Differences in contrast sensitivity measurement with letters and gratings.

Purpose: To evaluate the difference in measuring contrast sensitivity (CS) with gratings and letters. Methods: A randomized crossover study has been carried out on a sample of 43 healthy subjects (15 females, $23.1 \pm$ 2.1 ${years}$). CS function was measured by the Vision Chart LCD (CSO, Firenze) with two stimuli: sinusoidal gradings (Gabor patch) and letters (Sloan) both at five angular sizes ({1.3, 1.0, 0.7, $0.4$ and $0.22 {logMAR}$). At every stimulus size, CS has been measured monocularly with the best subjective correction by the Bayesian adaptive psychometric method "QUEST". Results: In both eyes, at all stimuli sizes tested, CS for the two different types of stimuli resulted significantly different ($p<0.001$), with a ratio between CS for gratings and letters ranging between 1.2 and 1.4. Conclusions: The study showed as it is possible measuring the CS function also by letters as well as the more traditional gratings. However, the two stimuli are not equivalent leading to different threshold evaluation in clinical setting.