D65 - Brightness (%) ISO 2470
ISO - Brightness (%) ISO 2471-1
Pulp is never entirely white but has a natural yellow colour because of the wood itself. Because of that the brightness is measured.
D65 Brightness is measured by illuminating a paper sample with an instrument that emits D65 light, i.e. a light that is related to daylight with the same amount UV light as in a winter day. The reflected light is filtered through a glass filter that only allows blue light to leave. The brightness is measured by indicating the amount of incident blue light that is reflected from the paper. With bleaching the yellowness is reduced and blue will be more expressed. The wavelength of the blue light is 457 nm.
A related term to D65 Brightness is ISO Brightness. In this method the paper sample is illuminated with the light of a bulb, instead of D65 light. The method is used if the paper sample not contains any fluorescent compounds.