Brightness is defined as the reflectance of a thick stack of paper by a visible light having a mean wavelength of around 457 nm and bandwidth of about 40 nm(blue light).
TAPPI brightness also known as directional brightness is measured by illuminating the product with a fixed angle of 45◦ relative to the angle of observation [5].
Measurements were performed following TAPPI std 452 om- 08 and shown in Fig. 7.
The data show the average brightness of each coating for different coating weights.
The reflectance of blue light by the PVA coated samples is the greatest resulting in higher brightness values than base paper.
DG gum and modified DG gum amples exhibited brightness values similar to base paper,
however with higher coating weights a reduction in brightness values were observed.
This reduction can be attributed to the brown color of DG gum due to the presence of residual lignin