Lg. Thygesen et So. Lundqvist, NIR measurement of moisture content in wood under unstable temperature conditions. Part 1. Thermal effects in near infrared spectra of wood, J NEAR IN S, 8(3), 2000, pp. 183-189
Thermal effects in near infrared (NIR) spectra of solid wood samples of Nor
way spruce (Picea abies Karst,) were studied between -20 and +25 degrees C,
When the temperature increases, the two main hydroxyl absorbance bands at
approximately 1450 and 1930 nm (here denoted bands A and B) shift by about
0.4 nm degrees C-1 towards shorter wavelengths. For sapwood, there is a mar
ked peak shift of both peaks of about 25 nm (band A) and 20 nm (band B) whe
n part of the moisture in the wood freezes/thaws at 0 degrees C. For heartw
ood, which has a much lower moisture content, the shifts are only about 5 a
nd 3 nm, respectively. If ice crystals form on the sample surface, a light
scattering effect is also seen in the spectra, causing a multiplicative dif
ference between spectra of samples with and without surface ice. It was dem
onstrated that it is possible to predict the moisture content even of froze
n wood using partial least squares (PLS) regression models. It is, however,
necessary to take the temperature into account, especially if the temperat
ure fluctuates around 0 degrees C.