The IOL Master V 3.1 added such features to the industry standard unit as "white-to-White", a key parameter for the Holladay 2 formula, as well as improvement of K readings, measurement in both + and - cylinders and several other improvements in areas such as data transfer, data import, etc. This was the latest software prior to a major hardware upgrade in which a faster processor was needed for V4 and V5 using Windows XP.