Saturday, 18 December 2010

Snow and the Wood Effect

As snow falls here in southern England for the second time this December I thought it appropriate to say a little about the Wood Effect. Named after Prof RW Wood, this is the brightening of foliage seen in infrared photographs that can often look like snow, first seen in his photographs from 1910.

The phenomenon had been observed before Wood, but not in an image, by Ogden Nicholas Rood, professor of physics at Columbia University, in his book on colour published in 1890 (or earlier). However this was based on spectrographic measurement of 'green' leaves and not photographic observation.

Willstatter and Stoll produced their theory of leaf reflectance in 1918 in their book Untersuchunger aber die Assimilation der Kohlenseture published in Berlin, and much of the literature on this was published in Germany. Not much is freely available on the web, although Mecke and Baldwin wrote about leaf reflectance in infrared in a 1937 paper which is available as a PDF (from Die Natlirwissenschaften in German).

Basically, the W-S theory says that light is reflected around within the internal structure of a leaf. Some eventually passes through and some is reflected back. Near infrared is not attenuated by the chlorophylls in the leaf as they are transparent at these wavelengths. The physical structure of the leaf is similar to that of snow, which is why infrared photographs can often look like snowy scenes. Unhealthy or dead leaves do not reflect so much because of changes in the internal structure, so the amount of infrared reflected by foliage can be used as an indicator of plant health.

Allen, and Richardson (Applied Optics Vol 12 #10 1973 behind paywall) explored the Willstatter-Stoll theory using ray tracing. This was a cumbersome process with 1970s computers and they were unable to do much, but they concluded that the theory predicted more transmission of light through the leaves than ray-tracing predicted. They concluded ...
The W-S model can be easily improved, however, by introduction of more intercellular air spaces. The modified W-S model promises to be an excellent representation of physical reality. Accurate predictions, however, require an inordinate amount of computer time.
Jacquemoud and Ustin, from Paris and California respectively, looked at modelling the optical properties of leaves in a paper from 2008 that is available online. They refer to a near-infrared plateau between 700 and 1100nm. Here ...
biochemical absorptions are limited to the compounds typically found in dry leaves, primarily cellulose, lignin and other structural carbohydrates. However, foliar reflection in this region is also affected by multiple scattering of photons within the leaf, related to the internal structure, fraction of air spaces, and air-water interfaces that refract light within leaves.
They don't call this the Wood Effect because, like most of the earlier papers I mention, they consider the optical properties across more wavelengths than infrared. However Mecke and Baldwin refer to the brightening of foliage in infrared as the Chlorophylleffekt.

So where is the earliest reference to this as the Wood Effect?

Strangely, this also seems to be in a German publication, in an article by someone named Marmet in a journal called Photofreund, (reference 18: 289-90, No. 15, August, 1938). I have seen this cited in a literature list produced by Kodak at the time, but not the original article itself.

The hunt continues.

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