Author Topic: discrepancy between my results and optometrist's results  (Read 3649 times)

Offline caimanjosh

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Re: discrepancy between my results and optometrist's results
« Reply #15 on: October 13, 2014, 11:36:45 AM »
I suppose the question is, what lighting conditions are to be considered "standard" for testing vision on a Snellen chart?  Is direct sunlight on the chart standard (probably not), bright daylight, bright artificial light, or moderate artificial light?  I'm not sure.

This inspired me to do a little research.  I don't think this is a definitive answer, but from a website that sells illuminated vision charts for eye doctors (http://precision-vision.com/Articles/b-letterchartmeasurements.html#Viewin_distance), I found the following guidance on lighting:

"What should the illumination be?

Visibility of objects is determined by their size, their illumination and their contrast.  In visual acuity measurement we want to determine a size threshold.  We therefore have to make sure that illumination and contrast are at levels where a ceiling effect is reached, so that small variations in illumination and contrast do not influence the measurement.  This leaves a fairly broad range of acceptable values.

Clinical tip:

For routine clinical use of front illuminated charts, a rule of thumb can be that contrast should be maximal and that the charts should be illuminated well enough so that extra illumination will not improve the visual acuity readings."

This would suggest that bright daylight is probably a reasonable standard (for those of us that don't have own our backlit eye charts!).