Imaging characteristics of photogrammetric camera systems

Photogrammetria, 29(1973):1-43 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

IMAGING CHARACTERISTICS SYSTEMS 1

OF PHOTOGRAMMETRIC

CAMERA

R. WELCH and J. HALLIDAY

University o/ Georgia, Athens, Ga. (U.S.A.) U.S. Geological Survey, McLean, Va. (U.S.A.) (Accepted for publication November 24, 1972) ABSTRACT Welch, R. and Halliday, J., 1973. Imaging characteristics of photogrammetric camera systems. Photogrammetria, 29:1-43. In view of the current interest in high-altitude and space photographic systems for photogrammetric mapping, the United States Geological Survey (U.S.G.S.) undertook a comprehensive research project designed to explore the practical aspects of applying the latest image quality evaluation techniques to the analysis of such systems. The project had two direct objectives: (1) to evaluate the imaging characteristics of current U.S.G.S. photogrammetric camera systems; and (2) to develop methodologies for predicting the imaging capabilities of photogrammetric camera systems, comparing conventional systems with new or different types of systems, and analyzing the image quality of photographs. Image quality was judged in terms of a nmnber of evaluation factors including response functions, resolving power, and the detectability and measurability of small detail. The limiting capabilities of the U.S.G.S. 6-inch and 12-inch focal length camera systems were established by analyzing laboratory and aerial photographs in terms of these evaluation factors. In the process, the contributing effects of relevant parameters such as lens aberrations, lens aperture, shutter function, image motion, film type, and target contrast were determined. The methods for assessing these individual effects were then integrated into procedures for analyzing image quality and predicting and comparing performance capabilities. M a p p i n g c a m e r a s are n o w being u s e d in jet aircraft at flight heights a b o v e 70,000 ft. and are being c o n s i d e r e d for use in orbiting satellites. C o n s e q u e n t l y , metric p h o t o g r a p h s at scales several times s m a l l e r than the c o n v e n t i o n a l 1:20,000 to 1:50,000 are being m a d e a v a i l a b l e for m a p p i n g applications. T h e suitability of s m a l l - s c a l e p h o t o g r a p h s for m a p p i n g , however, d e p e n d s on a n u m b e r of factors, including the degree to w h i c h i m a g e quality can be i m p r o v e d o v e r c u r r e n t s t a n d a r d s (of 2 5 - 3 0 l / m m resolving p o w e r ) t h r o u g h systematic analysis a n d o p t i m i z a t i o n of the c a m e r a systems. A s an e x a m p l e of the growing n e e d for optimizing p h o t o g r a m m e t r i c c a m e r a system p e r f o r m a n c e , c o n s i d e r the e n o r m o u s costs of r e p e a t i n g a satellite mission b e c a u s e of u n s u i t a b l e p h o t o - i m a g e quality. T o m i n i m i z e the risk, all the c a m e r a system p a r a m e t e r s m u s t b e carefully established, t h r o u g h systematic analysis, b e f o r e the mission. 1 Presented paper for Commission I of the 12th Congress of the International Society for Photogrammetry, July-August, 1972. Publication authorized by the Director, U.S. Geological Survey, Washington, D.C.

R. W t ' L C H A N I ) J . IIAI I tl)~,)

Resolving power alone is not well suited for systematic analysis nor is i~ a completely satisfactory measure of image quality, so that additional performance criteria are needed (Brock. 1967). Typically, they should include modulation transfer functions (MTF's) or squarewave transfer functions (STF's) for lenses.. shutters, films, and environmental conditions. When the functions are supplemented by information on film granularity and resolving power, image-motion tolerances, and other conditions, system performances can be compared and predicted in terms of total system MTF, resolving power, and the detectability and measurability of small image detail (Welch. 1972a, b). Effective evaluation techniques also permit operational photographs to be analyzed in a way that indicates the conditions responsible for any degradation so that corrective measures can be taken This practical consideration is especially important in contract photography. RESEARCH PROJECT

The U.S. Geological Survey began a research project in the summer of 1969, with the specific objectives of: (l) evaluating the imaging characteristics of modern photogrammetric camera systems; (2) developing methodologies for analyzing and predicting photo image quality; and (3) developing techniques for comparing the characteristics of existing systems with new or different systems. In attaining the first objective, studies were planned to assess the contributing effects of individual conditions, such as tens aberrations, aperture setting, shutter function, film type, target contrast, and image motion, of selected camera systems currently used by U.S.G.S. The techniques for determining individual effects could then be integrated into methods for comparing and predicting the capabilities of camera systems and evaluating photographs according to the other two objectives. It was apparent from the start that both laboratory and aerial photographs were needed. The laboratory photographs, taken under controlled conditions, would be analyzed to determine the contributing effects of the various components of the camera system. The aerial photographs, on the other hand, would represent operational conditions. By comparing their image quality with that of the laboratory photographs taken (with the same cameras), the degrading effects of operational conditions, such as image motion, vibration, and environmental elements, could be determined. For valid comparisons, the conditions for taking and processing both series of photographs had to be as nearly identical as possible. LABORATORY PHOTOGRAPHY

To obtain representative laboratory photographs of uniformly high image quality, the photographic equipment had to be carefully selected, a camera test facility had to be designed and constructed, and optimum exposure and processing procedures had to be specified.

3

IMAGING CHARACTERISTICS Photographic equipment

T w o U.S.G.S.-owned aerial cameras were used, a Wild R C 8 with Universal Aviogon lens (f = 6 inch), which is a standard wide-angle mapping camera, and a Zeiss R M K A 30/23 with T o p a r A lens (f = 12 inch), which is considered representative of the cameras that m a y be used at ultrahigh aerial and orbital altitudes. T h e only available data on the imaging capabilities of the two cameras were given in the manufacturers' reports on resolving power (Table I). Due to TABLE I RESOLVING P O W E R S

(l/mm)

O F C A M E R A S USED

(manufacturers' data)

Wild RC8 camera, Universal Aviogon lens no.251 (c.f.1.

Field angle Radial Tangential

0° 53 53

5° 53 53

152.27 ram) 1

10°

15 °

20 °

25 °

30 '~

35 °

40 °

45

52 51

51 49

49 47

43 44

51 44

49 40

26 35

8 17

Zeiss R M K A 30/23 camera, Topar A lens no.92 962 (c.[.L ~ 305.60 nun) 2

Field angle Radial Tangential

0° 40 40

7° 40 39

14 ~

21 °

30 37

28 27

1 Conditions: high-contrast targets, f/5.6, Agfa Isopan IFF 13 Planfilm. '-' Conditions: low-contrast targets (1.6:1), f/5.6, B Filter (480 nm), Aviphot Pan film, Perufin developer. differences in films, targets, and development conditions these data were not directly comparable and did not necessarily indicate the typical performance characteristics of mapping camera systems. T o o v e r c o m e this problem, a standard 4-rail polyester-base (Estar) m a p p i n g film, K o d a k Plus-X A e r o g r a p h i c (2402), was selected for use in both cameras in the laboratory and aerial experiments. The results obtained with this film were to be considered as baseline data representing the capabilities of currently used camera systems. Additional K o d a k films, all on 2.5-mil Estar base, selected for use in both laboratory and air included Pana t o m i c - X Aerial (3400), Aerial Color (SO 121), and E k t a c h r o m e Infrared Aerial (SO 180). T w o other films, High-Definition Aerial (3404) and D o u b l e - X A e r o graphic (2405), were selected for laboratory experiments only. All camera magazines were checked in the laboratory, and any magazine that p r o d u c e d anomalous resolution values on test photographs was rejected. The camera shutters were calibrated (Fig.l), and any discrepancies between nominal and actual shutter speeds were taken into account in exposure settings. Collimators

Three collimators, (f/15, f = 26 inch) that had been originally used in the U.S.G.S. camera calibrator were adapted for the research. A l t h o u g h preliminary tests indicated that the effects of chromatic aberrations on focal plane settings

4

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~ 600

700

i/8oo

i/9oo

INDICATED SHUTTER SPEED Isec) Fig.1. Shutter speed calibrations for the W i l d and Zeiss cameras. w e r e m a d e using the U , S . G . S . electro-optical shutter testing method.

T h e calibrations

were very shght, the collimator focal planes were adjusted to a white light source plus Wratten 15 filter combination that approximated the conditions of minusblue aerial photography. Later tests at the National Bureau of Standards (N.B.S.) confirmed the focal plane settings. In addition, N.B.S. determined the collimator response functions (Fig.2) and established a maximum resolving power of 108 l/mm, indicating that the collimators are nearly diffraction limited. 1(

0.( 0.7 o~

0.~ 0.5

0.3 0.2

0.1

-

I

0

I

20

I

30

t

40

I

50

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60

I

70

I

80

I " II-'~'

90

100

t10

SPATIAL FREQUENCY (cycles/mm) Fig.2.

R e s p o n s e functions for U . S . G . S . collimators.

Collimator reticles

Special collimator reticles (diameter, 7/8 inch) were manufactured by Itek Corporation to U,S.G.S. design and specifications. Each reticle (Fig.3) contains long-bar resolution targets and graded-square targets in 10:1 and 2"1 contrasts,

IMAGING CHARACTERISTICS

5

Fig.3. Specially designed target reticle used in the U.S.G.S. collimators (3 X). with a background transmittance of about 1 0 % ; the values were selected as representative of the average ground scene. Each reticle also contains as seven-step gray scale.

Light source For proper film response, particularly with the color films, and in order to encompass the wide range of Aerial film speeds, a 20,000 candlepower Sylvania movie lamp rated at 6,000°K was used as the light source for each of the collimators. New lampholders and cooling systems had to be designed and installed to solve the problems of lamp size and heat. Light levels were regulated with neutral-density filters, and target luminances were measured with a Minolta spot meter. Test stand A laboratory test stand (Fig.4) was constructed to mount the camera and hold the three collimators at fixed angles of 0 °, 15 °, and 30 ° to the camera axis. Photographic procedures Before the laboratory exposures, preliminary trials determined the target luminance levels for optimum exposure of the various films at 1/300 sec with the 2 aperture settings. Then a series of laboratory exposures from 1/100 to 1/700 sec (1/1000 sec for the Zeiss camera) was made for each of the camera/film/filter/ aperture combinations listed in Table II, to insure procurement of photographs

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[:ig.4.

L a b o r a t o r y Icsi , l a , d .

at optimum exposure despite any inaccuracies in procedures or variations in film processing. All the exposures with the Zeiss camera were made at f/8. A larger setting would have made the camera aperture larger than the clear aperture of the collimator, a condition which might have resulted in partial coherence. The exposed films were sent to an Eastman Kodak laboratory for processing according to prearranged specifications. Step tablets were exposed on both ends of all rolls of film before processing to provide the sensitometric data for quantitative evaluation of image quality.

IMAGING CHARACTERISTICS

7

TABLE II CAMERA/FILM/FILTER/APERTURE

C O M B I N A T I O N S USED IN LABORATORY E X P O S U R E S

Wild RC8 camera

Zeiss R M K

A 30/23 camera

/ilm

filter

aperture

/ilm

filter

aperlttrc

2402 3400 SO 180 SO 121 SO 121 * 2405 * 3404 * 3404 * 3404 *

500 nm 500 nm 500 nm 450 nm none 500 nm 500 nm WR15 ** WR15 **

1"/8 f/5.6 f/5.6 f/5.6 f/8 f/8 f/5.6 f/5.6 f/8

2402 3400 SO 180 SO 121 SO 121 * 2405 * 3404 * 3404*

D D D B none D D WRI5

f/8 f/8 f/8 f/8 f/8 f/8 f/8 f/8

**

* Combinations not used in the air. ** Filter in collimator. AER1ALPHOTOGRAPHY The project required aerial photographs

r e p r e s e n t a t i v e of o p e r a t i o n a l c o n -

ditions and suitable for objective and subjective compartsons with the laboratory photographs.

T h e v a r i o u s e l e m e n t s of a e r i a l p h o t o g r a p h y

h a d to b e p l a n n e d to

i n c l u d e s e l e c t i o n of a test site, use of t h e s a m e c a m e r a s a n d films, c o n s t r u c t i o n of g r o u n d targets, d e t a i l e d p l a n n i n g of the flying m i s s i o n , a n d c o n t r o l l e d p r o c e s s ing of the e x p o s e d films.

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~26~

120'-

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3- BAR RESOLUTIONTARGET HIGH CONTRAST

J~ J

3 - BAR RESOLUTIONTARGET LOW CONTRAST

]

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'1

k'-2°~

DIRECTIONof OVERFLIGHTS 1329° ) EDGE TARGET

~'~

Fig.5. Schematic representation of U.S.G.S. ground target anay.

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A N D ,l. H A l 1.IDA5

Ground targets

The ground targets (Fig.5) were designed to correspond with the laborator~ targets and permit objective and subjective evaluations of image quality at phot~ scales as small as l:50.000. The target material was either white or medium-gray vinyl plastic. Three principal types of targets were painted on the plastic: 3-bar resolution targets with element widths of 6, 4, 2.5, 1.5, 1.0, 0.75, and 0.5 ft. and a constant length of 26 ft.; squares in graded sizes of 4, 3, 2, 1, and 0.5 ft.: and an edge target. The resolution targets and graded squares were constructed in contrast ratios of 9:1 and 2.5:1, with a background reflectance of about 9%, taken as representative of average ground conditions. The edge target was painted off-white and in two shades of gray to provide edge contrasts of about 7:1 and 2.5:1 in two directions. Other targets included panels painted in shades of gray to complete a sixstep gray scale; red, green, and blue panels; and various small line patterns and letterings. Test site

With the assistance of the U.S. Army, the U.S.G.S. ground targets were installed adjacent to the permanent resolution test site at Fort Huachuca, Ariz., (Fig.6) in mid-November 1969. The site offered several advantages, including proximity of the military resolution target, relative freedom from disturbance, and excellent weather. Photography was limited to the middle part of the day to take advantage of maximum sun angle of about 45 °, and flights were planned (Table III) so as to obtain photographs at 1:12.000 and 1:24,000 scale with both cameras. Additional photographs at 1:48,000 scale were taken with the Wild camera. Each camera/film combination was flown twice, first at optimum exposure and then at half-stop overexposure (or underexposure for the reversal films) to make sure that the targets would be imaged on the straight-line portions of the D-log E curves (Fig.7). During the flights, the ground-target luminances were measured and recorded (Table IV) for use in determining contrast values and evaluating the effects of atmospheric luminance. The exposed films were sent to Eastman Kodak laboratories for processing. After the U.S.G.S. photomission a NASA RB 57 aircraft equipped with NASA-owned Wild RC8 and Zeiss R M K A 30/23 cameras flew over the target area at approximately 40,000 and 60,000 ft. and obtained photographs on 2402 film (RC8) and SO 117 film (RMK A 30/23). It should be noted that SO 117 as a color infrared film on 4-mi! Estar base with characteristics similar to those of SO 180, 2443, and 3443. The films were processed by NASA and sent to us for use in the research.

IMAGING CHARACTERISTICS

9

Fig.6. U.S.G.S. ground target array laid out adjacent to the resolution test facility at Fort Huachuca, Arizona.

Effects of atmosphere and image motion For most aerial photographic conditions, atmospheric luminance, which reduces the contrast of the ground objects, is the most significant environmental parameter affecting image quality. Atmospheric turbulence becomes a factor

10

R. WELCH AN[) J. HALLll)A'~

T A B L E I11 FLIGHT PLAN FOR AERIAL PHOTOGRAPHY

Flight height (]t.)

Film

Filter (nm)

Exposure

Wild R C 8 camera 6,000

(sec) 2402

500

3400 SO 180 SO 121 12,000

450

SO 121 SO 180

500

3400 2402 24,000

2402 3400 SO 180 SO 121

450

2402

550 (D)

f/8 f/5.6 f/5.6 f/5.6 f/5.6 f/5.6 f/5:6 f/5.6

t/500 1/700 1/225 1/200 1/250 1/350 1/150 1/200

f/5.6 f/5.6 f/5,6 ff5.6 f/5.6 f/5.6 f/8 f/8

1/150 1/200 1/300 1/400 1/300 1/200 1/700 1/500

f/8 f/8 f/5.6 f/5.6 f/5.6 ff5.6 f/5.6 f/5.6

1/700 1/500 1/400 t/250 1/400 1/500 1/150 1/200

f/8 if8 ff5.6 f/5.6 f/5.6 f/5.6 f/5.6 f/5.6

1/700 1,/500 1/400 1/250 1/300 1/400 1/150 1/200

f/5.6 f/5.6 f/5.6 f/5.6 f/5.6 f/5.6 f/8 f/8

1/200 1/250 1/400 1/600 1.'400 1/250 1"800 1:500

Zeiss R M K A 30/23 camera 12,000

3400 SO 180 SO 121 24,000

480 (B)

SO 121 SO 180 3400 2402

550 (D)

only when unusually tong focal lengths or hypersonic platform speeds at low altitudes are involved (Nielsen and Goodwin, 1961; Hufnagel, 1965; Hulett, 1967).

11

IMAGING CHARACTERISTICS

3oI 3400 2402

i

i ,

0

20~-

lO~i

--'

,~

J

RELLOGEXPOSURE

Iio

20

RELLOGEXPOSURE

\ 2op-

i

i

~

SO-18Q

i'

z !

O0

REL.LOGEXPOSURE

RELLOGEXPOSURE

Fig.7. D - l o g E curves f o r the f i l m s e x p o s e d in the air. T h e range o f i m a g e d target densities is indicated on each curve.

The effect of the atmosphere on ground-object contrast can ~be described according to the equation given by Brock et al. (1966): Ca --

(Bol" T.) -k B~ (Bo2.

Ta) + B.

where Ca = contrast at the camera lens; Bol, Bo,z = luminances of highlight and lowlight objects; Ta = atmospheric transmission factor, and B,E = atmospheric luminance. In this equation the two parameters significant in determining the image contrast at the camera lens are the atmospheric transmission factor T, and

]9

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t i . \ [ I,ll)A~,

I'ABLE IV ,tVERAGE LUMINANCE OF GROUND -[ kRGIU[S DURING PHO10 FI IGHI S

U.S.G.S. targets

Lanlim~t2ce (/ t . L i

Military targets

~; t . L )

t.ut~Hmm,. <

White vinyl Off-white paint Lt. gray paint Med. gray vinyl Dk. gray paint Black paint

4,900 3,900 2,700 1,300 500 131/

White bars Lt. gray bars Lt. gray background Dk. gray bars Black background

4,500 3,300 2,500 1,500 250

atmospheric luminance B,~. According to Boiteau (1964), T, does not exceed about 0.8 and B, generally adds a few hundred foot-lamberts to the highlight a n d lowlight luminances of the ground scene. Photometric analyses of the aerial photographs obtained at different altitudes, as specified by methods described by Brock (1952) and Specht et al. (1966) yielded an average transmission factor T, of 0.75 and curves showing the increase in atmospheric luminance with altitude (Fig.8). The curves indicate that, with the clear atmosphere of Arizona, atmospheric luminance does not increase significantly at flight heights above 12,000 ft. The contrasts of the ground targets at the camera lens are given in Table V. TABLE

V

AVERAGE CONTRASTS AT THE CAMERA LENS FOR GROUND TARGETS

U.S.G.S. targets

Contrast

Military targets

Contrast

High-contrast Low-contrast

5.6:1 1.8:1

High-contrast Medium-contrast Low-contrast

7:1 2.8:1 1.26:1

The motions that degrade the images of aerial photographs include the translational motion of the camera and aircraft relative to the ground during exposure and the periodic vibrational motions that may occur in a camera because of incompletely damped aircraft vibrations. Of the two, the translational motion normally introduces the more significant degradation (MacDonald, 1952). Modulation transfer functions (MTF's) are commonly used to describe the effects of both types of image motion (Scott, 1959; Rosenau, 1965). The normal (sin x)/x relationship was used to calculate the MTF's for the translational image motions (Fig.9). To obtain more exact values for the effects of translational image motion, a small representative correction function which takes into account the noninstantaneous opemng and closing of the camera shutter was computed (Hendeberg and Welander. 1963). This correction function (dashed line in Fig.9) was

IMAGING

]3

CHARACTERISTICS

24,00C --

18,00C - 5

.,c 12,00(

6,00(

I

I

200

I

I

I

400

I

600

COMBINED ATMOSPHERIC LUMINANCE rind CAMERA FLARE (Fh Lrimberts)

Fig.8. Combined atmospheric luminance and camera flare vs. altitude at the time of the overflights.

based on average conditions of an aperture f/5.6, shutter speed 1/300 sec, and image motion of 20 ,urn. The MTF's for vibration of the Wild and Zeiss cameras (Fig.10, 11) were

06 ~)

O4

t~

O2 1 0

--

10

20

3O

L_ ac,

t

J 5D

SPATIAL FREQUENCY (cycles

' 60

L 70

80

mm)

Fig.9. Modulation transfer function ( M T F ) curves for image motion and representative shutter operation. Shutter M T F based on f/5.6, 1/300 sec, and 20 ,urn of image motion.

determined from tabulations provided by Scott (1959) on the basis of a typical angular velocity of 0.0I rad/sec and entries for focal length and exposure (Brock, 1952; Welander, 1960, 1968). When the curves are compared with those derived for translational image motion (Fig.9), the relatively minor effects of vibration are apparent.

1~1.

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~ - ~

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37~

4,:

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400

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SPATIAL FREQUENCY (cycles:mm)

Fig.10. MTF curves for vibration in the Wild camera. Assumed angular velocity = 0.01 tad/see. 1.( - -

I0

2o

30

40

50

60

70

80

SPATIAL FREQUENCY(cycles/mm)

Fig.ll. MTF curves for vibration in the Zeiss camera. Assumed angular velocity = 0.01 rad/sec. RESPONSE FUNCTIONS T h e laboratory photographs used to determine response functions I (and resolving power, detectability, and measurability) were the o p t i m u m exposures f r o m each series. Targets imaged at the 0 °, 15 °, and 30 ° f o r m a t positions were analyzed. T h e aerial p h o t o g r a p h s selected for analyses were those on which the targets were imaged within 10 ° of the principal point. Response functions were determined f r o m traces of the low-contrast bar targets on both laboratory and aerial photographs. Additional determinations were m a d e f r o m traces of the high-contrast bar targets and the high- and lowcontrast edges imaged on the aerial photographs (Welch, 1971). All traces were obtained with a Joyce, Loebl M a r k I I I microdensitometer with an effective slit size of 1.5 X 115 , m . F o r the response functions derived from the laboratory photographs, it was necessary to apply a correction to take into account the degradation introduced by the collimator target, collimator lens, and microdensitometer response. A l t h o u g h the correction was very small for the shorter focal-length Wild camera, it became significant for the Zeiss camera, particularly at the higher frequencies. Consequently, the laboratory response functions deter1 The general term "response function" is used in this report to designate either the modulation transfer function (MTF) or the squarewave transfer function (STF).

IMAGING CHARACTERISTICS

15

mined for the Zeiss system are considered less reliable than those obtained for the Wild.

Analysis oJ laboratory photographs The laboratory-derived response functions (Fig. 12-14) vary greatly according to the film employed and also indicate the significance of lens aber-

ZEISS CAMERA

WILD CAMERA la

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SPATIAL FREQUENCY :¢ydes mml

SPATIAL FREQUENCY (cyeles,'mm)

\\\\

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SPATIAL F~EQUFNCY :cycles ram]

SPATIAL FREQUENCY [cycles ram1

LABORATORY P H O T O G R A P H S - - R A D I A L . . . . .

L.C. B A R S A T 0 ° , 1 5 ° , 3 0

TANGENTIAL

L.C. B A R S

°

AT 0°,15°,30

°

Fig.12. Square-wave transfer function (STF) curves obtained from low-contrast (2:1) bars imaged on laboratory photographs using 2402 and 3400 films. Exposure data given in Table II. rations. For example, if the various camera/film combinations are rated according to the laboratory-derived response functions, the systems using 3404 film rank highest, with 3400, 2402, and 2405 next with similar values, then SO 121, and finally SO 180 the poorest. The low response of SO 180 may have been due in part to the characteristics of the camera lenses; however, it also indicates that the system imaging capability is severely limited by the film. With respect to aberrations in the Wild lens, the curves derived from radially-oriented targets at 0 °,

t(3

R. \VEI.CH AND I. t I A I i IDA':

15 °, and 30 ° field angles are similar when the aperture setting is f/8. H o w e v e > those derived from langcntialiy oriented targets become progressively lower wit1) increasing field angle. Enlarging the Wild camera aperture to f/5.6 results in reduction of the on-axis response function but does not cause a significant change in the response functions at the 15 ° and 30 ° positions. The Zeiss camera len> with its narrower angular coverage would probably have been less affected by

'~'~~

,r,~, \ "\\\

WILD CAMERA

~

SO 121o°~

\

ZEISS CAMERA

x

SPATIAL FREQUENCY cycles mm

SPATIAl. FREOUENCYicyc~es mm

i

~!! i

\ I

z ! so 18O~o, ==

\\

\\

I SPATIAL FREQUENCY (cyclesm m

SPATIALFREQUENCY icydes ram)

LABORATORY PHOTOGRAPHS - - R A D I A L . . . . .

L.C. B A R S A T 0 ° , 1 5 ° , 3 0

TANGENTIAL

L.C,

Q

BARS AT 0°,15°,30

~

Fig.13. Square-wave transfer function (STF) curves obtained from low-contrast (2:1) bars imaged on laboratory photographs using SO t21 and SO 180 films. Exposure data given in Table 1I.

changes in aperture. This was confirmed in other tests. At the same f/8 settings the Wild and Zeiss cameras display similar characteristics for the same field angles and types of films, indicating that ~ e i r imaging capabilities are comparable. Manufacturers' filters were found to have a negligible effect on the response functions for the Wild camera. The response functions of the camera lenses independent of the rest of the system were required for further research applications and were derived by dividing out the M T F for 3404 film from the laboratory-determined M T F system

17

IMAGING CHARACTERISTICS

WILD-2405 f/8 500nm

ZEISS-2405 f/8 D

"~\\\ \\\ L

\\\

g

x

0 \\\

~o

"\ \ \ 20

lO

40

50

SPATIALFREQUENCY(cycles,ram)

SPATIAL FREQUENCY {cycles m i n i

LABORATORY PHOTOGRAPHS RADIAL L.C. ~ARS AT 0 ° , 1 5 ° , 3 0 ° TANGENTIAL L.C. BARS AT 0 ° , 1 5 ° , 3 0 °

Fig.14. (continued on p.18). Square-wave transfer function (STF) curves obtained from low-contrast (2:1) bars imaged on laboratory photographs using 2405 and 3404 films.

response for the Wild/3404 and Zeiss/3404 combinations. The resulting M T F curves for the Wild and Zeiss camera lenses (Fig.15) proved to be of satisfactory accuracy for this research and other experimental work.

Analysis ol aerial photographs STF curves for the aerial photographs are shown in Fig.16-19. Note that the curves for the various Wild camera/film combinations include predicted functions, obtained by cascading MTF's for image motion, vibration, and shutter operation together With laboratory-derived MTF's for the particular camera/film combinations and converting the predicted MTF's to STF form by the normal 4/~ relationship (Scott et al., 1963). Like functions for the Zeiss photographs were not derived because of the questionable accuracy of the laboratory-derived response functions. For an overall indication of the consistency of the response functions irrespective of the film employed, a mean square-wave function for all Wild camera/film combinations used in aerial photography was computed by averaging the range of values obtained for the edge-derived and bar-derived functions at selected spatial frequencies (Fig.20). The standard deviation from the mean, also shown in Fig.20, varies from about + 0.08 M at 10 I/mm to about ± 0.13 M at 40 l/mm. Even narrower limits might have been obtained if a slightly wider microdensitometer slit had been used to trace the exposures on 2402 film (to reduce noise) and if the Wild camera/3400 film combination had not been overexposed. For comparison an average predicted STF for all Wild camera systems is also shown in Fig.20. The correspondence between predicted and actual curves is considered remarkably good. In addition, the reliability of response functions derived from bars as com-

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Fig.15. M o d u l a t i o n t r a n s f e r f u n c t i o n ( M T F ) curves. A. F o r t h e W i l d c a m e r a lens (on-axis at f/8). B. F o r t h e Zeiss c a m e r a lens (on-axis at f/8).

pared with those derived from edges was determined. As shown in Fig.21, the mean edge-derived function averages about 0.03 M-0.04 M higher than the mean bar-derived function, a trend which may reflect the influence of the Wild/3400 combination. However, the results do indicate that: (1) the method of predicting operational performance based on laboratory data is quite feasible; and (2) the functions derived from bars and edges have about equal reliability as indicators of system performance. Judging from the response functions derived from the aerial photographs, camera system performance can be ranked as follows according to the type of film used: 2402, 3400, SO 121, and SO 180. The systems with 2402 film (f/8) outperformed those with the higher definition 3400 film (f/5.6) because of the smaller aperture used with the former, a trend also noted in the analysis of laboratory photographs. RESOLVING POWER

With the laboratory photographs resolving power (RP) was determined only for the optimally exposed long-bar, low-contrast (2:1) targets, whereas with the

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aerial photographs RP values were determined for U.S.G.S. long-bar targets of 9:1 (high) and 2.5:1 (low) contrast and military targets of 18:1 (high), 6:t (medium), and 1.3:1 (low) contrast. (Refer to Tables II and III for exposure data.) All readings were taken through a zoom stereomicroscope (to 60 ) and the magnification levels selected by the observer were recorded. The optimum magnification factor averaged about 0.75 times the resolution in l/mm. Selwyn (1948) obtained a factor of 0.8. The readings have been plotted logarithmically in Fig.22--24 to reflect the finding (Coluccio et al., 1969) that photointerpretability tends to vary logarithmically with resolution.

Analysis of laboratory photographs Readings obtained from the targets imaged on laboratory films (Fig.22) indicate that the Wild and Zeiss cameras have approximately equal RP when the same films and exposure conditions are used. As with the response functions, however, off-axis resolution levels appear to be strongly affected by lens aberrations. The Wild camera, for example, produces similar resolution values for radially-oriented targets at the three format positions whereas tangential resolution decreases to a limiting value of about 20 l/mm at the 30 ° field angle. An exception to the trend can be noted for the SO 121 photographs, which retain relatively high resolution levels for the tangentially oriented targets. Apparently the characteristics of the emulsion layers in the SO 121 film tend to mask the effects of lens aberrations. A reduction in resolution with increasing field angle was also noted for the Zeiss camera, although it is obviously less serious because of the camera's narrower angular coverage. The resolution values derived from the laboratory experiments are considered representative of the capabilities of the camera systems tested. It is particularly significant that, when photogrammetric cameras are used with very-highresolution films such as 3404 and SO 121, primarily designed for reconnaissance, they can produce on-axis low-contrast resolution values approaching 100 l/mm in the laboratory. Unfortunately the practicality of using the films operationally is currently limited by the relatively small apertures available in photogrammetric cameras.

Analysis of aerial photographs The bar graphs in Fig.23 and 24 and the photomicrographs in Fig.25 and 26 indicate the comparative resolving power of the Wild and Zeiss camera systems under operational conditions. The range of values for targets of different contrasts obtained from bars oriented parallel to the flight line (the direction of minimal image motion) are further summarized in Table VI. It is again evident that the Wild and Zeiss camera systems produce approximately the same resolution values when used with the same film under the same conditions. In Fig.23 and 24 the effects of image motion on resolution values obtained from bars oriented perpendicular to the flight line are most noticeable in the

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Fig.2l. Average square-wave transfer functions obtained from low-contrast (2.5:1) bars and edges as imaged by all the Wild and Zeiss camera/fihn combinations used in the air. l:12,000-scale photographs on SO 121 film, in which the more than 40 , m of image motion reduces resolving power to less than 20 l/mm. Generally, however, low-contrast resolving power was not noticeably affected by image motion until it exceeded approximately 20 um. Based on resolving power alone, camera-system performance can be ranked according to the film employed: SO 121, 3400, 2402, and SO 180, corresponding to that based on the manufacturer's low-contrast resolving power data for the films alone (Eastman Kodak, 1968). Hence it can be concluded that even under operational conditions the films rather than the lenses of modern photogrammetric camera systems determine maximum resolving power. It must also be noted that resolving power and response functions do not necessarily rank systems in the same order, particularly when systems using both black-and-white and color film are compared (Welch, 1969). For example, the

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camera/SO 121 (color) combinations produced very high resolution values but rather low response functions, and yet there is no question as to the superior interpretability of the color photographs. DETECTABILITY

Small squares in graded sizes imaged on the laboratory and aerial photo' graphs were examined under variable magnification to determine the detectability

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Fig.24. O n - a x i s resolution readings on U.S.G.S. a n d m i l i t a r y b a r targets i m a g e d on Zeiss aerial p h o t o g r a p h s ( H C , M C a n d L C -- high, m e d i u m a n d low c o n t r a s t targets).

29

IMAGING CHARACTERISTICS

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TABLE VI RESOLUTION RANGES (l/11111!1 GN ~ [ R I A L PIIOTOGRAPHS

Film

W i l d catJTet'a

Z e i s s ¢amera

2402 3400 SO 121 SO 3[80

16-49 15-55 17-62 13-27

19~-4 20--44 22-61 15-3[)

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Fig.25. Photomicrographs (27 X) of the military medium-contrast (6:1) bar targets as imaged on l:12,000-scale aerial photographs. Arrow indicates 40 l / m m resolution level.

IMAG1NG CHARACTERISTICS

31

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Fig.26. Photomicrographs (27 X) of the military medium-contrast (6:1) bar targets as imaged on l:24,000-scale aerial photographs. Arrow indicates 40 l/mm resolution level• t h r e s h o l d s ( M a c D o n a l d , 1958; C a r m a n camera/film combinations. The contrast the U.S.G.S. r e s o l u t i o n targets; however, those o b t a i n e d on the l o w - c o n t r a s t (2:1) of the average g r o u n d scene.

a n d C h a r m a n , 1964) for the different ratios of the squares m a t c h e d those of the d a t a p r e s e n t e d b e l o w are limited to squares, which are m o r e r e p r e s e n t a t i v e

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Analysis of laboratory photographs Detectability threshold values for the different laboratory camera/film combinations are summarized in Table VII. From the tabulated data and the photi~T A B L E VII DETECTABILITY THRESHOLDS (L/Ill)

Camera

Fihn

Angle of~ axis O: 15

30 35 35 35 35 35 35

Wild

2402 34O0 SO 121 SO 180 2405 * 3404 (f/8) *

30 2O 30 30 30 5

30 3O 30 30 30 15

Zeiss

2402 3400 SO 121 SO 180 2405 * 3404 (f/8) *

20 20 20 30 30 15

30 30 311 30 30 15

* These camera/film combinations were not used in the air.

micrographs in Fig.27-29 it is evident that films such as 2405 and SO 180 limit the detectability threshold to about 30 urn, whereas a fine-grain reconnaissance film such as 3404 permits the detection of low-contrast objects as small as 5 to 15 ~tm. The lens aberrations at maximum aperture and at the edges of the format also noticeably affect the thresholds determined for the Wild camera. For example, the Wild/SO 121 photographs exposed at f/5.6 provided on-axis detectability ot about 30/~m, which is about 10 /~m larger than that obtained with an aperture of if8. By using an optimum aperture (f/8) and a fine-grain film (3404), a 2-3 )< improvement in detectability is possible in the central portion of the photograph (Fig.29). At the corners and edges of the format, however, the detectability threshold is reduced to about 30 ,um regardless of the film used. Based on the foregoing analyses, the average detectability thresholds that can be expected for properly exposed photographs are as shown in Table VIII. TABLE VIII AVERAGE DETECTABILITY THRESHOLDS

Film

Threshold (,urn)

3404 3400, SO 121 SO 180, 2402

10 20 30

IMAGING CHARACTERISTICS

DD

24 02

3400

SO 121

SO 180

WILD CAMERA Fig.27. Photomicrographs ( 5 0 ) ~ ) of low-contrast (2:1) square targets as imaged on Wild laboratory photographs.

Perhaps coincidentally, the numerical values for the thresholds closely correspond to the listed RMS granularity values for the films (Eastman Kodak, 1968).

Analysis of aerial photographs Visual evaluations of the low-contrast targets imaged on the aerial photographs indicated an average detectability threshold of about 25 !~m. The highgamma, low-granularity films such as SO 121 and 3400 produced detectability thresholds of about 20 /am, whereas those for 2402 and SO 180 were closer to 30 ~m. Scale and format position did not appear to seriously alter the threshold values. A series of photomicrographs produced from aerial photographs taken on 2402 film at a range of altitudes (Fig.30) indicates the increasing effect of contrast and film grain on detectability as photographic scale is reduced.

34

P,. W E L C H AND J. HAI.I.II)A',

2402

3400 i

SO 121

SO 180

.ii; ¸ :

ZEISS CAMERA Fig.28.

Photomicrographs

(45

X) of low-contrast

(2:1) s q u a r e t a r g e t s as i m a g e d on

Zeiss laboratory photographs. MEASURABILITY

The measurability of small detail was investigated thoroughly, including (a) the effect of target contrast on measurement error, (b) the effect of image size or spacing on measurement error, and (c) the threshold image size required for a reasonable determination of the size and shape of a small object. Because slight imperfections in the collimator targets would have affected measurements on the laboratory photographs, the investigations were restricted to the aerial photographs, for which the large reduction factor between ground and photograph also reduced unintentional discrepancies in bar spacing or edge characteristics to a negligible level. E f f e c t o] c o n t r a s t o n m e a s u r e m e n t

error

Monocomparator pointings (at 35-40 X) were taken along the edges of high, medium, and low-contrast bars oriented parallel to the flight line and wide enough (typically 1 to 2 Umm) to overcome the effects of the system spread

IMAGING

35

CHARACTERISTICS

2405

2405 5 ~ ~.

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.

,. . . .

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Fig.29. Photomicrographs (47 X) of low-contrast (2:1) square targets as imaged on Wild and Zeiss laboratory photographs. function. A straight line was fitted to each series of pointings by the method of least squares, and the mean distance between lines was computed to obtain the imaged width of a bar (Fig.31). The absolute measurement error was then taken as the difference between the imaged and the ground width (reduced by an appropriate scale factor)• The precision of measurement was indicated by the standard error of pointing after the least-squares fit. Fig.32 illustrates the relationship between measurement errors and density differences of images on aerial photos taken with the Wild and Zeiss cameras. A number of parameters have influenced the shape of the curves. For example, in the contrast-exposure-gamma relationship, as the density extremes of an imaged bar approach the toe and shoulder of the D - l o g E curve, image spread increases and the measurement errors rapidly reach a maximum, These factors account for the errors that increase to the left in Fig.32. The errors that increase to the right are due largely to the operator's uncertainty in setting the index mark, a trend that increases quite rapidly as the density differences become less than 0.2. The

3(~

R. WELCH AND J..HA.I.t ID~

6,000 ft.

12,000 ft.

24,000 ft.

40.000 ft. 7~

000 ft.

WILD CAMERA Fig.30. Photomicrographs (36 ~ ) of the U.S.G.S. target array as imaged on Wild aerial photographs taken at the indicated flight heights using 2402 film.

37

IMAGING CHARACTERISTICS

degrading trends at the lower and upper ends of the scale indicate the desirability of maintaining correct g a m m a and strict exposure control for optimum measure-

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Fig.31.

Schematic representation of the procedure for measuring the imaged width of

a bar. inent. F r o m these experiments, linear dimensional errors of 3-7 l~m apparently can be expected on panchromatic and high-resolution color films and about 15 , m on color infrared film. The precision of pointing (curves 5 and 6 in Fig.32) averages about 1.5 u m for midrange density differences, increasing to 3 - 4 , m for a density difference of 0.1. As might be expected, precision is slightly poorer for the color infrared films. 50

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Fig.32. Measurement error vs. density difference (AD) for large bars of various contrasts imaged on Wild and Zeiss aerial photographs using: (1) 2402 film, gamma 1:3; (2) 3400 film, gamma 2.7; (3) SO 121 film, gamma 2.4; and (4) SO-180 film, gamma 2.4. Precision of pointing to target images is indicated for (5) 2402, 3490, and SO 121, and (6) SO 180. EJfect of image size on m e a s u r e m e n t error The images of high-, medium-, and low-contrast bar targets were measured over a range of spatial frequencies by the methods described above. F r o m the measurements it became evident that the errors were affected by contrast, exposure, gamma, and image density differences as well as by target size and spacing. For example, in Fig.33 the decrease in error as bar spacing is reduced can be explained by the compression of the high and low densities onto the linear portion

,~

R. WELCH AND ], HAl I.ID;\3

of the D-log E curve. Consequently, as might be expected with high-contrasi targets, a medium-gamma film such as 2402 (7.... 1.3) produces the smallest error~. tINES mm I0

15

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RESOLUTION BAR SPACING { urn)

Fig.33. Measurement error vs. bar spacing for high-contrast (9:1) and medium-contrast (6:1) bar targets imaged on Wild and Zeiss aerial photographs. Curves 1-4 were obtained from high-contrast targets o n films as listed in Fig.32. Curve 5 was obtained from low, contrast targets imaged on 2402 film.

For medium-contrast targets the measurement error (curve 5 in Fig.33) remains nearly constant or may even become greater as bar spacing decreases. Note that the resolution limit to which the operator could measure on the panchromatic and color films was about 32 //mm, despite observed resolutions of 40 and 60 l/mm respectively.

Threshold image size for reliable measurement To investigate 'the threshold image size required for reliable measurement, comparator pointings were made to the edges of images of high- and low-contrast square targets of graded sizes. The pointing coordinates were plotted at an enTABLE IX AVERAGE

MEASURABILITY

THRESHOLDS

Film

Threshold (,um)

3404 SO 121 3400 2402 2405 SO180

20 20 40 40 60 60

IMAGING CHARACTERISTICS

39

larged scale by means of an IBM 360/65 computer (Fig.34), and visual judgments were made to determine the threshold sizes at which the correct shape of the imaged object became apparent and reasonable linear measurements could be obtained. Average results are given in Table IX. In general, the measurability threshold values were about 1.5 to 2 times the linear dimension of the detectability thresholds. It is interesting to note that the empirically determined measurability thresholds closely approximate the objectively determined bar widths required for maximum response (Fig.35). The similarity indicates that the image size must at least ap-

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Fig.35. Normalized single-bar response functions obtained for photographs taken with the Wild camera. Laboratory photographs: (1) 3404 film at f/8. Aerial photographs: (2) 2402 film at f/8, (3) 3400 film at f/5.6, (4) SO 121 film at f/5.6, and (5) SO 180 film at U5.6.

proximate the width of the system spread function before object shape can be defined. Trinder (1971) discussed a similar relationship based on laboratory experiments for determining optimum target size. CONCLUSIONS

Analyses of laboratory and aerial photographs have demonstrated the feasibility of specifying the imaging characteristics of photogrammetric systems on the basis of response functions, resolving power, and detectability and measurability thresholds. The methods used to derive the data also appear suitable for analyzing and comparing camera systems. Typical methods include the determination of response functions from microdensitometer traces of edges or bar target

40

R. W E L C H AND J. HALLIDAY

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patterns, visual analysis of bar targets and squares to ascertain resolution value~ and detectability thresholds, and derivation of measurability data from comparatol measurements along the edges of imaged bars and squares. System performance can be predicted by extrapolating from laboratocx data. In particular, the system response function can be reasonably determined by cascading the laboratory-derived response function with the appropriate functions for image motion and atmospheric effects. The total system response function can then be compared with those given in this paper, for which corresponding resolution, detectability, and measurability data are also specified. If the control of photographic quality is required, response functions of aerial photographs can be determined by edge trace methods and the derived functions matched against predicted values. The reasons for significant discrepancies between actual and predicted values often can be isolated if sensitometric dala are available. Although the comprehensive analyses which have been discussed may seem unnecessary for normal aerial photography, in which photographs at conventional scale are obtained, they are important to the success of high-altitude, small-scale photographic missions, for which image quality and hence camera system performance is critical. Since the missions are costly, the time taken to analyze and select the optimum camera system parameters appears justified. ACKNOWLEDGEMENTS

The research that formed the basis for this paper was conducted under a Postdoctoral Research Associateship granted to Dr. Roy A. Welch by the U.S. Geological Survey in cooperation with the National Research Council and National Academy of Sciences. The authors are grateful to the many people in the U.S, Geological Survey, National Bureau of Standards, National Aeronautics and Space Administration, U.S. Army, Eastman Kodak Company, Itek Corporation, Keystone Aerial Surveys, and Kucera and Associates whose invaluable cooperation and assistance contributed so much to the success of the project. The reader should note that the names of products and manufacturers are stated as information directly and inextricably related to the research project and not as an endorsement by the government or the Geological Survey~ Anyone wanting to utilize these results, or replicate them, is entitled to know the facts on which they are based, without wasting time and effort on rediscovering what is already known. All are free to use alternative products and equipment, and we hope and expect that they will report their findings in equivalent detail.

IMAGING CHARACTERISTICS

43

REFERENCES

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