Yu-Chi Tai, PhD, John R. Hayes, PhD , James E. Sheedy, OD, PhD Text legibility: measurement methods, letter superiority, and acuity reserve. Yu-Chi Tai, PhD, John R. Hayes, PhD , James E. Sheedy, OD, PhD College of Optometry | Pacific University | 2043 College Way | Forest Grove | Oregon 97116 ABSTRACT RESULTS RESULTS (cont’d) CONCLUSIONS Threshold legibility predicts supra-threshold RT Threshold legibility of the 5 font conditions Purpose. The relative legibility of different text designs can be measured using techniques that reduce the angular size of the text to detection threshold. Do these measures of relative legibility relate to performance with text normally read at 3-4 times the threshold size? We investigated whether threshold legibility measures predict performance at normal supra-threshold sizes. Fonts detected at smaller threshold sizes are also recognized faster at supra-threshold sizes. This suggests that the same design features are involved in enhancing both threshold detection and supra-threshold RT. This validates threshold legibility measures to performance at larger sizes. Individual letter performance is better than word performance for both threshold measures and RT with smaller characters. This letter superiority effect largely disappears with larger letter sizes, most likely because of the fixed distance over which lateral interference operates. These results suggest that the common method of maintaining character spacing proportional to character size may not be best for smaller font sizes; greater spacing at smaller font sizes might enhance word legibility. Figure 1. Threshold legibility of letters and words in 5 different conditions. LogMAR is log of the minimum angle of resolution, lower numbers indicate smaller letters and better acuity. Figure 3. Line plots showing the effects of threshold legibility at each angular size on supra-threshold response time to letters. Response times (RT) to supra-threshold sized letters and words in same font conditions as above To note in Figure 3: Threshold legibility is related to supra-threshold RT for all sizes (p<0.001) The predictability diminishes with larger sized letters. Nearly identical results found for words MATERIALS & METHODS Methods. Forty-five adult subjects with 20/20 monocular visual acuity participated in a two-phase study. In the first phase, visual acuity testing of subjects was used to determine threshold legibility for both letters and words for 5 font conditions selected to provide a range of legibility: Verdana, Consolas, Times New Roman (TNR), TNR with ClearType rendering, and TNR with low contrast). In phase 2 subject response times (RT) to the same font conditions at normal supra-threshold sizes were measured. Acuity reserve Acuity reserve is the ratio between the actual size and threshold size. It is desirable to know the smallest size, or minimum acuity reserve, which attains optimum performance. The letter size for which no further RT improvements occur (Figure 2) can potentially be used to establish the minimum acuity reserve. Calculations on the data indicate a minimum acuity reserve of 2.93:1, or approximately 3:1. Figure 2. Average response time (RT) for individual letters and words for the 5 font conditions. RTs are shown for several supra-threshold sizes (20/80 is largest). ACKNOWLEDGEMENTS The Advanced Reading Group of Microsoft Corporation supported this research with a grant to James E. Sheedy, O.D., Ph.D. To note in Figure 2: RT is faster for larger characters “Letter superiority” – RT to letters is faster than for words. RT for letters and words are similar at larger sizes Letter superiority effect likely due to lateral inhibition of neural networks in retina CONTACT INFORMATION Principal Investigator Yu-Chi Tai T: 503.352.2289 E: tai1853@pacificu.edu