An optoelectronic instrument, known as a plant fluorescence sensor, is being developed for use as a working tool in agricultural settings. This instrument is a remote, passive monitor that provides a means of discerning plant stress at very early stages. With sufficient warning, the user could provide timely applications of fertilizer, water, and/or pesticide to achieve maximum crop yield at minimum cost. Figure 1 presents two views of the plant fluorescence sensor. The instrument is the subject of U. S. Patent 5,567,947.

Measurement of steady-state plant fluorescence offers the possibility of determining the physiological status of a green plant. The magnitude of plant fluorescence and its spectral (color) distribution is sensitive to a number of factors which are related to the ability of a plant to perform photosynthesis (the process by which green plants convert atmospheric water vapor and carbon dioxide into sugars and oxygen, using sunlight as fuel). For instance, the light capture efficiency of the plant is dependent on the type and amount of pigment molecules (such as chlorophyll) which in turn is dependent on adequate fertilization. Plants stressed from a lack of fertilizer will limit chlorophyll production and exhibit both lower overall level of fluorescence and shift in spectral distribution compared to healthy plants. Another factor, such as lack of adequate water, can serve to limit the rate of photosynthesis by causing the plant to close its stomata (the openings which allow the leaf to draw in carbon dioxide and water vapor); when this happens, the level of plant fluorescence will generally increase. Thus, measurements taken with this sensor can guide growers in the allocation of resources such as irrigation water, fertilizer, and pesticides.

Figure 1. The Plant Fluorescence Sensor in the left view is lying, with its casing removed, on a utility cart. In the right view, it is installed on a high-clearance vehicle for monitoring a Nebraska corn field.

Under sunlight, the chlorophyll in plants fluoresces at wavelengths from about 660 to 800 nm. The major problem in measuring this fluorescence is to discriminate against scattered sunlight, which can contribute a spurious component to the measurement. The present sensor is of the class of apparatuses known as Fraunhofer-line or spectral-line discriminators, but this sensor differs from others of its class by virtue of a unique design that exploits the spectral absorption lines of oxygen in such a way as to obtain enhanced spectral discrimination at lower cost. The desired spectral resolution and discrimination are achieved without need for the highly precise, expensive optical components with critical mechanical adjustments (e.g., Fabry-Perot cavities) that are used in other spectral-line discriminators.

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 open, the oxygen in the bulb is illuminated by plant fluorescence plus scattered sunlight in the chosen subband of A or B band, but the oxygen absorbs only the light at its characteristic absorption wavelength in this subband. Because the atmosphere has already filtered out sunlight at these wavelengths, the light absorbed by the oxygen consists almost entirely of fluorescence emitted by the plants. While the entrance-chopper aperture remains open, a silicon photodiode measures the total A- or B-band radiation received by the cavity.

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.

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This article first appeared in the May, 2000 issue of NASA Tech Briefs Magazine.

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