Abstract:
To derive sunlight induced fluorescence from radiance measurements, a first radiance measurement is taken inside an atmospheric absorption band, and an additional radiance measurement is taken outside of the atmospheric absorption band. Images of sunlight-induced fluorescence are obtained with the aid of a camera. Air- or spaceborne spectrometers and image points on non-fluorescent objects are used to determine the radiance conditions on the ground and the influence of the atmosphere.

Description:
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 γ  at the detector is formed as follows: 
     
       
           L   80 =( R·L   λ   0   +L   fluorescence )· T   λ   +L   λ   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 λ   0 ; L fluorescence  represents the fluorescence radiance, which is also assumed to be wavelength-independent; T λ  represents the transmission factor of the atmosphere between the object and the sensor; and L λ   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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects and the nature and advantages of the present invention will become more apparent from the following detailed description of embodiments taken in conjunction with drawings, wherein: 
     FIG. 1 is a graphical view of the invention in the form of a straight line on which radiances of non-fluorescent objects are shown, while fluorescent objects are shown in the half-plane above the straight line; 
     FIG. 2 is a schematic view of a first embodiment of a device for detecting fluorescence from a distance of only a few meters in the range of the O 2 A absorption band; 
     FIG. 3 is a graphical view of the course over time of reflected radiances measured with the aid of a spectrometer in or before the O 2 A absorption band; 
     FIG. 4 is a graphical view of the course over time of a fluorescence derived from FIG. 3 according to Eq. (2) and measured with a PAM fluorometer, with the time being shown in seconds (s) on the abscissa and the fluorescence radiance being shown in mW/(m 2 nm sr) on the ordinate; 
     FIG. 5 is a schematic view of a preferred embodiment of a device for carrying out the method of imaging detection of fluorescence; 
     FIG. 6 graph of a typical reflectance spectrum, with the wavelength being shown in nm on the abscissa and the reflection being shown on the ordinate; and 
     FIG. 7 is a graphical view showing a radiance measurements from which the reflectance spectrum in FIG. 6 is calculated by computing the quotient and subsequent multiplication. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Depending on the aerosol content, the influence of the atmosphere between the measured object and the sensor from measured distances of up to a few hundred meters can be disregarded. This means that the transmission factor T λ =1 and the path light L λ   path  are both equal to zero. The resulting evaluation equation is as follows:                L   fluorescence     =         L   1     -         L   1   0       L   2   0            L   2           1   -       L   1   0       L   2   0                   (   2   )                                
     As the evaluation equation (2) shows, a relative calibration suffices for two detectors that detect the fluorescence radiances L 1   0  and L 2   0 . If the absolute value of the fluorescence radiance is to be determined, the two detectors that detect the fluorescence radiances L 1  and L 2  must be calibrated absolutely. Otherwise, a relative calibration of the detectors with respect to one another suffices. 
     In remote sensing, the influence of the atmosphere between the object and sensor cannot be disregarded. Furthermore, in most cases it is not possible to take a direct measurement of the sunlight incident at the object. The method described below must be used. The following evaluation equation results from Eq. (1):                    k   3     ·     L   fluorescence       =       L   1     -       k   1     ·     L   2       -     k   2                           k   1     ≡         L   1   0     ·     T   1           L   2   0     ·     T   2                   (   3   )                   k   2     ≡       L   1   path     -       k   1     ·     L   2   path                             k   3     ≡       T   1     -       k   1     ·     T   2                                                    
     In FIG. 1, a radiance L 1  is shown on the ordinate, and an radiance L 2  is shown on the abscissa. The radiances of non-fluorescent objects lie on a straight line. The fluorescent objects lie in the upper half-plane. The distance in the ordinate, or L 1 , direction is equal to k 3 ·L fluorescence . If the radiance L 1  is shown above the radiance L 2  of non fluorescent objects, k 1  corresponds to the slope and k 2  corresponds to the offset section of the fitted straight lines. The fluorescence radiance of fluorescent objects can then be calculated form the above equation. 
     The value k 3  cannot be derived only from the measurements of the radiances L 1  and L 2 , however. The Fluorescence radiance can therefore be derived only in relative units. For an absolute value, however, ground measurements must be taken, or the transmission factor of the atmosphere T 1  and T 2  must be known from other measurements or simulation. 
     FIG. 2 show an embodiment of a device for executing the method of the invention with respect to a detection of fluorescence over a distance of several meters in the range of the O 2 A absorption band around 762 nm. With the aid of a optical waveguide  31 , a lower detection unit  3  in FIG. 2 records light emitted by a measured object  2  illuminated by the sun  1 . This optical waveguide  31  comprises a plurality of individual multimode fibers that are statistically distributed onto two detectors  34  and  35 . As a result, any image information is blurred and the two detectors  34  and  35  “see” the same regions of the measured object. 
     A detection unit  4  for detecting irradiated light, show near the top of FIG. 2, has basically the same design as the detection unit  3 , namely a optical waveguide  41  and two detectors  44  and  45 . In Addition, however, a diffusor  46  is disposed in front of the optical waveguide  41  for increasing its acceptance angle. 
     Interference band-pass filters  32  and  42  having a central wavelength of 752 nm and a bandwidth of 5 nm are disposed in front of the detectors  34  and  44  for the spectral selection. Interference band-pass filters  33  and  43  are likewise disposed in front of the detectors  35  and  45 , the filters having a central wavelength of 762 nm with a bandwidth of 5 nm. A precision of ±2 nm is sufficient for the position of the central wavelength and the bandwidth of the interference filters. Only the accordance of the spectral transmission characteristics of the filters  32  and  42 , sharing one pass-band and of the filters  33  and  43  sharing the other pass-band respectively is important. 
     To verify the functioning capability of this device, it was simulated with the aid of a spectrometer. The spectral selection of the channels was achieved by a grating. (In principle, any arbitrary spectrometer having a sufficient spectral resolution could be used.) The measurement was taken from a distance of about one meter. This corresponded to a measured-point diameter of 20 cm. 
     To verify that the fluorescence is actually detected, the fluorescence kinetics of the photosynthesis apparatus of a green plant (Kautsky effect) is recorded. At the same time, a currently-conventional, active measurement method, PAM fluorometry, was used to take the measurements. The measuring distance was about 1 cm, with a surface of 0,2 cm 2  being detected. 
     FIG. 3 shows the course over time of the reflected radiances of vegetation, which were measured with the aid of the spectrometer, in (L 1 ) or before (L 2 ), of the O 2 A absorption band. FIG. 4 shows the course over time of the fluorescence derived from this according to Eq. (2) and measured with a PAM fluorometer. The two measurements matched very well, despite the differently-sized measuring surface. 
     FIG. 5 schematically shows an embodiment of a device for imaging detection of fluorescence. The lower detection unit is replaced by a CCD camera  5 . To avoid the cost of a second CCD-camera, only one CCD-camera equipped with a filter wheel  6  is provided. The interference band-pass filters, not shown in detail, are accommodated on the filter wheel  6 , permitting switching between the two detection channels. 
     According to the invention, a method is created for deriving sunlight induced fluorescence from reflection measurements. Thus, it is possible to derive the fluorescence from passive measurements with simple spectrometers. This in turn permits the use of air- or spaceborne spectrometers for large-surface detection of the chlorophyll fluorescence and therefore the photosynthetic state of the vegetation. 
     The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without undue experimentation and without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. The means and materials for carrying out various disclosed functions may take a variety of alternative forms without departing from the invention. 
     Thus the expressions “means to . . . ” and “means for . . . ” as may be found in the specification above and/or in the claims below, followed by a functional statement, are intended to define and cover whatever structural, physical, chemical or electrical element or structure may now or in the future exist which carries out the recited function, whether or not precisely equivalent to the embodiment or embodiments disclosed in the specification above; and it is intended that such expressions be given their broadest interpretation. 
     LIST OF REFERENCE CHARACTERS 
       1  Sun 
       2  Measured object 
       3  Detection unit 
       31  Optical waveguide 
       32 ,  33  Interference band-pass filters 
       34 ,  35  Detectors 
       4  Detection unit 
       41  Optical waveguide 
       42 ,  43  Interference band-pass filters 
       44 ,  45  Detectors 
       46  Diffusor 
       5  CCD camera 
       6  Filter wheel