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Lasers offer unique view of pine needles

29 Aug 2007

The first study of laser backscattering from single pine needles has been carried out by Finnish researchers.

Measurements of monochromatic backscattering from individual Scots pine needles, approximately 0.5 mm wide, have been made by a Finnish team. (Applied Optics, 46, 4916.)

"Information on backscatter from coniferous forests is needed to develop models of forest reflectance and canopy radiative transfer which can then be used in forest monitoring," Sanna Kaasalainen of the Finnish Geodetic Institute explained to optics.org.

This is the first time individual needles have been examined in this way. Broad leaves have been studied in some detail as part of various climate studies, but data on conifer needles has been scarcer simply because of the difficulty in studying such small objects.

The team's solution involved a specially constructed instrument, based on standard techniques used for backscattering measurements and CCD photometry but modified to suit such a small target. A diffuser smoothed out the laser beam, and a 2 mm iris in front of the diffuser enhanced the angular resolution sufficiently to allow accurate measurements.

Kaasalainen and colleagues identified strong backscattering peaks, ie. coincidence of the incidence and scattered light paths, from both sides of a needle when illuminated by a 1064 nm 10 mW laser. Such backscatter enhancement is called a "hot spot", and the team measured an increase in brightness of up to 40%.

"It proved easy to measure the changes in intensity, but accurate calibration was a challenge due to the small size of the object," he explained. The team's technique used a 50% reflectance test target for calibration, but they are working on more accurate methods for radiometric calibration, to measure the reflectance of a single needle accurately and not just in relative terms.

"Our purpose was not to present a data bank on hot spot signatures of a range of pine needles, but to test a measurement technique and obtain preliminary results," said Kaasalainen. "We feel these results are unique."

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