EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 267)

Measurement Definitions
Optical Modulation Analyzer 259
Measurements for Eye Diagrams
Contrast Ratio
The contrast ratio (CR) is only defined for RZ formats as the ratio between
the one level and the level between the pulses in the magnitude graph, that
is .
Crossing
The crossing percentage (NRZ formats only) is a measure of the location of
the eye crossing points relative to the separation between the one and zero
levels. Horizontal time histograms are constructed, and the histogram
having the smallest variance determines the crossing point.
CR E
one
E
contrast
 =
One level
Contrast level
Most narrow histogram
Crossing = x %