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Message: <blockquote data-quote="typo" data-source="post: 3147251" data-attributes="member: 83808">Ron,As you pointed out to me a number of years ago cd/m2 is a measure of luminance or how 'bright' a surface is and lux is a measure of illuminance or the amount of light falling on a surface. They are not convertible. However if you misuse a light meter and use the lux range to measure surface brightness there is about a 10 fold difference in the units. So 1cd/m2 reads as 10 lux and vice versa.When I started this stuff I was in the habit of taking illuminance readings of the light falling on the target, whereas now I find illuminance at the surface as a better measurement as direct sunlight or scattered light can result in different luminance values.Nearly all the information I've found on the subject references Koenig's study in the 1880s where the peak acuity for that single subject was around 200cd/m2 in SI units. According to my light meter (which is very cheap and may not be accurate) I peak at 300-400cd/m2 and with a pronounce decline below this range that has most certainly got worse over the years. There is a more modest reduction above this level with only a pronounced decline over about 3000cd/m2. One thing that I know I need to pay better attention to is the whole view illumination as this better controls the pupil diameter and therefore acuity. In the kind of experiment we've been discussing here placing a 300-400cd/m2 luminance target against a dark background can still result in glare and poor results. Of course if it occupied a high proportion of the view there is no problem. It might be with more uniform illumination I would get a different peak value.Cheers,DavidPS. The illumination levels recommended for normal acuity testing varies from country to country and may differ with diagnostic practice but the USA normally uses 85cd/m2, Germany 300cd/m2 and Britain 'at least' 120 cd/m2 in standard tests as far as I can work out.</blockquote>

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