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Research | Fluorescence

Fluorescence

We have been taking underwater photographs and video of reef fluorescence at night for Dr. Elizabeth Oh of the Tropical Marine Science Institute (TMSI). Scientists are investigating fluorescence for spotting new coral recruits on reefs. Coral recruits are tiny (1-5 mm) and hard to see under normal light, but fluoresce nicely under blue light.

First we take a photo of an area of coral illuminated with a powerful bright broad-spectrum dive light to record what is there, a normal photograph. Then we put a blue 'exciter' filter on the dive light to limit the illumination to a narrow range of frequencies at the blue end of the spectrum. A yellow 'barrier' filter is fitted to the camera to block the blue light from the dive light, passing only those frequencies near the yellow part of the spectrum. If we point the blue light source at an inanimate object (like the sand seabed) and photograph with the yellow barrier filter we get a black image, because none of the light from the dive light can get through both the blue exciter filter and the yellow barrier filter.

So why take a photograph if it's going to turn out black?

The blue light excites some coral to fluoresce, generating its own light that is radiated at different frequencies (colours) than the blue excitation illumination. With the yellow barrier filter on the camera we capture only the light that the coral itself is emitting due to the fluorescence in the yellow band of colours. The fluorescing coral generally shows up glowing green.

Here is a series of photographs with the left column showing coral illuminated by ordinary broad-spectrum light and the right column showing the fluorescing light the coral is emitting, photographed using our blue exciter and yellow barrier filters. As you can see, the inanimate seabed does not show up in the right-hand pictures, only the active flourescing coral shows brightly.

On some coral groups, only part of the coral fluoresces. We noticed that coral that was bleached or was being exploited by algae did not fluoresce. Sometimes only the 'pits' in a 'massive' coral (like brain coral) fluoresce, rather than the whole body. Also, in the same colony of coral there might be areas fluorescing brightly and other areas less or not at all.

It seems there's a lot to learn about coral health and growth by looking at fluorescence!


 
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