FED/MAC Data
Visible/Infrared Intelligent Spectrometer (VIRIS)
The Visible/Infrared Intelligent Spectrometer (VIRIS), developed as the IRIS
Mark IV by GER, Inc., and a portable illumination source (a spectralon coated
hemisphere/baffle system employing 4, high-intensity, quartz line, 30 W bulbs)
were used to characterize the reflectance of branch samples with needles
attached (spruce and hemlock).
Instrument Measurement Geometry
The VIRIS provides 2 nm spectral resolution from 0.4 - 1.1 um, and 4 nm spectral resolution from 1.1 - 2.5 um. The
portable illumination source is placed at a standard distance (50 cm at an
incidence angle of 35 degrees) from the branch samples.
Single aged branch segments (1st-year and 2nd-year) were arranged in standard
sample trays placed at a distance of 75 cm from the optical head of the VIRIS,
yielding a field-of-view of 1.8 by 4.8 cm.
Processing Steps and Data Sets
A total of three spectral scans were
acquired and averaged to give a single VIRIS reflectance data set. Multiple
data sets (i.e., each consisting of different branch segments) were acquired
for each age class, using branch segments from the upper canopy of each tree.
Derivation Techniques/Algorithms
A software program known as the
Integrated Data Management Module (IDMM) was developed at the University of
New Hampshire and used for the analysis of VIRIS data. This program divides
each spectral curve into a series of smaller, more workable spectral regions.
From each spectral curve, 81 different reflectance, derivative and wavelength
parameters were extracted (not all were included in this data set collection).
Special Corrections/Adjustments
The individual spectra were smoothed using
a nine-point smoothing function. The formula for this function was:
Y(n) = 0.04R(n-1)+0.08R(n-3)+0.12R(n-2)+0.16R(n-1)+0.20R(n)+0.16R(n+1)+
0.12R(n+2)+0.08R(n+3)+0.04R(n+4)
where, Y(n) is the weighted reflectance of the target
for a particular channel,
R is the reflectance measured by the instrument
for a particular channel, and
n is the channel number.
Although this function altered the absolute reflectance
values of individual spectra to some degree, the procedure reduced instrument
channel-to-channel noise and produced more easily interpretable derivative
spectra.
For optical reflectance data acquired with the VIRIS outdoor data, all scans
were acquired from the tower at 70 feet. No scans were averaged, and all times
are local. A comparison of 1st- and 2nd-year branch segments in hemlock showed
visible reflectance features similar to those seen in the needle optical
studies: i.e. higher green peak reflectance (16%) in the 1st-year samples
compared with (13%) in 2nd-year samples. A significant drop in NIR
reflectance is seen between 1st- and 2nd-year branch samples, a feature not
seen in the needle optical studies of hemlock.
Spruce reflectance data for 1st- and 2nd-year branch samples showed features in
the visible region which are similar, with 1st-year branch samples having a
higher green peak (18% vs. 13%) and a more narrow chlorophyll well than 2nd-year
samples. A less dramatic drop in NIR reflectance is seen between the age
classes in spruce than was seen in hemlock.
Data Available:
Dr. Barrett N. Rock
Inst. for Study of Earth, Oceans and Space
University of New Hampshire
Durham, NH 03824
(603) 862-2949