . ' We now come to the question of what can be done to avotd the occurrence of specular reflectioh. Let us look at this In two parts, the general and the particular. As a general subject in photography we need concern ourselves very little about Its occurrence. All of us as experienced ground photographers know that, except for special lighting effects and with great discretion, In order to get good photographs we turn our backs toward the sun and by observa- tion guard against including overbright objects. By this same deduction we could also conclude that aertai photography taken with the same caution could equally well avoid specular reflection. But aerial * Photography Is not quite so simple. a in order that a photographic mission be most Successful, It is carefully... programmed before the mission ts flown, ; eee ‘ira ttds daagtieine Tov, more | FEEB Se: ce. "4 And because there are no possible means : OF ‘knowing, ‘all the reflecting surtaces tn the path of the mission, the laws of chance practical ly guarantee that for 2a long mission some smooth-surfaced object will be fmaged _ Jn the very angle that will produce specular reflection. However, If the — object Is known to exist, and an Image of such an object Is a purpose of the mission, there should be no reason why the photography could not be securéd from a satisfactory angle. EO ee it could be posstble that,| . there Is only a single chance to acquire a photograph ot"? @ desired object, and that, regardless of the uncertainty as to the sun, object, and camera, it is deemed best to take that chance. In this case an exceedingly small variation in the relative position of the three could insure a good view or an unsatisfactory . one. It Is worth the try. But if the object in question, an airfield for “ example, Is one of the prime targets [:. . __ \ and the presence or probable presence of highly reflective objects (In this case elther oi! tanks or air- craft) ts expectable, the programming should take account of possible specular reflection. to the point of guaranteeing egainst it by careful placement ; oa "tha position which not necessarily approaches the angie of incidence “of “ttght’ 3 on the object but which Is In the same azimuthal orientation. This would minimize the probability of good . - + vm tee. FO ee ee SE fee re wage : a Fue h wens . ; 88 en ay Oy Store
shadows. It Is In sharp contrast to the long-standing belief that objects are most clearly seen when’the shadow falls toward the observer. The really .-+- analytical photoanalyst finds that.a good look at the ‘sunny side of an . object provides details usually associated with obliquity, and often revealing of Image details not captured on the shadowy stde. No matter how careful ly .---. euch programming ts established there may still be times when an aircraft, ortented at an arbitrary angle and with a curved surface reflecting sunlight rom a fortultous portion of Its complex exterior, will direct a bright enough ‘amount of- specularty reflected IIight to cause poor Ifneation over a fraction — - -of the Image. If such overbrightness destroys a critical detall of the _. 7: -, |mage {t+ may be Impossible to. distinguish that aircraft as balng one or the ~ " "other of two closely Ike models, or to Identify.a known model from a possible --z:-«.¢losely resemblIng prototype that has not been previously observed. Although ~ _ Such an Instance seems wholly unlikely, the possibility should not be over= _- --- . - -*fooked, and cannot be completely avoided. . _. rr oc ee eee eemadie wasp tn most Instances where clear photography Is vital It Is possible to... -— .-arrange for more than a single frame of coverage of the vital object. Det mae s --- the chances are very good -- vee 2. that an at feast one frame If not both will have the object clearly Imaged. If nelther Image Is clear or- only” -: _ “g stngle poor Image Is acquirad, there Is some value In knowIng that an...) * Ttnteresting object ts at least poorly Imaged and that. its Identity may be _ clarified when another mission Is flown. nt re . - +e ve - ° - - were ee Fe ae TREN tins weve: 2 5m. ara “ATS iy) ee ET CSE a ce, . ‘ Pee Bet ea Bree
Problems of Orientation and Viewing Angle _ Overhead photography provides the analyst with imagery that varies with lighting angle, viewing angle, and target orientation. First of all the geographic location of an object determines whether, on a particular _ date, there can be sufficient light at a site to allow clear photography. . - Snes come Prue Tog tes RATE, . wo, eee a eh es mentite os ne nad Exceptions made to this condition are tncéiided in missions with the full knowledge that certain targets are better viewed under poor light than not at all Go - wametrsti to A= “Ig eases where obscurity occurs the reasons are os LL ge. generally other atmospheric conditions or terrain or structure shadows. _ The matter of sun angle’ should not be left without pointing out’ the ~~~ -“geasonally greater quantity of light in the sky. In summer than ta winter.” ~~ _ | the sun's course were related to a-cylinder Instead of to the spherical “shape of Earth there would be no differences in this respect. But anyone reflecting on the subject recognizes that the immediately pre-suzrise and post-sunset light in summer Is far greater than in mid-winter. soe In fower latitudes it can happen that with high seasona! sun mid-day photo-coverase yields imagery tnat is almost shedowless, generally an undesirable feature. On the other hand, In the latitudes between the Tropics (Cancer and Capricorn), coverage can be secured seasonal ly with . shadows falling either north or south as well as dally either east or west. This allows unshaded views of al! sides of an object of interest and provides ' guidelines for targeting in order to acquire the desired look. } Ves gs Eee wow Sree ne . “ . red A wees ce te ee ee A full discussion of these properties of the imagery should also Include unit target shepe and complexity, and the nature of its surface. But these are so intimately related to the basic aspects of vision and imagery that, If discussion is deemed desirable, it should follow after a full reading _of the second section of this report. FO 7 LO
‘ : - \ ¢ A photoanalyst has mixed feelings ebout shadows. He learns early that photography Is easier to look at if oriented so that shadows fall toward ¥ him. For vertical photography this Is simply done, but for oblique photo- _ graphy, where obliquity should also be slanted In the direction of the viewer, | there are times when a choice has to be made between the two. And then there . - 18 10 single answer or simple rule to follow. The situation varies not only “with the obliquity and shadow angles but with Image quality and even the “nature of the Image Itself. The analyst In such cases may find It useful + to look at the photography, the site location, from’séveral angles, .and often _. finds something reveating In viewing the imagery from different angles. , .- The angle permitting the most penetrating analysis Is often found to depend ... .. -- on-target orientation, or on whether or not the object to be Identifled.is....._.. " @ single unit or a smal! factor In a more detailed and complex site. Lt 7°. Target orientation ts also a dual or triple concept. ‘In the first + place It may make a difference in judgment whether one views a 30-foot -~-missile or a freight car from the end, the side, or some point in between. ' §¢ may or may not be Important whether It fs seen from the direction In a which the shadows fal! or in the direction of obliquity. If the target =~ ~>* has a highly reflective surface the IIght ‘return from it will depend on ...the relationships of the éamera's look angle to the angles of incidence —s = -and reflection (Figure.7). These are ali orlentation factors that.aid or... 2° *. - hamper the look at the Imagery. © But target orlentation with respect to - .. -- acquisition of the imagery is also important. This ts immediately obvious _ .”... 7. In -‘consideration of ground resolution targets, where tests show that-the  - -._..- =~ target orlented.parailel to the flight path.of the vehicle hes a..lesser .:.. 2-:-<=:- “-pesolution than the taraet oriented perpendicularty, the reason’ being thet --7--.. (—there Is a slightly greater sensitizing of the emulsion in the direction of -- - -: . fllght, with the film traversing that flight path. in some Instances the - - difference may be Important. 7 = ome oe + em eee ene » » _ 2 . ° . . . ad - . ed - * e ° ee ete a Pee TT al aR ee italia, ST) . @¢ ocr: fs :
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----becowes isolated, 2 hermit if you will, lacking the support of discussion ee oe al Pee _—_ PERFORMANCE SKILL Imagery analysis i is not just a matter of “object definition. As much as Its concern over objects may be emphasized, the imagery analyst is responsible also for recognizing action or the Immediate condition of a --- -- - capablitty that Is belng developed. He Is most often expected to be able to draw on an Intensive background knowledge of some one or several diverse and technically complex fields of man's industrial, political, economic and military capabilities. To the-degree that it is pertinent, he.acquires a . wide acquaintance with the geographic assets and limitations of a site, and - - _ithe history of the activity there. I+ is expected that he keep himself - te Informed of whatever may transpire ‘there, .or elsewhere, that may be. related --~ mo “to the function in question. In the process he becomes skilled in the techniques he finds most 7 ” productive of satisfaction in his work. ‘Such skills may take the form.of — -- fllustration_of a single site or feature, of photographic comparisons between similar but unrelated sites. Or he may have a special interest in verbal . communication, in which case his word pictures become increasingly apt and meaningful. But in ali probability he will not be highly skilled in all Fae: collateral techniques. Lack Most particularly he wit probebly not-be-extrenely- “tanerestee ative | - highly competenfzin the mechanical aspects of imagery derivation and presenta- - tion. He=may=do -it+-and Fiany analysts are graphic ertists, but the -everace - analyst.wilil neither enfoy nor perform.well at graphic support. or ‘the .mensura- tion of his own smali oabjects® analysis problems. This is not “because he lacks ' Interest in the data such manipulation-can provide for him but because he is ~ then moving from the subdjective analysis, which he enjoys and .therefore-does~ ~~ : “well, to objective skili that by comparison is a subdominant accompli shment-————- “and therefore. not able to command the place in,his interest which is -- necessary for real skill. in the process of becoming self-sufficient he a ee ee ae wees te oe and differences of Opinion and purpose ™ which in photoanalysis are highly productive. ue The techniques of. subjective-and objective analysis of photography are— "7 ..distinctly separate concests, both as to philosophy and practice. ~The ee individual who is.concerned with the objective techniques will become a . ere eeke aa a photo-led technician, a photogrammetrist,- -or-an opticai----- - Specialist perhaps, bat he wilt not be interested in. in-verbelly describing the -.-
- ° tame * ok eed ee ee ew me tee mee oF fe SO RN ES ow MRE + ee ee eee we ee ee ‘leg of an electronics ‘site or the chemical output of a rallside Installation. His tnterest Is In the mechanica! perfection of the photographic product, and having achieved the highest quality under. a’ given. set of mechanical conditions, he then attempts to improve some small component of these ~~ . ‘conditions for another go-around. The imagery analyst, on the other hand, Is Interested In the game of deriving expertise from the visual potentials. '“ His skill differs from the mechanical or mensurable, and when he is asked to do both he ts ina position analogous to that of ‘a test pilot being asked | . espatpte @ place in the assembly line of an aircraft “factory. “there is a {' need-for.objective analysis of-.small-object photography, -it- should not be be expected that this be done by the subjective analyst. True, he will, or - should, understand the objective background, but he needs the support of the skilled and knowledgeable objective analyst to produce the best products. . . In addition, ‘It wil! probably be discovered that the ‘objective work can be done detter by tha objective specialists, that they will do it faster and . less expensively than fhe subjective analyst can, that he thereby will be - free to accomplish a greater amount of the work ha does best, and that as a_ _ total result more work will be done more economical ly and more People wild be ‘happler. - . e - oe -- 5 we oe me ee ee Ee ee oe 8 ee ete en ee ee Ee
. > ft £75 COMMUNICATION Communication of Ideas ts mainiy a language skili. Our words must be well chosen to say exectly what we think If we are to convey to others... precisely what we mean. The phrase, “be specific," rings fa our ears. This does not mean that graphics, elther annotated photographs or objective Itlus- ‘trations, do not have a place in putting across salient points in presentation, . or that numbers are not highly effective In conveying magnitudes. These are ” useful after the fact, Ths concepts they convey must first have had to be verbally defined. In company with visual aculty and enalytical skill in studying.the Imagery, idea formulation and vocality are the means by which ‘the analyst communicates. To do this well he must know the nature of the media with which he works, and the descriptive terms which effectively carry.-—-— ; his message to others. ~ The first of these Is. covered in the second section .--. . : “Of "this report. Here we wil{ concern ourselves with Introducing the analyst's -- - , practical problem of communicating what,he knows. - re - | As In any profession, the body of photoanalysis terminology has been. . _- -. >". “accumulated slowly as the sclence has grown.-.Certain terms thet-have been —-~. - well chosen stand the test of time. Others, less apt, may be carried in active... use because of priority, because their real meaning is overlooked, because =<"? thetr use is expected and even requirec,-or because-more-adequate- terms have-- —-- . not been Introduced.- The photoanalyst, as a vocalizer, might-sharply Increase - - - bhis-ability to handie-words if this phase of his activity’ were brougnt to | attention as a whole field of habit that needs evaluation. Unless a good look __ “-=--and on open'mind are brought to focus on the problem it might better.be dis=.._7- ' regarded. . ; . , ae a The prime weakness, poor word handling (which here also Inciudes -. :~—---- phraseology), resolves itself into two parts, words poorly used and words *- --poorly:- understood. Words poorly used are most commonly the general or non-professional terminology that has been Incorporated and thereby endowed___:_ . with pseudo-scientific status. "Poor image quality" is such @ term. Many of these terms are ambiguous and misleading. They include terms which mean | one thing to one group of analysts,f ~~ but something else ---- oer me- 1i—— ce "-Ferms which connote different things in several aspects.of Image-production, ---- - guch as speed or flare, most commonly are used precisely in one sense, which » ": . should be the only- usage, and In all other cases should be replaced by terms - which really mean what Is implied. In a few cases, as contrast and exposure ‘used by photogrammetrists In one sense and photoanalysts in another, two Lees ' different usages are justifiable. . - ° P| i oe - ; . : we -* .
- + o- ” . . : any Among words poorly understood we find most commonly the borrowed sclentific terms, which in the borrowing-have been poorly chosen, and in the retention have come to denote something outside the original meaning which may be a valid term within the bounds of ‘the profession. Halation fits Into this group.- We find a simple term, cefined precisely in- physical sclence, used loosely and conveying no useful Impression when substituted. Into the position of ‘a related, significant icea. There is, for Instance, ° a custom of using “resolving power" as a criterion In Imagery when "ground resolution" -Is the Important value. _On the other hand, we may find-.ourselves -.— poorly understood because our audience or readership falls to catch the full .-stgnificance of terms we choose. tn this cease we; however, have the choice _. of using simple vocabulary so carefully presented as ‘to convey the more _ complex meaning, or of. explaining exactly what an unfamiliar term means. =. The photoanalyst.should be.aware of these weaknesses In verbal * 7. communication, He may find value-in this amount of definition. Togo.” -peyond~this point “Is -not- within-the scope.of.an.Introduction. tothe problem. ..._. ~ se ° . . ad ae eS - . « . ad - ne a an oe ee ee ee ee ee . -- . . - es =. oe : . : - - we oe - . 7. . . Pee Tee 86 ° " a — 7 a r Cre wr ate. FEE SOF me oe Pe EE OR CN SOOT RE tie CH OEE Gal 5 * tear em mate Oe OO a Bag FE Oe Weeeee tet ee eee : - oe - . ?
Se see 9 BSS ee ES She OK oe. nw el tal Al ae Steg MARANA £ ied Bz SWS RIS aes ee ea a lar ee Cone lassen SREARYS . . . . ese o . == Given two reels of photography from film with equal resolving power, used In two different but supposedly equal cameras, exposed under two different (supposedly equal) sets of atmospheric conditions, photographing two actual (and ‘supposedly equal) sets of scenes, developed under supposedly equal conditions, and viewed by two cifferent (supposedly equally capable) . Imagery analysts, It Is difficult to conceive that definition and description -~ Of Identical smal! objects would be equal. There are too many variables for - full equality. Si aa a a ie oe eg A ROR sale Selo, point here fs that if everyone concerned with photoanalysis, and with its. — products Ese Sake eae comprehended that imagery. analysis is not an exact science, and the reasons therafore, our effort could be better... , ‘concentrated into doing what we can instead of being too concarned witn ‘~ what we cannot. .In the following section are analytical data on which a Be oa? such comprehension mus* be based. : . 1 wee . a ‘ wee bad - q 48 mee om 28 Pd PERO et ee te one.
es 7 SECTION 11 mo BASIC IMAGERY RESEARCH In the transmission of an object's appearance on the ground to its description In a photo analyst's report, the descriptive data sre forwarded through a serles of media, none of which functions perfectly and each of which degrades the image or robs It of certain detail which is not recoverable. The media in turn are the reflected light carrying the image details, the atmosphere which attenuates the light rays enrouté to the camera, the lens which focuses light on the film, the roll of film on which the latent image Is recorded, the negative or processed film and the duplicate positive or reproduced positive image, the mechanical equipment used to light, magnify, and measure the image, the human eye, and the analyst's mind wherein the imagery is translated to a facsimile of the object on the earth's surface. _.Between the object and the report the image can be said to have one stable- or real stage and one instable or Imaginative stage. The stable stage is that recorded on the original negative which, once processed, remains the same until or unless emulsion deterioration sets ia for organic reasons. Reproductions from that negative are essentially also parts. of the stabie stage although one may differ in quality from another. The instable stage is that recapitulated: in the mind of the analyst; It occurs fleetingly in single recognition fragments the total and composition of which becomes not so much an image as a reasoned _eonclusion, whose recording depends on pre-knowledge anc descriptive ability. it Is assumed that we are sufficiently versed in the art and purpose of under- standing to seex explanation for whatever knowledge is necessary for insight to occur. The physical media and means which contro: the transmission and reproduction are discussed as they pertain to the present state-of-the-art of Imagery analysis. | o ; : ye + re og ed 7 Tem
THE FIRM Composition and Characteristics Loe The requirements specified for films to be used in the procurement of serial photography are necassarily very stringent. The reliability of eerste “Information extracted from the photography is dependent upon “the” film meeting these requirements. _— A film ts ‘composed of two basic parts--the film base end the photographic emulsion. There are other components required but they are directly related to one of the above or to their combination. _ The film bases used predominantly 2 are solyesters, or "safety" film, so called because they do not support combustion. The two most important, charac- pteristics, other than stability, are greater tear strength ; tinue’ Ve covy> These films are much more expensive than the cellulose acetates “available, but are necessary for: | _ ot cartographic purposes | bacause of their high dimensional stability. .. °° The emulsion is a suspension of silver halide crystsis within a gelatin mass. The gelatin serves as a protective colloid. Without this protective ' colloid, the developing of an image would be impossible because the silver halide, whether it is exposed or not, would be reduced to silver on contact with the developer. The gelatin is also essential for uniform dispersal of -— the crystals of silver halide. Strict quality controls must be.exercised ._. . when the emulsion Its appliec to the film base. It must be smoothly-and uniformly distributed over. the base; otherwise a lens effect will probably result. A lens. effect produces 2a spreading of light within the image, $ solely as a result of a non-uniformity of enutsion “application. . - “" The emulsion and the film base are joinec by an achesive. -Additionally |... . -an anti-halation coating is most often applied to the non-emulsion side of. the. . - film base. This coating is water-soluble and is removed during processing of . the film. Some fila bases because of their color qualities, do not require © anti-halation coatings. |. oe + eee : . we .. a o “SE ; - o ie ote a : . 2° There are many fectors which must be considered in the ultimate ‘selection. . afia particular film type in order to obtain the most-desiradle image for - -.. _ maximum Interpretability. These include film speed, granularity, contrast and definition characteristics, spectral response,et¢. 4 -_. seco ABB)
oe. “se . . ee . c. Re \ Speed, as used when speaking of film, Is merely an expression of the sensitivity of the emulsion. This must not be confused with spectral response which Indicates ‘the range of the visible portion of the electromagnetic . spectrum that a film emulsion will record. Although commonly referred to .as film speed, It might more IIkely be called emulsion speed. It Is a ‘relative quantity, used to Indicate emulsion sensitivity to Ilght. The speed depends In part upon the size, quantity, and dispersal of the silver halide crystals in the colloidal suspension. These factors equete to the granularity _ and graininess of the film. | Film speed may be thought of more simply as the - *‘time required for Incident IIght to. produce the desired chemical reaction _within the emulsion, thus creating a condition of exposure. Speeds are expressed In general terms; f.e. fast, medium, slow, however they do have definite numerical values which are more often used by the photogrammetrist. o The granularity of an emulsion Is an expression of ‘the graln size and the density of thelr dispersal throughout the solution. Graininess is a psychophysical quantity referring to the visual appearance of the granular ‘structure In a developed photographic image. Graintness is evident as-a result of an agglomeration of developed graiis or an overlapping pattern of grains. Slow emulsions In general have finer, granularity anc better resolution ‘than the faster types, but they are not necessarily the best for aerial photography on that account. In past years fina-grained emulsions were excluded from use in aerial photography by the need for high contrast and full emulsion speed. of a film for certain contrast characteristics.does. relate directly to granularity and film speed. Contrast of images is a function of maximum and minimum-density levels af granularity and exposure. ——S~ - Definition may be stated as the degree of clarity and sharoness apparent © . In a photographic image. Restricting discussion of this subJect to film ~ characteristics only, definition is more dependent upon granularity of the emulsion than any. other factor. Although. a certain level of sharpness Is necessary, it should never be forgotten that sharoness, as such, without - any specification of focal length, conveys no cuarantea of good resolution” — . Of ground details. aa Fs
a: . ae . Soe . Spectral response indicates the range and ‘peak of. that portion of the visible spectrum to which a given emulsion Is ‘sensitive. Many other experimental film types are available and peIng tested. a 7 4
Processing Techniaues and Procedures But first, reference to the characteristic curve must precede detailed discussion of variable factors in development. It is the most common see reference point utilized in describing variations In density scale (or - Gontrast), and gamma rating (or reiative contrast). The characteristic curve ~~ (Figure 8) Is used in this text merely’as ‘a graphic reference tor clarification of statements written in the foilowing paragraphs. ” _ Density, by definition in the photogrammetric sense, is & measure of the =...degree of blackening of an exposed film or of the direct image in a positive -. print-out ‘material. Density scale indicates the range between the highes? and lowest densities in negetive images. The term density scale or range is “often reterred to as the contrast capability of the negative or positive materiai-. - This must not bé'confused with the usege of contrast as it Is applied to imagery _. analysis.. In this iatter instence, contrast is intended to incicate the relative ..... physical appearance of an object agaiast its background. | There the term has a subjective application. OS — -— A graphic representation of density range as-applied to The characteristic. . curve Is shown in Figure 9. {tf is shown here as a function of the -relationship- - ... bestween exposure.and the gerzae value of a given emulsion. | og ee ce ee ; : . . 2 The density scale of the negative is most Influenced by three variable ~ conditions:. the brightness scale of the imaged objecy, the exposure level of - “the camera, and the degree of development. After the imagery has been obtained, onfy one of the three variaSle conditions listed can be altered--the degree of | development. It also is probably the most important single factor which can affect the density scale. SPP peepee creas oun n- we See, TEE, Lee! LUM OU OAT TERA Ra MOD WARGO EP ETT Oe, ree See . + Oe 2 a Ve week a ore Ard aE Aaah wre ee . aaa > ai fe et te ’
_° <- , ° . o . ms ° . “: -"Pigure 8. Typical characteristic curve. a, on , ° “ . ° e “s ° "« oe ° *. . e . - . . an ‘ . , ut . og + | 7° BZ - . Se o. - + Figure 9. Gazsa-density relationship. | e . - @-. or . . : , . : ; . PT) ° e e .
* Of, the many development changes possible, variations in devalopment time epparently offers the most satisfactory method of controlling the negative density scale. The range may be regulated toward efther maximum or minimum density as shown on Figure 9 Gamma Is ¢ term used to denote Inherent relative contrest of 2 given . emulsion and also the degree of development. It. may be expressed more , Clearly as the relation of a negative contrast (or density scale) to subject contrast or brightness range. Gamma is synonymous with the angular measure- ment formed by the slope of the straight {ine portion of the characteristic curve (8-C, Figure 8). As long as most of the negative density range falls within the bounds of this straight line, gamma can be a reliable guide to contrast. If most of the density range registers on tha curvec toe portion of the curve (A-B, Figure 8) it then becomes an unreliable guice to contrast. The preceding description of density, density scale, and samma is, by necessity, an oversimplification of each. In reality, the terms are - Interrelated to such an extent that each Is dependent upon the variable conditions created by the others.
Resolving Power and Ground Resolution There. Is a general consensus that the primary limiting factor In Identification.and mensurazion of small objects is resolution. This being the case, It is well to taxe a critical look at resolution, for the photoanalyst Is concerned with photogrephy mainly as a working tool In order to identify, describe, and measure an object. through tha medium of the photographic Image. Again, as in the problem of optical acuity, we have to consider the camera, the film, and the eye. For any optical instrument a theoretical limit of resolution may be calculated on the basis of the wavelength of light and the diamater of thd lens. It is sald“that, for the eye, the Image of an object must have a diamater of at least 0.002 mn on the retina (46). A circle 0.1 nim in diameter at a distance of 12 Inches from the eye should be Ccistinguishaosle. For the eye we have, whenever necessary, additional magnifying devices beyond the present capabi lity-of any film. ° _ soot _1# we are to work up to that limit we should have 2 clear understanding of what Is meant by resolution on photography, how resolving power is derived and measured, and just what vzlues and problems it contributes end leaves unsolved tn the analyst's quest for knowifig what he sees and his attempt to mean what he says. How Is a certain resolving power applied to 2 particular .- ..« -ftlm, and of what practical use is such knowledge to the anaiysv? Is there . possibly a better or siapler way to define that quality of .the film which .. - Is paremount In Its use? ‘ Photographic resolving power, as an Industrial conceat, is the capability of the cemara system, tens plus emulsion, to resolve ans reprocuce fine black ard white iines. Tne finer the lines resolved the higher the resolving power of the photographic system in use. Resolving power Is expressed in lines per millimeter. . . These three statements generally constituTe a photoanalyst's introduction to the capability of the photographic maverials which ere to be the tools of his trade. If he Is Inquisitive he may attempt to learn more, and what he . usually finds out before ceasing to be concerned is that the-Lines/mm-.is... -.. convertible to the ability of the photography to reveal the imege of an object _~ of certain dimensions at a certain scale. He discovers thev, when working «°° - ‘with. a certain photographic system, he should be able to see, under magnifi- ‘ catian, objects that are, say, ten or five feet in width, whereas with another —-- -systam he should see something only one foot wide.- As he becomes fami lar. a
{ . . . < } with more and more photography he realizes that In many Instances he sees ’ perimeter’ fences around secured sites, that such smal l-scaled objects as electric power lines and guy wires of towers are clearly ciscernible on. coverage which Is not black and white but actually a motley assortment of grays. He discovers that, stereoscopical ly, he may see through haze or overcast sky, and that lines/mm is most often a completely disregarded term. He Is not necessarily disturbed by the Increase of knowledge, but he {ts by the lack of any clear explanation of why and how there seem to be. sr only vague connections between theory and practice. Let us Investigate. the former and attempt to’ put the latter on a reliable base of reasonabie terminology. rs . .. Le The concept of resolving power and tne tests ¢evised to determine the resoiving power of an optical system originated in astronomy. There, where the photographic telescope Is turned to the night sky, I7 is always possible - - to see the Individual smaller stars, although it can be known beforehand, that _. _ acertain instrument is capable of detecting stars of Certain megnitude. It — Is sometimes difficult to separate the Images of two distant points of light, _such as double stars. A standard tes+ for determining the qualities of a iens capable of resolving such stars was developed by G. 5. Airy In the 19th | century. Deane ee tee sate A lens, whether sing!e or complex, does not produce a perfect imase. This means that a point object will not produce a point ime but a central - . prominence surrounded by a blur or diffraction pattern. A single poiat Image, If large enough, is distinguishable as a blurred-entity. Innis ~~~ * studies of jhe size and relative fuminence of the image center and The ...  .... surrounding zones formed at a focal point by a beam of lignt passing through a lens, Airy proved that the pattern was tne function of the wavelength of light and properties of the lens in use. Because the central point in such 8 pattern is so great in comparison to the diffraction pattern surrounding ne ' it, the fatter was disregarded end the central disk is said to constitute ------ ~~ the image of a point object. This is known as tne Airy cise (Figure 10a). oan Unless centers of two such points ere separated by a distance equal to the. radius of a single point Image the two points are not separately distiaguish~ able by that particular lens system, in which case they.are said to be not-: .-= --". ‘ sesolved (Figure 10b). When a series of discs of known size is rated egainst | a certain lens the fesolving power of that lens or its amplitude reduction. ----~ | curve is cetermined along with its opticum focal. engtin 2. mine tee As the science of photogreahy developed, involving lens. capabiiitles and object sizes, the concept of resolving power was applied to it anda similar test was devised that is usec to measure the resolving power of . . . > . . - . . . (C) teow . , ‘ es : 7 ey ten cea: be tne . a
Figure 10. ae Dirtraction pettern ‘produced by perfect lens b- Resolusion between two close images. @. ; ‘. . . ° a er e - . . ; . . . . - . . eo . . . . . . . . : ; ° - vee - ° . . t. . . .. : . , - ee = ec . s * . ; oe =. o mare Cd $ . . ry Ad ‘ iad ee . ct ~ Tt i.e. oe » ° ~ a : . ones
‘“ ‘the system used for aerial photography. Lenses used for aerial photograahy . ere subject to tests, both visual and photographit. The test with which we are concerned here is the resolution test, in which standardized charts are . photographed at different focal settings for the lens. When images on the finished negative ere found to have the best overall definition of the smallest possible test pattern, the equivalent focal length of the lens is determined. . The resolving power ot the lens Is cetermined by the rating of lines per millimeter of the finest line pattern which is fully resolved into separate fines on the negative. The exposure and development of the film must be adequately controlled to conform to the test situeetion. If the “ width of a line plus an adjoining space in the patvern is 0.5 mm wice, and the rate of reducticn from the photographed chart to its image Is 40 to one, the actual width of the finest Ine and space on The chart is an elishtieth of a millimeter, and the lens Is said to be capesle of resolving 80 lines/mm, This Is lens resolving power rather than photegraphic resolving power which has a test of Its on. For resolution of two image points from a good camera lens, the necessary separation Is such a small quantity that it causes little difficulty in the production of good aerial photography. This is because the size of silver halide grains, the active Ingredient in the protograpnic emulsion, Is so such larger that granularity of the emulsion is the present . rt ~ i. a ee va “ae mse “a wee tm ae ee . By Its very nature, camera film, which is made up of a more or less - thin coating of emulsion on a fixed base, cannot capture the full integrity of an Image which comes to it from tne camera lens. When the image talls on the photographic saulsion it suffers a certain diffusion which is relatively more serfous ine smalier the defail.  . » : . . -_— 4 In order thet an image be visible when a photograph is enlarged or ‘magnified there must Se a higher poventiai of visibility than that limited by the scale of the original negative. for the fiim, the upper limit of effective magnification usually occurs at the point when The diffraction . blurring of the image on the film becomes a distraction sufficient to offset ' the gain in visual acuity. In other words, the photoenalyst is accustomed — _to using film under higher end higher magaiticavion untii if "falls apart," which Is when the graininess of the photograph interveres with furtnaer ‘analysis of the image. The present iimiz of visibility under. magnitication Is thus seen to be set by the crain structure, or granularity, of tne emulsion, the indivicual silver halide grains. The film therefore has limived ° ‘resolving power which is a ‘limiting factor in photoanelysis. Typical values of film resolving power are supplied by the manufacturer. . . . a ate le : so cae
‘ 1 om ao a one as iaueoasenal - @ ews a ~ a Lote - . * e ve. arr . - oe =. . -* - « e -" ¢ eons . - - wo ae A ee -For typical aerial camara film there ara two defined sets of resolving power presented In the IInes/ma framework. One is the total overall resolution and the other a contrast. factor relating to bri These I tnes/mm limits are Innate to tha film emulsion, image quality sets In. factors, gamma and contras jens and film In resolving power. flim emulsion that can be evaiuated ' capabl itty of a camera le two. activated In-flight. when they should be using the latter. of Its component parts, Is known or can be determined: before the.system, Is: its meaning Is of real significance to photogrammetric 9. ee Ae et, CST RRS within the limits set by the resolving powar before undue ceterioration of ocessed nesative Image can be megnified only ghtress 2nd tone, nd It Is not essential to think of tha two emulsion . +, separately in order to-see the relation between ‘Thare are extractable qualities of the | against the focal fength-resolving power ns which permit most effective combinations of the : The ‘terms resolving power and ground resolution differ In meaning but In discussion of photo Interpretation analysts, often unknowingly, use the former The resolving power of a system, or any and optical design engineers decause it relates to the objective capabilities of the averail photographic system. On the other hanc, sround resoiution is. a measure of the capability of tne system as expressed by resuits obtained on the Imagery, which are affected both by in-flight-conditions such as light attenuation, contrast ratio, anc image motion camaensation, and by the - reproduction and processing of the film In the laboratory. . . oe The photo analyst ts most concerned witn the ground resolution of a SYS ‘a a fee, Petr al Fe Sa "Bye ‘Dy practica however, this does not hold true for many of the & precisa meaning of the statement ots: faccnwe This is a measure of the ability . size and separation with an object=- obserwad. eke ate ge ei teace es 9am Feguirements. Fo When the photo analyst is in 6 ; LOM iia To OSTA : a to rasolve a resolution target of uniform to-background contrast ratio of five-to-one (Figure 4 aa One must uncerstend 7h Items fm a natural setting have, at b3s7, a3 tvo-To- would saduce one's ability to resolve a Target witTain the stated one-foot . figure. This does not mean, however, objects much smaller than udincus a meaningful degree of accuracy. see and identify power lines when the cb to-cna coatrast rario.. This ... and/or light Is reflected from the object In a specular man (43 + . ager a0 OST Oy Os . sages UE eee be ste ee ree 3, PX Ne pee, ~~ wee”, : . , . ° * se om My wee. PMieipeaet fe ae tem and what {ts Implications ara in ‘regard to fulfilling nis specific . r this reason it is felt thet sround resolution, | es ie warrants more detailed discussion. . formed that the ground resolution.obtained, af is one foot, It is generally his conciusion that one-foot _ eee the imagery, are icentifiable and measuray le. . $0 items +hat one cannot detect and identify but onty thet they cannot be measurec to A perfect example of this Is the ability to ject-to-background contrast Is favorable | ner (Figure 4). - . toy . eh EE Or ey, | |
Dan J _ Resolution, by definition, indicates the ability of a@ system to render a sharoly defined image. Many Items, sr2ifer than the statec ground resolution, are detected and Identified even though they are cefinitely, not sharply imaged. The reason for this Is most-often the result of identifica- -- tion by association; aided by an analyst's skil! and experience in a particular speciaity. Object pattern and the knowlecce that, in a given set of circum- stances, @ particular Item could be present, [s also an Immeasurabie aid in detection and identification. est el wots In further assigning the proper perspective +46 a statement of ground resolution It must be understood that, two adjacent targets, one oriented in ‘an X-direction, the other In a Y-directton, will have different resolution values. This occurs because all Image motion is not compensated: aa Bee Fd ae See tee CE mete Geegeep ee nnate eg ’ This is nat necessarily a detrimental factor in Cevection and icenvitication, but It must be considered In mensuration. Rather than applying 2 resolution | value to an entire mission, it should be used as a comparative figure, ; " Indfeative of relative resolution between two or more missions. + It may eventually be possibte to derive factors that can be apolied to - “ aerial image characteristics to determine photographic resolution of chosen emufsions. But because resolving power cepends on contrast the resolwing power Is lowered under flelé conditions such as haze and other factors thet tower the tone of an object. Resolving power actually performs no function — In definition of an object by virtue of distinguishing its contrasting parts. Higher resolving power tn a system is a laboretory or comperative factor only. In review, it should be emahasized that ground resolution in aertsl photography is not to be recgarced as 2 characteristic of tha system itself. It Is a threefold combination depending on the Iight source anc its energy;. the Instrument and Its capabilities; and the receiver and its sensitivity. It becomes rather meaningless to spezx of resolving power unser these : - cireumstances, but the practice is convinued. Coaveational resolving power concarns only calculated images and has very little meaning compared with the ground resolution to be apatied to detected images, which is jthe value of practical interest for those who work with seriai photogrephy. Boo Sr . CF Wits. . . . is a. re . . ‘ Photoanalysts are realty interested tna scalar value that cen be . apptiied directly to the film and used to deriva actual dimensions of the object Image. At present this value is best: expressed in teras of. around axe resolfation, which is minimum object size measurable, —. oo. .- wen Mesias ote . ot " ~?, + “, But even grounc ~ssolution > i hot constant because landscapes are h6T plaae surfaces, photogreshy is most often not vertical, and the Image may not be at the nadir of the, or photograph. ; a eee EPL
FIELD CONDITIONS ee The Nature of Licht pes oo ee Light is radiant energy evaluated in proportion’ to Its 5 ability to ‘stimulate our sense of sight. Through space it travels In straight lines, commonly referred to as rays. There Is a repeated pattern of e:ectric and .magnetic forces along the path of such a ray. -The unit of the pattern in | this repeated sequence is the wavelength of light. Light waves approximately 400 to 700 Millimicrons long are normally sensed by the humen eve (Figure 11), and are said to be the link between the object which Is seen and the viewing eye which sees the object. For a given amount of light iatensity the eye is _ relatively insensitive to the red enc of the spectrum. Sensitivity increases through the spectrum from red to yellow-green (about 554 millimicrons) and “geyond that point diminishes to reach insensitivity in the violet region : (Figure 12). The exact .wavelength of light which Is most readity visible aan changes with the inteasity of light-as is shown In Figure 13. Light perception and color sensitivity vary from one person to another; the variations in these, however, are due to human factors and are not physical’ attributes of the ‘Ught, “= * aS such. ‘The scattering of sunlight in the aimosphere results In-the phenomenon. ‘we know as daylight. When the air is sufficiently clear we recognize direct sunlight and extended visibility. When air contains epprecladie amount of water vapor, dust, smog, or other parvicles, differentiated as Rayleigh or Mees atmospheres cepending on elavation, known coilectively es haze, there Is weakening OF suniigny as 3a result of loss of IIght scattered skyward ac2in,. | -- as well as limitation of distant visibility which results from actual inier- ference of particles and from diffusion of light in the surtece atmosphere. Light rays that undergo either refraction or refiection in the course of -_ ‘passing through the atmosphere experience scme ,polarizat Ton before reaching a _ an object on the earth's surface. Because blue light survives the journey better than the other wavelengths we heve the impression that the sky is blue. The deptn of blueness is a measure of the amount of {ight broken.up ...--- chromatical ly. ta the varfous shades of blue the discerning incivicual ls recognizes certain seteorological or atmospheric conditions. To such an | 7 Individual even tne elevation of the sky is not a constant. "“iiow high is the sky?" is not an idle question. oo - ~~
. MUSREOn mos  Prequeneyo}20 20 8? 8 ms 5 74. Ste pikes , L . . ‘ : . ; ‘ Soe, re Figure ‘D. “Wave length-frequency relat tionship for the visible and - “adjacent spectrums for ight traveling | in free ‘Space. Poa ee a eee = wee bey . wie RIE . a, e , : : a ° . e ‘40 . . lege ie . “ . . c e. _— ry ~ oe . , . 2 «wun - ". ° ~ - - mee ee ; ceo wl ge, meee ee ec ee . ° zg - Visibility amve ot = soo plistoility curve for - . ~ ° very low light ieee! —— rormol ight levels . ; :7 ° *€7er1 : 7 iy . - . , - oe -- Os ° ; . a) Po] me ee 3 0.6 - $ 6 e "e Ps e 5 _ . a2 - og . ° a es 350 400 459 560 550 699 6359 700 759. . , ’ ° . Woveiength in Millimcrans ae . . “4. . ts ot ‘: ‘ "7 Miatet 1 Btue breenbeielpetied Figure B. Relative visibility of the normal human eye ‘at normal and - Greatly reduced light levels, showing the Purkinje shift. ~~ ee ” - \ -. 4 a i ev, . ‘ se tat oe OU "bs . eer Pe oteeee wD mo ec * an 7, Ly
r . Cy Light in the Atrosphere tee The Usefulness of aerial photography always depends on atrospheric and | lighting conditions at the time ang place. * an é | ‘graphy of great clarity, of complete lack of any imagery frem the earth's surface, or of some Intermediate state. To the photoanalyst the exact cause of the obscuration is of Interest mainly as he seeks to fully uncerstand the whole condition under‘which the achieving of photography is limited. ‘ _ In photographing: the light | reaching the camera from the ground through clean eir is aopreciably attenuated. The !ong paths of light waves suffer more from ettenuation. Obviously, the greater the distance the light must travel through the - | atmosphere, the greater the variety and emount of attenuation produced. All particles in the atmosphere contribute to some degree to the scatter-' Ing of light. Scattering occurs as either reflection or refraction end is one of the major processes involved in the attenuation of light waves, end, conse- quentiy, In the procurement of aeriai photography. It Is manifested by tha ‘ scattering of different wavelengths of light in direct prozorvion to the size of the atmossheric particles which they may strike. Therefore, the denser the atmosahera and the more diverse its molecular cemposition, the : smaller the aumber of light waves properly reaching the desired object on _— | the ground. , 7 Playing a less dominant role in the attenuation of light weves fs a absorption. In absorption, light waves are completely assimilated by eee atmospheric particles equal to or greater In size than the wavelengths of light attespting to penetrete. A high content of moisture in the etmosphere — Is the most absorptive condition usually encountered and the most damaging |. Jn the procurement cf usable photographic images. The common relegation | of absorption to a fess dominant rote in attenuation Is cue to the fact - Smee that it is significantly minimizeé on a seemingly clear cay, wheres scattering always occurs, regardless of atmospheric conditions. 2...) Ll. _ The majority of the light waves which ultimately reach the object on the ground is reflected beck into the avmosphere, either in a diffuse or . _ specular manner, and is subject to further atmospheric attenuation elong Its path to the camera. In addition, the effect of light radieted upward | from the atmospheric impurivies may be even more important. This light ~~ plays no part in the Image formation but, falling upon the catera lens . °
. tone . aw of ee seems . . Pre res ere! along with the image-forming light, It weakens the image and further lowers _ the contrast of. the scene. Tha term haze ig used for both the minute particles In the atmosphere which impece light anc vision and for the lack of transparency of the atmosphere. Its effects are irportant. faze mocifies _ Imagery both before and after the image-creating light falls on the object of _taterest. As light ts tiltered earthward through haze It becomes diffused ‘and scattered instead of direct, with an apparent tencency towere scetrer in a forward direction. Fully diffused light Is Incapable of creating shadows; ‘It limits variations in reflectivity from unlike surfaces, anc is devoid of | ‘gtrong contrasts. CS mea th tog elta ee’, oF “a ° ‘e we te eo ele THe De a a + Pe ery : . ° . s ‘oe 378, tor oye. 8 , The scatter of blue light by thin. mist or haze is Sixteen times the scatter of Infrared light. ®8y epplying an Infrarad filter to screen out the scattered blue and other visible light waves, a clear picture may be - obtained through mist or haze. However, the desirability of this procedure ‘fs dependent upon the sensitivity of the recording film to Insure that there _ [s rm appreciable loss of resoiution.  Ultra-violet light, which is said to + be scattered by haze In the atmosphere +o a much greater extent than is light In the visible spectrum, “is particularly effective In causing deterioration of the photo negative, which degrades the picture contrasv. ‘There adoears to be ro way to measure the amount of atmospheric attenuation encountered on photo imagery. At present the only way to quantity atmospheric conditions is to use verbal! description. Such terms as clouds, haze, fog, and poor imsge cuality are the most commonly used. If Iv has been - Impossible to circumvent uncestredle atmospheric effects in the camera end film systems, there is nothing tne analyst can do to increésa the information transmission capacity of the Imagery. - a —— - - e- . se a ro * 4 SN ORERE one ee were ePre swe eeee Nee gee
Reflection an¢ Imacery Light. reaching through tha atmosphere to an object Is absorbdad,- transmitted, or reflected according to physical laws governing light propagation (Figure 5). The absorbed portion turns Into heat exergy which may be detected by special non-visus!l sensors. If some of the light Is “transmitted It passes through the object In obeyance with laws concerning the density of the materlals through-which it passes. The reflected light, the portion In which we are interestec in aerial photography, is reflected Into the atmosphere, carrying potential Images of the odjecis from which It Is reflected. SO , ‘ tee a 0 Ly Only by being reflected from the.object does light convey the image to the camera where {t is captured by the film, or to the eye whence It is conveyed to the brain and transiated to a mental image. As the atount of light or energy Increases, tiie ability to capture an image or stimulate eee the sense’of sight varies from zero to a certain maximum and beck to zero. -- again. With complete lack of radiant ereroy, image projection is Impossible. ‘Optimum light falling in clean straignt lines on common surfaces creates --.. — sharp contrasts, strong shacows, end clear visual images. Gree ver absorption. .... *- of ‘light by an object results in darxer Imagery; greeter reviection causes.  -+— lighter values. OQverabundance of light reflected from a surface, whether _. diffuse or speculer reflection, obstructs the image and prosuces in its - place a formless biod of Light (Figure 7). Within the Image precucing range. _. the actu2! amount of light reaching the camera flim or the retina of the - ~~ eye ts less Important than the clarity of the Image which it trensmits,: tne sharpness of the Image boundaries, the strength of its contrests, the perfection of Image transmission. . __ The reflecting capability of an object or surface veries with the nature of the material from which the object is mace, with its surface, and with the angles that its verlous portions, large and small, subternc to the direction of the sun, the eye and camera. Where the surface of an object absorbs and transmits much of the incident tight a lesser amount of Jight will te reflected (Figure 5a). If the object surface is rough in comparison Te the wavelength of light, Its elementary sections lie at various angles to the direction of . Incident light, and the reflected rays are scattered In many Cirections. - If a the surface is minutety rough so light is uniformly scattered tn-all directions the reflection is sai¢ to be diffuse (Figure 5b). A matte surface is diffusely reflective. Diffuse scattering of light seriously affects object definition on & photograph. On the other head, a smooth metal object or water surface - reflects light from a single plana, the surface, and mainly In a single direc- - tion. This Is called specular or regular reflection (Figure 5c). oan
e it Is Importeat to note that for each ray of light the angle at which It strikes thé surface, the Incicent angle, determines the angle at which It Is reflected (Figure 14). if the sun is high in the sky It will be necessary to ba closely above an object to catch strongly reflected IIght. We earth-Sound creatures have more occasion to experience Licht reflected ~__ from low objects, such as house windows facing s sunset, or headlights ~ from oncoming cars shining on rainy pavements. Very IIttle effort hes been made to clarify what actually happens when ; specular reflection. strikes camera film. To fill the, ghotoanalyst's needs the explanation Is simale. in aerial saotography of highly polished or smooth surfaces, the amount of direct reflection may be so great as to ‘completely destroy the Image of the area from which the reflection comes. .OCestruction of such an Image results not from the actual faiiure of tha light - to carry details or outline of the Image, but from the incapability of: the film coating to absords tmzectataly the overabundant light. Tne resulting Irradiation within the emlsion has the same net effect as the inability of the buman eye to see throush 2 blinding light. . __. The quality of an Image Is determined by what hespeng to the Light refiectec from an object as a whole and from its component parts. Image ~ sharpness and object definition on a single photosrephic missicn with a given quality of film may vary from excealient to useless, Cesencing upon, the clarity of the light which reaches the object, the reflecving capability of Its surface, and the perfection with which the Image is transmlived through the atmosphere to the camera and film. If the surface which faces the camera has variable light absorbing capacity, this shoui¢c be revealed by variations In gray shades, from black to whita. The shades and variable qualities of images are referred to as tone. Distinction between tonal values, their contrast, [s as essential to photographic Imagery anatysis as light is: to vision. . Myc ee we mem ee
Figure. 1k. Rat + Ret om! ~*%s . oe . o Z s = = “% . oe ad = eer e-memeae - ationship between € ray incident on a plane sur fece and the reflected and refracted rays which result. oe ‘Wm, we A -¢@- -@ 7 bad e ~ . - neiedind a e 2? ' - se eo. e ~. REFRACTED . RAY 3 . . ey - . . “. . - . . « ° : . a - ; . - . . - - . a ° 8 - . . « o 4 . . e . -"* te bd . e e ° — ro ° A . ° . . . ° Ss .e - . - e e a « . . - . . . . ® ‘ e . . . . = ° . . + bad . e -* e ° nd CY . . . ee = . ee . . - . > . . : ce * 2 8 eos ‘ae . .8 se . . e woes mee. . -? - . * - 20? . . . . : e tf een ee » ° e . e re a. Se ire % mas . . . e * e ' ° . ee ee? . . "om = «® . i . . a ee 8 » - wh te oe 8 e * cy e . ‘ . ( o: . * . . - € oo? ° - ~~ +? - ae *s . - . ° . . od ee
VISION - | Its Relation to Licht , Astronomers have found that If an observer is placed In a dark room having only a very small opening towards the night sky, he can see stars of magnituce 7, 8, and even 8.5. This is so becausa the stars are then seen against a dark background. It is the brightness of the sky on which the stars appear which prevents us from seeing these dim stars uncer ordinary conditions of observation. The Illumination produces by the faint- est star we can see with the naked eye is about 107% times that produced by the sun. This means that the earth receives from the faintest ster, per unit area, an amount of light which is one hundred million million times Less than that received from tha sun. In tests determining the threshold of vision the amount of radiant energy or light required is taken as the enercy content of the flashes which are seen In a proportion of the trials equal to 55 percent fraquency of seeing range from two to 5.6 times 107 erg. The mechanice! energy of a pea falling from a height of one Inch would, if. transformed into lusinocs energy, be sufficient to give 2 faint impression . of light. These figures tell us something of the high quality of our visual sensitivity. , The deciding factor [n vision is not the amount of light but the | . wavelength. The sagnitude of the enersy quentum is inversely prosorticnal to the wavelength of the rediation, as is: shown in the following exemple. _ The energy of @ quantum of vioiet light of 400-millimicron wavalength Is twice as large as that of a quantum of red light of 800-miliimicron wave- length. A flux of light does not behave as a stream of Individual quantum particles, lixe a series of bullets. It is only when fight inverscts with matter, for instence when it is absorbed by the retina, they its quantum properties come into evidence. . » . . tn the case of a human eye, vision Is achieved by its refractive system. — Light coming from all parts: of an object falls on ali the external perts of the eye. As @ result of the passage of light through the transparent media of the eye, the rays originating froa the same soint of Tne object are, iceally, reunited at ore point of the retine so that to each point of the object which - ‘emits light there corresponds a point on the retina which receives some of this light. These corresponding points are arranged in the same order on _ the object and on the retina. But no optical ‘imege Is ever completely sharp and clear. ‘Because of light diffraction and optical aberration the average eye does not reach the theoretical limit in its functional ectivity. in | Pere mee cng es, . Sve eee eee ee, + :
“, ery ’ - *, ft seeing, tha sharpness of the image Is determined to some de sre by the accuracy . with which convsrgence of light onto the retina takes place. Normal visual aculty approaches the limit imposed by size and spacing of receptors in the fovea. The sensitivity of the eye is so high that it comes vary clese to the absolute limit sat by tho quantum props Ties Of light. Its maximum. accuracy In the discriminetion of form is high enough for the weve propertiés of light to have a strong. bearing on the sudfect. It Is expectzdle therefore that the theory of the natural limit of sensitivity and accuracy of physical Instruments be apolied to the study of the physiology of vision. But it is equally expectable that not all attributes of vision are directiy cemparable to those derived as either attrioutes. or limitations: of. such physical instruments . - Bed rn 3
ey Visual Acuity ; , Acutty,.a dictionary synonym of sharpness, is also cetined as the ability to see fitre details of an object, or 2s Keerness of sense perceastion. To the . degree that it is mechanical, visual aculty is comparable to the resolving - power of & camera lens. As 2@ physical phenomenon, visual acuity Is measured in terms of a critical dimension (ciameter, width, or orher scaiar value) of a black test object just berely detected on a white dackgrouns. It is defined as the reciproca! of the visua! angle subdtended by the smallest details which the eye can distinguish, in this case the thickness of the dDleck bars, simiiar to those In the camere and film tests. This angle is expressed in minutes of arc, sa that if the thickness of the bars which are Gistinguished corresponds to an angle of one minute, the visual acuity is unity; if it corresponcs to 60 minutes, the visual acuity is 1/60. .Standzard visual acuity is cetined as the ability to see an odject so smal! that its image subtencs aa angle oF only one minute of arc at the eye. This one-minute stendard is said to be used in optomatric tests invoiving letters on wall charts. . - Tests of visual acuity 2re comzonly nade with black or colered test forms on a white or luminous tecxground. Recognition of such objects depends on other variables besictes size amd shape, and on the whole is thought fo depend ‘on diistinction between object and backgrovtnad over 2 sufficient distance to permit discrimination. Wher Steck test objects are usec with a sutticiently wide, luminous backsroumd or surround, If is found thay visu2l acuity increases with Increasing backgrounc fesinance until it reaches e maxinum valve after which It.rises no farther. This is in keeping with the everycay observation that in dim light we ere unabie to read fine print which is eesily legible at higher liluminations, whife om the other hand there is 2 limit to the fineness of detail which can be seam with the naked eye even in sunlight. Oojects of | lower contrast aeed to be iarges than 2 black-and-white tesT object in order” to be detected. . - In one field exoerimant ft was found thet a wire wes seen egeinst a sky of high brightness when its dianeter suotenced only heif a second of visual angle. For this it was fousc that the wire nad to be et leest one cegree long. Shorter wires had to be thicker. Silhouetted squéres were seen at angles of only 18 seconds. Consitering length and width, squeres were more efticient visual targets than fine fines by about a fector of three. In another test It was’ demonstrated <net tha seeing of fine wires in horizontal or vertical meridians was superior to such seeins in oblicue orfentations. This may be of Interest to anyone concerned with imagery because there are, apparently, no dioptric fectors to account for such’ selectivity. In another study of visual texture discriminetion and death perception, results are belleved to wwe 5
a soe ~ e De $ * é show thet connectivity detection is dasic to both of these visual tesks and that Wt Is a more primitive process than form recognition. From such tests as these one may anticipate furvher understanding of The visual process which may be of service to.the imagery analyse. on . . -. : ‘ Te tn actual ‘practice, tha ‘ability to see and the objects ‘that one sees co not necessarily conform to theoretical findings. This is.particulariy evident in photoanalysis where much of what Is written on the susject is reported as the results of tests Involving either purely mechenical ‘bases or human vision under prescribed conditions. There is no doubt that much of the testing and reporting is both valid aad valuabie, and that the better reac analyst is the most versatile in appreciation of his own visual processes. However, it must be realized that the experience of photoanatysts coes not occur under controi ied conditions but rather over the full renge of light, contrast, oSjecy variety, and scale of necessary definition. The purpose In cartograshy is to reproduce as faithfully es possible exdetly what is seen on the phovegrapny used for that purpose. That cartography and imagery analysis are two cifterent fields of competence Is strongly | avoweg by anyone who has changed fron The former to ‘the latter. There seems to have beea litvie investigation o7 whet an snalyst really sees ca photo imagery and the mantal comparisons and processes by which -he produces his findings. It Is not surprising thet the meture analyst hes . developed reflective and reesoning abilities that are not verified. oor - en gare ° 7 Ne —— - Senet ee aaneen: ed oe Ree me, SPE ree Mrmr ee geen On EE rer ere one
wn . ° “we Stereoscopic Vision There appears to be some confusion as to Just how stereosconic vision takes place, both as a visual activity and with ressecT To the attrisutes of the two images that enadle it to result In three-dimensional perception. The exact nature of the seeing process Is obscure end Is not a pert of the current discussion. But the attributes of the two imases thet ere detected and analyzed In the visual process which results In stereoscosy are tha subject of some concern and ¢cifference of opinion. On the one hand it is sald that stereopsis will occur when correletable stimuil are out of phase with respect to some reference system such es another set of correlated objects. Any mechanism which can detect the correlation betweén the binocular stimuli and also detect a difference in their pheses can yiel¢é representation of depth. Fusion is not necessery. A slightly ditferent dimension is given to the problem In consicering disparity, prodedly the most responsible factor in stereo acuity. it ts sald that disparity exists between two points when they occupy different visual dfrection. Disparity is an adequate stimulus for stereopsis when there are other points of different cissarities in the flel¢; only similar objects can fuse and therefore only similar objects cen be seen in captn when thay ere disparate. The fusion concept fails to precict outcoma where depth eccurs without disparity excest on the outer ecge of the patterns. Elsewhere it hes been said thet monocular acuity cesends on a process of boundary recognition, whilst stereoscopic acuity relies on patrera recognition over large areas. it ts not exsectec here fo expiore tha subject further - except to observe that tne exact nature of the process is important to the photoanalyst because of its bearing on the processing of film To be used in stereasconie study. If ft is not known whether the effect of fusion or disoa-ity, or the recognition of edges of areas, Is the more imsortent, there is a question asto the desired processing of an indivicual negative or positive for special study purposes. Solution of such problems es these should be sought from actua! operational material and varied processing rather than from laboratory test objects because the latter do not incluce the nature. variables which are found in the former. - K clearer, more meantagful concest of stereoscopy 2s the photoanalyst makes use of It is contained fm racognition that the abso!:7e convergence of a single point never determines its azoerent depth in a stereoscopic Image. its relative convergence with other scints Is used by the brain to locate it with ragard to those other points after the geseral position of the Image Is fixed. In binocular vision there is a fixed tie between accommodation and convergence. Thus in looking at a point two feet away the eyes are focused for a two-foot distance and the axes of the eyeballs intersect et two feer.. . e . a : a
i . 4 . t Regardless of the distance of the fixetion point, nearly he seme engle of convergence always goes with a given focus, only varying siightly for objects not directly In front of the observer. Soth change togethsr as objects of . different cepth are ‘viewed. In stereoscozic vision this established tie is broken. The two views of tha point on the stereocraph determins the conversence. This convergence changes with changes of seperation between different points on the two views. However, the accommodation remzins constant, 1.e., the focus of the eyes Is either fixed for the Gistance from the eyes to the stereograph or by a distance cictated by the lenses of the stereoscope--usually infinity. This spilt of the acconmoda7 ion-convergence habit is the outstanding difference between binocular and steresscopic vision. Stnce the eyes ere presented with a situation foreign to binocuiar vision, the psychological Interpretation tends to difter from.that of binocular vision. . . In very simple stereogréphs containiag only separave points and lines and viewed without a stereoscope, the resultant intarpretation tends to place one prominent point or plane of the pleture--called the "fixation" point or plane--In or near the plane o .the plate. Thus one point is | approximately determined from the accommodation. The death of the rest of the points of the picture are interpreted from this point in eccorcence with~ | both the relative convergence with this poimt ance the other cepvn factors refating the points and objects to each other. ° ~~ mame,
t r, 4, . “2 CHARACTERISTICS OF THE IMAGERY ~r 7 — Tone ell The tote! Image on overhead photography exhibits a variety of related conditions: ‘tone, brightness, surrounds, sharpness, end contTrés 7: whose effects must be evaluated In order that object recognition end csseriation may be accomalished. Of these, tone and brightness. are direct aspects of object appearence and result from light modifications as the object moculates or attenuates the Incident radiation both in intensity and color. The quantitative relationship between the light or photometric Input to a photographic system and the output of thet system is expressé. ia terms of tone of the developed photograph’ image. an Tone, generally defined 2s color quality or value, or a cistinguishadle shade variation, Is detectible as visible differences or texturai values in the gray scale or density level on black-and-white phorograpny. iv is the overall tonal quality that maxes one photograph immediately more attractive than another, and the textura! quality of individual portions cv the photograph that give It character. A very smal! inequelity in the tonal ‘evel of a positive may play a significant part in the information quaiity of aa Image ‘reproductions, and careful contro’ in tha photographic laporatory Is essential to first class Imagery. For specific purposes, such as imege enhancement, the tona quality of adjacent portions of the film mey be varies by controls In the reprocuction process. ; _ Tone reproduction is concerned with the relationship thet exists between the object and its .lmage. There are two distinc? phases in the convenviona! approach to the solution of tone reproduction probiems. Tha first phase deals specifically with the objective or physical relationships of the tone repro- | duction cycle. ‘The second deals with the sudjective or asychocshysicel factors which relate to the human observer. It is in connection with the first, or objective, prese that:analytice! methods of perticular interest to engineers have been developed. for the photoanalyst the main concern is the relationships between ‘tone differences on the photograpiic copy in use anc the assurence with which he can derive the image they represent in order to isenvity end describe the original represented by thet imzse. By tozal and textural evaiuation he distinguishes between a flat, ‘smoot, hare survace and one with notable death or minor Irregularities. A functioning railway (rigure 15) apsears cifferent. from a rail line under construction where the rails are lacking. Hard topped roads differ from dirt surfaces, grasslan¢s from shrubbery as 6 ground cover, acorn fleld from a meadow. , oa
° “. i . ~ i eee ~ . of‘ ° e . ne os ve ” y - ce FIGURE 15 BS . Photograsch showing rails and ties on railroad bed. ° » - . an po OTN 8! - S
Fation) . : "w= ol om Brighsness As changes in wavelength of light cause changes In tone, so changes In Its amplituce resu!+ in luminosity or brightness changes. Erishtness of Individual colors Increases with intensity or amplituce of iignt and is highest in yellow light. Eut tn tnis senso it must be cistinguished from. the curve of energy of the spectrum which rises, with wave:ength, trom the violet to the rec end. Brightness is The attribute of a lignt source by which It appears to emit light from a’ minimum for very dim to a maximum for very bright, the measurement seiag in lamberts or cerncle power per square centimeter. With the use of a photometer it Is possibla fo measure cifferences In brightness varying on the orcer of one to several million. Vision is said to be at its best when brightness contrasts are about 1:20, anc without visiole discomfort if less than 1:100. Much iower contrasts are discernible. Flash glare or spot glare causes strained brightness accommscaTion. The optimum level of brightness varies with the image. ; ° Actual leveis of brightness vary with clarity of the avmosshere, tie time of dey, angle of tha stn, and tae seésoa. Variation with clarity of the atmosphers [Is well illustrated in citferences in photograshy on clear and hazy days. Objects of different srightness In clear light sre said 'vo suffer the seme proportionai recuction in brightness by haze. With increasing light scatter the difference tends toward unity of brigntaess, and Taereby to decrezsed contrast. ° ~ Differences resulting from time of day are well recognizes Sy eircraty ‘crews who fly mapoing coverage, their flights generaliy 5e:ng restricted to the time of cay tn a desert region when the 2ir is calm, or in a hunid region before air humidity reaches uzcesiradle concentretion iater in the day. The relation of photogreahic orightness To angie of Tae sun is iflus- trated by the cesiradiiity of shadows sufficient to show some derail su7 ror to obscure brightness and outline of related images. Seasonal cifferances cut across ell of these. One investigator found, for example, that ina .. Washington June morning lignt is brighter anc June atverncon ligay ig culler than December's. Strictly seasonal cifferences include the cern quelity of burgeoring vegetation or The a.seco oF snow cover. The range of brightness which cen be acceptad.y recorded resresents the usefu! range of a photographic meterie!. invisidilivy is experienced when emulsion sensitivity is insufficient to recerd an overweak Light source. ; Irradiation results when the ugoer cimit of brightness .is overreecned. When specular reflection or overbrightness cecurs the emuision suffers excessive SO GO.
particie activation and the resulting Image Is only a nominal representation of the object. There are instances when specular reflection from smal | objects such as wires or other smooth metal objects thus bring Into the range of detection Images that could not otherwise register within the IImits of the resolving power of the film. The useful range of negative film is greater than the range of brightness Ia the vast majority of subjects =~ photographed. The exposure scale of positive materials ts much fess, and ~ In general the higher the gamma to which It Is developed the lower {fs the materlai's latitude of capability. — ee 7 ‘oat ~ Ideally, 8 photographic objective’ sho Id be capable of reproducing - accurately the full range of brightness in the object photographed. In_ order to do so, It Is necessary that each area In the Image receive light” — - only from the corresponding area In the object. When other than true image- . formiag light falls upon the image area, an adulteration of brightness occurs . and the. photographic reproduction lacks the brightness quality possessed by ._... -the arlgInal subject. If the negative Image were perfect,-and the-brightness —~. of the photo copy, translucent or opaque, varied exactly with the. brightness of the negative, the photographic image brightness should be comparable to  _ . - that of the object as seen by the human eye. Because exact rendition of --..... ) stage tn the process,-the [deal_image Is_. ¢ eet brightness fatis to occur at each never fully realized. . o-
Surround queer "5 f The surroundings, generally called the surround, of an object are Influential tn Its detection, enhancing the possibility when differences ere notable, minimizing it when they are minimal. Probably the most notable difference, and one often overlooked, ts the general site location. Aman- . made site Is most apparent In an otherwise natural setting (Figure 16). A misstle or Industrial site In the wilderness, a frequented waterhole; |. Eau), 8 trail across the desert: these are most obvious not because of... size, luminance, or tone, but because of differences In context. While ft Is true that what the analyst sees depends on the perception of differences $n luminance and tone, and that these are used to distinguish an object from Its surround, It ts Important to bear In mind that the term surround encompasses an overall view of the site In which anvobject occurs. in this respect we distingulsh between surround and contrast. ° --An object. anywhere ‘Is recognized because. It has different tone or a brightness from Its. surroundings. Brightness difference allows a distinction -- - of the border between an object and its background, which, according to tests, - ‘can be rated as.a product of the area and the luminance, one decreasing as the --- -- other Increases. With low contrast object and background the size of the object must be greater in order that the brightness differences can be detected. - It has been sald that larger targets require less contrast but more total flux -—- es the area Increases unt!! a certain stage Is passed when the required contrast becomes Independent of area. Although It Is also reported that the greater the ratio of tength to width the greater contrast required, It must be realized that under fleld conditions this ts not necessarily true. The relationship also vartes notably with shape and fine detail of a complex object. It Is difficult to compare profitably actual photographic analysis problems with such baboratory test serles on object and surround. In test studies the objects are most often simple geometric shapes of different sheces from black to white viewed under different brightness conditions. Under field conditions it ts only rarely that the analyst seeks a simple geometric object or a unl form tone. The surround of an object Is stable on any single or stereo view, but the contrast between the two parts of a stereo pair may be revealing. | This Is matnly as a result of the luminance and tone and not because of the change In the surround as such. The effects of variations tn luminance on recognition “of an object one dark or light ground is difficult to equate with the effect of object size, shape, and composition as it actually occurs on éne set of - coverage In comparison to another. It Is the analyst's feeling that there needs to be a clarification between the objective concept of surround and the analyst's Interpretation of the word. ene Le . . : ——g ie hie . - . Stree pee,
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eS Se . —~ Sharpness of Definition ne 20 Sharpness of definition relates to Image clarity rather than to contrast es 8 term denoting differences tn elther brightness or tone. Thus It Is not precisely synonymous with contrast, although the two are known to act together ~ to enhance or degrade the value of the photo coverage. For example, a hazy atmosphere degrades the definition of the object by diffusing the IIght before it strikes the object, as Is exemplified by lack of shadows. At the same time the atmosphere lowers the photographic contrast, reducing both brightness and tone. But the two differ in thelr effects on Image clarity. Although the general effect of flare (non-Image-forming tight) Is to Increase Ii luminance of every polnt of the camera Image and to reduce the contrast, it Is said that over a limited range of exposure ‘a certaln amount of flare, or for that matter haze, can actually Increase contrast In the negative. It Is possible for the overlay which affects Image quality to be efther lighter or darker than the object of attention. a Se . ° In addition to brightness contrast and tone contrast, Image clarity Is affected by the depth of focus of the lens, by the position of the object Image on the film frame, and by the relation between sun angle and camera. In optical terminology the term depth of focus means the total allowable varlation of the sensitive surface, from the position of best focus, within . which the Image Is toferably sharp, according to a given standard of perform- ance. In aertal photography the range Is understandably wide and the image . clarity varlable. To oo ; as CTT tents : ‘Object distortion can be thought of as variation of magnification with fleld angle. Alf other things belng.equal, an object Is most perfectly defined at the nadir of the frame. It tends to be degraded with position . ~ elsewhere by tmperfection of the camera fens (no lens Is perfect), by the Increasing distance of the object, by attenuation of the image, and by the look angle of the object. The desirable look angle of an object Is not necessarily the same as the desirable mensuration angle. The look angle on vertical photography has been sald to be best If shadows fall toward the — rae " But the presence of shadow often obscures detalls which are more ve. me a _ wg ~ “ . . . wy . a = . - oe Or ar LA a ERNE omer Ne ree rir oe mera Sey. Me!
subjective sensation, re “s to the quality aspects of p° “fographs associated with which ease of visibiiity occurs, while sharpness of om object Image refers to ‘the degree of resolution of element detail as Is determined by. _ - granularity and resolving power of the flim. —_ a * Mechanically, the outline of a black object on a white background. fs _ . pg sharp tine, and the object's dimensions can be precisely determined an Theoretically, the contrast In photography of a black and white test object “> should be equally distinct. But, because the photographic emulston Is made” . .up of myriads of miniscule Irregular particles, the line is not a-sharp cut batween the white area from which afl color -has been removed and the black .~2° area which has suffered no effect from exposure.- -Instead, there Is a zone, however narrow, of gradual change between the black and white. The quality- a of a picture Is closely related to the shape of the curve which describes ——— the brightness transition from a light to a dark area, the-edge gradient.. _. aa, a eames tee eee FT. ~-. The-character of that zone, the steepness gradient of the curve reveal ing__ fhe remaining emulsion density contrast, or the- sharpness of the Image outline, Is referred to In photogrammetry as the edge gradient or the spatial--induction,- depending upon whether the Investigator Is concerned with the steepness or == 2“ declivity of the most nearly vertical portion, the “edge gradient; or- the width... of the zone, measured horizontally, between the density plateaus of the pure” ~~ -. white and pure black, which Is referred to as “spatial Induction. "In the , . mechanical derivation of ar obfect Image on photography -the.edge. gradient may—- be employed to distinguish the most probable locations of object perimeters, -~~ :- from which mensuration Is possible; or the width of the zone of gradual change, by means of which..a. longest or : shortest distance-between object perimeters ~ permits .a greatest or least size to be attributed to the object as a whole. —~ The procedures, howovyer, Is not attogether that simple because. : data. for the photographic mission, as well as object position on the trame, are_ entered. Into the machine calculation to account for additional mechanical =~ varlations in Image response. f And; -Just-as the visual_analyst's photographic ~ . .dmage clarity -or.sharpness of deftnition Is subject to environmental conditions” and ‘optical Itmitations In determining the object's-absolute position and size, - go the photagrammetrist has prattems of equal complexity In his determinations-- _ made from’ mechanical resporse to the. photography.) ~ - toe cep ee wel ~o ne - eof - o Pied . . . al * 7,
Contrast _ ye wes Contrast Is the expression of difference In brightness, color, or tone, ond tn aertal photography analysis It Is used to denote recognizable differences ‘$n both mechanical and visual stimulation. {!n mechanical considerations, negative contrast Is defined as the difference between maximum and minimum. ° densities of the Image. In this context the contrast of a duplicate positive may depend on the brightness range of the subject, the degree of exposure of the film, or the degree of development, often confused with gamma which Is ~ retative contrast. The terms contour, border, and outline are equated to - contrast location. in. that they are.used to Indicate “the narrow region that .. -- seperates something from something else on a photographic Image or other two-- -- _dtmanslonal representation. In photogrammetry contrast differences are measured --Im a variety of concepts such as spatial Induction, edge gradient, object . ma madulation, modulation transfer function, and others. Each of these Is basically. ae analysis of the horizontal or spatial difference between two adjacent, _ - weasurably or visibly different: fractions of--the Image, or between two recog- —~ nizable differences In density of the emulsion. Unfortunately, In the emphasis ‘on mensuration If fs frequently the deriving of -the--exact-location of a -- ----- _ boundary tine that appears to be the most Important contribution expected of _ “=~the- photo analyst: =. That. analyst: makes. a distinction between.a target which ~— ° can be discriminated and described because of contrast and the evaluation of -- = - a minute area which out!ines differences of brightness or tone, such as are oe “The subject of laboratory studies of contrast. a “et 4 : Veen - tol ae —--->—Contrast-has -been.equated. with. brightness range of a subject; completely- disregarding tone. But contrast may also vary from weak to strofig according >=— to the degree: of differentiation between-tones of.a photo Image. Pattern -... .._. recognition by the visual analyst depends as often as not on non-adjacent — ‘ variations. In contrast, elther luminance or tone. tn this context contrast has .long been used_In a descriptive sense rathey than as a mensural quality... And, as the Imagery analyst uses ff, Contrast cam mean the comperisen-of ~—-—— relative luminance and tone differences betwéen two or more sequential Images - —- of “an-ob fect. ---— ~—-- -- be nee. : coon Visually, though not for mensuration purposes, the exact edge of a contrast read not be precisely located, but the concept Is confused by statements which. .... mete distinctions between surface contrasts on opposite sides ofa fine of sepsration, and the Implication that contrast occurs directly at the border. It Is true, photogrammetrically, that a contragt effect takes place at a IIne of separation, but In fmagery analysis the actual contrast between two things Involves two areas rather than IImitations of the edge or space separating
ee aay . 5 ‘ . em - the two. When sharply outlined edges or contrast borders eccur In photo "ona bhack background, for example, will be resolved if they are about.tive ~. Imagery there Fs no detectible Increase In contrast across the borderline as distinct. from the contrast of adjacent areas. As a matter of fact, In analysis experlence on aerial photography there Is, probably more often than not, a natural decrease of contrast near the edge. A few examples - willl serve to Illustrate. A concrete highway does not Immediately abut the . grass or forest cover of the area It transits. Invariably there are ; shoulders or disturbed areas with Intermediate tone. A sianted or rounded ae roof loses contrast. near the edge, due either to differential reflection — of_ light or to diffraction. A pond of water Is almost Invariably surrounded by a molst but not surficially wet shore zone which Is frequently undiscernible on black-and-white coverage but nicely detectible In 'certain-chromatic — aaa * photography. - — . , Te ee The behavior of object contrast, both as to brightness and tone, must... -be considered In the recognition or detection performance of a photographic . system and the resolution-of a‘standard film type. According to a report ==: on tests on object recognition it has been found that if a tens Is found to - on have an angular resolving power of, say, $:2;000 for.detail of contrast 0.2, ~ _ -~ it wlll resolve from 20,003 feet details ten feet apart on the-ground-If_.| ----.~: ene ce their contrast with their surroundings Is 0.2, but much smaller objects will ~~. ‘be seen In the ‘negative If their contrast ig-high enough. ‘White objects .... _.-| feet apart. Conversely, objects of lower contrast than 0.2 will.be seen LT nae the Imagery only if they are targer and more widely separated than ten feet. In this case the contrast differences are due to object and background tone + ~~. and not to changes In Suminous flux or brightness from test to test. 02 wae see ae ae -~- It should also be remembere¢ that single objects will often be recorded —— at sizes smaller thar. the Limit of resolution for two adjacent objects. -As —--: +. the camera gets- farther away from an object, its.Image at first gets smal ler-—"7~ -. In accordance with the faws_of geometrical optics, but eventually a stage Is>->--: reached at which the-patch of Light is the smallest that the lens will. glve./.> - -If the camera continues ta receda the size of the image remains about tthe---  - same, but Its contrast with the background diminishes, since less flux-enters -=~. the lens -to be.concentrated In-the luminous patch. The distance at which ~ 2-2 ~ the object finally vanishes in the negative depends mainly on its contrast” © .:- ‘with the background, though the contrast of the emulsion also has=some-.- -. Influence. The shape of the Image at this stage is. malnly determined by- The aaa characteristics of the feas and bears little relation to the shape of*the . --- -.. object, but It will be recorded in seme fashion from much greater distances ‘~*--—- than might be expected from the resolving power as determined-by- standard sets ° . . + tay 2 . te P . ~ ee .Y . “ . . . . eee. . a, thee ae, . of ° re . - ; re ett . TR ee eres ome ST oo. - . car “le ws,
. . ( . ae ew te Leggy . ~ over ¥ - ; e . tests. As an example of this kind of effect, negatives taken from 10,000 feet with an elght-Inch fens wlift often show the white broken lines painted on roads for traffic control at corners; the Individual white patches are of the order of three Inches wide, while the ground resolution from that distance with that length lens Is approximately two feet for black-and-white- - {Ine test. The nominal resolving power of a lens-flim combination has .0 ‘therefore to be used with care In attempting to predict the size of objects - which can be recorded from a given helght; some thought has first to be given — - to thelr nature and the contrast with their background. : One of the greatest probléms In photo reconnalssance Is caused by low levels of contrast. After Identifying the existence of a contrast there is concern, photograrmetricaily speaking, with the linear shape of the line of differentiation, or its acuity. in much of the test work reported on photo- *: grammetric studies, emphasis has been put on ‘the premise that visual aculty depends In .large measure on the ability to perceive contrast at the border of the object. {t has been suggested that the critical region of a target - - - ts a ribbon Just Inside its perimeter. Tost Senemen mame oe ‘~ —- me 9 + emer eee "~~ -When sensuratioa Is attempted the-sharpness of a contrast becomes~-——-- objectively important, as does’ the abruptness with which the vartation takes place. Attempts made to correlate sharpness ratings with physical measurements . -- of some aspect of ‘the developed Image have not: deen entirely successful. Nelther " ‘resolving power nor simple density relationships across an abrupt boundary - . between light and dark areas result In satisfactory correlations with. sharpness ratings. Because the existence of contrast..is the more clearly discernible and theless adapted to determination of Its origin, the greater-emphasis In photoanalysis has been placed on its quantification.and,_as 8 result, the ~ term contrast is mow most commonly used to Indicate the density dif ference-or— — edge gradient. Whether, photogrammetrically, a contrast is a contour to be ; perceived, a spatial deviation -to_be_resolved, or an edge gradient, there Is ~*~ more to the concept of visual analysis than 3 mere dimenstonal_value, and = : more’ to interpretation of photo Imagery than definition of perimeters. “oe -- The detatis of an object, as they appear In duplicate positive-trans- - -. - parencles, are often incapabte of..definition. They may have suffered se reduction of contrast which-always obscures some of “the smaliest details of - . | any object.. In visual analysis-that -obscurifg is “not-necessarily synonymous ~~ - with lack of object identification. Mensuration of the minute detail.would > ~ truly depend on sharpness of contrast between its area and Its-surround. 0 But this mensuration activity should occur only after recognition has taken , place; and, from the photoanalyst's point of view, the need for mensuration must depend on determination, by other aspects of the Imagery, otherwise derived, that the minute detail Is worthy of mensuration ee eo Py . . : cnr . : : . . f:. . 1 7, een ase e wee «ok getty cee e . a PTE O me we cee ee igs ar . . : : TAS ates eae oe . ae aa . a aehiidiie’ 2 . . . . : The foo . : cee . _ . : i - . - * . * st of - oe ee ° “ tos woe a atte . . We i 3 . oo .
j Lighting Defects oo ; Other than poor contrast, IIghtIng defects are usually referred to T as flare, tight scatter, poor Image quality, or halation, which plays - little, If any, part In modern photography. Any report, written or verbal, - .-- which falls to use these or other terms adequately also falls to convey | I to Its reciplent actual conditions which may be of significance. Any explanatory term should convey a message. Transmission of ideas can be notably Improved, and even reasonable understanding of the Imagery may cm 2. 40 benefit, by knowing ahd using each term to mean what others expect It to. 7. | signify. Poor Image quality, a catch-all! phrase, If used only for the “~~ 2s oriefest reporting, may suffice as a general excuse for !Imlted review......2 22 -- Light scatter, used to indicate diffusely’ reflected light, actually says- ~:~ --.-.: ] nothing about an Image nor Its quality.’ Light scatter and flare should 3 cotta be distinguished from both diffuse and specular reflection. The latter.” ase ts also sometimes referred tod as glare, because It comes from particularly — ” > ] highly reflective surfaces and results tn excessive Irradiation. Much of ae what Is commonly referred to as alther flare or. halation Is also Irradiation., os “ee oo . FO In professional usage flare refers only to the non- Image - forming. “Light coming to the fI!m from Itcht reflection within the camera. .Non-Image-_ forming I Ffght from extraneous sources has Identifiable characteristics: Tang “should be Identified by other terms. Light scatter results from atmos- -----— . - we eam pherlc conditions and manifests itself In the reduction of. contrasts In7 = "=" the Image because of. excessive amounts of light belng reflected from the_-: ras atmosphere Into the camera. These two effects are clarified in the following. | explanation. _ Although the man-Image-forming light must be considered. as | == part-of the Image which acts espon-the- negative material, it: ts convenient - in the treatment of the tone reproduction cycle” to refer to the camera: Image -. _ as distinct from the ‘flare Image. In this sense the camera Image may .be | ca - _- attributed only to the Ifaht coming directly from the object by virtue_of | —- =~ the refractive characteristics of -the lens system. -The flare Image Is not — “ae an Image In the conventions! sense, s since It conforms to no geometrical ___ Ta ~ pattern, but a veil over the entire-Image plane. Its effect Isto Increase .-... the Ii tumi nance of every pohat of the camera.Image and to reduce. the sontrast. oo coe TO iia j One of the simplest explanations ‘of ‘Irradiation ‘is given tna dictionary. definition, which states that it is the spreading of ‘Sight by the grains of . = @ photographic emulsion causing the developed ‘Image to be larger and more_-. ee diffuse at the edges ‘than the optical Image. At is differentiated from) - —-——- halation In Figure 17. Yo 2 eormal degree Irradiation Is’ caysed whenever a...” ~ beam of Night enters an emstsion-because the light Is always scattered or |"... diffused to some extent by successive reflections from the s! iver at ide 2 . a
i ee el oe an + eaacrarmmes ane name tlhe Pape  eerumtn, ecantnhinttainaatiem = cane re nee ten Te nnn a aaa cor eats ’ - ee ’ . . ae _ me le at . . . . . 3 i. %° . ° fo 8 . : - ed igure 37. Diagram illus re teresa 8. irradiation and halation. ae ne See ee te ae oe ° - . ot oe - a faee . oe we te hoe WTRF. - ape LAdndthenmteitnatmeg eaten ae ee Selene . seen . 2 —e = . Pia . “' - pee mace AT - a mee . a oe SwWee. —_*-e ovo we + . + th ewe - canes ” . oe eas SN Mee | - : z s _oo'* * r2 . pads — TS Pw we HS 7 oc = oe - oe oy or et ee ie eteitenared et eee, oe owe. ~ ne De ws - FF, . . Ce eS ai * rY owe nein - Ty rae "> weet oe. or ai Doris eee , nn * : =~ : OO PEO a he ° . +t o -- = . . . To z . wee . 2 —_ Soe erm [5 SO Re see me ee, eens | meee ome meh, or : e . . . soe oo: , +. ae yal . 7 te Ye og .° . a er an s ion 7 . . a rn 1. o- bs re othe . . +o ” . . . . . — ve tee wee eee -_ - =: 72° - . ‘. ° .- Poe . tee i ah - alas . : . . /? . -- . ime ee. 8 os comm @ 7 me. wey owe 7 ee ow TN te tte. ed zs - we. ne eam, - maw aw me . we . - * e . . . ~ . . . e . bd m- pee AO | 0 mR sae - RRS emcee —. M . : . , owe -. a oa ee T ete ened men eee ome & Su te ge fee ape ee a Stew ene ae ee le ee . we some co apene OF toa - . * : —_ a. oe! . . - — Soares - re ee cme ee oe - - ype LL oe Cceeee Serene. PEL OT l Lit e cs 4 bee . od . "ee id . le ° woe. * . . . - . . ° ts . - es _— aa Daaitanente? g . ned . a oe a . . ‘ 7 6 . s . . ~ oe we ste — ° - . _ . meee ° — 7 . ee es | , = - - . . we te ew ed . . ° "See ~ Sa Sa rr eens pe pee, . Tie 6 tee ne Pan cena me - - — Cred ee . ‘ ‘ - . ~ Tht as eee a 2 -. tm ee moe wee ee oe wae ™ patina” . . . — —yo oe aye oon SE RD Oe SRS Te EY PCS Sees oe kee meee ee eee ete? game cee "stove - : — ae . ' - . omee ese 4 oP am. oe ww ce ee. ord . ‘a .. ve . . . ve - . yet , rain eae oe 2 hee we Cem mer ar aR pe ene en wer wee ae ny ena « DT amie ee mcrae = 6 see! meee le Pee sey . . . . : oe -" «6 . a Te mere eet een oe - (te es a a ae ene ee oe cae . ° te . =: . . ‘kot, .- “ % 8 : . ve: “ee SoM Dh fo . ° e *; -_ — ee enced son 2 mee . ” : ‘ . wr : : : _ pane nanertiiiaen rot . oe. STD mre cee n amen een ee eon : - eee es .tae Da - > nd omar = : \ e ‘s . Pet fener me me oe ae oo ete meee 8 ee om. aaa yee. Pont mmn e sem eee eg. egstatergmen ee ee 1 oe - a i ~ ’ ’ . «tone iin “bs 7 Rete se ee — to eoemee oe - TOS ey ee ao cee ee « * . . . . . 4 CT tee emmy oe 28 een ees ow omy te eee ao meeps oe Stee + Bee . . - ee a Tey “ee .. - hhh a On oe cme oe . eet — me. . 7° : . . ee . - + ames . ~. 7% . 7? woe sos a a ; lace Om Zane an . ; Figure 18. Imagery of the edge of a razor b ‘fr nd a . ~ emulsions. =: . ° . o«. @oarse grained i Le . . ia : bd , . . . . . . e . . . s ' . «é e . =6 ° . . o . . ° a
p | | . . te abe me we : . . . . a ae : v : wane. . grains In the emulsion. Ne greater the exposure the mo: ,pronounced the ‘spreading of the Image beyond the boundary of the exposure on the surface. irradiation Is the term which fully covers both the cause and effect of "ves such diffustan of. light. ce FR The amaunt, charactor, and dispersion of IIght are not alone responsible _ “for Irradiation effects In photography. Layer thickness of the emulsion ‘and its grain stze also affect light distribution at the time of exposure. -.~—- .. Resutts of one study, investigating these factors In connection with image - --- °-— _. definition, indicate that the exposing Light diffuses more the longer it... travels through the emulsion layer, i.e., the thicker-the layer. It-appears._ that definition decreases with-increasing emulsion-thickness, possibly -  .-. --. because of increasing light diffusion for thicker iayers where the total._———— amount of scattered light is the same.— The-heavier-scattering in high. 2.7. stiver halide coating weights offsets the advantages of increased contrast.--s0. ~. Really fine grain emulsions show little Irradiation (Figure 18).._The. — 2-5 +. _ process of light attenuation in the deveitopment of dup!icate positives -._. o> from original negatives-is ordinarily explained by. absorption and. scatteringo— ~~ of the fiight by the substance of the image. Coe, a *-"*  AvpAenomenon that-must be understood If. analysis of lighting defects 2. 2. are to be meaningful is halation. It was coined In the early days of . oo, photography when glass plates were used as-backing-for emulsion. It.was.....-2 shown to arise from light being reflected from the far side of the glass ~~ 27" _- plate, or the standard film backing, and thus affecting the emulsion © 20 o> beyond what should have been the normal boundary of the image (Figure 17). -°--—- It occurs when Image forming Ifght passes through the film. Some of the ~*~" - IIght Is reflected back into the emulsion and, because of—Lightrdiffusiony:--— _ this light spreads sidewise beyond the boundaries of tne image and thus. . reduces the sharpness of the latter. if a small, strong source of ‘light is ------ ___.10 the field, and an especially targe proportion of light is reflected at an=-+-: approprfate angle the image of ttie point of light “is surrounded ‘by-a circte---—— or halo--whence the name. Halation is comparatively smal! when the thickness _ cf the emulsion is so great tat the emulsion absorbs most of the image=-+----——~- forming |Ight,-or when--the emutsion contains. the diffused. light-so well that =. -- little passes through the fiJm. In current practice-an opaque backing is. .——-.. used for both glass plates and camera film. In aerial photography nonha lative ~-:: film ts now used exclusively and with such success that-film experts: report*---:: = that hatation fs no longer a probiem. However, those who have not investigated -:. its meaning persist fn using-halation as an expression for light defects of. --*- all sorts, of which Irradiation, flare, Sight scatter, and specular reflection =; are Important In present photographic practices. Each of. these has a -. 22:22 garticular meaning which fs as Important to the photographic analyst as are: =.~- = tha exact meanings of tone, contrast, and other terms which give his work a ..-... scientific base. ee ee ¢ ‘\ F . . we , * ¢ .. ae TR ey .
Serta tee ete pee eee a rn . . 5 oom . . - : ° oe . * a ee Oo eS . ’ . . - . . 3 ant! — - . os : Sf GBJECT RECOGNITION ANO IDENTIFICATION os 3 Ae ° . . : . ‘ ‘“s° A principal conclusion of objective Interest Is the observation that under uniform Illumination, such as found under cloudless daylight conditions _ tn aerial photography, the. luminances of a group of subjects will be directly proportjonal to thelr reflectances. A principal alm of the several technical, photogrammetric and photographic skills Is to provide aerial coverage in which the reflectances will be proportionately imaged. This-is a polnt_of contact In imagery between the objective or engineering aspects and the _ subjective role and operations of the imagery analyst. .. 2 2. | OTN en 5- The recording or definition of an object on photography ts obviously not the same as object recognition as a visual performance. It has been said that the most urgent task in the general problem of qualitative oo . evaluation of a photographic system or of Its separate elements--photographic _ material, lens, etc.--Is the establishment of a clear- relationship between the: results of objective measurements and the visual perception of the image. In photography the capability of defining the object depends on the sufficiency of film quality and processing for adequately registering the object. “Vi'sual~ --- ~_ recognition requires an object size and clarity that are within the range at ; which the eye, with or without_supporting optical lens, can detect the image because it exhibits a sufficient latera! $tzes—The- object. must_be above the limit of registration of the photographic system currently in operation. 77° ~~ Recognition of smal! objects is enhanced by sharpness of.outlines, closely related to graininess of the film, and by the clarity of detail, a subjective - term that signifies overall appearance or_good seeing quality of a piece of _film, which Is quite often related to-contrast. The objective basis of smal} vetallreadibtlity-is. in_the photographic capability of the combined film and camera system. The problem has becn serarated-into_jts component parts | in the observation that, with highest,. black-and-white contrast, tné ‘smaliness—  ~ of detectible objects-is~a measure of the capability-of the photographic system or the absolute or differential- sensitivity cf the eye... These conditions are ~~ readily differentiated in photographic analysis. ~ COTTE eee ee " etme j- For the Imagery analyst the maln concern. ts the assurance with which he distinguishes character differences of the photographic copy in'use and with. __- which he recognizes the details of the. image In order to identify the object ----.-~- represented by that image. For him imagery analysis consists of -two different .- .. processes, Image GetectIon and recognition and object tdentification and ~~ Cescription. As the’new analyst familiarizes himself with imagery he draws on several’ unrelated experiences. He has looked at ground photography and so lis acquainted with landscape imagery as such. He has looked down on landscapes ~ ee a - . : Cae at ~ Teal , / J 2 tome eke oe a . . 7 | & . N mo . re - “a seria are Ose
. - vse - ee ; ; . ; ‘ . wee ee tt : ner atenm © wo te a elon them, ~ Se ee . . oe ee ae Oe er a) ee hd o . : . « omy elther from a h7Fl or mountain or from an aircraft and thus recognizes layouts and tonal differences. He realizes that many elements of a Jandscape may be perfectly familiar and Identifiable, but, If put to a test, he knows that in no view will he be able to Identify everything. Some of the elements will be as unfamiliar to him as a word on paper Is to the lil lterate. And some of the elements will be too small scale for. . seeing. — 7 —Hls greatest hetp | ‘tn early Stages of training comes from looking at ; Imagery~and. having pointed out to_him the Images of things with which =~. :-..+, he Is familiar, roads, bul ldings, “alréraft, trucks; and representative © 2001". patterns. ~ This makes him a photo reader: But he becomes an-independent  ~:—:: analyst only when he studies the unverified coverage and discerns something ae a for which he has had no previous evaluation. Only experience, a conditioning: - - process, tan make a dependable analyst of a photo reader, but he’can be ~— -—_ amazingly.assJsted along the’ way If he knows why he- discerns only so much. eT F at any sitage lo_his training. fn analyst acquiring skill in Identifying Le and descrribing.speclaity” Gbjects, ‘Industrial sites, military tnstal lations, ales: See or whatever, will learn more ‘stowly than the analyst-who- eapeemes aie already has competence In another speclalty field because the latter will” nos know from experlénce what specialty competence Involves. Detection requ lres — =- -> =o " only-the -recognitjion that there may be something of Interest at.a~ particular... spot om a photograph. Recognition requires areal Jimitation; tdentification_ means astabI ishing the essential character; and description Invol ves- ‘Tecount~.. “me Ing or classI fying critiéal features. in reflection, it seems rather ° . incongruous that such competence stems from such a simple thing as _the-. cs aor woe ee. oe appreciation of tonal varlatfons. Pe OT ees ed A wide varlety of_ tests. has been made to measure an 1 individual ts ability ~ to see as a physical performance ard as a mental perception. When this | neo. ablilty Is apptied to-sk£fl._ fin reading photography the question becomes - -- — terrifically involved because very little serious attempt has been made to - °:.”": distingutsh- Just what the anatyst can be expected to see physica! ly from - —_ what he may. be able to Interpret.and rationalize because of special. knowledge or skijll In a certain fletd which may be of:photographic_concern. Jt Is eto doubtful that the quantities derived from any number of relatively. simple ~~ ores viston,, detection; perception and recognition tests can add up to an adequate | 7 conclusion of the total quality of an analyst's-performance. HIS recognition - --~ or detection Performance ® may rate nigher or lower Than his ability. to Identify. 3 e Site, . - Let u us 5s toe, as ‘an ‘éxanple, the recognition of an Instat lation made up ; of six or elght cozvonent parts in more or less well defined arrangement. .. aa . if | photo: scan reveals most oft them, the expert analyst, wut know where to—" -—--- --- aa - "teme e e.
search for the missing bits and recognize ili-defined but nevertheless egtectible indicators near the threshold of vision. Falling this he may unow whether there Is enough assurance In the parts he sees, and the con- gitton of the photograph, to postulate that the installation can be verified without them. At this point the seeing, or visual aculty, mus se distiaguished from the mental, or subjective, activity. gene 8 eaip er ent Throughout reports on vision the term threshold Is used In trrecon- clladle connotations.- It Is commonly used In a step-sequence sense such a3 orightness difference or contrast thresholds, which are only detectible . verlations of 8 quality ratier then the Introduction of-a new effect. The.  _. sirolest use of threshold, In its unique meaning as. @-polnt of beginning,... .—.--. Is the threshold of vision, which has been defined as the minimum oe porceptible luminance after adaptation -to utter darkness in test situations... .. nas pecome complete. ‘Such a test.Involves.a person sitting in a dark room, anowing that, after his eyes’ have become accommodated to the dark, a faint -—==--— Light wiil come on In small step Increments and that he Is to Indicate” ~~" irrediately his awareness of the light at whatever stage he first’ detects ~" °— it. This Is a measure of a person's brightness perception, but as it Is , —_ neither detection nor recognition of an object Image, this threshold has TOTS tte Nomen ~) real place tn.photo analysis... ne Pe ed - - - 2 TTS eee —_—_ . Another use to which threshold has been put Is the threshold of recognition, or of recognition or detection, as tf the two were tdentical.” Kere thresho!d Is used In a meaning of significance to the photo analyst. .. tasce detection Involves simply the awareness of a presence, such as. fs _-.-. owe Indicated In the statenent that objects In tests be large enough so that-- —--.-.-- the detection threshold Is not a limiting factor. In other tests where  —_ _ . ‘rn thresholds are to be compared there Is not a real threshold but a " 7° rezalrerant of recognition, or Identification, which are goals of the photo . eratyst. “In the form test-cited It was found that the form threshold varied Sirertly with the ratio of perimeter -to area and-.Inversely_with the magnitude: —-~> of critical detall. Pat In practical terms this means that with’ given area . 9 circular .form Is more easlly distingulshed than a square, rectangle, triangle, _ er Ireegular form, and an Intricate desfgi woutd-bemost diffticult;—this-does. — . rut esan that a photographic image of one form could be detected more readily ‘san that of another but, rather, that In real ilfe. simple forms are resolved. .--- 0.3 recognized more readily-than-compfex ones.--— © =~... 2 ee et: . ae es sce Shem ON re ener oe ne Image outlines In a camera film are dependent on what has been called *e curulative effect of everything affecti Tight from the object to the s4,ctollogical system of the observer. The ability to see or to distinguish =| - seal! detalis of an: Image always remains a goal of the photo analyst because = + Nn ial ei ee a) ee es Ee Ue - ow +
the more he can extract from a photo Image the more re.tIned his Interpretation can become. -The photo analyst recognizes the small details both objectively | and subjectively. If a simple Image Is large enough to be reproduced the task Is a matter of observing shape, area, and other attributes that can also be measured and described equally well by another photo analyst or a photo; .. — grammetrist working from the same plece of flim. However, the imagery --- eee analyst Is rarely concerned with Identifying 3 simple. shape or-.image....He. ~~. . ts geserally-concerned In what may be called-instal lation analysis -to-differ-. “me antiate It from component. analysis or area analysis. True, he must evaluate i. the area or site location;-and afso-the minor. compqnents or-smatt -ttems that .--- - are defined by-contrast perimeters... But his most important function’ ts the -.. - knowledge or Judgment he uses tn evalyating -the--Imagery- to compare such -.-.----- - attributes ‘as -site-size, location,.and ‘layout or pattern withthe various::. 2° .:. components whose relative arrangement and proportions, are colfectively . 9... 2... Inticative of the Installation purpose and capacity. The evidence of con- stsuction or operational activity at the site or In proximal or. | ike_-locations__. Is also .tmportant. ‘Only the-well!l ‘qualified anatyst-:can:fully. equatethese ——~-.. attributes with the performanca such an object.would have. And if the out! ines>.~. andi detail! are vague he ray discern. sufficient detail. to deduce whether what .--. -- _-he-Is viewing has In fact certain features that he-knows. should or.should. not... . be there. , . . . oy kh “ . i? a vs wot eae oo . i . 7 wee
cow ee ee eg ate ee Riv rene ° “ ° . £N s sBUARY Pt a Dis te, in this study attempt has: been made to discuss plioto. analysis In -eneral and Its problem areas In particular from the viewpoint of the visual s-styst, and to Include basic factual! or experimental data on which that acalyst cam draw to understand why and how problems and imitations In --  — -=.. scatysts occur. The scope, depth, and treatment of the material Is expected *> serve three distinct purposes. First, as a link nk between. + the active... +. . snatysts and the non-Imagery analysts@Me ec ee, It Is -scessary that certain ground rules, so to spk 10k,.be recognized In order enat both groups know why sometimes the Impossible is faced and why it Is 77-7 - unattainable. Second, as a Ink between the Imagery analysts and the - ssatogrammetric, photographic and engineering professions, It has been 7S saggested: ‘that. such a discussion of visual ana: ysis oroblems may be useful . in "directing Improvement In both the Imagery ard its. content and tn the eajective support provided to the visual analysce i a cme) ee ame .- oe ame : Last but not least, as a fairly ‘complete coverage of analysts!. problems____ eng activities;—it ‘should serve_as a_usefui reference. It may be read simply ‘se Informational purposes. it presents summary Tdeas-and~-a wide -variety.... .. of topics that are open to elaboration, discussion, and probably to disagree- ront. It may serve as an Invitation and a starting polnt for airing and cachanging Ideas. [it should evoke suggestions and means for Improvements | in what the analyst does as well as what he produces. It should prove -eful In helping anyone Involved In Imagery analysis or Its benefits to _ tece problems more realistically. I+ is meant to explain the basic sifference and resulfing conflict between visual analysis and the subsidiary sctivities that go tmto the analysis as such and Into its exploitation and results. Most of all, I¢ ts meant as a catalyst to clear up static concepts ‘n order that the dynamic functions of the art may be used 7 a basis for  velId procedures. ee