That's a good point, Jimmy.
Ishihara tests are designed to be conducted in Northern Daylight. Most Optometrists have a lamp which simulates this condition but as with all things mechanical they will all be subject to degradation over time.
According to an Optometrist friend of mine, the Ishihara tests are only ever truly reliable if they are conducted in good daylight conditions.
As for the Ishihara tests being different, I was informed that the only differences were between the 24 and 36 plate editions . JAA requirements are based only on the first 16 plates of the 24 plate edition.
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