TWO-STAGE RECTIFICATION SYSTEM 605

lamp advance (abscissa) and time delay inseconds (ordinate). For any given inter­mediate tilt the exposure program can be in­terpolated with sufficient accuracy. Experi­ence will show that these programs canreadily be modified whenever the densitydistribution in the aerial negative deviatesfrom uniformity, and therefore requires adifferent rate of exposure increase in fore­ground or background zones of the terrainimages.

In lower tilt photography U< 20°) theprogram curves straighten out and have asmall slope angle. The total exposure timedecreases rapidly and may be as short asfive seconds for near-vertical photography,while high tilt photography at 75° may re­ljuire up to eight minutes.

The entire procedure of rectification con­sists of relatively few manual steps of align­ing the photograph, film-stage and easels byprecomputed data, selecting the punched

program card. inserting it in the cardreader (Figure 7), operating a fell' sll'itches onthe control panel, energizing the powerfulvacuum unit which holds the screen andphotomaterial flat on the easels and cools theXenon arc lamp, and finally pushing the pro­gram button which lights the lamp and setsthe exposure program in motion. At the end ofthe cycle, the lamp is turned off automaticallyand returns to its starting position, ready forthe next run. The final product is a positive ornegative transparency, or a paper print ofcompletely uniform exposure and, naturally,of high geometric fidelity.

The project of building a series of instru­ments of this kind was sponsored by theUnited States Government. They will beused by several Government Services as ameans of rapid rectification by precomputedorientation data of large quantities of obliqueexposures ranging from verticals to 76°obliquity.

Contrast Control for Diapositives*

JAMES G. LEWIS,

l'opographic Division,U. S. Geological Survey

ABSTRACT: The objective of contrast control or dodging in diapositive printingis to permit a transfer of photographic images in which the contrast betweenminute contiguous images is retained and, at the same time, the maximum andminimum densities of the diapositive are limited.

Contrast control in diapositive printing improves the acwracy of stereoscopicpointing in projection-type plotting equipment. The improvement is greatest foraerial negatives hav1:ng extreme density ranges. Diapositives for four repre­sentative stereoscopic models were prepared in a 153/55 ratio-printer using aninfrared quenching-type contrast control, an electronic feedback-type control,and using no control. The standard deviation of height readings expressed as afraction of the flight height was 1/14,360 for the infrared-type control, 1/12,670for the electronic control, and 1/11 ,140 for no control.

OBJECTIVE OF CONTRAST CONTROL negative to the diapositive, as completely asAT PRESENT (1961) almost all topographic possible, all of the imagery appearing on then. maps are compiled from aerial photo- original negative. To achieve such a transfer,graphs through the use of stereoscopic plot- some means of controlling image-contrast isting instruments. Most of these instruments necessary, particularly for aerial negativesutilize glass-plate diapositives prepared either having extremely large density ranges.at negative scale or a reduced scale. Insofar The objectil'e of contrast-control or dodg-as image-quality sen'es stereoplotting effi- ing in diapositil'e preparation is to permit aciency, it is desirable to transfer from the transfer of photugraphic images in which the

* Presented at the Society's 27th Annual Meeting, The Shoreham Hotel, Washington, D, c., March19-22, 1961. Publication authorized by the Director, U. S. Geological Survey.

606 PHOTOGR.-\MMETRlC ENGlNEERING

PHOTO SENSOR'

tion. A brief explanation of those Il'hich maybe used in diapositive printing, follows:

Hand dodging.-Exposure is varied be­tween relatively large areas by a hand-heldmask placed between the printing light andthe diapositive emulsion being exposed. Al­though judgment is permitted, this is alaborious and time-consuming task. Smallarea contrast control is not obtained.

Unsltarp-mask techniques.-Light modula­tion is accomplished by placing a previouslyprepared unsharp positive of the exposure be­ing printed, between the printing light andthe negative. In this process light modula­tion is applied to somewhat smaller areasthan in hand dodging. The system is time­consuming because of the need for carefulregistry bctll'een the unsharp positive and thenegative.

ELectronic feedback type.- Diagrammedschematically in Figure 1 is a velocity modu­lation electronic system as installed in a153/55 diapositive printer (153/55 = ratio ofcamera focal-length to projector principal-

FEEDBACKSIGNAL

'''''''''T,,,AriI

C'"'~At1/11

/I/'1/11III'iii IIII '

W I,AERIALNEGATIVE

JAMES G. LEWIS

EXISTING CONTRAST-CONTROL SYSTEMS

All-contrast-control systems which are ap­plied during exposure utilize light modula-

contrast between minute contiguous images isretained and, at the same time, the maximumand mi ni mum densi ties of the dia posi ti ves arelimited. The range of density should be suchthat it can be accommodated in the viewingsystem of the stereoscopic plotter. When such"high-contrast" transfers are effected, theidentification characteristics of images are re­tained and stereoscopic plotting efiiciency isincreased.

The value of the contrast-control in dia­positive printing depends on the characteris­tics of the diapositive emulsion and its expo­sure and de\·elopment. The emulsions gen­erally used for diapositives are Class I (me­dium) contrast and Class I! contrast. A ClassIII contrast is nOlI' available but was notused in the tests to be described. The emul­sion characteristic, exposure, and develop­ment determine the contrast of the smallestcontiguous images. The modulated printinglight of the control system provides the meansfor limiting the integrated maximum andminimum density of somewhat larger areas ofthe image. In the ideal case, the "meticulous­ness'" of the modulation of the printer light issufficient to provide an optimum contrasttransfer for all small contiguous images.Meticulousness, as applied to contrast con­trol, is defined as the degree to which smallareas of the image are controlled.

1 Jackson, K. B., Faclors Affecting the Interpret­ability of Air Photos: The Canadian Surveyor, Vol.1-+, No. 10, October 1959, pp. 454-464.

FIG. 1. Schematic diagram of 153/55 diapositiveprinter equipped with electronic feedback typecon trast control.

CONTRAST CONTROL FOR DIAPOSITlVES 607

~ OIAPOSITIVE

II'~L'"I~\II \I '~ BLUE PRINTING LIGHT

TUNGSTEN SOURCE~ I \RED FILTER ----(f\'\ I/~/~ R"O QUENCHINO LIGHT

\\/ AERIAL NEGATIVE

PHOSPHOR COATINGRED QUENCHING TYPE

FIG. 2. Schematic diagram of 153/55 diapositiveprinter equipped with an infrared quenching typecontrast control.

distance). The scanning spot of a large tele­vision or cathode-ray tube provides theprinting light. The photo-sensor provides afeedback signal to the cathode-ray tubewhich causes a simultaneous and controlledvariation in the speed of the moving spot onthe face of the tube. The scanning speed atany given instant is inversely proportional tothe density of the aerial negative in the areaof the scanning spot. Automatic contrast­control is thus provided in areas as small asthe moving spot.

Infrared qnenching type.-Figure 2 illus­trat~s an infrared quenching system installedin a 153/55 diapositive printer. An ultra­violet light-source excites a Huorescent screencoated with an infrared quenching-type phos­phor. The screen emits a blue light to whichthe diapositive emulsion is sensitive. Thebrightness of the screen is varied by thequenching action of the infrared light, thequenching being greatest in areas where thenegative is least dense. The screen thus emitsa modulated blue light as though an unsharpmask had been inserted between the film andthe screen. The size of the smallest area inwhich contrast control can be obtained is pro­portional to the distance between the aerialnegative and fluorescent screen.

The Geological Survey implemented theelectronic feedback type of control for dia­positive preparation several years ago in oneArea office of the Topographic Division. Theinfrared quenching type seen here was in­stalled recently in one diapositive printer forproduction use, and already shows consider­able promise.

RESEARCH IN CONTRAST CONTROL

In order that a sound judgment could bedeveloped as to the value of contrast-controlsystems, it was felt that quantitative testsshould be conducted. The tests were plannedso that evaluations could be made for identi­cal models utilizing diapositives printed \Yith­out dodging, and those printed with both theelectronic feedback and the infrared quench­ing-types of control. The planned analysiscalled for determinations of relative consist­ency of height reading under the three condi­tions of contrast control, and for statementsof preference by experienced stereocompilersbased on stereoscopic viewing of the severalmodels.

Eight aerial negatives forming four stereo­scopic models were selected as having therange of negative densities normally en­countered in map compilation. ER-55 dia­positives were prepared for the four stereopairs using the electronic and the infraredquenching types of contrast control and alsousing no control. The three stereo models wereset up on adjacent ER-55 compilation units.Stereoscopic ele\'ation readings were repeatedon five selected points in each model, by 10 ex­perienced people. The stereoscopic readingson any single point were not repeated until allother points in the series had been read. Thestandard deviations given later are thereforenot comparable to so-called consecuti\'epointings of stereo observation. Mr. T. J.Blachut2 has commented on this importantfactor of stereoscopic measurement in a reporton "The Second International Mapping Ex­periment, Renfrew Test Area." T\\'enty addi­tional experienced stereocompilers examinedthe models and gave judgments as to com­parative quality for map compilation.

The points selected for stereo measurementrepresented four problem areas encounteredin production. Deep-shadow areas, largewhite-glare areas, timber, and dull low-con­trast areas were considered.

Model set No.1, shown in Figure 3, depictsrolling terrain having a large deep-shadowarea completely covering the north side of alarge granite mountain, namely, Stone Moun­tain in Georgia. This area also was about 90per cent covered with timber, and containeda number of small ponds and several roads.The points selected for reading are numbered1 to 5. In this model, note that no point was

2 Blachut, T. ]., Results of Experimental Plot­ting for 1: 50,000 maps. Second InternationalMapping Experiment, Renfrew Test Area: Reportto the IX International Congress of Photogram­metry, London, 1960.

608 PHOTOGRAMMETRIC ENGINEERING

FIG. 3. Terrain covered by stereomodel set 1 with5 points where measurements were made.

TABLE 1

RESULTS OF STEREOSCOPIC MEASUREMENTS

AND OPERATOR OPINIONS

read in the shadow of the mountain, an areawhere contrast control was an obvious asset,as is shown in the next two ill ustrations. Thestandard deviations of the readings, expressedas a fraction of the flying height for all ob­servations in this set of stereomodels aregiven in Table 1. (In all cases, control-type Arefers to the electronic-feedback system andcontrol-type B, to the infrared-quenchingsystem.) Stereocompilers' preferences for con­trast control are given in Table 1.

Contrast control

Stereoscopic observationsH -7- (Standard deviation)

The total range of density in the aerialnegative was 0.4 to 1.92. The range of nega­tive-density in the areas where points wereread was 0.98 to 1.92. It can be"concludedthat for all areas except in the mountain shad­ow, contrast-control has only slightlyimproved, but has definitely not limited theaccuracy of stereoscopic pointings.

Figures 4 and 5 show the definite advantageof contrast-control in the mountain shadow.No points were selected for measurement herebecause of no identifiable imagery in the un­controlled model. All diapositives used inmodel set 1 were Class II contrast, and weredeveloped in DK60a for 5 minutes at 68°F.

Model set 2 shown in Figure 6, again pre­sented an area of some relief, but with lesstimber cover than in set 1. In this model avery extensive white area on the side of alarge hill, and the pattern presented by alarge orchard, were of interest. The total

Type B No controlType A

Model setnumber

Stereocom piLers givi1W.first preference

For control99

A gainst control21 FIG. 4. Mouutain shadow area from model

set 1 with no control.

7o

104

21

10,7709,940

15,38010,42011,140

14114

2150

10,95016,56016,56015,38014,360

919165

49

12,19012,67016,15011,33012,670

Model selnumb",

OneTwoThreeFourAll models

OlleTwoThreeFourAll models

All model sets (4)All operators (30)

CONTRAST CONTROL FOR DIAPOSITIVES 609

FIG. 5. Mountain shadow area from modelset 1 with contrast control.

density range in this aerial negative was 0.4to 1.9. Points numbered 1 to 5 were selectedfor stereoscopic measurement. Note thatpoint 3 was selected in the large white areaat a location hardly detectable in the uncon­trolled model. In Table 1 the standard devia­tions of readings on all points are given. Thestandard deviations for point 3 were: Controltype A-l/7,260, Control type B-1/12,670,and No control 1/5,170.

Figures 7 and 8 show the area around point3, with and without contrast-control. Theadvantage of contrast-control is especiallyevident for areas such as that surroundingthis point where the aerial negative densityextended to 1.9. Stereocompilers' preferencesare given in Table 1. All plates were Class IIcontrast and developed in DK60a for 5 min­utes at 68°F.

Model set 3, shown in Figure 9 offered flatterrain with a 50 per cent cover of dense tim-

FIG. 7. Area around point 3 in modelset 2 with no control.

FIG. 8. Area around point 3 in model set2 with contrast control.

FIG. 6. Terrain covered by stereolllodeJ set 2.

610 PHOTOGRA MMETRIC ENGINEERING

FIG. 9. Terrain covered by stereomodel set 3.

her. The total range of density in this aerialnegative was 0.36 to 1.04, well within the de­sirable limits of 0.30 to 1.5, as specified foraerial negatives. The standard deviationsof readings on all points were: Control-type A-1/16,150, Control-type B-l/16,560, andNo control 1/15,380. These values show nosignificant advantage for contrast-control.The deviations, however, are the lowest re­corded for the four model sets. The limiteddensity range of the aerial negative no doubtcontributed to the low standard deviationsand to the lack of a significant difference be­tween controlled and uncontrolled models.

The number of preferences of stereocom­pilers for the different control types ara givenin Table 1. All plates were Class II contrastand were developed in DK60a for 5 minutesat 68°F.

Model set 4, shown in Figure 10, containedrolling terrain. This model is characterized bylarge leveled areas cleared for an industrial-

type activity. The total density range in theaerial negative was 0.4 to 2.8. The former oc­curred in the ri ver and the la tter only on thesunny side of the long buildings. The densityrange of the negative in the area used formeasurement was 0.8 to 1.62. The standarddeviations of readings on all points are givenin Table 1. The values show an advantage inthis model for type B can trol as compared totype A control. The reason has not beenconclusively determined. Microdensitometertraces on the diapositives across the pointsmeasured showed no significant differencesin contrast between the two types of control,but did show that for most of the points meas­ured, type A control produced a darker dia­positive. Strong preference for type B con­trol in this model suggests that type A con­trol produced a diapositive of too great adensity-range for best measurement with pro­jection equipment. The larger range was at­tributed to the lack of meticulousness of the

FIG. 10. Terrain covered by stereomodel set 4.

CONTRAST CONTROL FOR DL\POSlTIVES 611

type .-\ controL Stereocompilers preferencesare given in Table 1. All diapositive plateswere Class I medium contrast and were de­veloped in DK60a for 5 minutes at 68°F.

Table 1 gives the combined results of thcstereoscopic measuremen ts, and stereocom­pilers' opinions for the four model sets. Thestandard deviations are: Control-type A­1/12,670, Control-type B-l/14,360 and Nocontrol 1/11,140. Forty-nine stereocompilersexpressed first preferences for type A and 50for type B control; 99 preferences for a con­trast-controlled model and 21 preferences forno controL

CO:-;CLUSIONS

It can hc stated that contrast-control indiapositivc printing improves the accuracy ofstereoscopic pointings. The percentage of im­provcmcnt obtained is directly related to thedensity range of the aerial negative. The nega­ti"es used in the research reported herein\\'ere not extreme or unusual examples, butwere those which might normally be encoun­tered in mapping operations. The percentageof im pro"emen t for the con trast-con trolledmodels therefore is not nearly as great as

would ha,'e been obtained \\'ith extreme den­sity conditions of the type that \\'ould exist innegati,'cs of snow scenes or rugged mountainterrain.

V. S. i\lilner and r. N. Tsygan03 of theUSSR recently reported on the improvementof accuracy of stereoscopic measurement us­ing the unsharp-mask technique. The areaschosen for study and the points used for meas­urement were extreme and, therefore, the re­sults are not directly comparable to those ofthis study. It is interesting to note, however,that their results are similar to those reportedin this paper. They reported an increase inthe accuracy of stereoscopic measurements offrom H to 2 times (34 to 50 per cent). Notcin Table I that in model set four, typc B con­trol showed a 40 pcr cent improvement overno controL

It is hoped that this study may benefitothers by furnishing quantitative data in anarea where pictorial quality is often the onlyguide.

3 Experiment:; in the Use of the Unsharp-MaskMethod of Preparing Contact Prints and Dia­positives: Geodesy and Cartography, Nos. 3 and 4,USSR Ministry of Internal Affairs.

Autofocus Rectifier Modified forElectronic Dodging andAutomatic Exposure Control*

[{EX R. MCH.\IL,

Photogrammetric Engr.,Bausch & Lomb, fncorp., Rochester 2, N. Y.

(A bstract ,is on next page)

T HIS paper discusses the basic constructionand operational principles of a Bausch &:

Lomb Autofocus Rectifier that has been mod­ified to provide electronic dodging and auto­matic exposure control. The rectifier is illus­trated in Figure 1. This development pro­gram was supported by the U. S. Navy, Bu­reau of \\,'eapons, under the direction of theI. S. :'\aval Photographic Interpretation

Center. The instrument is currently beingtested and evaluated at its facilities in Suit­land, Maryland. It is important, therefore,that it be understood that, \\'hile this paperdoes not necessarily reAect the official vie\\'sof the Defense Dept., it does reAect their con­sideration to permit Bausch & Lomb. Inc. asprime contractor to present this report of thedevelopment program. Credit for the eIec-

* Presented at the Society's 27th Annual Meeting, The Shoreham Hotel, vYashington, D, C, March19-22, 1961.