Video imager detects stress in plants

17 Jun 2002

Scientists at NASA's Stennis Space Center have developed a spectral reflectometer for quantifying stress in plants and a portable video imager to detect stress in plants.

The hand-held spectral reflectometer exploits the spectral reflectance characteristics associated with chlorophyll contents of healthy and unhealthy plants while the video imager acquires multispectral images of a scene which contains plans, digitizes the images, and processes the data to generate a new image which maps chlorophyll loss.

The spectral reflectometer consists of two optical systems which measure radiances reflected by the plant target and a reference reflector through narrow-band optical filters centered at 695 nanometres and 840 nanometres. The target and reference radiances are captured simultaneously and the physiological stress is determined by the ratio of radiance values acquired the different spectral bands and its product with the ratio of two reference radiances acquired in the same spectral bands. The energy is focused on discrete photodiodes from which the output is digitized and routed through a microprocessor. After processing a numeric value indicating the degree of plant stress, or chlorophyll loss, will appear on an LCD display.

The optical system consists of a primary lens to intercept light reflected from the plants and focus the energy on the detectors, a beamsplitter to divide the energy and direct half to each detector, bandpass filters to pass only the wavelengths of interest, and detectors which detect energy from samples and reference.

A detector converts light to electric current while a transconductance pre-amplifier converts current to voltage and provides gain. A microcontroller controls the multiplexer, sample, and hold, along with the analog/digital converter.

The reference is selected while the instrument is viewing the reference target. Five data samples from each channel are taken and stored. The samples of each wavelength area averaged, dark value is subtracted, and the final value of each wavelength is stored for later use.

The sample is selected while the instrument is viewing the target of interest. Five samples are taken and averaged in the same manner as the reference sample, and percent reflectance is computed for each wavelength by dividing the sample by the reference.

A dark sample can be taken as often as necessary with an opaque cover over the collection lens; the dark reading is stored and subtracted from each reference or plant sample taken.

The portable video imager consists of a three-channel multispectral telescope with one narrow-band optical filter centered at 695 nanometres, one narrow-band optical filter centered at 840 nanometres, and one unfiltered image. The three images are collected by three identical video cameras. The video signals from the three cameras are sent to the video processor, as are the two reference samples taken through filters with the same spectral bands. The output of the processor is combined with the panchromatic image and displayed on a video monitor. If desired a synthetic image indicating local contests of chlorophyll could be overlaid on the panchromatic image.

The research was headed by Bruce Spiering and Gregory Carter of the Stennis Space Center. NASA has filed a patent application and plans to license the devices for commercial development.