5.3.8 Optical PropertiesParticles with diameters less than one tenth of visible light wavelengths are not expected to cause light scattering [341]. CNF are typically in this size range and therefore, unless significant nanofiber agglomeration occurs, CNF suspensions are highly transparent. Therefore, films of 20 μm thick produced with TOCNs obtained from hardwood and softwood have 78% and 90% light transmittance at 600 nm, respectively [166]. In addition, light transmittance at 600 nm of 20 μm thick films could be improved from 61% to 82% by subjecting the initial CNF gel suspension to three additional homogenization passes before film preparation [318]. Carboxymethylated CNFwith low hemicellulose content tend to form large fiber fragments and aggregates of micron size, which can compromise film transparency.Some researchers found that film surface roughness significantly reduced the light transmittance of nanocellulose films. When film surfaces were polished or impregnated with an optically transparent polymer layer (e.g., using an acrylic resin) the total light transmittance couldbe increased up to 89.7% [342].BC has been presented as a promising material for optically transparent composites. Patchan and Graham [343] tried to prepare a material suitable for use on the ocular surface. For this purpose, BC in the form of hydrogel, with high transparency, high mechanical strength,biocompatibility, and wound-healing enhancement are critically important. Enough moisture on the ocular surface is also key for conjunctival and corneal wound healing. BC has been shown to be both biocompatible and biodegradable (biosorbable) in vivo [344]. Patchan and Graham [343] used cellulose materials from wood pulp, cotton, and bacteria and dissolved them in lithium chloride/N,N-dimethylacetamide to form regenerated cellulose hydrogels. They found that overnight activation time increased optical transparency of the hydrogels from 77% to 97% at 550 nm.