Brightness and whiteness are two very different ways to measure the appearance of White paper.
Brightness is a traditional measure that still appears on most packaging in the U.S. The TAPPI standard (GE brightness) measures the ability of a paper to reflect blue light. On a scale of 0 to 100, the higher the number, the brighter the sheet.
Whiteness measures paper in the same way the eye sees it. As you’ll remember, light is actually made up of all colors combined. When light strikes an object, the object absorbs some colors and reflects others. Whiteness is really the absence of color. The higher the CIE Whiteness the better the contrast of the printed colors. (PaperSpecs PRO members, you can find out more here.)