BRIGHTNESS AND OPACITY
Brightness measures the amount of light reflected back from the paper. It is measured on a scale of 0 – 100, 100 being the brightest. The higher the brightness of the sheet, the more vibrant your images will appear.
When it comes to printing, opacity determines how much ink shows through on the reverse side of the sheet. The opposite of opaque is transparent, so the higher the opacity of a sheet, the less show through.