A SLIT-SCAN ELECTRO OPTlCAL RECTlFIER 747 light level during scanning. The common point. In addition the recording could not be corrected by an iris adjustment plane is rotated through the angle p. during at the because the resolution would then rectification. These additional motions would become diffraction limited. all be obtained by servos programmed from Tipped panoramic produces punched tape. A mathematical development geometry in the in which there is and proof of this method of operation has no set of lines that are linear and perpendicu­ been accomplished but is much too lengthy lar to a principal line. As previously explained to present here. this was a requirement for rectification by this machine. However, there are special OBLIQUE SLIT PHOTOGRAPHY adaptations for handling this type of photog­ Oblique slit photography rectification is raphy. An analysis of the transformation essentially a simplification of the oblique equations indicates that one method of recti­ single-frame case. Obviously the scale re­ fication which may be used is a two-stage mains constant for all lines in the photograph process. The copy is first recti fied as though it parallel to the line of flight. It is only neces­ were a vertical panoramic photograph. The sary to rectify in the direction perpendicular resul t of this transformation is an oblique to the flight vector. This is accomplished by single-frame photographic equivalent having having a fixed lens to copy and lens to re­ a -angle equal to the tip-angle of the pano­ cording distance for the required scale change ramic photograph. The single-frame oblique and obtaining the transformation in the equivalent photo which results is then recti­ perpendicular direction by relative motion fied in the machine as previously described. between scanning speed and recording speed. If the size of the rectified photograph in the first stage is too large, it must be reduced in CONCLUSION size during the first rectification and then As may be seen the machine as described in enlarged in the second stage to the required this paper can be used to rectify the various size. types of oblique photography discussed. By By modification of the machine it is pos­ using optical projection, high-resolution and sible to rectify the tipped panoramic photo­ relatively high operating speed is obtained. graph in one step. This is indicated in Figure By using high-precision servomechanism 7. The scanning slit and copy, the lens, and drives driven by digital computer derived the recording plane are angled with respect punched tape, high-precision in metric trans­ to each other so that the Schei mpflug condi­ formation will be obtained. For special re­ tion is satisfied. That is, lines through the quirements of rectification, the machine may scan-slit, the plane of the lens, and a line be modified for optimum operation at the through the plane of recording intersect at a cost of introducing additional complexity.

Panoramic Progress-Part I

ITEK LABORATORIES, Lexington 73, ~Mass.

THE ADVANTAGES OF PANORAMIC and more areas of our world and-soon, PHOTOGRAPHY perhaps-of other worlds as well. In order to GENERAL cover the large areas involved, and to resolve HE development and increasing use of the desired ground detail, present-day recon­ T in the field of naissance systems must operate at extremely aerial reconnaissance has resulted primarily high-resolution levels. Unfortunately, high­ from the need to cover in greater detail more resolution levels and wide angular coverage

EDITOR'S NOTE: This paper which is to be supplemented by Part 2 in the March 1962 issue includes the contents of a brochure by [tek Laboratories entitled "Panoramic Progress." A reading was convincing that the contents were so valuable and helpful to photogrammetry and photogrammetrists that repeating in this JOURNAL was highly advisable. For permission to take this action, thanks are given to Itek Lab­ oratories. 748 PHOTOGRAMMETRIC ENGINEERING

are basically counteracting requirements. resolution decreases with increasing f-number, The maximum theoretical resolving power the use of huge is generally unsatis­ (Nm ) of a lens, along its optical axis, may be factory for aerial reconnaissance. expressed by the equation. BASIC TYPES 1426 lines/mm. Nm =------There are three basic camera types that f-number may be considered: frame, strip, and pano­ However, off-axis resolution diminishes for ramic. The ground coverage obtainable with tangential lines as the cos3 of the angular each of these is described below. separation from the optical axis of the poin t considered. For radial lines, resolution off-axis Frame cameras diminishes only as the cosine of that angle. In In a frame camera, the lens forms an practice this means that where wide-angle of the scene to be recorded over the full area coverage of a lens is required, the resolution of a single frame of film. If a between-the-lens of tangential lines at, for example, 45 degrees is used, the entire frame is exposed off-axis would be down to about 35 per cent simultaneously; with a focal-plane shutter, a of its on-axis maximum; even for radial lines, slitted curtain sweeps across the frame, ex­ resolution would be down to about 70 per posing the film as it travels. When using a cent. frame camera in aerial reconnaissance, ex­ Practical considerations make wide angular posures are made at regular intervals as the coverages even less desirable. Various aberra­ vehicle passes over the terrain. The intervals tions such as coma, lateral , are so timed that a sufficient overlap is re­ field curvature, and astigmatism are all much corded on the consecutive frames to record more difficult to control in with wide complete coverage, without gaps, of the area angular coverage than in those with narrow of interest (A, Figure 1). coverage. Thus, narrow-field angles are very preferable for high-resolution photography. Strip cameras An important consideration in discerning A strip camera records a continuous strip of small ground detail is the optical system's terrain as the aircraft flies over it (E, Figure focal-length. For a given altitude, ground 1). In its simplest form, this type of camera resolution can be increased by the use of consists of a fixed lens mounted in the camera longer-focal-length optics, but as the focal­ which records on the film that which is length becomes longer, generation of the wide directly below. The shutter is kept open dur­ glass areas required to keep a reasonable low ing the entire recording period, the exposure f-number becomes increasingly difficult. Since being controlled by a slit of variable width.

A. Frame Camera B. Strip Camera

~ C. Panoramic Camera

Direction or Scan • ~

~ I -

Direction of Direction of Direction of Flight Flight Flight

FIG. 1. Ground coverage obtained with frame, strip, and panoramic cameras. PANORAMIC PROGRESS-PART I 749

During exposure the film is moved in the case to cover a reasonable swath. Based on direction of flight at a rate equivalent to the state-of-the-art considerations, a single pano­ rate of image motion. ramic camera can not only handle the type of problems outlined here, but it will always Panoramic cameras weigh less than a group of frame or strip A panoramic camera is also a scanning type cameras with comparable resolution and of camera; unlike the strip camera, however, coverage capabilities. it sweeps the terrain of interest from side to I t is often thought that panoramic cover­ side across the direction of flight (C, Figure age is excessively wasteful at the edges of the 1). This permits the panoramic camera to re­ frame, i.e., at the limits of the lateral cover­ cord a much wider swath of ground than age. However, an array of single-frame cam­ either frame or strip cameras. As in the case of eras would cause the same overlapping un­ the frame cameras, continuous cover is ob­ less they were cycled non-simultaneously. In tained by properly spaced exposures timed to panoramic photography, the picture is taken give sufficient overlap between frames. This at a known time and in such a manner that all means of exposure limits the effectiveness of points in the picture can be related directly to a panoramic camera for photography at low the principal point. altitudes and high speeds; since the area is The panoramic camera concept has in covered by relatively narrow camera angles many cases been selected as the most promis­ in the direction of flight, a fairly high cycling ing for meeting design goals. Even a one-foot rate is required when the vehicle is traveling focal-length, coupled with 5-inch film, re­ at a low altitude. Moreover, the duration of a quires a lens performing to only about 11 de­ complete sweep cycle from side to side is usu­ grees off axis. This relatively narrow-field ally longer than the cycle of a frame camera. angle makes it possible for the lens designed Panoramic cameras are thus most advantage­ to achieve an image close to perfection, for the ous for applications requiring the resolution focal-length and required, over the of small ground detail from high altitudes. total width. If frame cameras with focal-lengths com­ parable to those employed in panoramic THE DESIGN OF PANORAmc CAMERAS cameras are used for wide-area coverage, BASIC PANORAMIC CONFIGURATIONS either a large array of cameras must be used The numerous mechanical approaches to to cover a reasonable swath (90 degrees or the design of panoramic cameras may be more), or a single camera must be rapidly divided into two general categories: (1) direct­ cycled and indexed from one position to an­ scanning cameras with swinging (or rotating) other. Indexing of an entire camera in a lenses, and (2) cameras that scan by means of vertical configuration requires power and may rotating mirrors or prisms. disturb the uniform motion of the vehicle. Equally difficult, especially from the stand­ Direct-scanning camera point of resolution, is the indexing of a mirror A typical direct-scanning camera is the for a camera in a horizontal configuration; Hyac type shown in Figure 2. The design in­ e.g., if a swath 140 degrees wide is to be volves rotating the lens about its rear nodal­ covered with a 24-inch lens, an array of seven point and accurately positioning the film in frame cameras, or a single frame camera in­ the proper focal-plane position by means of dexed into as many positions, would be re­ two small rollers mounted on the end of a quired. Since the frame camera using a square scan arm which rotates with the lens. format would cover the same forward dis­ The scan arm also carries the scanning sli t. tance on the ground that it covers laterally, it With the lens sweeping at a given rate, the can be seen that a single frame camera would width of this slit (in the direction of scan) de­ have to take pictures at a high rate. Seven termines the amount of exposure given each cameras could be used, but this would require picture element; the wider the slit, the longer an equal number of lenses and the total weight the exposure. Ina typical application this would become excessive. width may be varied from 0.07 to 0.04 inch. If strip cameras are considered for wide The length of the slit across the scan direc­ area coverage and if, for instance, a one-foot tion must, of course, cover the width of the lens were used to cover a strip of film 4t inches image strip to be obtained. The maximum wide, it is obvious that the ground coverage scan which can be obtained with a camera of for a single camera per pass would be only the type shown is necessarily somewhat less about 21 degrees. Therefore, a number of than 180 degrees, since at that angle the lens such cameras would also be required in this would already be looking back onto the film 750 PHOTOGRAMMETRIC ENGINEERING

I II I I --1--- I I DATA I RECORDNG TUlE F1~ l FILM GUIDE JOy.- - 2 I "_M I PO$lTIONltlG I ROLLER I SCAN ARM I I

I IMe - - _IMe MCNA6LE _ M(lVEM!::NT ~ (.AM P...ATE "- '- 12- flS '-- LENS 8ARREL \~IO'l.z· HALF ANGLE \ \ \ \

FIG. 2. Hyac 140-degree camera. platen (the example illustrated here was direct scanning system if the optical system is designed for 140 degrees of angular coverage). offset by means of two mirrors. However, as can be seen in Figure 3, a full I t should be noted that, in the direction of 360-degree coverage can be obtained with a scan, only the lens and scan arm move while the fil m remains stationary. Furthermore, since the center of the lens rotates as a unit with the scanning slit, the sharpest possible image is always projected onto the film even if irregularities should develop in the scan rate. I n the worst case there could be some variation in the exposure because of these irregularities (resulting in corresponding vari­ ations in the film density) but the resolution would remain essentially unaffected. The first successful high-resolution airborne panoramic camera was designed in 1957. A direct-scanning camera, the Hyac I had a 12­ inch f/5 lens and covered a total scan angle of 120 degrees. In-flight performance of the Hyac I resulted consistently in resolutions of 100 lines per millimeter on SO 1213 film. Rotating prism camera Figure 4 is a schematic of a rotating-prism scanning camera employing the concept of a continuously rotating lens. A full 180-degree scan, or even more, can be achieved with this FIG. 3. Direct scanning system. configuration. A double-dove prism, the PANORAMIC PROGRESS-PART I 751

DANCER ROLLERS

27 ; ---r------~ ---.-~ t~==---=-=-=--=--===-==.....:=..:=-4Z+=:....==-=--=.:::=-=--=-=-=-=---="':=-~I I I I I I I I I : I I I I I I I I I

I FEED SPOOL TAKE -UP SPOOL ! I (1000 FT) : 31 I Ii I I I'------,

1 I I I I i I I ':', _--//- - - - I I '~, ~-- '/ I ", '"," \\ //"/ / "- " "I -,- -:--'T------/ \ "// "" '""" \ 1// /' " '- ' ",,~" " h/~-/ /' '-.. " v/ - --~__ ,~- --''------=-O::----D~~L?'-----. __~_'==F-~:::.::::-::~-- /1/_ --- .--- I I I 1 I I I I ! I I I FIG. 4. Rotating prism camera, diagonal faces of which are aluminized and The diameter of the drum as shown on the cemented together, is placed in front of the drawing is such that it must make exactly lens, This prism must rotate 90 degrees for two revolutions for one 90-degree turn of the each l80-degree scan, The light path is di­ prism. The film passes from a supply spool verted by a diagonal mirror and imaged on around an idler roller, over a pair of loop­ the film which passes around a rotating drum. forming or dancer rollers to another idler The rotation of the drum must be accurately roller, and then to a roller which guides it coordinated with that of the prism in order to around the drum. The path to the take-up achieve good image quality. spool is similar. Because of the fast motion of 752 PHOTOGRAMMETRIC ENGINEERING the recording drum whenever a picture is REPRESENTATIVE PANORAMIC DESIGNS being taken, enough space must be allowed so The Hyac camera (Type I Figure 5) that a full frame can be stored by the dancer The simplest and most reliable of the pano­ rollers. ramic types considered here is the Hyac In the ready position, the prism is turned a camera. This camera is characterized by rota­ few degrees beyond the point which allows it tion of the lens about its nodal-point, by a to look 90 degrees out to one side of the curved focal-plane which does not move dur­ vehicle. When the mechanism is released, the ing scanning, by a non-folded optical path, prism and drum are both brought up to the and by the provision of image-motion com­ desired speed. The prism turns 90 degrees pensation (IMC) through translating the lens clockwise at a uniform rate and the drum along its axis of rotation with a flat cam sur­ makes two revolutions in order to complete face. This design avoids the complexity of the l80-degree scan. The prism then travels a synchronized film motion; the absence of fold­ few degrees beyond for deceleration. When ing mirrors and the structural of a the prism is turned in the counterclockwise rigid tie between-the-lens axis of rotation and direction in preparation for the next photo­ the platen minimize the degrading effect of graph, the drum remains stationary. To main­ vibration. tain theoretical resolution, the synchroniza­ tion must be significantly more precise than Modified Hyac camera (Type II in Figure 5) the final resolution to be achieved. If resolu­ This is similar to the original Hyac camera tion on the order of 100 lines per millimeter is in that the film remains stationary in a platen desired, relative motion in the image plane while a lens scans through the required cannot exceed approximately 1/200 mm. In panoramic angle by rotating about its nodal­ addition, systems employing dove prisms point. The difference is that the nodal-point cannot be successfully applied where large of the lens is located at a considerable fraction are required. The large glass mass of of the focal-length in front of the lens. This is the prism is extremely sensitive to tempera­ a long known for its compact­ ture changes, and any distortion of the prism ness. Thus, the lens translates along an arc produces image degradation. The fact that the concentric with the platen for scan. This is reflection comes from a reflecting surface in­ particularly adaptable to reflective optical side the glass adds to the seriousness of this systems such as a Cassegrainian. (For ex­ effect. Image doubling from the two prisms ample, a 240-inch f/20 Cassegrainian mirror takes place at the slightest provocation; even system has been designed and constructed in under the best laboratory conditions, the which the front mirror is located only 40 theoretical resolution of the lens is cut in half inches from the focal plane.) in one direction by the aperture-splitting ac­ The advantages of this type are the light­ tion of the double-dove prism. weight mirror system and the ability to pack­ I t should also be noted that the configura­ age a long focal-length in a configuration of tion, as shown, will result in positive prints small volume. The mechanical problem is that are mirror of the scene they rep­ somewhat more difficult than that of simply resent. If properly oriented are rotating a lens, but it is by no means insur­ required, an additional mirror must be in­ mountable. The power requirement will be serted in the optical path. relatively high because of the large masses Comparison of the two schemes that must be moved, with an attendant higher friction bearing than for a rotary Hyac. Comparing direct scan with rotating prism scan, it can be said in general that, where Modified Hyac camera (Type III in Figure 5) highest possible resolution for a given lens is of This type features a flexible platen and a paramount importance, the direct scanning rigid scanning arm, compared with Hyac's camera is to be preferred. This is primarily rigid platen and spring-loaded scanning arm. because of the need for extremely fine co­ Since the film is not restrained by the platen ordination of film and prism motions which, to lie along the locus of the focal plane, it is in the latter system, must be to a few seconds necessary to meter the film past the slit. This of arc. design is considerably more complex than On the other hand, a direct-scanning cam­ that of the basic Hyac camera because the era, especially if it is equipped with scan arm, film must be kept in precise synchronism requires more space than does a rotating with the regular rate of the scanning lens. prism system. This becomes particularly true Such synchronization would probably be in systems using very long focal lengths. more difficult to achieve with this particular PANORAMIC PROGRESS-PART T 753

PANORAMIC CAMERA TYPE

MODIFIED. MODIFIED MODIFIED OSCILLATING ROTATING NODDING HYAC HYAC HYAC MOVING foe AL CAMERA HYAC MIRROR OPTIC BAR LENS CAMERA CAMERA CAMERA PLANE CAMERA CAMERA CAMERA CAMERA

VERY VERY FAIRLY FAIRLY LOW RELIABILITY RELIABLE RELIABLE REliABLE LOW RELIABLE RELIABLE RELIABLE RELIABILITY RELlABILlTY

INHERENT RESOLUTION EXCELLENT EXCELLENT FAIR FAIR GOOD FAIR GOOD POOR CAPABILITIES

WEIGHT 6 LOWEST MOOERA"!'E LOW MODERATE MODERATE HIGH HIGH HIGH POWER

FILM ADAPTABLE FLOATING SCANNING FILM FIXED SLIT 8 LENS CONTINUOUS FILM, LENS. a STIIT10NARY. TO CA5SEGRAIN PLATEN-RIGID ARM POSITIONS TO PLATEN- STATIONARY. ROTATION MIRROR MOVE NO LENS ROTATES ARM FILM 8 METERS FILM PLATEN MOVES FILM MOvES FILM MOTION SYNCHRONOUSLY. FEATURES SYNCHRONIIATIO ABOUT NODAL P~SITIONED SYNCHRONOUSLY. 'N SYNCHRONOUSLY SYNCHRONIZED. REourRED. POINT a METERED NO PLATEN. SYCHRONISM. WIT~ MIRROR. MODlFIEO E-2 EXTERNAL. AT SLIT.

TYPE NUMBER I n m I'l :l[ lZI :m 12m

FO~ ,PLANE ROLLERS F"lLM FOCAL PLANE MIRROR FOCAL PLANE MOTION FILM O~;~C ~ ~ jE.~~ MOTION STATlONAR~ I\tILEN" ,\ \ ~\ I'I~ ~-" ~s ' "\\ SLIT TECHNIQUE {'ON, - 7{ iI' \ 111/ 1/ " \\ STATIONARY SKETCH , I Lt?EiJ~ I II NODAL \~\ SLIT ", ,;,;: " ,j POIN::J1 " LE J :;- '~~~ !, ~ i:i/I I MIRROR ,; / f\~ NODAL tJ II \\ ROTATION ,; 1/ MOTION :;t(.LENS POINT III LENS MOTION LENS " \I NODAL ;Ii'MOTION MOTION ) '1 NODAL POINT POINT iNODAc .:,NT NODAL POINT NODAL POINT NODAL POINT FIG. 5. Representative panoramic configurations. design than with the moving focal-plane lens are rotated about a common center. Such camera since the film rather than the platen a design is one step removed from the sim­ would have to be driven, and its much smaller plicity of the basic Hyac camera in that there mass would make it more susceptible to are two moving elements which must be kept vibration. The flexibility of the film and the in synchronization. The reliability of such a need for precise synchronization will also device should be reasonably high, but its in­ affect the inherent performance capability, as herent performance capability is not as high will the necessity of driving the film. It as the basic Hyac system because of image should be noted that driving the film by quality lost due to vibration of the film when means of sprockets has been rejected in all moving. considered designs since it is not feasible with The weight of such a camera is slightly such a drive to obtain the resolution required. more than that of the Hyac because of the necessary platen-moving mechanism; power Modified Hyac camera (Type IV in Figure 5) requirements are somewhat higher for the I n this design, the lens rotates about a same reason. The over-all dimensions of the point lying between the nodal-point and the moving focal-plane camera are comparable to focal-plane. The film lies unrestrained in the those of the basic Hyac. A slight advantage in camera and is posi tioned and metered by roll­ adaptability is achieved by the ability to ro­ ers at the slit. In these respects, this modifica­ tate about a variety of axes, but this ad­ tion is similar to that discussed in Type III, vantage is lost in the present case because of the principal difference being the rate at the necessity of enlarging the vehicle cutout which film is metered. The advantage of as the axis of rotation moves away from the smaller volume for a given focal-length must node. be weighed against the disadvantage of not having the lens rotate about its nodal point. Oscillating-mirror camera (Type VI in Figure 5) Moving focal-plane camera (Type V in Figure In this design, the lens remains fixed with 5) the optical axis transverse to the direction of In this design, the film is held in a curved flight. A mirror in front of the lens oscillates focal-plane and both the focal-plane and the and the film is moved past a fixed slit at a rate 754 PHOTOGRAMMETRIC ENGINEERING suitable to synchronize with the image move­ inherent performance potential are considered ment. This is more complex than the designs to be low. discussed previously, although it is still only necessary to synchronize the motions of two MOUNTING AND STABILIZATION elements, the oscillating mirror and the film. As with other types of , Since this must be done with considerable dis­ the full potential of panoramic photography tance between these elements, the inherent can only be realized under conditions of performance capability would be low. proper stabilization and image motion com­ The weight would probably be high since an pensation (IMC). additional element, the scanning mirror, has Because of the wide angular coverage of the been added. This mirror will of necessity be panoramic camera, the relative effects of quite large and heavy. Power requirements pitch, roll, and yaw very considerably over will also be large since this mass will have to be the picture area. For instance, near the hori­ accelerated and decelerated quite rapidly for zon the effects of roll and of yaw are maxi­ scanning. mized while effects of pitch are relatively less. When scanning directly below the vehicle, on Rotating optic-bar camera (Type VII III the other hand, effects of roll and pitch are Figure 5) much more important than are effects of yaw. Thus, for maximum information acquisition, This is a modified Air Force E-2 camera de­ a mount that will stabilize the camera in all signed and built by the Boston University three axes is important. Physical Research Laboratories and Vectron Of equal importance is IMe. With the use Inc. This design is more complex in that two of faster vehicles and requirements for finer mirrors, the lens, and the film must be in ground resolution, the camera will move over constant rotation. A mirror at 45 degrees, the several times the resolution width even dur­ lens, and a second 45-degree mirror are dis­ ing short exposure times (1/400 to 1/2,000 posed with the optical axis along the longi­ second). To compensate, the lens must be tudinal axis of the vehicle. The entire bar is moved back, opposite the direction of flight then rotated at constant speed about the (i.e., at right-angles to the direction of scan). optical axis. If the second mirror is placed at A certain amount of error is unavoidable one half the angular speed, IMC is accom­ since the height and ground speed of the plished by shifting the entire bar longi­ vehicle are usually known only imperfectly. tudinally. Obviously, this is a more complex This IMC error can be further compounded if device than those previously discussed; be­ the vehicle, because of weather conditions, is cause of the concentric arrangements, how­ doing more than a negligible amount of ever, reliability can be high if care is exercised "crabbing." If it is, the vehicle is not actually in fabrication. traveling in the direction of its heading, and The weight is unusually high since two IMC, operating along the vehicle's longi­ extra mirrors are involved. Power require­ tudinal axis, will compensate in the wrong ments are held to a minimum because of the direction. Thus, azimuth correction in the continual rotation characteristics of the sys­ mount is also important for maximum resolu­ tem. tion. A third requirement for all types of pho­ Nodding-Lenscamera (Type VIII in Figure 5) tography is exposure control. In a panoramic In the nodding-lens design, the lens as a camera with a fixed scanning speed this is whole is rotated about a point on the surface most readily accomplished by varying the slit of a mirror any place between the rear vertex width. Required exposure is a factor which and the focal-plane. The mirror is rotated at usually changes rather slowly and therefore half the angular speed of the lens, and the film presents a relatively minor problem. Standard is driven at a rate equal to the focal-length techniques are available for measuring avail­ times the angular rate of the lens. This is the able light and setting the required exposure most complex design we have considered, and automatically. because synchronization must be maintained among all three elements, its reliability and (To be continued and completed in March 1962 issue)