Spectral Reflectometer for Quantifying Stress in Plants

Spectral measurements of chlorophyll loss indicate stress levels.

Stennis Space Center, Mississippi

A hand-held optoelectronic instrument has been designed to generate a quantitative indication of the loss of chlorophyll, and thus the level of stress, in plants. The instrument exploits the known spectral-reflectance characteristics associated with the chlorophyll contents of healthy and unhealthy plants. In particular, the instrument indicates the ratio between the reflectances of plants in narrow spectral bands centered at wavelengths of 700 and 840 nm, respectively. This ratio ranges from about 0.1 for a healthy plant to 0.4 for an unhealthy plant. [Other similar instruments have been based on fluorescence (as distinguished from reflectance) ratios, but the intensity of reflected light is much greater than fluorescence intensity at the wavelengths used in this instrument.]

The instrument is operated in the following procedure: Readings are first taken with the instrument aimed at a standard reflectance target. Next, readings are taken with the instrument aimed at the plants of interest. Both the plants and the target could be illuminated by sunlight or artificial light. From time to time, readings are also taken in the dark to obtain corrections for nonzero components of photodetector outputs ("dark currents") at zero illumination.

In the instrument (see figure), light reflected from the plants or target is intercepted by a lens, then split into two beams. One beam is band-pass filtered at a wavelength of 700 nm, the other at a wavelength of 840 nm. Each beam then impinges on a photodetector, which is located with the center of its input face at a focal point of the lens.

The outputs of the photodetectors are amplified and offset as needed. A 2:1 multiplexer selects whichever of the two amplified, offset photodetector outputs is to be fed to a sample-and-hold (S/H) amplifier followed by an analog-to-digital converter (ADC). A microcontroller controls the multiplexer, the S/H amplifier, and the ADC. The digital sample put out by the ADC is fed to the microcontroller for further digital processing in coordination with the acquisition of samples, as described next.

With the instrument aimed at the reflectance target, the operator presses a pushbutton switch marked "reference," causing the microprocessor to command the acquisition of a digital sample, first at the wavelength of 700 nm, then at the wavelength of 840 nm. Five samples are taken and averaged automatically on each channel with the single push of a button, then the sample from the dark reading at that wavelength is subtracted to obtain a corrected reading, which is stored. The instrument is then aimed at the plants of interest, and the operator presses a pushbutton switch marked "plant sample," causing the microprocessor to command the acquisition and processing of readings from the plants in the same manner as from the reflectance target.

At any time before or after acquiring the reflectance-target or plant readings, the operator could place an opaque cover over the lens and press a pushbutton switch marked "dark sample" to acquire the dark readings. In the same manner as for the reflectance-target and plant readings, five dark readings would be acquired at each wavelength and averaged. The average values would then be stored and used to correct the reflectance-target and plant readings as described above.

The microprocessor divides the corrected plant reading for the wavelength of 700 nm by the corrected reflectance-target reading for that wavelength to obtain the reflectance of the plants at that wavelength [r(700)]. The microprocessor also divides the corrected plant reading for the wavelength of 840 nm by the corrected reflectance-target reading for that wavelength to obtain the reflectance of the plants at that wavelength [r(840)]. Finally, the microprocessor calculates [r(700)/r(840)], which is the desired ratio indicative of stress in the plants.

  This Instrument Measures the ratio between the reflectances of a plant sample at two wavelengths; one red, the other infrared. The ratio indicates the level of stress in the plant sample.

This work was done by Bruce A. Spiering and Gregory A. Carter of Stennis Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Physical Sciences category.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Stennis Space Center; (228) 688-1929. Refer to SSC-00050. Download detailed Technical Support Package for this Brief