The contrast ratio is the difference between the Background Ra and Legend Ra:
Estimated Contrast Ratio = Largest Estimated RA/Smallest Estimated RA where
- Largest Estimated Ra is the bigger of the two Estimated Ra values.
- Smallest Estimated RA is the smaller of the two Estimated RA values (not necessarily Background or legend just whichever is smaller)
If the value is 0 the contrast ratio is empty.
If Estimated Legend Ra or Estimated Background Ra is empty the Estimated Contrast Ratio is empty.