  The optical response of hierarchical materials is convoluted, which hinders
their direct study and property control. Transparent wood (TW) is an emerging
biocomposite in this category, which adds optical function to the structural
properties of wood. Nano- and microscale inhomogeneities in composition,
structure and at interfaces strongly affect light transmission and haze. While
interface manipulation can tailor TW properties, the realization of optically
clear wood requires detailed understanding of light-TW interaction mechanisms.
Here we show how material scattering and absorption coefficients can be
extracted from a combination of experimental spectroscopic measurements and a
photon diffusion model. Contributions from different length scales can thus be
deciphered and quantified. It is shown that forward scattering dominates haze
in TW, primarily caused by refractive index mismatch between the wood substrate
and the polymer phase. Rayleigh scattering from the wood cell wall and
absorption from residual lignin have minor effects on transmittance, but the
former affects haze. Results provide guidance for material design of
transparent hierarchical composites towards desired optical functionality; we
demonstrate experimentally how transmittance and haze of TW can be controlled
over a broad range.
