OTTER Spectrometer Data INVESTIGATOR: Lee F. Johnson Research Scientist TGS Technology Inc. NASA/Ames Research Center Moffett Field CA 94035 Requested Acknowledgement: Co-Author or Citation INTRODUCTION: Laboratory bidirectional reflectance spectra throughout the region 450-900nm. have been taken as an aid to understanding remotely sensed spectral data collected at approximately the same point in the growing season. Of particular interest is examination of the spectra for the influence of absorption by biochemical constituents in the vegetation, particularly nitrogen, lignin, chlorophyll, starch and cellulose. Also of interest is examination of more gross spectral properties such as the near-IR/red ratio. Obtaining spectra of vegetation samples in the laboratory reduces or eliminates the confounding effects of atmosphere, platform instability, understory, exposed soils, mixed species, and canopy architecture which are present in aircraft data. Laboratory data may identify spectral regions upon which to concentrate analysis of aircraft data, or may provide a check against aircraft data which has been processed for removal of confounding effects. Thus, the shape of these spectra is of primary interest, as opposed to the magnitude of the absolute reflectance values. It is also anticipated that these data may be input as optical parameters to a radiative transfer model. Vegetation samples were collected from various OTTER sites: Scio (control + fertilized Douglas fir), Metolius (control + fertilized ponderosa pine), Santiam Pass (subalpine fir), and Juniper (juniper). All samples were collected 15-August-1990, inserted into plastic bags, placed on ice in the field, and subsequently stored in a refrigerator at the laboratory. The spectral measurements were made on 17-18 August. The date of sample collection corresponds well with airborne data acquisitions (AVIRIS, ASAS, TMS, TIMS, Spectron) which occurred 13-14 August. EQUIPMENT: Instrument Description: Platform: Laboratory Key Variable: Bidirectional Reflectance Principles of Operation: Dispersive grating spectroradiometer Instrument Measurement Geometry: The spectrometer head was mounted on the end of a boom approximately 1.5 meters above the stage, nadir-looking, with a 6-degree FOV. Two stabilized halogen lamps were used to illuminate the stage, each with a look-angle of approx. 45 degrees. The lamps and the stage thus approx formed an equilateral triangle, one meter on a side. Manufacturer of Instrument: The instrument used was a Spectron SE590 (Spectron Engineering, Inc., 1990), equipped with a CE390 vis/IR head (aka: Ames #2), owned by the Ecosystem Science and Technology Branch at NASA/Ames. The instrument was manufactured by: Spectron Engineering, Inc. 255 Yuma Ct. Denver CO 80223 303-733-1060 Calibration: A spectral calibration was performed on this instrument in June 1990 (Dungan, 1991) using a helium neon laser, with light output at 632.8 nm. The published channel/wavelength mapping for the CE390 head was established as correct. Radiometric calibration has not been performed on this instrument (other than to establish linearity of response), so radiance measurements are not possible. Rather, measurements are reported in terms of reflectance by comparison to a highly reflective reference panel. Scans were taken of an 18"x18" spectralon reference panel and from the stage (each with scan average=8) at approximately 45-minute intervals over a total time period of 3.5 hours. The ratio of reference scans to vegetation scans was 3:1. PROCEDURE: All measurements taken under artificial illumination in an office at the OSU/Forest Sciences Dept. The office was a windowless interior room. A stage upon which to measure the foliage was constructed so as to minimize reflectance. A cardboard box, approx. 3" x 2" x 1.5", was painted with Krylon flat-black spraypaint, inside and out. Also painted were dividers used to separate liquor bottles, to act as baffles. A 4" diameter hole was cut in the top of the box. The resulting stage produced a flat spectral response approximately .02x that produced by the reference panel. A boom-mounted tripod was situated adjacent to the box (stage), which was placed on the floor. The boom extended over the stage at a height of approx. 1.5 meters. Stabilized halogen lamps were established to either side as described in the above section on geometry. The Spectron head was affixed to the end of the boom, directly above the stage. The field-of-view was 6-degrees, approximately circular, as defined by the entrance slit of the camera. To improve the signal/noise, the data-logger was set to a scan- average of 8 (ie: each spectrum was an average of 8 individual spectra). The scan-averaged spectra were themselves combined and averaged to formulate the means and standard deviations reported to PLDS. Five western-hemlock branches were available from the fertilized plot at Scio, and five from the control plot. The five branches were stacked over the hole in the stage, and scanned. Three (scan- averaged) spectra were taken of all samples from each plot. The branches were manually re-arranged for each measurement, to decrease bias introduced by architecture of the assemblage. A similar procedure was followed for three branches subalpine fir collected from Santiam Pass, and three Juniper branches collected from the Juniper site. Five bunches each of ponderosa pine needles, excluding stems and branches, were collected from fertilized and control plots at Metolius. The needle length was sufficient to span the diameter of the hole in the stage. Two criss-crossing bunches, forming an assemblage approx. 4 needles thick, were measured at a time. Three scans were taken from fertilized samples, and three from control samples. Each scan represented a different pair of bunches, except one which was necessarily repeated (though once on top and once on the bottom). The halogen lamps were activated only during the actual scanning process, to minimize the denaturing effect of the heat. Of course, the (fluorescent) room lights were turned off during data acquisition. DATA MANIPULATIONS: Target reflectance was calculated from each vegetation response spectrum (it should be recalled that each of these three spectra is in turn an average of eight spectra, due to the instrument scan-average operation) as follows (Daughtry et al., 1989): reflectance (scan) = [response(scan) - avg bkgd response]/ [avg panel response - avg bkgd response] The means and standard deviations represented herein for each site were subsequently formed from these reflectances, corrected for the true reflectance of the reference panel. Wavelengths <450nm and >900nm were deleted due to noise considerations. ERRORS: Sources of Error / Quality Assessment: Four main sources of error serve to reduce confidence in the absolute reflectance values. First, the stage was not enclosed by a black curtain or other blocking device, therefore allowing a small yet appreciable impingement of stray light reflected from surrounding objects into the Spectron field-of-view. The amount of stray light should be about equal for all scans. Second, the amount of biomass per sampling unit was not constant. The signal and therefore the computed reflectance varies with amount of biomass. As the primary goal was examination of the data for absorption features, a decision was made to maximize the signal by measuring all available sample foliage rather than attempting to control for biomass. This may account for rather large discrepancies between the magnitude of the reported mean reflectance of control vs. fertilized samples at Scio and Metolius. Third, variance within the sampling units was large because of differences in canopy architecture resulting from branch rearrangement. Finally, although efforts were made to minimize exposure of the vegetation to the rather intense radiation of the lamps, it is possible that some changes in spectral response may have occurred between the first and third sample scans. REFERENCES: Spectron Engineering Inc., SE590 Field-Portable Data-Logging Spectroradiometer Operating Manual Daughtry, C., Ranson, K., and Biehl, L., A New Technique to Measure the Spectral Properties of Conifer Needles, Remote Sensing Environment 27:81, 1989. Dungan, J., Field Spectroradiometer Calibration Progress Report, Interoffice Memorandum, NASA/Ames Research Center, 5 January 1991.