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We present preliminary design studies and modeling results for a new system for the assessment of vegetation
photosynthetic function, especially carbon uptake.
Plant health and carbon uptake efficiency are of key consideration in assessing global productivity, biomass, changes in
land cover and carbon dioxide flux. Chlorophyll fluorescence (ChlF) measurements are critical for understanding
photosynthetic functioning, plant environmental stress responses and direct assessments of plant health. Plant ChlF
occurs predominately in two broad emission bands in the red and infrared regions of the spectrum. Unfortunately, the
fluorescence signal from vegetation is much weaker than, and obscured by, the reflected signal. This limitation can be
overcome by acquiring ChlF measurements in atmospheric absorption lines.
The Interferometric Sensor for Plant Fluorescence (ISPF) will measure plant ChlF using the Fraunhofer Line
Discrimination approach. Fabry-Perot (FP) etalons will be used to restrict the measurement to radiation in the Solar
Fraunhofer lines (SFL). An advantage of the proposed sensor design is that it will collect measurements using two sets
of SFL at the same time. This technique increases the optical throughput producing a larger signal to noise ratio (SNR).
The instrument is designed to have two channels for two different spectral regions. Each channel will have two sub-channels,
one defined by a prefilter (Reference, Ref) and the other having a tunable FP etalon. The first subchannel (the
Ref) will cover a relatively broad spectral range to include at least two Fraunhofer lines but for which the fluorescence
signal will represent only a small fraction of total reflected light. The second subchannel will use a FP interferometer to
restrict the detected light to include only the selected SFL where the ChlF in-filling is significant. A small change in the
fluorescence will then produce an insignificant change in the Ref subchannel but a relatively large change in signal from
the FP subchannel. Changes in albedo or clouds will affect both subchannels proportionally so that the ratio of FP/Ref
will be sensitive only to ChlF and almost insensitive to other parameters.
The ISPF sensor will measure the fluorescence energy emitted by vegetation under natural sunlight. Advantages of the
sensor over other designs are that it is passive (i.e., does not require an external illumination source), has simple structure
and can be manufactured in a rugged, monolithic form that has no moving parts.
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