This is a linear transformation of Yxy, in an attempt to produce a chromaticity diagrams in which a vector of unit magnitude (difference between two points representing two colours) is equally visible at all colours. Y is unchanged from XYZ or Yxy. Difference non-uniformity is reduced considerably, but not enough. A third coordinate, w, can also be defined but is redundant.
u = 2x / (6y - x + 1.5) = u'
v = 3y / (6y - x + 1.5) = (2/3)v'