Fig. 1.

(a) Typical Badal optometer consists of two focusing elements (L1 and L2) and flat mirrors (M1–M4) that are together used to form an afocal telescope. M2 and M3 are placed on a translation stage. Moving the stage (Δd) can alter the physical distance between L1 and L2 (M2’, M3’ and the dashed line) introducing extra vergence (ΔV) at the exit pupil P₂ (dashed line), which leads to a focal plane change in the eye (ΔV). Note that ΔV here shows the defocus change in diopters and not physical distance. (b) When two wavelengths (say, a red and a green) are used together in a Badal system, the eye’s LCA forces the two wavelengths to be defocused relative to each other. (c) Two long-pass filters (LP1 and LP2) inserted between the mirror pairs M1, M2 and M3, M4 separate the longer and shorter wavelengths so that they can have tunable relative defocus at P₂ as a function of Δd. L, lens; M, mirror; LP, long-pass filter; LCA, longitudinal chromatic aberration; P, pupil plane; ΔV, vergence change; Δd, distance change in (a) the afocal telescope and (c) in the transmitted beam.