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Barrier Filters

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[Barrier Filters]
Photo © 2012 S. Beyer

General:

Barrier filters are usually made from acrylic/plexiglas, but sometimes they are also made from glass, mostly for camera filters.

The advantage of plexiglas 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 that depending on the precise spectral properties of the filter material used, the images you will see or obtain may differ significantly.

Photo:

The photo at the top right shows how colourful 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.

Comparison:

The following pictures were taken with a glass filter from a different manufacturer and a plexiglas filter such as those provided by FluoMedia.org:

[Glass filter]
Glass filter (other manufacturer)
[Plexiglas filter]
Plexiglas filter (FluoMedia.org)
[Glass filter]
Glass filter (other manufacturer)
[Plexiglas filter]
Plexiglas filter (FluoMedia.org)
[Glass filter]
Glass filter (other manufacturer)
[Plexiglas filter]
Plexiglas filter (FluoMedia.org)
[Glass filter]
Glass filter (other manufacturer)
[Plexiglas filter]
Plexiglas filter (FluoMedia.org)
[Lightsource]
White light source spectrum (no filters)
[Superposition of spectrographs]
Superposition of spectrographs

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 (see also images 16, 27, 28, 29, 30, 31 from our photo gallery).

The plexiglas 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 (see also images 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 from our photo gallery).

Moreover, as can be seen in the two topmost photos of the example images above, the tips of the Acropora coral are different from the rest of the coral (right image, plexiglas filter). This can also be seen with UV light (e.g. 365 nm); under UV light the tips are dark blue whereas the rest of the coral is light blue. This information is completely lost with the more selective glass filter (left image). The more selective glass filter thus filters out too much.

Complete system:

The following spectrographs show what the effect is of an excitation filter in a complete system with barrier filter:

[Blue light without filters]
Blue light without any filters
[Blue light with barrier filter]
Blue light with barrier filter
[Superposition with/without excitation filter]
Superposition with and without excitation filter
[Blue light with excitation and barrier 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.

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