Quote Originally Posted by Ken Nadvornick View Post
While this is undoubtedly true, in isolation it can only give the tester a binary result. Either the safelight is safe within the parameters tested, or it is not. It says nothing regarding why the safelight may not be safe. And more importantly, if the reason the safelight is unsafe might be an easily and inexpensively correctable flaw.

... It's not that testing isn't the final word, it's that more information is always better.

Ken: I agree.

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.