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mays L.), sorghum (Sorghum bicolor L.), soybean (Glycine max Merr.), and wheat (Triticum aesth'um L.) plus 40 
agricultural soils. As crop residues decomposed, their fluorescence values approached the fluorescence of the soils. 
Moisture quenched fluorescence, but the relative difference in fluorescence between crop residues and soil remained 
fairly constant. Greater than 90% of the crop residues less than 2 years old could be discriminated from 35 of the 
40 dry soils and 39 of 40 wet soils using fluorescence. Daughtry et. al. (1993) concluded that fluorescence 
techniques were better suited for discriminating soils and residues than reflectance techniques. 
Our previous research involved non-imaging techniques to measure fluorescence. Advances in low light 
imaging technology convinced us that it is feasible be possible to capture an image of the fluorescence from crop 
residues. Video imaging of crop residue fluorescence could provide an intuitive understanding as to the amount of 
residue cover which was not possible using traditional non-imaging techniques. Our objective was to demonstrate 
the feasibility of fluorescence imaging to quantify crop residue cover. 
2 - MATERIALS AND METHODS 
Video images were acquired with a Xybion Intensified Multispectral Camera 1 (Model 201, Xybion Electronics Corp.. 
Cedar Knoll, New Jersey, USA). This camera has a filter wheel, located in front of the intensified camera assembly 
that contains six 1-inch (25 mm) optical filters. The filter wheel can run synchronously with the video scan rate to 
provide a repeating 6-image sequence or the wheel can be locked at any one filter position to provide continuous 
imaging in a selected spectral region. For the fluorescence images, we operated the camera in the locked filter mode 
and integrated over 6-120 video fields (0.1 -2.0 sec). For the reflectance images, integration was not necessary' and 
we operated either in the run mode or the locked mode depending on our objectives. Video images were grabbed 
using the Xybion Image Capture and Analysis System (X1CAS) on a PC microcomputer. 
The filter wheel contained the following band pass interference filters: 453-488 nm, 535-570 nm. 650-685 
nm, 735-750 nm. 775-795 nm, and 840-870 nm. Although the 453-488 nm band was not optimal for measuring the 
broad band fluorescence of crop residues (Daughtry et al. 1993), it was satisfactory for this proof-of-concept 
demonstration. 
The video camera was mounted on a frame 0.7 m 
above the surface (Fig 1). The ultraviolet illumination 
source was four 12-volt, 6-Watt, longwave UV lamps 
(Model ML49, UVP, Inc., San Gabriel, California, USA) 
arranged in a square pattern 0.3 m above the soil surface. 
The lamps were angled toward the center to provide nearly 
uniform illumination over the scene. Radiation from the 
lamps was filtered with Schott UG-1 glass to minim ize 
radiation greater than 400 nm that would interfere with 
measurements of fluorescence. Visible and near infrared 
illumination was provided by two 120-volt, 60-Watt 
incandescent lamps positioned to provide nearly uniform 
illumination over the scene. 
To calibrate the video data, three aluminum disks 
of known area that were painted white were placed on a 
gray background. The number of pixels occupied by the 
disks were determined by e xaminin g a histogram of the 
video scene. This distribution was distincdy bimodal, i.e., 
bright disks on a dark background. The area per pixel was 
calculated and later used to determine the fraction of the 
scene covered by residue. 
A series of test scenes was created using two 
distincdy different agricultural soils and weathered soybean 
stems collected 8 months after harvest. The soybean stems 
were mottled with dark-colored areas caused by microbial 
colonization and were selected as a representative of crop 
residues that fluoresce moderately (Daughtry et al., 1993). 
The projected area of the soybean stems was measured with 
an area meter (LI-3100, LICOR, Inc., Lincoln, Nebraska, 
Figure 1 Xybion intensified multispectral camera 
mounted over a tray of soil and soybean residue. 
Four UV lamps with filters illuminated the scene. 
1 Company and trade names are given for the benefit of the reader and do not imply any endorsement of 
the product or company.