The optic normal or biaxial flash figure is not useful for determining optic sign, because the 2V cannot be determined and it cannot be distinguished from a uniaxial flash figure.

Estimates of 2V angles can be obtained from a centred Bxa figure, for 2V < 40°, and from a centred optic axis figure for 2V = 0 to 2V = 90°, (See Figure 7.32. in Nesse).