Radiant energy – Luminophor irradiation – Methods
Reexamination Certificate
1999-04-20
2001-12-11
Epps, Georgia (Department: 2873)
Radiant energy
Luminophor irradiation
Methods
C250S458100
Reexamination Certificate
active
06329660
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of deriving sunlight induced fluorescence from radiance measurements, and devices for executing the method.
REVIEW OF THE RELATED TECHNOLOGY
In remote sensing, reflection measurements are often used to determine the status of vegetation. In the process, the light reflected by the vegetation is detected in more or less narrow channels (e.g. frequency bands). For verification, and to preclude for atmospheric influence, measurements are also taken with spectrometers a few meters above the ground.
Within the scope of a measurement campaign, these types of measurements are taken for wheat cultures (e.g. fields), for example.
FIG. 6
shows a typical reflectance spectrum. A notable feature is a peak at 762 nm in the so-called infrared plateau, indicated by a circle. This peak never occurs in laboratory measurements using artificial lighting.
To ascertain the cause of this peak, the following procedure must be considered in the determination of a reflectance spectrum: First, the light reflected by a white reference standard (i.e., a surface having a known reflection factor of nearly 100%) is measured. Then, the light radiated back from the test object is detected.
FIG. 7
shows these two measurements, from which the reflectance spectrum in
FIG. 6
was calculated by computing the quotient and subsequent multiplication with the reflectance spectrum of the white reference standard.
FIG. 7
clearly shows the o
2
A absorption band of the atmosphere at 762 nm. Because the measurements contain a larger relative error due to the lower signals, measuring errors were determined to be the cause of the peak. The peak, however, was intended to be statistically distributed once with an upward orientation and once with a downward orientation.
In the course of further measurements, however, the peak was always upwardly-oriented. A systematic error must therefore have been present. Because the error only occurred in green vegetation, however, the cause could only be chlorophyll fluorescence.
In biology, chlorophyll fluorescence is used to characterize the state of the photosynthesis apparatus of plants. Up to now, active measurement methods have generally been used to measure the fluorescent light emitted by some materials, particularly green plant parts, under daylight conditions. In these methods, the material to be tested is irradiated with a pulsed or modulated light source in addition to solar illumination. The fluorescent light additionally generated by this light is detected with the aid of the lock-in measuring technique. For short distances and point measurements, LEDs are used as the light sources (refer to the periodical “Rev. Sci. Instrum.” 1975, Vol. 46, No. 5, pp. 538 through 542).
LASERS are used for larger distances and imaging detection (refer to the journal “Remote Sensing of Environment” 1994, No. 47, pp. 10 through 17). In these active methods, however, only the additionally-generated fluorescent light, and not be fluorescent light caused by solar irradiation, is measured. Hence, information can only be obtained regarding the fluorescence quantum yield.
SUMMARY OF THE INVENTION
It is the object of the invention to further develop a method of deriving sunlight induced fluorescence from radiance measurements, without using an additional light source, and to provide devices for carrying out method of detecting sunlight induced fluorescence, particularly chlorophyll fluorescence of green plants, from a distance of several meters, and for providing on imaging detection of fluorescence, especially chlorophyll fluorescence of green plants.
Thus, a method of deriving the fluorescence from a reflection signals was development. This method is described below.
The present invention provides a method and devices for carrying out a method of detecting sunlight induced fluorescence, especially chlorophyll fluorescence of green plants, from a distance of several meters, and for imaging detection of fluorescence.
The method of the invention is based on the “filling” of broadband atmospheric absorption bands in the reflection signals with fluorescent light. The starting point is that the radiance L
&ggr;
at the detector is formed as follows:
L
80
=(
R·L
&lgr;
0
+L
fluorescence
)·
T
&lgr;
+L
&lgr;
path
(1)
Here R represents the unknown reflectance factor of the material to be tested, which is assumed to be wavelength-independent in the tested spectral range. The radiance of the sunlight at the object is represented by L
&lgr;
0
; L
fluorescence
represents the fluorescence radiance, which is also assumed to be wavelength-independent; T
&lgr;
represents the transmission factor of the atmosphere between the object and the sensor; and L
&lgr;
path
represents the radiance of the so-called “path light”, i.e. the light scattered directly to the sensor from the atmosphere between the object and the sensor.
The radiances for a least two wavelengths located closely together, namely a radiance inside an atmospheric absorption band and a further radiance lying outside of the atmospheric absorption band, are measured, so the wavelength independence of the reflection factor R and the fluorescence radiance L
fluorescence
is assured.
To detect the chlorophyll fluorescence at 762 nm, a distance between the two measured wavelengths of, for example, up to 10 nm is possible. Knowledge of the exact position of the wavelengths is not critical. The only condition that must be met for solving the resulting system of equations is different radiances of the illumination for the different wavelengths. This condition is met in the range of the atmospheric absorption bands of water vapor and molecular oxygen between 660 nm and 1000 nm.
In most cases, the prerequisite of a wavelength-independent reflection factor R and a wavelength-independent fluorescence radiance L
fluorescence
can be circumvented by an advantageous modification of the method of the invention, in which a measurement is taken on both sides of the absorption band instead of one measurement being taken outside of the absorption band. In this way, for the same wavelength, two measurements having very different incident radiances are obtained, and the fluorescence radiance can therefore be determined.
In comparison to the fluorescence-measurement methods used up to now, the advantages of the invention are that an additional light source can be omitted, and it is therefore possible to detect the sunlight induced fluorescence of larger surfaces. In addition, fewer requirements are placed on the detection system with respect to spectral resolution.
Hence, a spectral resolution of, for example, 10 nm is sufficient in the range of the O
2
A absorption band around 762 nm. This is turn permits the use of less-sensitive detectors, or measurements over larger distances. Moreover, it is possible to take a fluorescence measurement in the spectral range of 650 nm to 800 nm, which is important for a detection of the chlorophyll fluorescence.
In accordance with an advantageous embodiment of the invention, image points of non-fluorescent objects are used to determine the radiance conditions on the ground, and the influence of the atmosphere, for deriving sunlight induced fluorescence light from image data obtained with an air- or spaceborne imaging spectrometer.
REFERENCES:
patent: 3598994 (1971-08-01), Markle
patent: 4671662 (1987-06-01), Zupanick et al.
patent: 4708475 (1987-11-01), Watson
patent: 5062713 (1991-11-01), Farquharson et al.
patent: 5298741 (1994-03-01), Walt et al.
patent: 5567947 (1996-10-01), Kebabian
McFarlane et al., “Plant stress detection by remote measurement of fluorescence”,Applied Optics,vol. 19 No. 19, pp. 3287-3289, (1980).
Browdy and Niemark
Dutsches Zentrum fur Luft-und Ramfahrt E.V.
Epps Georgia
Hanig Richard
LandOfFree
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