Band-stop filter in the context of "Q factor"

A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.It is the inverse of a band-stop filter.

Didymium (Greek: δίδυμο, romanized: dídymo, twin) is a mixture of the elements praseodymium and neodymium. It is used in safety glasses for glassblowing and blacksmithing and filter lenses for flame testing, especially with a gas (propane)-powered forge, where it provides a filter that selectively blocks the yellowish light at 589 nm emitted by the hot sodium in the glass without having a detrimental effect on general vision, unlike dark welder's glasses and cobalt glasses. The usefulness of didymium glass for eye protection of this sort was discovered by Sir William Crookes.

Didymium photographic filters are often used to enhance autumn scenery by making leaves appear more vibrant. It does this by removing part of the orange region of the color spectrum, acting as an optical band-stop filter. Unfiltered, this group of colors tends to make certain elements of a picture appear "muddy". These photographic filters are also used by nightscape photographers, as they absorb part of the light pollution caused by sodium street lights. Didymium was also used in the sodium vapor process for matte work due to its ability to absorb the yellow color produced by its eponymous sodium lighting.