Atmospheric oxygen strongly attenuates incident sunlight in spectral lines in two groups known as A band (wavelength ≈ 760 nm) and B band (wavelength ≈ 688 nm). Each of these bands includes about 40 spectral lines with such strong absorption that the atmosphere can be considered opaque at the middle wavelengths of these lines. Chlorophyll fluoresces strongly at these wavelengths. Thus, if light from plants at these wavelengths is measured, one can be assured that the measurement represents only fluorescence from chlorophyll, without contribution from scattered sunlight. Of course, the measuring instrument must be close enough to the plants in question that atmospheric oxygen does not also appreciably absorb the fluorescence from the chlorophyll, along with sunlight at the affected wavelengths; the resulting practical limit on the range of the present instrument or any similar instrument is 100 to 200 meters.

The present sensor (see Figure 2) is based on this spectral-discrimination principle. Light from sunlit plants is focused by a lens, then made to pass through a filter that passes wavelengths in a chosen 10-nm-wide subband of A or B band. The band-pass-filtered light passes through an entrance chopper, then along a light pipe to a spherical integrating cavity. Centered in the cavity is a quartz bulb that contains a gaseous mixture of oxygen and argon at a total pressure of about 100 torr (about 13 kPa).

Figure 2. This Spectral-Line Discriminator measures fluorescence from sunlit plants. Discrimination against scattered sunlight is achieved by a design that utilizes the absorption spectrum of atmospheric oxygen in combination with absorption and fluorescence in a bulb that contains oxygen.

When the entrance-chopper aperture is closed, the oxygen in the bulb emits the absorbed light energy as fluorescence in A band (regardless of whether the illumination is in A band or B band). Because the entrance-chopper aperture is closed, the remaining light in the cavity consists entirely of this secondary fluorescence, proportional to the fluorescence from the plants. An exit chopper synchronized in opposite phase with the entrance chopper opens to allow this light to pass to a photomultiplier tube cooled to a temperature 40 K below ambient. The output of the photomultiplier tube is processed and fed to a data-acquisition system.

This work was completed by Paul L. Kebabian, Herman E. Scott, and Andrew Freedman of Aerodyne Research, Inc., for Stennis Space Center under 1996 SBIR Phase II: NAS #NAS13-707. For further information, contact Herman Scott of Aerodyne Research, Inc., at (978) 663-9500 extension 267).

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Laurie S. Dean, Commercialization Manager
Aerodyne Research, Inc.
45 Manning Road
Billerica, MA 01821-3976

Refer to SSC-00037, volume and number of this NASA Tech Briefs issue, and the page number.