Ken: I agree.
Originally Posted by Ken Nadvornick
If a safelight tests "safe", fine. However, if there's a failure, there could be any number of reasons why. 0C filters can fail, for instance, because they have faded to the point of being ineffective, or they may not be suitable for the paper in question, or it may be something else. It is entirely possible that a safelight test can fail when the safelight itself is OK, but there was stray light from the enlarger, or even another source that was not accounted for.