## 2 A Review of the Steps Necessary for Collecting Science Quality Spectral Data in the Field  
A number of authors have suggested a variety of procedures and considerations for collecting spectral data in the field [2; 3; 4; 5; 6; 7; 8; 9]. The recommendations which follow are drawn from those authors and also reflect our own combined 110 years of field experience:  
1. Configure the instrumentation slowly and carefully; focus on every step in the set-up. Test each piece of equipment before beginning the data-collection exercise. Carefully document instrument conditions (orientation, environment, _etc._) and settings either manually or, if able, automatically as part of the data record. 2. Begin collecting data no earlier than 2.5 h before solar noon; stop no later than 2.5 h after local solar noon. 3. Clear skies are best for collecting spectral data, especially if one is using a single field radiometer and calibrating with a bright-white reference surface. It is sometimes necessary to collect data under diffuse conditions, but do so only when you are using a spectroradiometer system (or systems) that allow for the simultaneous measurement of the downwelling (incident) irradiance and the upwelling radiance from the target of interest. You may be able to execute target scans during periods when the overhead sky is partly cloudy by waiting for intervals when the direct-beam conditions prevail, but be aware that reflection from bright-white clouds may actually increase the intensity of the incoming radiation. 4. In the case of acquiring spectral data over a vegetated surface from a nadir position (the usual approach), orient the field radiometer so that you have a field of view at the top of the canopy of at least one meter (and more is generally better). It is important to collect spectral data from an area large enough to be truly representative of the target vegetation, and not just one plant, or a portion of one plant. In fact, if the fraction of vegetative cover is not 100% and the sensor field of view is inadequate, you run the risk of scanning the soil background, and even measuring no vegetation at all. Therefore, it is imperative that one measure the exact distance from the top of the vegetation canopy to the sensor optic, and calculate the precise field of view at the top of the target location. 5. With regard to the use of a single field radiometer and a bright-white reference surface for calibration on a clear day, it is advisable is to scan that panel at least once every 20 min. When collecting spectral data when the incoming solar energy is being diffused by clouds from time to time, one must calibrate even more frequently. The goal is to scan both the target (e.g., a vegetation canopy) and the calibration panel under the exact same illumination conditions. The frequency of calibrations can be minimized when using two field radiometers (or some similar derivation), with one looking upward and one looking downward, and operating them concurrently. This arrangement, however, is not a trivial matter, and it requires considerable work to precisely match the two sensors. 6. A bright-white (nearly 100% and diffuse) reference surface should be used to calibrate when making spectral measurements of vegetation. The reference panel should be in a level position during scanning, and one should watch for and eliminate shadows which may be cast on the surface of the panel. One may have to either increase or decrease the distance between the sensor optic and the panel itself to avoid shadowing. Where possible, attach the calibration panel to a tripod, and once it is positioned beneath the sensor optic, step away at least 5 m during scanning. If the calibration panel must be held by an individual, instruct that person to hold the panel level above his/her head. In any case, be sure the exact same procedure is followed with every calibration scan; this includes people standing nearby being in the same position each time.