By far the most important thing one must do when one specifies a filter's performance is to determine one's absolute "needs" and to distinguish them from one's "wants" or desires. The needs relate to the functional aspects without which the filter and the rest of the optical system will not function properly. These needs do not necessarily have to be optical in nature: the tolerance on the size of the filter or the environmental parameters might be very important needs.
The wants, on the other hand, are additional features that add value to the system but are not critical to its fundamental operation. A bandpass filter may be adequate with a certain tolerance on its bandwidth, but the system's performance might be significantly better if it were made narrower: this would be classified as a want.
There are always gray areas that need to be resolved subjectively on an individual basis. Hopefully, these guesses are minor when compared with the total number of decisions required in the system. Usually, more information or a better understanding of the system in which the filter will be used can help to clear up ambiguities.
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