When compared to a point source the surface source has slightly poorer thickness uniformity. For better uniformity, one needs to decrease sample size (l) and increase distance to substrate (h). This situation calls for a possible in a bigger chamber and with better vacuum and it means that lot of evaporant is wasted. Better uniformity can also be improved by the use of multiple sources, moving substrate during deposition and using rotating crucible to reduce evaporant near center. If source and substrate are kept on same sphere surface as given in Fig. 3.26, then the equations become

and the dependency on ,or r on is avoided.