Illumination level for the Snellen chart?
 

I recently had my eyes tested by my GP and the result was 6/6 in the right and 6/9 in the left. I was slightly disappointed by this since I was expecting 6/6 in both.

However I later read somewhere that the minimum illumination for the chart (if it's not a back lit type) is 480 Lux.....which I understand is quite bright compared to say an average room. The room my GP used (the corridor of the surgery) wasn't very well lit at all with only a moderate amount of daylight present and no lights on.

Is that correct and how much of a difference does the illumination make to the result?