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Green plants, far more efficiently than animals or molds, are strong reflectors of infrared light. It seems to be related to membrane-related phenomena that is abundant in chloroplasts. Infrared imaging of plants always gives a "shiny, snowy" appearance to them. In contrast (pun not intended) "dead" plants or parts of plants (like older branches or bark, or dead wood) are much more dim in infrared imaging, as they tend to absorb infrared, or poorly reflect it. Thus, dark roots and branches yield very bright leaves!
In addition, because clouds reflect infrared (as they do visible light) while blue skies carry very little infrared light, clear skies will have a dark appearance when imaged in infrared. Finally, because water absorbs infrared light, bodies of water appear dark.
Images taken with infrared-sensitive film have a very "distorted black-and-white" appearance to them. because of the phenomena described above. Ansel Adams was infamous for demonstrating this effect in infrared photography. Digital cameras, on the other hand, show the images as purple natively because both red and blue sensors in the camera are sensitive to infrared light. Not only because we do not see infrared light but also because it is far for visually appealing, it is customary to re-color the purple light as white light, to match that from infrared film.
Enjoy!
Visible color can be included with infrared imaging to obtain interesting effects, especially with the sky and other reddish objects.
Waiting at a red light. Note that the red light is barely visible.
A palm tree in a nearly cloudless sky.
Mickey welcoming us to the Epcot Food and Wine Festival, in infrared (left) and visible color (right).
Here is a pest-damaged leaf. Notice how the infrared-darkened damage is limited to the interface between the intact leaf and the fully-damaged leaf, both of which are fully infrared-reflective.
Houses along one of the barrier islands of Florida's Atlantic coast.
These photos demonstrate how dead wood loses its ability to strongly reflect infrared light. The dead trees really are "ghosts in the bramble".
Is that a cell phone tower or a palm tree? Infrared imaging gives it away immediately!
One can almost imagine with infrared imaging how the Coast Guard can scour dozens of square miles of ocean quickly yet find lost people....
I decided to learn what aspect of plants reflect infrared light so strongly. So I reflected infrared light through a microscope containing a sample of pond algae (most likely Spirogyra sp.). Although there is a weak correlation between the presence of chloroplasts (the "green" in plants) and some infrared reflection, the strongest reflection seems to occur then light is traveling through large amounts of plasma membrane (for example, tangentially or the through the edge of a cell or through the chloroplast).
The chloroplast-independant aspect of this reflection may explain why recently-killed plants (with membranes still present), as well as fungi and animal skin, all of which have intact plasma membranes, reflect infrared light so well as well (see the people in the above image).
Here is a false-color image in which near-infrared light is resolved spectrally by using two different filters, and combining the composite aligned image. Bluish colors represent high-energy infrared (almost visible red - between 650 and 750 nm) light presence, while yellowish colors are richer in lower-energy near-infrared light (between 800 and 1100 nm). One can see the Rayleigh scattering of atmospheric light and the fact that water does not strongly reflect infrared light (it in fact absorbs it strongly).
