Colors of the Reef |
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Underwater Photography
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Why do the Colors Disappear when Diving?Even the clearest water absorbs visible light, although this absorption is very weak. The maximum of this absorption lies in the red at about 740 nm, but is tailing far to the blue region of the spectrum. Figure 1 shows the absorbance of water for ultraviolet and visible light. The lowest absorbance is found in the blue region around 400 nm, for light of this color water is much more transparent than for red light above 650 nm. This becomes more clear in Figure 2, where the transmittance of water is plotted for several distances in water. As you can see, even a water column of 1 m absorbs more than 20% of the red light above 650 nm. A water column of 10 m transmit less than 10% red light, and this is the reason why first red and later orange an yellow light disappear even in quite shallow water. This becomes a big problem in underwater photography. Without external flashes all pictures have a blue and green color cast. And since the light of the flash has to travel twice the distance - from the flash to the object and then reflected back to the camera - it helps only for short distances. This is the reason why macro pictures look much more colorful than pictures taken from distant objects underwater. The situation is entirely different for blue light. Even a 10 m water column transmits 90% of blue light at 400 nm. Thus, on a sunny day in clear water the amount of light is enough for taking pictures in moderate depths - the lack of red is the problem. This can partially be solved by changing the white balance of the camera so that the red is enhanced. In addition a red filter can be used which suppresses the blue part of the spectrum. The filter basically allows longer exposure times to increase capturing the red while not overexposing (due to the limited dynamic range of film materials or digital sensors) the blue. However, all these tricks don't help if the transmittance of red approaches zero and thus no red light reaches the camera.
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