Sponsored by:  NASA – Instrument Incubator Program
Principle Investigators: Zbigniew Kolber and Paul Falkowski

This project seeks to develop an airborne Lidar-based system to measure photosynthetic performance and primary production in terrestrial ecosystems.The proposed method is based on measuring laser- induced fluorescence transients (LIFT) in response to a 50 millisecond excitation sequence at energy levels of 30 to 50 W/m². Specifically, this project seeks to:

• develop a Laser Induced Fluorescence Transient (LIFT) methodology to remotely assess photosynthetic performance from fluorescence transients induced by weak excitation light;
• proof and validate LIFT methodology in laboratory conditions;
• implement this methodology in a compact prototype instrument capable of airborne operation;
• field-test the LIFT method in relevant environmental conditions, operating from an airplane platform;
• specify the engineering design of the LIFT instrumentation for the next stage of Announcement Opportunity (AO) development.

The project proposes to excite chlorophyll fluorescence of the plant’s green tissue with a 50-millisecond-long excitation sequence of controlled intensity averaging 30 to 50 W/m². This will expose the photosynthetic reaction centers to about 20 quanta, causing up to 60% saturation of the photosynthetic electron transport and inducing transient changes in the chlorophyll fluorescence yield. The functional character of the measured fluorescence transient is controlled by the excitation signal and by a set of photosynthetic parameters, such as photosynthetic light utilization, the efficiency of photochemical conversion, and the rates of electron transport in photosystem II. All these parameters can be calculated by fitting the measured fluorescence transients into a mathematical model describing the relationship between photosynthesis and fluorescence. The project develops this model, determines the experimental protocols satisfying the optimal conditions for LIFT measurements under a limited signal-to-noise ratio, and conducts laboratory studies to verify this approach.

The proposed instrument uses a rectangular array of individually modulated laser beams to produce a wide excitation beam. Moving along a flight path with a typical speed of 135 mph (60 m/s), the beam “paints” a spatially modulated excitation image on the ground. This excitation pattern will, in turn, produce a fluorescence image, modulated spatially by the photosynthetic light utilization of exposed plants. The fluorescence image will be collected by a telescope and acquired by a red-sensitive microchannel plate image intensifier. Photosynthetic parameters will be calculated by fitting these two images into a numerical model describing the functional relationship between light, fluorescence, and photosynthesis. The same model will be used to calculate the rates of primary photochemistry.

The excitation beam are generated by an array of blue, blue-green, and red laser diodes. Commercial solid-state laser diodes in the red region (640-670 nm) of appropriate power rating are currently available. Blue, and blue-green solid-state laser diodes are at an earlier stage of development, and should become commercially available within the next 1-2 years. Using an array of laser diodes instead of standard Q-switched YAG lasers has several advantages: compact size and low power consumption; ability to generate an arbitrary excitation sequence; multiple-color excitation, and ability to generate a spatially modulated excitation pattern. The project also investigates an option of using a frequency-doubled YAG laser operating in a cw mode, equipped with a beam-expanding and beam-forming optics.

The project includes a field-test of the prototype instrument conducted in collaboration with the NASA’s Lidar group (Goddard Flight Space Center.) The results of these tests will specify design and performance requirements of the instrument that can be made mission-ready at the AO stage of the LIFT project. The system will comply with ANSI Z-136.1 guidelines on eye-safe laser radiation.