"SOME CHARACTERISTICS OF THE VISUAL PROCESS AFFECTING THE 
OBSERVATION AND MEASUREMENT OF ARTIFICIAL PASS-POINTS" 
Desmond O’Connor 
Research and Analysis Division 
GIMRADA 
Fort Belvoir, Virginia 
ABSTRACT 
This paper describes an experimental study of factors affecting 
the precision of centering black circular measuring marks in sharp, 
high contrast targets with homogeneous backgrounds, subtending visual 
angles up to 45 minutes of arc, in photopic vision. 
The results supported the concept of a retinal zone approxi 
mately 4 minutes of arc diameter over which acuity is constant, but 
further suggested that this may be dependent on the type of task in 
volved. The maximum information content for the pointing appeared 
to be contained in a ribbon approximately 1 minute of arc wide around 
the edge of the target. The most precise pointings were made by 
selecting a measuring mark to give a minimum annulus within this 
ribbon irrespective of the target size in the range examined. 
I. INTRODUCTION 
This investigation has as its principal aim a study of the pre 
cision of centering black circular measuring marks in sharp, circular 
high-contrast targets with homogeneous backgrounds, imaged on the 
fovea centralis. 
An earlier investigation by the writer (10), carried out on a 
precise stereocomparator, suggested that there was a target visual- 
angle of approximately 20 minutes of arc, below which the pointing 
error was constant, and above which the pointing error gradually in 
creased in accordance with Weber's law. The measuring mark size did 
not appear to be important in the small size - ranges usually en 
countered. The threshold pointing error in the constant region was 
of the order of 1.7 microns. 
A review of the literature in acuity, however, suggests that 
the eye is remarkably sensitive to tasks of this type, thresholds 
of 1 second of arc or better having been obtained. Independent de 
terminations verified that 1 micron in the photo-plane of the Wild 
Stk stereocomparator under magnification 12X subtended a visual angle 
of the order of 10 seconds in the eyepiece. The 1.7 microns referred