Purpose The authors applied partial coherence interferometry (PCI) to estimate the thickness of the human choroid in vivo and to learn whether it fluctuates during the day. Methods By applying signal processing techniques to existing PCI tracings of human ocular axial length measurements, a signal modeling algorithm was developed and validated to determine the position and variability of a postretinal peak that, by analogy to animal studies, likely corresponds to the choroidal/scleral interface. The algorithm then was applied to diurnal axial eye length datasets. Results The postretinal peak was identified in 28% of subjects in the development and validation datasets, with mean subfoveal choroidal thicknesses of 307 and 293 μm, respectively. Twenty-eight of 40 diurnal PCI datasets had at least two time points with identifiable postretinal peaks, yielding a mean choroidal thickness of 426 μm and a mean high-low difference in choroidal thickness of 59.5 ± 24.2 μm (range, 25.9–103 μm). The diurnal choroidal thickness fluctuation was larger than twice the SE of measurement (24.5 μm) in 16 of these 28 datasets. Axial length and choroidal thickness tended to fluctuate in antiphase. Conclusions Signal processing techniques provide choroidal thickness estimates in many, but not all, PCI datasets of axial eye measurements. Based on eyes with identifiable postretinal peaks at more than one time in a day, choroidal thickness varied over the day. Because of the established role of the choroid in retinal function and its possible role in regulating eye growth, further development and refinement of clinical methods to measure its thickness are warranted.

PURPOSE. This study sought diurnal variations of eye length in human subjects, analogous to those reported in laboratory animals. METHODS. Seventeen subjects, ages 7 to 53 (median 16) years and mean spherical equivalent refractive error Ϫ0.68 D (range, Ϫ3.00 to ϩ1.00 D), underwent axial length measurements at multiple times during the day between 7 AM and 1 AM the following day, using partial coherence interferometry (PCI), a highly precise, noncontact method. Diurnal axial length measurements were obtained on two or more days in 10 of these subjects. RESULTS. During at least 1 day, 15 subjects showed a statistically significant (ANOVA, P Ͻ 0.05) diurnal fluctuation of axial length, with a magnitude generally between 15 and 40 m. From the diurnal tracings that fit a sine curve using statistical criteria, the mean period of fluctuation was 21.6 Ϯ 4.33 hours (SD), the mean amplitude was 27.1 Ϯ 11.9 m (SD; range, 12.8-41.4 m), and the maximum axial length tended to occur at midday. Each of the subjects with multiple daily measurements showed axial length fluctuations on at least 1 day, but there were day-today differences in the diurnal variations: most notably, four subjects showed axial length fluctuations on each day; in others, the fluctuations were not observed on each testing day. CONCLUSIONS. The human eye undergoes diurnal fluctuations in axial length, with a pattern suggesting maximum axial length at midday. Based on repeated measurements, these daily fluctuations may not appear regularly in all subjects, suggesting the possibility of physiologic influences that must be defined. (In

Both axial length and IOP fluctuate during the day much of the time in most subjects. However, diurnal IOP fluctuations do not appear to cause diurnal axial length fluctuations.

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Two experiments were conducted in the laboratory to evaluate potential benefits of different retroreflective markings for nighttime pedestrian visibility. Video recordings of a jogger wearing four different markings were made from a vehicle in four different road environments. Subjects viewed composite tapes that included each of the 16 jogger marking/road environment combinations as well as travel with no targets. The task was to step on a pedal immediately upon seeing a jogger, which had no effect on the flow of the video playback. The time between depression of the pedal and the point of "impact" was the major dependent variable. Experiment I showed that performance was better for all retroreflective markings than for the dark control and that it was better with markings of the limbs than of the torso. Experiment 2, which included a secondary video tracking task, showed that performance was better for markings that incorporate biological motion than for a vest or arbitrarily positioned stripes on the limbs. Questionnaire data indicated that 85% of the subjects judged the biological motion markings to be "easiest to see." Also, subjects reported more conservative estimates of nighttime visibility and greater willingness to take personal precautions at night after participating in the experiment.