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NIR and water

by cfastie |

Above: Pure NIR photo of white water lily (Nymphaea odorata).
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The interaction of infrared light and water can influence our interpretation of near infrared photos taken to investigate aquatic or marine plants. This question most recently came up last week when @code4maine asked about using Infragram cameras to map eelgrass beds along the Maine coast. I was thinking about this issue as I was testing some new infrared cameras this weekend. This pair of photos was somewhat revealing:
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PoolNIR2k.jpg
A swimming pool in normal visible light (left) and in pure near infrared light (right).
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Two differences between water's interaction with visible and near infrared (NIR) light are conspicuous in these photos:

  1. Absorption: NIR light is absorbed by water more strongly than visible light. The pool looks brighter in the visible light photo because more of the light is transmitted through the water and less is absorbed. The longer the path through water that any light has to follow, the more of it is absorbed. But NIR light is absorbed faster, so the pool steps get darker faster with depth in the NIR photo. It looks like very little NIR light can pass through more than a meter of water (the pool bottom is not visible).
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    774px-Water_absorption_coefficient_large.gif
    Light absorption coefficient of pure water. The absorption coefficient (y axis) is a log scale so NIR light (e.g. 760 nm) is absorbed 10 times faster than orange light (625 nm).
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  2. Reflection: You can barely see some clouds reflected in the surface of the pool in the RGB photo, but the clouds and tree are clearly reflected in the NIR photo. This is not because NIR light is reflected more strongly than visible light. I think the reflection is just easier to distinguish because so little NIR light passes through water and it appears dark.
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    reflectance-graph.gif
    NIR is reflected less by water (the "Sea") than is visible light. But reflections are easier to distinguish because so little NIR light passes through the water and it is much darker than the reflections.
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    PoolReflectNIR2k.jpg
    Here is another example of the apparent difference in reflection from the surface of water recorded in visible and NIR light.
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    Aquatic or marine plants that are floating or emergent can be studied with aerial infrared photography, but plants under more than about a meter of water are probably not very visible in NIR photos.

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1 Comments

I think part of the problem is that infrared light and visible light (and each subdivision thereof) are reflected differently at the interface of air and water.

When imaging plants in a strictly air-based substrate, you don't have to worry about dispersion. Unfortunatey, the interface of air and water disperses light unequally.

If you were to measure strictly 589nm yellow light, the standard example used to discuss refractivity, you'd not experience any problems with dispersion. If you're measuring both an infrared wavelength and, say that 589nm yellow wavelength, for the purpose of comparing them to each other, then you end up with 8 scenarios that change how your data is going to appear.

Consider these 8 scenarios:

  1. the common light source is in the air, the subject is in the air, and the observer is in the air. This is the normal use case for NDVI.
  2. the common light source is in the air, the subject is in the air, and the observer is in the water. You're taking picture of land plants lit by the sun with an underwater camera for some reason.
  3. the common light source is in the air, the subject is in the water, and the observer is in the air. Photographing aquatic plants using the sun and a dry camera as suggested here.
  4. the common light source is in the air, the subject is in the water, and the observer is in the water. Underwater photography of underwater plants using the sun.
  5. the common light source is in the water, the subject is in the air, the observer is in the air. Now we're getting silly.
  6. the common light source is in the water, the subject is in the air, the observer is in the water. Silly again.
  7. the common light source is in the water, the subject is in the water, the observer is in the air. Capturing an underwater plant using underwater LEDs or bulb with a dry camera.
  8. it's all under the water.

The first and last are the only two that are directly relatable using different wavelength comparison (like NDVI), because there is no water/air interface to speak of to cause dispersion. http://en.wikipedia.org/wiki/Refractive_index#Dispersion

The scenarios where the camera is underwater but the subject is above water (2 and 6) will be hampered by total internal reflection of light traveling from under the water to up in the air. http://en.wikipedia.org/wiki/Total_internal_reflection#Examples_in_everyday_life

I'm not going to analyze each scenario, but if you consider each, you'll find that the interface between water and air is really going to change how much IR versus how much other colors are present. If you were to come up with some fixed thresholds for comparison between plant species in scenario 1, those values would get totally messed up when applying the thresholds to data captured in scenario 3.

It's not to say you cannot photograph aquatic plants using the sun with a camera above the water. It's to say that such data is not comparable to similar data collected of land plants on land. So if you do try to capture data across a barrier which causes dispersion, know that your data is not comparable to similar data captured where no such barrier existed. You can't use NDVI on underwater plants and discuss it as you would NDVI of land plants. The intrinsic quality of the data would be different.


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