Trip 9: More filter tests

Grabbing the only really nice day of the week, we have managed to complete 6 lower resolution scans today (two in each plot). As we have discovered that the 1040 laser is weaker than the 1550, different optical filters have been used: ND0.6 for the 1040 and ND 1.6 for the 1550.

SALCA at plot 2

Focusing on the Gap Fraction Analysis, there are a couple of points of interest:

1) SALCAs measurement of the Gap Fraction under-estimates the gaps compared to the hemispherical photography: This follow previous work completed at the University of Salford and arises due to the fact that hits and misses are only currently counted as either full hits or full misses - there is no allowance for partial or minimal hits.

Plot 4, where the ferns have reached head height and are showing up on our scans

2) At higher zenith bands (closer to the ground) we have found that the Gap fraction appears to increase, where the hemispherical photography analysis does not. We believe that this is a result of the range of the scanner only being able to pick up the nearer targets, whereas the hemispherical photos are guided by the number of photons hitting the lens.

Gap fractions for plot 4: Red lines indicate the 1040 scans and Blue lines show 1550 scans, with different noise thresholds measured of reach. The green line is the Gap Fraction from the hemispherical photographs

These experiments are to compare against the experiments we conducted last week to find an acceptable level of repeatability. There have been some issues with the current mount that is used to hold the filters and this is definitely something that needs work, but our results largely follow what we expected. Another trip will be needed to ensure that our results are acceptable. In the meantime, further tests will be run back at the university.

Oliver Gunawan
14th June 2011