Optotype recognition under degradation: comparison of size, contrast, blur, noise and contour‐perturbation effects

Abstract

Background

Visual acuity is measured by diminishing letter size till recognition threshold is reached, letters varying in legibility. In this experiment, size reduction was compared with other means of handicapping letter legibility.

Methods

In five normal observers, discrimination thresholds for 13 sans‐serif capital letters in a 5 × 4 format were obtained by a staircase procedure for size reduction, as well as for 20 minutes of arc (logMAR = 0.6) letters subjected to four kinds of image degradation: reducing contrast, convolving with blurring spread, embedding in noise and perturbing contour smoothness.

Results

Threshold correlation and distribution of response errors show prominent differences and indicate the degree to which the visual processing of the various modes of image degradation is distinct.

Conclusions

The validation of four other ways of impairing the recognition of optotypes in addition to size diminution reveals their potential in the differential diagnosis of defects in pattern detection, in evaluating therapeutic regimens and in developing concepts of form perception.

Key words:

• contrast sensitivity
• form perception
• image blur
• visual acuity

This manuscript marks the 70th anniversary of the author's first scientific publication, also in this journal (CXO), then named Australasian Journal of Optometry, and also a single‐author contribution at the Interface of Clinical Optometry and Vision Science (Westheimer, G. Analysis of measurements of the interpupillary distance Austral J Optom 1945; 28: 531–533).

This manuscript marks the 70th anniversary of the author's first scientific publication, also in this journal (CXO), then named Australasian Journal of Optometry, and also a single‐author contribution at the Interface of Clinical Optometry and Vision Science (Westheimer, G. Analysis of measurements of the interpupillary distance Austral J Optom 1945; 28: 531–533).

Acknowledgements

Ryan N Lee's effective contribution to the acquisition and presentation of the data is gratefully acknowledged. The author benefited from helpful discussion with Ian Bailey and Andrew Carkeet.

Notes

This manuscript marks the 70th anniversary of the author's first scientific publication, also in this journal (CXO), then named Australasian Journal of Optometry, and also a single‐author contribution at the Interface of Clinical Optometry and Vision Science (Westheimer, G. Analysis of measurements of the interpupillary distance Austral J Optom 1945; 28: 531–533).