Contrast, Edge-gradient, and Target Line Width as Factors in Vernier Acuity

Experiments are described in which vernier acuity performance is measured for two subjects using target lines in which the line width, overall contrast and edge-gradient are varied . The results show an optimum line width of about 15 min arc for achieving a best performance . Contrast is shown to be a significant factor in vernier acuity in agreement with other acuity measures . The results also show that edge-gradient is significant in vernier acuity contrary to earlier propositions . 1 . Introduction In carrying out vernier acuity tasks, the human visual system resolves displacements that are smaller than the limit of resolution for any other form of visual acuity . These displacements are typically of magnitude 5 sec arc compared with 1 min arc for most other forms of resolution . It has been demonstrated by experiments in which target line length has been varied that these achievements are largely mediated by a process of spatial integration of information received in the retina-Foley-Fisher [1] . Other parameters affecting vernier acuity performance have also been investigated . Baker [2] considered the effects of illumination, exposure time and wavelength of illumination . The last was further investigated by Foley-Fisher [3] who also considered the effect of interrupted illumination and of the time spent in making a setting . Although it has generally been accepted that target line width was of little consequence in vernier acuity performance, there is no direct experimental evidence to confirm or refute this . French [4] demonstrated that there might be a possible effect arising from line width when carrying out experiments on the separation of the target lines and in which he used two different line widths of 52 and 444 sec arc . He observed that the lower limit at which resolution fails when the length and separation of the lines was varied was more constant for the thinner line than for the thick coincidence line . Berry et al. [5], in experiments carried out on stereoscopic vision, found no variation in vernier acuity performance with line widths varying between 27 and 424 sec arc . Visibility, detection and visual acuities in general are largely determined by the contrast between a target and its background-Horowitz [6], Linksz [7], Ludvigh [8] and Cobb and Moss [9] . Whilst it was suggested by Luckiesh and Moss [10] that similar effects of varying contrast could be expected for vernier acuity, no experimental evidence existed . It might further be expected that not only would the contrast differential be significant but also the edge-gradient or blur function of the cross-section of the target lines . Hartridge [11] reported that blurred absorption bands could be aligned as precisely as sharp lines and it D ow nl oa de d by [ St an fo rd U ni ve rs ity ] at 0 0: 46 0 9 M ar ch 2 01 4 180 7. A . Foley-Fisher has been inferred that edge-gradients are not significant in vernier tasks . Despite the improbability of such a result, the assumption has been carried forward without any direct experimental evidence . In the present experiments, measurements of vernier acuity performance have been carried out on two subjects using a number of different combinations of line width and contrast . A further set of experiments were also carried out in which the edge-gradient was varied and as the targets used in these later experiments also included some variation in width and contrast, some compensating corrections had to be made . In all experiments the background intensity was maintained constant and the target line length was always the same . 2 . Method Experiments were carried out using apparatus and experimental procedures described by Foley-Fisher [3] . The contrast and background intensities were measured using an S.E.I. photometer, and for all target lines a microdensitometer trace was taken of the cross-section at three points along its length . These traces were obtained using a Joyce Loebl double-beam microdensitometer and verified the contrast measurements made with the photometer . From geometrical measurements on these traces the line widths and edge-gradients were calculated . A typical microdensitometer trace is shown in figure 1 and the measurement of line width was taken as W, whilst the edge-gradient was determined as the value of HI W as shown in the figure . Figure 1 . Typical cross-section of microdensitometer trace of a target line . D ow nl oa de d by [ St an fo rd U ni ve rs ity ] at 0 0: 46 0 9 M ar ch 2 01 4