Definition of visual acuity.

IT is well known that different types of test object will give different measures of acuity in terms of the size of the angle subtended at an observer's eye by the critical detail in the test object. Acuity of an average observer may vary from 1" arc, the thickness of a single black line just discernible on a white background, to about 1' arc, the size of a gap in a black ring viewed on a white background. Other objects may give values between these two extremes, but in general, acuity is considered to be defined by the broken ring type of test object, rather than by the presence of a single thin line. A grating pattern is frequently used to measure the acuity of the eye or the resolving power of a lens system or photographic process. The definition of what is considered to be the fine detail in this pattern is not, however, always clearly stated. It may be defined as the width of one single black line, or of one single space between the lines, or it may be defined as one pattern width, the width of one black line plus one white space. In the measurement of visual acuity the definition of one line width is frequently used (e.g. Pirenne, 1948; Hecht, 1931; Bartly, 1941). The resolving power of lenses (Houstoun, 1930) and film resolution (Mees, 1942) tend to be defined in terms of the pattern width of the grating. Schlafly (1951) states fairly clearly that the film industry uses the pattern width, whilst the television industry uses the single line element width. Because it is not always clearly stated which definition of fine detail of a grating pattern is being used, and because of the existence of the two different definitions, confusion is apt to arise as to which definition is intended in any particular context. In view of this confusion, it was decided first of all to make a subjective comparison between grating patterns and the broken ring pattern in order to determine a method of defining the grating pattern detail, so that acuity measured on it would be the same as acuity measured on the brokenring pattern under normal conditions of observation. Since televisionimages are superimposed upon a grating pattern or raster, an attempt was then made to apply the result to television systems.