What is Rd and +b in cotton and its definition?

Both Rd and +b are the color grade parameters for cotton. Rd describes the reflectance in cotton. Higher rd indicates the brightness in cotton while lower rd reflects the dullness or grey look in cotton. Cotton having higher rd is considered the better cotton and it also adds to its value and price.
While +b in cotton indicates the yellowness in cotton. Both these units are tested or measured on fiber testing machine HVI (High Volume Instrument) Earlier Fibro graph machine was used to test the cotton results but now a days majority of labs and mills use HVI machine with latest features.