Contd...

These are computed as follows:

The difference of heights between two points on adjacent fringes of an interferogram corresponding to a growth hillock or dissolution valley is given by

If the total variation in the crystal thickness normal to the crystal face is in time , the normal average growth rate is given by

where. The symbol is the total number of fringes that have crossed the point of observation on the interferogram. Thus the normal average growth rate becomes

If is the physical distance between the two points lying on adjacent fringes, the slope of the hillock is given by

The normal growth rate and the slope of the dislocation hillock are related to the tangential step velocity as follows:

Thus by calculating and , the tangential step velocity is obtained.