lakes and IR

["Lee Tarpley" <tarpley@xxxxxxxxxxxxxx>]

Several interesting questions have arisen during the recent discussions
concerning the interaction of water (and other substances) with the very
near infrared radiation to which films such as HIE are sensitive. These
include the absorption and scattering of various wavelengths of radiation by
water that is pure or that contains solutes or suspended particles. The
subject of surface reflectance has also come up. The interaction of
radiation and natural bodies of water is a subject not just of concern to
photographers, but has also been extensively studied as a part of limnology
(the study of freshwater ecosystems). For example, the book "Limnology" by
R.G. Wetzel (1975) devotes a chapter to the subject of Light in Lakes that
discusses these topics and more. One note of interest concerns the effect of
coloration due to organic matter concentration of various lakes: the
absorption of radiation in the visible wavelengths is increased relatively
much more with greater lake coloration. In 1 m of distilled water, the
absorbance by blue light is less than 5% compared to about 90% at 800 nm
(very near infrared), wheras in 1 m of highly colored lake water, all the
blue light is absorbed while that at 800 nm is still less than 95%.
Lee Tarpley

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