A recent chat with a colleague about cheap sources for optical filters and a similar conversation on facebook reminded me of an old trick for getting cheap fluorescence filters: theatrical gels. These are the filters used to give theatrical lights their color and they are sheets of dyed plastic. While they don’t have the performance of dichroic filters (or even absorptive glass filters), they are much cheaper – a 2 foot square piece costs less than $10. Conveniently, Rosco, one of the major manufacturers of gels, provides absorption spectra for all of them. They also have an easy-to-use web tool for browsing through different color filters.
For example, here’s the spectrum of Roscolux #15, which makes a not bad long pass filter with a cut-on wavelength of about 550nm:
Transmission spectrum of Roscolux #15.
Their low cost makes them useful for situations where you need a large area filter and don’t need the performance of a dichroic filter. For instance, I originally discovered them for screening GFP libraries on a 22 cm plate – we plated out thousands of colonies, illuminated them with a filtered arc lamp, and then covered a pair of goggles with gel filters to see the fluorescence of each colony. They’ve also been used for building cheap blue transilluminators for visualizing DNA gels.
These gels aren’t going to replace dichroic filters, but have their place. If you have a clever use for them, post a comment.