The axial nonuniformity is calculated as follows:

Light output per unit pathlength in the crystal (s/d) is plotted as a function of z-position in the crystal. The front face of the crystal corresponds to z=0 cm, while the back of the crystal is z=22 cm. The nonuniformity constant is equal to the slope of this scatter plot, divided by the average value of s/d in the region of interest. Using this definition, a negative axial nonuniformity corresponds to a decreasing value of s/d from z=0 to s=22 cm.

The transverse nonuniformity is calulated as follows:

"s/d" is plotted as a function of x-position in the crystal. Looking at the back face of the crystal, the positive x-axis points to the right. The nonuniformity constant is equal to the slope of the scatter plot, divided by the average value of s/d in the region of interest. A negative transverse nonuniformity on the right side of the crystal corresponds to a decreasing value of s/d from the center to the edge of the crystal, while a positive transverse nonuniformity on the left side of the crystal means that s/d increases toward the center.