Fiat Lux: the effect of illuminance on acuity testing.
OBJECTIVE
To determine the effect of changing illuminance on visual and stereo acuity.


METHODS
Twenty-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.


RESULTS
A 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).


CONCLUSIONS
The 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.
Directorate of Orthoptics and Vision Science, University of Liverpool, Thompson Yates Building, Brownlow Hill, Liverpool, L69 3GB, UK.
Laurence P
LP
Tidbury
Department of Biostatistics, University of Liverpool, Liverpool, UK.|Department of Eye and Vision Science, University of Liverpool, Liverpool, UK.
Gabriela
G
Czanner
Directorate of Orthoptics and Vision Science, University of Liverpool, Thompson Yates Building, Brownlow Hill, Liverpool, L69 3GB, UK. d.newsham@liv.ac.uk.|Department of Eye and Vision Science, University of Liverpool, Liverpool, UK. d.newsham@liv.ac.uk.
David
D
Newsham
D000328:Adult; D005260:Female; D006801:Humans; D008029:Lighting / Q000592:standards; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000503:physiopathology; D014787:Vision Tests / Q000295:instrumentation*; D015348:Vision, Binocular / Q000502:physiology; D014792:Visual Acuity / Q000502:physiology*; D055815:Young Adult
Illumination; Lighting; Myopia; Stereoacuity; Vision; Visual acuity