Practical recommendations and limitations for pushbroom hyperspectral imaging of tree stems

In this short communication, we present a pilot study testing a new close-range sensing technology - a portable, pushbroom hyperspectral camera - in varying field conditions in forests. We evaluate how measurement conditions affect the in situ collection of stem bark spectra. In situ spectral libraries of woody elements are needed in, e.g., physically-based remote sensing applications, biodiversity mapping, and 3D vegetation modeling. Recent technological advancements bring portable and close-range capable sensors, such as small pushbroom imaging spectrometers, for consumer and research use. However, it is important to investigate the strengths and limitations of sensors utilizing pushbroom technology. Spectral measurements under forest canopies are challenging due to varying illumination conditions, which can have a significant effect on the quality of the data. We acquired hyperspectral reflectance images of Norway spruce (Picea abies (L.) Karst), Scots pine (Pinus sylvestris L.), and silver birch (Betula pendula Roth) stem bark directly in the forest. For each tree we collected reflectance images at 30-minute intervals throughout a day from a fixed view angle. The most significant change in the measured spectra occurred due to spatially varying irradiance between the white reference panel and the bark surface. The spatial variation of irradiance had the largest effect on data quality in visible and red-edge regions, and the smallest in near-infrared. In non-diffuse conditions, changes in irradiance were often unpredictable as clouds or canopy elements moved in and out of the direct solar beams. Diffuse overcast days with clouds can extend the time window for measurements, making it a practical choice for acquiring hyperspectral images of stem bark. We concluded that with a well-planned measurement set-up it is possible to improve the precision of in situ collected spectra of stem bark.