Normal Angle Camera Calibrator*

GOMER T. Me NEIL,

Photogrammetry, Inc., Rockville, Md. t

T HE rapidly growing field of photographicinstrumentation requires the use of many

normal and sub-normal-angle cameras, as wellas wide angle aerial cameras. The NormalAngle Camera Calibrator was designed anddeveloped to feature the calibration of recon­naissance, tracking, still, and motion picturecameras with angular fields of 400 or less.

The Normal Angle Camera Calibrator,Model I, is a precision instrument that pro­vides a known angular array of targets locatedat optical infinity. These targets are photo­graphed by the camera under test. The imagesof the targets are then read and measured toyield the necessary data for the determina­tion of focal-length, location of principalpoint, distortion, and resolution.!

A brief introduction to the Wide AngleCamera Calibrator will assist in understand­ing the evolution of the Jormal Angle Cam­era Calibrator. The development of the \iVideAngle Camera Calibrator \I'as started in June,1955 with an award of a contract by theBureau of Aeronautics for the delivery of thecalibrator to the Naval Photographic Inter­pretation Center. The U. S. Navy presentedmany performance specifications; however,the significant requirements \I'ere:

a. 90 degree field of view,b. 2.5 inch clear aperture of collimators,c. relatively small,d. relatively inexpensive, ande. relatively simple procedure for the re­

cording and data reduction phases.

During a period of nearly six years, threemodels of the Wide Angle Camera Calibratorwere developed as the result of a close liaisonwith military and governmental personnel to

t "The 65 targets are nominally positioned to thedistortion free posi tion based on the da ta pre­sented in the NBS Test Report. The preciseangular relationship of the target array is deter­mined by a Wild '1'-2 theodolite as a final calibra­tion procedure."-Excerpted from Mr. McNeil'sletter of June 22, 1962.

GOMER T. MC\'EIL

incorporate modifications and refinementsthat \I'ere based on the practicalities of orera­tion and test.

Model III of the Wide Angle Camera Cal­ibrator took firm shape and was exhibited atthe l\1arch 1961 Annual Meeting of ASP­ACSl\I. Soon after last year's Annual l\1eet­ing, one of our customers con tacted us rela­ti\'e to a problem that \I'as experienced \I'hennormal or sub-normal angle cameras \I'erecalibrated on the Wide-Angle Camera Calib­rator. The problem consisted of an insufficientnumber of targest recorded by the cameraunder test to conduct a comprehensive datareduction. The relati vely small camera field ofview in conjunction \I'ith targets spaced atangular intervals of 7~·0 precluded the record­ing of su fficien t targets.

The Wide Angle Camera Calibrator withside panels removed is shown in Figure 1.The 7~0 interval between collimators is asu fficien tin terval for wide-angle ca meras.The angular interval should be decreasedhO\l'ever, \I·hen the field of view of the camerabeing tested is relati\"ely small. For example,only the central target of the Vride Angle

* Presented at the 28th Annual Meeting of the Society, The Shoreham Hotel, Washington, D. C,March 14-17, 1962. The Abstract for for this paper is on page 346 of the 1962 Yearbook, Vol. XXVlf!,No.2.

t 5454 Carroll Avenue, Washington-f{ockville lndustrial Park, Rockville, Md.

633

634 PHOTOGRAMMETRIC ENGINEERlNG

FIG. 1. Wide Angle Camera Calibrator.

Camera Calibrator is recorded by a 3S mmmotion picture camera with a S-inch lenswhile 29 targets are recorded wi th the usc ofthe Normal Angle Camera Calibrator.

The Normal Angle Camera Calibrator isillustrated in Figure 2. The basic conceptsupporting the design of the Normal AngleCamera Calibrator is the utilization of onespecially designed collimator lens to cover 65targest over a field of 40°, in place of installingan individual lens for each of the 65 targets.This optical design approach has two distinctadvantages: a) the targets can be spaced atrelatively small angular intervals since thereis not a space requirement for the installationof a lens and housing for each target, andb) the cost of one larger lens and housing issignificantly less than the cost of manysmaller lenses and housings. Figure 3 is anoptical schematic through one bank of tar­gets.

The collimator lens (Figure 4) was manu­factured by C. P. Goerz American OpticalCompany and was designed by Mr. K. E.Brandt of the same company. The lens, calledthe Geotar, is a four element air-spacedGaussian type wi th a focal-length of 20­inches, a clear aperture of 3t inches and a fullfield of 40°. The front and rear crown ele­men ts are rare earth glasses and the twoinner elements are high index Aints. The ab­stract of the National Bureau of StandardsTest Report on the Geotar lens is tabulatedin FigureS.

The targets are placed in the planar focal

surface of the lens. There are two banks oftargets at 900 to each other as depicted inFigure 6. Each bank of targets covers a fullfield of 40°. The interval between targets is2to. Additional targets are placed at half­angles of 15°, 17tO, and 20° on diagonals of arectangular format to accommodate amateurcameras with a 24X36 mm format. Twoadditional banks of targets are placed at 90°to each other. These latter targets are spacedat an interval of 10 and consist of a cen ter­cross only while all other targets contain reso­lution patterns and a center-cross.

The control panel is shown in Figure 7There are six rheostats to adjust the illumina­tion intensity of the various zones. Two inter­changeable plug-in timers cover an exposurerange from 1 second to 10 minutes. The panelcontains a display counter to indicate theexposure number. The display counter issynchronized with a similar counter in thefocal-plane of the collimator lens. The focal­plane counter is recorded on the calibrationnegative. A counter defeat switch is mounted

FIG. 2. Normal Angle Camera Calibrator.

NORMAL ANGLE CAMERA CAL! BIL\TUR

IMAGES OF TARG;TS ON NEGATIVE

CAMERA UNDER TEST

COLLIMATOR

30 26 22 18 14 10 6 2 0 4 8 12 16 20 24 28 32

o 0 SGH;:OU~~ 0 0 0 0 0 T~GE?NUM~R?"§2 0

FIG. 3. Optical schematic through one bank of collimators.

DISTORTION

635

RESOLVING POWER

a· S· ID· IS· 200

Tangential 46 39 32 23 23

Radial 46 39 32 19 19

Measurements were made photographically at aperturef/6.8 using a K-3 filter. a tungsten source and EastmanKodak spectroscopic emulsion Type V-F. Development

'was in 0-19 at 680 F for three minutes with conllnuQUS

agitation.

S· ID·

-0.01 0.01

IS·

0.03 O. J4

FIG. 4. Goerz Gcotar 20-inch focal length, f/5.7. FIG. 5. Abstract of NBS Test Report.

I

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