We have several indications that measurement noise was a minor contributor to the observed variability in aberration measurements. First, in a companion study
25 that used the COAS aberrometer to measure aberrations of model eyes, we found that repeated measures within a second produced almost identical aberration measurements (between frame variance of higher-order RMS = 1 × 10
−6 μm
2). Over this same time scale, variance of RMS in the human eye data (variance = 81 × 10
−6 μm
2) was 80 times larger
(Fig. 9) . This comparison shows that the short-term variability in the human eye data does not reflect instrument noise, but most likely reflects changes in the eye’s optics, such as microfluctuation of accommodation, instability of the tear film, or small fixational eye movements, all occurring over a very short time scale. Second, although both eye position and rotation vary during any series of measurements, our control experiments indicate that these two factors have little impact on measure variability in aberrations. Third, our previous measurements in model eyes show that operator realignment between measurements introduces a significant but still small amount of variability into the data (between realignment variance of higher-order RMS = 92 × 10
−6 μm
2 in the model eye), which is approximately 3.5 times smaller than the between-trial variability (variance = 324 × 10
−6 μm
2) in human eyes. Therefore, we conclude that 72% of the trial-to-trial variance of higher-order RMS can be attributed to the eye (again, due to accommodation fluctuation, tear film instability, and eye movements), and not instrument alignment. Finally, we note that the variability in the higher-order RMS wavefront error over multiple days and multiple months is even larger (e.g., 441 and 973 × 10
−6 μm
2, respectively) than within a day. It indicates that the optical fluctuations of the eye are larger over longer time scales, which includes not only all the fluctuations that exist in shorter time scales, but also fluctuations that may reflect the genuine long-term changes in the eye’s optics.