Comparison of scotopic pupil measurement with slitlamp-based cobalt blue light and infrared video-based system

PURPOSE: To determine the accuracy and reproducibility of scotopic pupil measurements with a slitlamp-based cobalt blue light. SETTING: University of Texas Health Science Center-San Antonio, San Antonio, Texas, USA. METHODS: In a prospective experimental study, 2 independent observers measured 16 subjects' pupils with a standard slitlamp-based cobalt blue light. In the same setting, the accuracy and reproducibility of the measurements were compared with those obtained with an infrared video-based system. RESULTS: Measurements of pupil size using the infrared video-computer system were highly reproducible. With the slitlamp-based cobalt blue light, the 2 measurements obtained by observer 2 (n = 16) were very close to each other (mean difference 0.09 mm +/- 0.23 [SD]; P =.12), whereas those obtained by observer 1 (n = 16) were slightly different from each other (mean difference 0.18 +/- 0.33 mm; P =.043). The mean pupil diameter measured with cobalt blue light was not significantly different between the 2 observers (P >.05), and the measurements were highly correlated with each other (r = 0.96, P <.01). The mean difference between the measurements using cobalt blue light and the infrared video-computer system was 0.21 +/- 0.54 mm for observer 1 (P =.47) and 0.06 +/- 0.47 mm for observer 2 (P =.85). Good correlation was noted between the measurements taken from each measuring device for both observers (r = 0.87 and 0.90, P <.01). CONCLUSIONS: Slitlamp-based cobalt blue light provided a reasonably good measurement of scotopic pupil size. It correlated well with the infrared video system when used at the lowest setting by experienced personnel. However, the observed difference between the measurements obtained by the 2 devices indicates that the cobalt blue light should be used cautiously in the refractive surgery preoperative evaluation of patients with large pupils.