APHA Q & A

HunterLab Technical Support representatives probably answer more questions concerning the APHA color index than any other color scale or index. This Applications Note is designed to answer some of the most frequently asked questions concerning this index. For more basic information concerning the APHA index, refer to the November 16-30, 1996 Applications Note entitled "APHA."

What is the Formula for APHA?
There is no published formula for APHA. The ASTM method on APHA (D1209) describes only visual evaluation of APHA, where samples are visually compared to known standards. While instrumental determination of APHA is allowed by D1209 and D5386, no formula is prescribed by either method.

In order to allow you to determine APHA with your instrument, HunterLab uses a correlation it developed by comparing solutions of known APHA values to instrumental measurements. Since APHA is a yellowness scale, HunterLab based its correlation on yellowness index (YI E313) values. This correlation was optimized for each of its instruments that is capable of measuring APHA. These correlations are proprietary and apply only to HunterLab instruments.