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Typically, measurements start with assembling the spectrometer, fiber-optic
cable, light source, and leaf clip or integrating sphere, turning on the instrument and
light source, and giving them some time (e.g., around 15–30 minutes) to warm up
and stabilize. Meanwhile, the user interface can be connected to the spectrometer
and folders can be organized. For instruments that are operated from a bench, it is
important to find a stable position; ideally, neither the instrument nor the fiber-optic
cable should be moved between measurements. The same applies to instruments
operated from a backpack; the setup should be as consistent and stable as possible.
To avoid damage to the fibers, it is important to avoid bending fiber-optic cables,
including at connections between the fibers and the instrument. Generally, it is good
practice to use and transport fiber-optic cables as stretched out as possible. A good
option is coiling longer fiber-optic cables loosely while allowing for enough play at
connections between cable and instruments and between cable and leaf clip/integrating sphere to keep angles around 180°. It is important to keep the fiber-optic
cable away from branches when walking through vegetation, because it can be
snagged and broken.
One way to test if the instrument has warmed up and is stabilized is to measure
an invariant surface, such as a reflectance standard. Before starting sample measurements, it is time to take reference measurements, including measurements of socalled dark current (the background signal from the instrument), which some
instruments take automatically, and white references (materials that approximate
Lambertian surfaces, which reflect light at all angles equally or are perfectly diffuse). Dark current measurements correct for instrument noise, while white reference measurements determine the light entering an instrument and allow the
calculation of reflectance (and transmittance). White references are usually made of
polytetrafluoroethylene, better known by its commercial name Spectralon. They are
available in different sizes and shapes; some leaf clips have built-in white references, but they are difficult to keep clean. It is important to keep white references as
clean as possible, because even small traces of dirt and oil affect the spectral signal.
Reference panels should only be held by their sides (touching the surface should be
avoided), and they should be covered after each measurement. Depending upon
usage and field conditions, frequent cleaning according to the manufacturer’s
instructions may be necessary.
Generally, a white reference reading should be made before the first measurement and whenever conditions (e.g., temperature, the arrangement of the fiber-optic
cable, instrument, or lamp settings) change. However, it might be more practical or
accurate to take white reference measurements at regular intervals, such as every
10 minutes, for each sample, or after a certain number of measurements, and to take
additional measurements when needed. It is good practice to plot the reflectance
spectrum of each white reference measurement and to save the spectrum. The
reflectance of the white reference should be around 100% for all wavelengths except
for the beginning and end of the spectrum, which are generally noisier; deviations
from 100% or excessive noise can indicate a dirty panel, or issues with the cooling
system, lamp, instrument setup, or a low battery. Measurements should be taken
immediately after sample collection, because leaves dry out quickly. For most
15 Spectral Field Campaigns: Planning and Data Collection
Typically, measurements start with assembling the spectrometer, fiber-optic
cable, light source, and leaf clip or integrating sphere, turning on the instrument and
light source, and giving them some time (e.g., around 15–30 minutes) to warm up
and stabilize. Meanwhile, the user interface can be connected to the spectrometer
and folders can be organized. For instruments that are operated from a bench, it is
important to find a stable position; ideally, neither the instrument nor the fiber-optic
cable should be moved between measurements. The same applies to instruments
operated from a backpack; the setup should be as consistent and stable as possible.
To avoid damage to the fibers, it is important to avoid bending fiber-optic cables,
including at connections between the fibers and the instrument. Generally, it is good
practice to use and transport fiber-optic cables as stretched out as possible. A good
option is coiling longer fiber-optic cables loosely while allowing for enough play at
connections between cable and instruments and between cable and leaf clip/integrating sphere to keep angles around 180°. It is important to keep the fiber-optic
cable away from branches when walking through vegetation, because it can be
snagged and broken.
One way to test if the instrument has warmed up and is stabilized is to measure
an invariant surface, such as a reflectance standard. Before starting sample measurements, it is time to take reference measurements, including measurements of socalled dark current (the background signal from the instrument), which some
instruments take automatically, and white references (materials that approximate
Lambertian surfaces, which reflect light at all angles equally or are perfectly diffuse). Dark current measurements correct for instrument noise, while white reference measurements determine the light entering an instrument and allow the
calculation of reflectance (and transmittance). White references are usually made of
polytetrafluoroethylene, better known by its commercial name Spectralon. They are
available in different sizes and shapes; some leaf clips have built-in white references, but they are difficult to keep clean. It is important to keep white references as
clean as possible, because even small traces of dirt and oil affect the spectral signal.
Reference panels should only be held by their sides (touching the surface should be
avoided), and they should be covered after each measurement. Depending upon
usage and field conditions, frequent cleaning according to the manufacturer’s
instructions may be necessary.
Generally, a white reference reading should be made before the first measurement and whenever conditions (e.g., temperature, the arrangement of the fiber-optic
cable, instrument, or lamp settings) change. However, it might be more practical or
accurate to take white reference measurements at regular intervals, such as every
10 minutes, for each sample, or after a certain number of measurements, and to take
additional measurements when needed. It is good practice to plot the reflectance
spectrum of each white reference measurement and to save the spectrum. The
reflectance of the white reference should be around 100% for all wavelengths except
for the beginning and end of the spectrum, which are generally noisier; deviations
from 100% or excessive noise can indicate a dirty panel, or issues with the cooling
system, lamp, instrument setup, or a low battery. Measurements should be taken
immediately after sample collection, because leaves dry out quickly. For most
15 Spectral Field Campaigns: Planning and Data Collection