Abstract
PurposeTo determine the effect of changing illuminance on visual and stereo acuity.MethodsTwenty-eight subjects aged 21 to 60 years were assessed. Monocular visual acuity (ETDRS) of emmetropic subjects was assessed under 15 different illuminance levels (50–8000 lux), provided by a computer controlled halogen lighting rig. Three levels of myopia (−0.50DS, −1.00DS & 1.50DS) were induced in each subject using lenses and visual acuity (VA) was retested under the same illuminance conditions. Stereoacuity (TNO) was assessed under the same levels of illuminance.ResultsA one log unit change in illuminance level (lx) results in a significant change of 0.060 LogMAR (p < 0.001), an effect that is exacerbated in the presence of induced myopic refractive error (p < 0.001). Stereoacuity scores demonstrate statistically significant overall differences between illuminance levels (p < 0.001).ConclusionsThe findings of this study demonstrate that changes in illuminance have a statistically significant effect on VA that may contribute to test/retest variability. Increases in illuminance from 50 to 500 lx resulted in an improved VA score of 0.12 LogMAR. Differences like these have significant clinical implications, such as false negatives during vision screening and non-detection of VA deterioration, as the full magnitude of any change may be hidden. In research where VA is a primary outcome measure, differences of 0.12 LogMAR or even less could affect the statistical significance and conclusions of a study. It is recommended that VA assessment always be performed between 400 lx and 600 lx, as this limits any effect of illuminance change to 0.012 LogMAR.
Highlights
The original LogMAR letter chart (Bailey-Lovie) was introduced in 1976 as an alternative to the ‘unsatisfactory’ Snellen chart [1], and has undergone one major revision to become the gold standard test for use in the clinic and for research (ETDRS) [2]
A total of 28 subjects were recruited with mean (SD) age 32 (11) years; no subject had visual acuity (VA) worse than 0.300 LogMAR and none had manifest strabismus
The test, re-test variability of ETDRS LogMAR charts, typically ranges from 0.01 to 0.18 LogMAR [2, 16,17,18,19,20,21], a contributing factor to which could be variances in illuminance, demonstrated by the significant relationship between task illuminance and score achieved during VA assessment
Summary
The original LogMAR letter chart (Bailey-Lovie) was introduced in 1976 as an alternative to the ‘unsatisfactory’ Snellen chart [1], and has undergone one major revision to become the gold standard test for use in the clinic and for research (ETDRS) [2]. The ETDRS study protocol stated that an illumination level between 807 lux (lx) and 1345 lx [3] should be used during testing. To meet this requirement, selfilluminated charts were developed to control the amount of light falling on the chart. Whilst these illuminated charts maintain the required chart illuminance in a dark room, any external illumination will influence the amount of light falling on the chart, and vary the level of visual acuity (VA) measured [4].
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