Barrier filters are usually made from acrylic but sometimes they are also made from glass, mostly for camera filters.
The advantage of acrylic is that it is easier to cut into arbitrary shapes, and that there is less risk of injury from breaking, whereas the disadvantage is that this material is more prone to scratching. However, this latter problem is alleviated by water, which usually fills the scratches under water and renders them imperceptible.
Beware of mixing filters and light sources from varying manufacturers. Your results can vary depending on the precise spectral properties of the filter material used, the images you will see or obtain may differ significantly from those made with “spectrally matched” components.
The technology of barrier filters has come a long way over the last few years. Below is an image of how it used to be done. Image courtesy: Larry Cohen.
Photo © 2012 S. Beyer
The photo above shows how colorful the experience of underwater fluorescence can be with the right barrier filter. Note that the tips of this coral fluoresce in blue with the blue excitation light. They also fluoresce in blue under UV light, which corroborates that these blue tips are stemming from fluorescence, not just from reflected blue light.
The following pictures were taken with a glass filter from a different manufacturer and a acrylic filter such as those provided by FireDiveGear.com:
As you can see in the superposition of the spectrographs of the two filters above (see the image immediately above to the right), a minute difference in the filtering properties (see red circle) can lead to huge differences in the images you will get, as shown in the three pairs of example images at the top above.
While the glass filter is more selective and lets less of the reflected blue excitation light pass through, which is the very purpose of the barrier filter in the first place, it unfortunately produces dull images with (predominantly) only reds and greens which lack brilliance.
The acrylic filter on the other hand is less “pure” and lets more of the reflected blue light pass through, but in compensation produces brighter and more colorful images, which most people find aesthetically more pleasing.
For the very technically minded see this White Paper detailing the analysis and comparison of 18 different barrier filters from different manufacturers.
This paper will explain the differences and why “not all barrier filters are created equal” and why it can DRAMATICALLY affect your image quality.
Spectral Analysis of the Excitation / Barrier Filter Matched System
The following spectrographs show what the effect is of an excitation filter in a complete system with barrier filter:
Blue light without any filters
Blue light with barrier filter
Superposition with and without excitation filter
Blue light with barrier and excitation filter
As you can see in the above images, the excitation filter further reduces the little rest of blue light that still goes through the barrier filter.
The line of the resulting spectrograph is almost flat, meaning that almost no blue light at all passes through the combination of excitation and barrier filter.
This is exactly what we want, because we only want to see the fluorescence, not the blue excitation light – except for a very tiny little bit of remaining blue light, in order to obtain a brighter and more colorful image, as shown in the three pairs of example images at the top of this page.