Filter construction

We were replacing the filter set in one of our microscopes and as we were taking them out, this excitation filter (a Chroma ET490/20x) fell apart:

Filter parts

Excitation filter in two pieces. The left piece is the interference filter, and looks reflective in this picture but transmits blue light. The right piece appears to be anti-reflection coated glass.

Apparently, sitting in front of an arc lamp for seven years causes cements to fail (although the other four filters we removed were all fine). Interestingly, the two pieces this filter separated into tells us something about how it works. Typically, the interference filter only has a finite blocking band. ┬áThis means that it will only reject undesired wavelengths within a specified band. To reject wavelengths beyond that, such as IR or UV, filter manufacturers add additional blocking glass to absorb these wavelengths (see page 11 of Chroma’s Handbook for more on this). I think what’s happened to this filter is that the blocking glass has separated from the interference filter. The right piece in the photo above, if you put it in a spectrophotometer, transmits from about 400-700 nm but absorbs outside those wavelengths. Presumably it’s the blocking glass that goes with this emission filter.