ERA EE RE 4 
  
  
  
  
SRE ds Ea 
18. 
Research programmes that have been undertaken to date have 
investigated the tasks of detection and recognition of simple shapes 
(Welch and Halliday, 1975) and the ranking of scenes according to 
their quality, for the purposes of interpretation (Frieser et al, 1963), 
Hufnagel (1965) and Scott (1968). Before considering this research, 
background information on visual acuity will be given. 
4.1 Definitions of Visual Acuity 
In visual research acuity, being the "capacity to discriminate 
fine details of objects in the field of view" (Riggs, 1965), is 
dependent on the type of visual task adopted for its measurement. 
Riggs refers to the four visual acuity tasks of detection, recognition, 
resolution and localization, each task resulting in different 
estimates of acuity. 
(i) Detection is the task of stating whether or not an object 
is visible. It is expressed in terms of the minimum size of object 
which is detectable and varies according to whether the object is a 
dark object or a bright background or a bright object on a dark back- 
ground. For the former case a disc of diameter of approximately 30 
secs of arc is a representative measure, while for the latter, an 
object, no matter how small, may be visible provided it is bright 
enough. 
(ii) Recognition is the task which involves not only detection 
of an object, but also recognition of its shape eg. the Landolt Ring 
or alphabetic letters. Acuity values of between 30 secs and 1 min of 
arc have been measured for this task. 
(iii) Resolution involves the descrimination of separate elements 
of a repetitive pattern such as in a resolution target. Values for 
this task depend on the type of object viewed. 
(iv) Localization is the task of discrimination of small dis- 
placements of one part of an object from another. Typical of this 
type of task is that of pointing with circular measuring marks, or 
graticules. Figures as small as several seconds of arc have been 
revealed for the type of task, while O'Connor (1967) revealed pointing 
precisions as low as 1 sec of arc for pointing observations to ideal 
targets. 
Acuity values expressed for the various tasks assume ideal high 
quality targets which would not occur in normal photogrammetric 
practice. However, the distinction between the tasks and the relative 
acuity values derived for them is important when undertaking research 
on one or more of these visual tasks. 
4.2 Research on Detection and Recognition of Photogrammetric Objects 
Welch and Halliday (1973) investigating the detectability of 
photogrammetric targets of contrast 2:1, found that targets between 
10um and 30pm on the photograph were visible depending on the compo- 
60 0