1. Does size-threshold text legibility relate to performance with suprathreshold-sized text? Yu-Chi Tai, PhD James E. Sheedy, OD, PhD, FAAO John R. Hayes, PhD Pacific University College of Optometry Forest Grove OR 97116,
Tai Y, Sheedy J, Hayes J. Can threshold legibility predict suprathreshold performance? 2007;84:E-abstract

2. Issue Threshold legibility measures
“Back-up” method used by Tinker
Visual acuity method
Similar
Follows standardized rules of visual acuity measurement
Do threshold measures of relative legibility apply to normally sized characters that are suprathreshold?

3. Methods 45 healthy adult individuals (18 men and 27 women)
Relative text legibility
threshold visual acuity
letters and words
Reaction time (RT) to suprathreshold letters and words

4. Font stimulus conditions
Five stimulus conditions for letters and words, used to evaluate threshold legibility and suprathreshold recognition time (TNR = Times New Roman).

5. Relative threshold legibility of letters and words across 5 conditions (n=45)
Significant differences across all condition pairs for letters and for words, except for letter legibility between TNR and Consolas (p = .081), TNR and ClearType (p = .182), and Consolas and ClearType (p = 1.000)

6. This validates the selection of the font stimuli as providing a legibility range
Note that words have more clearly defined steps than letters

7. Response time measurements
Voice response latency with voice key
Same font stimuli conditions
Letters and words
Each of the following sizes:
20/20, 20/25, 20/32, 20/40, 20/50, 20/62, and 20/80

8. Response time by font condition and size

9. Letter superiority appears to function only for small characters
The letter superiority effect, demonstrated for threshold legibility, is also shown for RT, but only smaller angular sizes show a faster RT for letters. At larger angular sizes, i.e. those commonly used for typical reading, the RT to recognize a word is about the same as for a letter.
The likely reason: lateral inhibition or lateral interference

10. Effects of spacing on word legibility (another study)

11. To note RT increases with smaller characters
Curves flatten for larger sizes
“Acuity reserve”
Longer RT for words compared to letters - with small text only
Effect disappears with larger characters
Potential explanation:
Contour effect is greater for words with smaller characters
The “letter superiority effect” applies only to small characters
Possiblities
A large benefit of greater size is defeating the contour interaction effect
Also, character spacing should not be proportional across character sizes – spacing should be a greater percentage of character size for smaller font sizes

12. To note
Does TNR perform poorly for words because of the serif contribution to lateral interference?
If lateral inhibition is responsible for letter superiority effect at smaller character sizes – then word legibility should be improved with greater spacing for small font sizes

13. Threshold legibility and suprathreshold reaction time
Letters
Two-way randomized block ANOVA significant effects on reaction time in recognizing suprathreshold-size letters
threshold legibility (F(1,1517)=1033.3, p<.0001),
angular size (F(6,264)=225.03, p<.0001)
their interaction (F(6,1517)=25.80, p<.0001).

14. Letters
Displayed conditions represent the following threshold legibility measures:
Verdana, ; Consolas ; ClearType ; TNR -.105; Low contrast

15. To note Larger RT with poorer legibility
Larger RT with smaller characters
Diminishing improvements with larger letter steps (“Acuity Reserve”)
Effect of letter size on RT is diminished for fonts with better legibility

16. Threshold legibility and suprathreshold reaction time
Words
Two-way randomized block ANOVA significant effects on reaction time in recognizing suprathreshold-size words
threshold legibility (F(1,1510)=781.97, p<.0001)
angular size (F(6,264)=489.81, p<.0001)
their interaction (F(6,1510)=29.31, p<.0001)

17. Words
Displayed conditions represent the following threshold legibility measures:
Verdana, ; Consolas ; ClearType ; TNR ; Low contrast 0.094

18. Letters
Words

19. To note Similar findings as for letters, except….
Improvement with letter size continues into larger letter sizes
i.e. greater acuity reserve
Longer RT for words with small text compared to letters
Effect disappears with larger characters
Potential explanation:
Contour effect is greater for words with smaller characters
The “letter superiority effect” applies only to small characters
Consequences of the above
A large benefit of greater size is defeating the contour interaction effect
Also, character spacing should not be proportional across character sizes – spacing should be a greater percentage of character size for smaller font sizes

20. Acknowledgment
These studies are supported by Microsoft Corporation