Comparison between the defocus curves measured by visual acuity analyses with either the red or the green background of the duochrome test, or the white background

The Duochrome Test (DT), based on ocular longitudinal chromatic aberration (LCA), is widely used to refine spherical refraction in the optometric practice. This study investigated the possible differences between the two DT background colours by analysing the defocus curves (DCs) measured on eleven young subjects (20-24 years). DCs were obtained by measuring monocularly their visual acuity (logMAR) with various dioptric powers of the lenses in front of the eye under either red, green, or white backgrounds. The positions of the minima of the three DCs were deduced to compare the three values of dioptric power corresponding to the best corrected visual acuities in the three conditions. Moreover, the DC slopes corresponding to the addition of negative lenses were compared to evaluate possible differences under different chromatic stimuli. A steeper slope for the red background indicated a greater difficulty compared to green and white in maintaining a visual acuity close to the best one. These findings highlight the need for further research to understand the reasons for this peculiarity of red.