A SLIT-SCAN ELECTRO OPTlCAL RECTlFIER 747 light level during scanning. The exposure common point. In addition the recording could not be corrected by an iris adjustment plane is rotated through the angle p. during at the lens because the resolution would then rectification. These additional motions would become diffraction limited. all be obtained by servos programmed from Tipped panoramic photography produces punched tape. A mathematical development geometry in the photograph 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 tilt-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 panoramic photography 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 optics is generally unsatis­ (Nm ) of a lens, along its optical axis, may be factory for aerial reconnaissance. expressed by the equation. BASIC CAMERA 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 cameras 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 image 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 shutter 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 color distortion, 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 lenses 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 aperture 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.