How Deep Underwater Do You Have to Go Before Red Becomes Blue
What this means is that if you're diving at 60 feet or so, you see mostly blues and greens. Yellow hangs around, but it's muted. No more red or orange. Except, sometimes there is. On this page I compiled a number of pictures of red at depth. The first time I saw it was in Roatan while diving at about 60 feet on a bright, sunny day with great visibility. In the midst of all that blueish-green I saw bright, solid red. Not just reddish or reddish-brown, but deep, solid red. I took a picture of the phenomenon with my Canon G10 digital camera, first without flash, then with flash. The camera saw things exactly as my naked eye had seen it. Below you can see what caught my attention:
I consulted with several experts on this strange occurence and also solicited opinions on scubaboard.com. Suggestions were that I had come across and example of either bioluminescence or fluorescence. I felt that bioluminescence should be ruled out since it was daylight when I saw the phenomenon. I also had my doubts about fluorescence as the red was still red in the flash picture.
A bit more research brought me to a National Geographic source where Luis Marden explained that fluorescent red would disappear in the flashlight. I then had an interesting email exchange with a true expert on the matter, Dr. Charles Mazel, who studies optical properties of marine organisms, especially corals, and also has a business called NightSea. I showed my pictures to Dr. Mazel and asked for his opinion.
Mazel said it was right to rule out bioluminescence, but not necessarily fluorescence. He explained that underwater we see color because of the differential absorption and reflection of broadband white light -- both sunlight and flashlight -- from a surface. We generally don't see fluorescence because it's not strong enough. He explained that what Luis Marden had probably witnessed was daylight-stimulated red fluorescence. The flash revealed the full-spectrum reflectance, which in that case was just brown. Dr. Mazel felt that what I had seen had both red fluorescence properties as well as red full-spectrum reflectance color.
I then read more of Dr. Mazel's research and findings as well as other sources, and from that my understanding is that fluorescence essentially means reflecting light not at the incoming, but at a different wavelength. So the substance I saw at 60 feet reflected incoming light, devoid of what we see as red on the surface, at a wavelength that both my eyes and the camera lens did see as red. And unlike what Marden found when he flashed at it and found its "real" color not to be red, what I saw would have looked red on the surface as well. The one thing that still puzzled me was that what I had seen in Roatan at a depth of 60 feet did not glow or anything, it was simply deep, solid red. This was in contrast with the suggestion that fluorescence in daylight is not easy to see as it is a weak effect and ambient light will usually overwhelm it.
Another exchange with Dr. Mazel helped me gain additional understanding of fluorescence. He explained that while we tend to think of fluorescence as an intense glow of very saturated colors, it doesn't have to be that way. Fluorescence is often used to make things more noticeable and stand out (like highlighter pens or safety vests). He suggested that what I had seen was fluorescence that made it red even when there was no red to be reflected, and so what I saw stood out clearly, even though the fluorescence wasn't strong enough for glowing. Had I gone back to that place and used special lights at night (something Dr. Mazel specializes in), I'd have seen the glow.
As it turned out, I should have purchased one of NightSea's special flashlights to screen for green and red fluorescence as I saw plenty more examples of red at depth on a subsequent dive trip to the Caribbean islands of St. Kitts and Saba.
Our journey on the live-aboard vessel Caribbean Explorer II took us first to St. Kitts, and some of our first dives there were at the wreck site of the M/V Corinthean at about 55 feet. As usual, colors disappeared, but then, as if by special order, I saw what appeared to be some sort of orange corallimorphs in an otherwise all blueish underwater landscape. A closer look revealed clear, strong orange fluorescence. No doubt there at all. This was strong fluorescence.
We then moved on to the island of Saba and spent several days diving there. And that's when I saw the deep, rich red again, in several places, and always at depths between 85 and 95 feet. The pictures below were all taken during daylight dives with a Canon G10, on the left without flash, on the right with the Canon's built-in flash.
At this point I was very tempted to procure some of the red whatever-it-was and bring it to the surface for closer examination. That, of course, was against my strict rule of leaving whatever I see during my dives in place and besides, the waters around Saba are a marine park. I did touch the red, though. It turned out to be a very tough crust covering rocks or corals, so tough that it actually couldn't even be scraped off. So I limited myself to record as many occurences of the red as I could.
In each case you can see the intense, rich color of the substance that makes it appear deep red at a depth where almost all other color is gone. It does not have what we usually associate with fluorescence, i.e. the exaggerated glow.
Note how the color of the crusty substance stays the same in natural light and when illuminated by the camera's flash. That's in stark contrast to the surrounding coral and other growths that seem to have no color in whatever ambient light there was at 90 feet, just to light up in brilliant red, orange and yellow under the flash.
The following pictures are all from dives off Saba, all at depths of about 90 feet.
And below, finally, another example of obvious, or shall I say "conventional," fluorescence.
Source: https://www.scubadiverinfo.com/2_red_at_depth.html
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