Kodak is notoriously conservative. Someone may have used a very sensitive instrument and found they could find the first detectable change after one minute in front of a carbon arc lamp or something, and made the recommendation.
Mmmm carbon arc lamp!! I'm told by many a gaffer on movie sets they still use them every now and then, but I have yet to actually have the privilege of seeing one in action.
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~Stone | "...of course, that's just my opinion. I could be wrong."~Dennis Miller