The following discussion was prepared to answer the question as to what the real observable effect is with and without the use of an excitation filter. Many people ask: "If I have a torch with blue LEDs, do I really need a blue excitation filter on top of that; what does it actually do for me for the extra cost?"
This report should lay this issue to rest.
Note that the images are individual frame shots (snapshots) taken from low resolution video. Unfortunately due to a problem with the autofocus of the camera, these images are all a bit blurred. The quality of these pictures is therefore rather poor.
For the purpose of this discussion however they are fine because all we are really looking for is the overall difference in color saturation and spectrum content.
The photo at the top right shows an anemone during the daytime with red color at a depth (about 18 meters or 60 feet) at which red is already almost completely absent through absorption by the water column. This red color must therefore be the result of fluorescence of the anemone. This is the only exception to the rule that you need special lights and filters to observe fluorescence under water .
The images shown below were taken on 16 September 2012 at Roman Rock, Mashraba, Dahab, Egypt (Red Sea).
The light source was a Hartenberger maxi compact LCD torch with 21 blue LEDs with an excitation filter mounted to the front such that it could be easily removed and replaced during the filming sequence.
What follows are side by side comparisons of the same coral blocks unfiltered and filtered. To the extent possible all other things (angle of incidence, framing, position, etc.) were all kept the same. Notice the richness of the colors with the filter and the clear presence of more red in the images. Without the filter, the images appear "washed out" with very little brilliance and color saturation.
One can clearly see the dramatic difference in the overall aesthetic quality of the color in the filtered images. Based on the above discussion it should be obvious why our torches are all equipped with such a filter.
The following images corroborate the above difference between a blue light with and without excitation filter in terms of spectrographs:
Sandwiched strobe filters:
The following images demonstrate the difference between a sandwiched excitation filter from a different manufacturer (left) and FireDiveGear.com's sandwiched excitation filter (right). Note however that this kind of filter can actually be used both for strobes and for white light torches. Clearly FireDiveGear.com's filter shows more of the subtle details of fluorescence (e.g. the finer details of the arrowcrab, the faint red fluorescence of the background) and provides for less distortions. The last image at the bottom left has been taken with just a plain dichroic filter in front of the strobe instead of the sandwiched excitation filter from the other manufacturer. Note the white light which illuminates the background. This is a clear indication of the benefit of a sandwiched excitation filter over just a dichroic filter. All of the pictures below were taken by Solomon Baksh (used with kind permission).
Mixing equipment from different vendors:
All the photos below on the left were taken with a Nikon D7000, 60mm lens and two Sea&Sea YS110a strobes, using a SuperBlue light for focusing, another vendor's excitation filters on the strobes, and one of our camera barrier filters on the lens.
All the photos below on the right were taken with a Nikon D7000, Sigma 17-70 lens and two Sea&Sea YS-D1 strobes, using a SuperBlue light for focusing, FireDiveGear.com's sandwich excitation filters on the strobes, and one of our camera barrier filters on the lens.
This demonstrates that excitation and barrier filters (or torches and barrier filters) from different manufacturers do not always give the expected results. Every manufacturer has a system of torches and filters which complement and match each other's properties. Mixing equipment from different manufacturers may therefore give suboptimal or even frustrating results.
All pictures were taken by Brian McHugh (used with kind permission).
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