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LED arrays excite fluorescence

17 Jun 2002

A fluorescence imaging system containing more than 1200 LEDs finds its first application.

By using arrays of blue and red LEDs, a collaboration of researchers from the UK and the US are developing a fluorescence imaging system to assess the damage insect bites have on plants and their ability to photosynthesize light. The latest results show that the effects on photosynthesis have been underestimated by as much as 3-fold ( PNAS 99 1088).

Photosynthesis involves the conversion of light into chemical energy, a process mediated by chlorophyll molecules in the leaf. Complementary to the complex photochemistry involved, chlorophyll molecules also fluoresce and studying chlorophyll fluorescence is now a standard technique for probing plant photosynthesis.

Evan deLucia, head of the research team at the University of Illinois, explained: "We have developed an instrument that spatially maps chlorophyll fluorescence and extends the capabilities of previous instruments by providing a complete fluorescence analysis and simultaneous gas measurements."

The imaging system contains an array of 1200 red and blue LEDs. The red LEDs emit at 660 nm and drive the photosynthesis while the blue LEDs emit at 470 nm and provide the measuring pulses for imaging the fluorescence.

Kevin Oxborough of Essex University, UK, told Optics.org: "Because the excitation is blue, it is easy to filter out reflected light when imaging the fluorescence from the leaf. The reason for using red LEDs to drive photosynthesis in between images is simply that they [the red LEDs] have a higher quantum output."

A progressive-scan CCD camera is used to gather real-time images over large areas up to 6 cm2. The camera is synchronized to the blue imaging pulses to detect the chlorophyll fluorescence at 682 nm.

Since the construction of this instrument, the team has been working on a more advanced commercial system. This new, patented configuration uses 1600 blue LEDs all pulsed together at different frequencies to provide the desired light intensity. Called the FluorImager, it will be distributed next month in the US.

  • The researchers tested their device by mapping the effects of caterpillar feeding on photosynthesis in wild parsnip leaves. They found large patches of photosynthetically depressed cells extending well beyond the area consumed by the caterpillar. On average, the indirectly affected area was six times that of that area affected by tissue removal.

    Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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