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the entrance point of the fiber-optic cable in particular needs to be flexible, yet stable. Furthermore, it is important that the fiber-optic cable can be stored securely
during transportation to avoid damaging the fibers and that the instrument is properly ventilated to avoid overheating. Changing the orientation of the fiber-optic
cable changes the amount of light reaching the detector. Thus, it is good practice to
ensure that the entire setup can be assembled easily in the same way every time. For
spectrometers that are being used outside, the exterior should be made of materials
that do not easily overheat when exposed to sunlight; polymeric surface films that
provide radiative cooling (Zhai et al. 2017) could reduce the problem of overheating.
Often it is easier to collect data with handheld spectrometers in two-person
teams, one person carrying the instrument and measuring and the other operating
the computer and checking the data. However, it can be advantageous to have a
system in place allowing one person to operate the spectrometer alone when needed.
Vendor trays are a good option for carrying laptops while taking measurements.
Small handheld devices can be very useful for collecting data, but their small screens
make data checks difficult, and it can be impractical to name and rename files on
small devices. Instrument software using voice control would be an advantage in
this regard. Furthermore, when one person is operating a portable spectrometer, the
white reference needs to be securely stored and in reach of the user. It is also important that backpacks fit comfortably, which means adjustable straps, cushioning, and
ventilation. As with leaf-level spectrometers, instrument and light source should be
switched on at least 15–30 minutes before data collection to allow the instrument to
warm up and stabilize. Like other passive RS systems, handheld spectrometers
should ideally be operated under stable illumination conditions, which is rarely possible. However, clear sky and no haze are a good place to start. Reference measurements (white reference and, if needed, dark current) should be taken before the first
measurement and whenever illumination or temperature changes (after breaks,
when adjusting the backpack, changing the sampling position, etc.). Again, it is
good practice to take references at regular intervals, e.g., every 10 minutes, because
of changing solar angle, ambient temperature, and sky conditions and to take additional references when needed. During measurements, the entrance optic of handheld systems should be positioned as far away from the body as possible (i.e., by
stretching out the arm holding the fiber cable) to avoid measuring one’s own shadow.
In flat terrain, measurements should be taken in nadir position (i.e., with the fiber
optic pointing directly down). In steep terrain, it is advantageous to point the fiber
optic at a right angle toward the slope. It is good practice to wear nonreflective
clothing and shoes in dark shades as stray reflected light off clothing can affect the
spectral measurements.
At midlatitudes, the best time for measurements, given clear sky, is around solar
noon (i.e., when the sun is at its highest point in the sky), whereas in the Arctic and
the Tropics, time of day is usually less important than sky conditions. Generally, at
midlatitudes, spectral measurements should be taken when the sun elevation angle
(the angle measured from the horizon to the center of the sun) exceeds 45°; angles
exceeding around 50° are better. For example, a good time window for canopy spectroscopy (sun angle >45°) for Minneapolis, MN, on June 21 would be from around
A. K. Schweiger
the entrance point of the fiber-optic cable in particular needs to be flexible, yet stable. Furthermore, it is important that the fiber-optic cable can be stored securely
during transportation to avoid damaging the fibers and that the instrument is properly ventilated to avoid overheating. Changing the orientation of the fiber-optic
cable changes the amount of light reaching the detector. Thus, it is good practice to
ensure that the entire setup can be assembled easily in the same way every time. For
spectrometers that are being used outside, the exterior should be made of materials
that do not easily overheat when exposed to sunlight; polymeric surface films that
provide radiative cooling (Zhai et al. 2017) could reduce the problem of overheating.
Often it is easier to collect data with handheld spectrometers in two-person
teams, one person carrying the instrument and measuring and the other operating
the computer and checking the data. However, it can be advantageous to have a
system in place allowing one person to operate the spectrometer alone when needed.
Vendor trays are a good option for carrying laptops while taking measurements.
Small handheld devices can be very useful for collecting data, but their small screens
make data checks difficult, and it can be impractical to name and rename files on
small devices. Instrument software using voice control would be an advantage in
this regard. Furthermore, when one person is operating a portable spectrometer, the
white reference needs to be securely stored and in reach of the user. It is also important that backpacks fit comfortably, which means adjustable straps, cushioning, and
ventilation. As with leaf-level spectrometers, instrument and light source should be
switched on at least 15–30 minutes before data collection to allow the instrument to
warm up and stabilize. Like other passive RS systems, handheld spectrometers
should ideally be operated under stable illumination conditions, which is rarely possible. However, clear sky and no haze are a good place to start. Reference measurements (white reference and, if needed, dark current) should be taken before the first
measurement and whenever illumination or temperature changes (after breaks,
when adjusting the backpack, changing the sampling position, etc.). Again, it is
good practice to take references at regular intervals, e.g., every 10 minutes, because
of changing solar angle, ambient temperature, and sky conditions and to take additional references when needed. During measurements, the entrance optic of handheld systems should be positioned as far away from the body as possible (i.e., by
stretching out the arm holding the fiber cable) to avoid measuring one’s own shadow.
In flat terrain, measurements should be taken in nadir position (i.e., with the fiber
optic pointing directly down). In steep terrain, it is advantageous to point the fiber
optic at a right angle toward the slope. It is good practice to wear nonreflective
clothing and shoes in dark shades as stray reflected light off clothing can affect the
spectral measurements.
At midlatitudes, the best time for measurements, given clear sky, is around solar
noon (i.e., when the sun is at its highest point in the sky), whereas in the Arctic and
the Tropics, time of day is usually less important than sky conditions. Generally, at
midlatitudes, spectral measurements should be taken when the sun elevation angle
(the angle measured from the horizon to the center of the sun) exceeds 45°; angles
exceeding around 50° are better. For example, a good time window for canopy spectroscopy (sun angle >45°) for Minneapolis, MN, on June 21 would be from around
A. K. Schweiger