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Fluorescence Imager

The flourescence imager (FI) provides visible and fluorescence images of targets underneath the rover, in an approximately 8 x 6 cm area (0.12 mm/pixel). The images are used to identify life by detecting naturally fluorescent biomolecules, primarily chlorophyll, but possibly other biomolecules such as carotenoids. Because the fluorescence signal is ordinarily overwhelmed by reflection at the fluorescence emission wavelength, it is crucial that there be no illumination at that wavelength; therefore, the instrument could only be used at night, when we had control over the illumination.

The bottom of the electronics box is a Fresnel lens, on which are mounted red, green, and blue LEDs. The lens focuses all of the LEDs on a central area underneath the FI. A hole is cut in the center of the lens, and three inexpensive off-the-shelf RGB webcams, sensitive to light from 400950 nm, are pointed down through the hole (circular green circuit boards). One camera is unfiltered; the other two have a 665 longpass and a 700/75 bandpass filter, respectively. IEEE 1394 (Firewire) outputs from the cameras are routed through a hub (circuit board at the upper right) to the roverís Firewire bus. The main rover processor could switch the LEDs and control image capture from the three cameras.

When all three LED colors are activated and an image is captured with the unfiltered camera, a visible reference image is obtained (this is important so that the science team can simply see what the FI is looking at). Using only a single color of LED and capturing from one of the filtered cameras provides a fluorescence image. Changing the LED color and filter bandpass allows detection of different fluorescence signals. The signal of greatest interest is chlorophyll, which returns a strong red fluorescence signal under blue illumination and the 700/75 bandpass filter.

Given the tight schedule before the first expedition, it was clear that it would not be possible to test and iterate the FI design to the extent that one could be confident of its performance in the field. Nonetheless, it was decided that developing the instrument was a worthwhile exercise in learning integration issues, and in building ties between the instrument team and the rover team.

Actual use in the field was limited. Because the FI could only be used at night, readings were not taken during daily field operations. Instead, some of the samples were manually collected after the fact and placed underneath the FI in order to test its performance. None of the samples returned a recognizable fluorescence signal. Aside from the fact that chlorophyll levels in the samples were likely very low, there were several problems that made much of the returned data unusable. Most of these problems could easily have been corrected if more time had been available for testing and design iteration.

ProblemPotential Solutions
Poor focusUse cameras with autofocus or a mechanism for raising/lowering the FI
Visible reference image saturatedMore precise control of LED light levels
Camerasauto-correct color balanceAvoid low-end consumer webcams
Color channel to wavelength mapping unknown / difficult to calibrateUse B/W cameras for fluorescence imaging
Limited to nighttime operationsIntegrate a shroud; use pulsed illumination; use brighter LEDs

After the field expedition, the FI was tested under lab conditions, separate from the rover. Its ability to detect a fluorescence signal was verified, but sensitivity was fairly low, which is not surprising given the low quality of the cameras.

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