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 hemispherical reflectance spectra throughout the 440-900nm. range have been taken as an aid to understanding remotely sensed spectral data collected at approximately the same time. 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, understory, exposed soils, mixed species, and canopy architecture which are present in aircraft data. Further, acquisition of spectra within an integrating sphere further reduces confounding effects such as stray light, and biomass and geometry differences which are encountered in a bi-directional configuration. 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 the OTTER Scio site (control + fertilized Douglas fir). Samples were about 20-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 4-December-1990. The date of sample collection corresponds with airborne data acquisitions (AVIRIS, ASAS, TMS, TIMS, Spectron) which occurred during the period 8-25 October. EQUIPMENT: Instrument Description: Platform: Laboratory Key Variable: Hemispherical Reflectance Principles of Operation: Dispersive grating spectroradiometer/Integrating Sphere Instrument Measurement Geometry: The sample port of the sphere was located at 180 degrees with respect to the illumination source. The camera observed the interior wall of the sphere at 90 degrees. (fully described in LI-COR, 1984). Manufacturer of Instrument: The spectroradiometer used was a Spectron SE590 (Spectron Engineering, Inc., 1990), equipped with a CE390 vis/IR camera (aka: Ames #2), owned by the Ecosystem Science and Technology Branch at NASA/Ames. Manufacturer: Spectron Engineering, Inc. 255 Yuma Ct. Denver CO 80223 303-733-1060 The integrating sphere was LI-COR Model 1800-12, also owned by the Ecosystem Science and Technology Branch at NASA/Ames. Manufacturer: LI-COR 4421 Superior Street PO Box 4425 Lincoln, NE 68504 402-467-3576 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 pressed barium sulfate reference. The ratio of reference scans to vegetation scans was 1:1. PROCEDURE: The Spectron 390 visible/near-IR camera was interfaced with the LI-COR integrating sphere, with the resulting geometry described above. Reference scans were taken by illuminating a pressed barium sulfate reference, background scans were taken by illuminating the flat black surface of the sample port, and vegetation scans were taken by placing the vegetation on the sample port. In all cases the camera observed the painted barium sulfate covering the interior surface of the sphere. Within a period of approximately one hour, the following measurements were taken: 3 reference scans, 3 scans of the same control sample, 3 more reference scans, 3 scans of the same fertilized sample, and finally 3 background scans. The samples were prepared as follows. One set each of about 10 needles was selected from the control and fertilized foliage. The needles were aligned vertically, and joined at the bottom by a piece of cellophane tape. The needle-bunch was then inserted into the sample port such that the tape itself was not present within the port. The camera 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. The illuminator was equipped with a heat shield to eliminate radiation at wavelengths greater than 1100nm. All measurements were taken in the Spectral Measurements Lab of the NASA/Ames/Ecosystem Science and Technology Branch. 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 (LI-COR, 1984): reflectance (scan) = [response(scan) - avg bkgd response]/ [avg reference response - avg bkgd response] The means and standard deviations represented herein for each site were subsequently formed from these reflectances. The pressed barium sulfate was assumed to be an ideal reflector. Wavelengths <440nm and >900nm were deleted due to noise considerations. ERRORS: Sources of Error / Quality Assessment: Due to the lengthy period of time between collection and measurement, it is likely that some denaturation of chlorophyll, protein, starch and sugars occurred, possibly to a greater extent in the control than in the fertilized samples. In fact, the foliage was visibly desiccated at the time of measurement, the control samples more so than the fertilized samples. To compensate, efforts were made to select needles with as fresh an appearance as possible. The measurements themselves are rather stable, as evidenced by the low standard deviation. Reflectance of the pressed barium sulfate was assumed to be 100 percent. The actual values range from 0.999 percent at 400nm. to .992 percent at 1100nm. This deviation is considered within the noise level of the data. REFERENCES: Spectron Engineering Inc., SE590 Field-Portable Data-Logging Spectroradiometer Operating Manual Dungan, J., Field Spectroradiometer Calibration Progress Report, Interoffice Memorandum, NASA/Ames Research Center, 5 January 1991. LI-COR, Model 1800-12 Integrating Sphere Instruction Manual, Publication No. 8305-0034, revised August 1984.