One thing about comparing used and unused bulbs of the same type is to keep in mind differences between lots. When GE or anyone else makes bulbs they do not make every lot of bulbs to produce identical light spectrums. When they test them they look at the relative K rating and the relative lums output. If they fall within a set +- tolerance then they are shipped.
You could possibly end up with a used bulb from one lot and a new bulb from a different lot and there difference in K rating initially being so far different that your data now would be considerably misleading.
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Originally posted by The Grim Reefer
Even uncalibrated you should be able to check for a spectrum shift. Don;t you have some GE 6500's? If you have a new one and some you've been using you should be able to see a difference if my source is correct. Like I said according to him at six months there is a noticable shift. If you can test used and unused lamps from the same lot it should be the best way to see if there is a noticable shift.
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