The present paper introduces a new method to characterise internal stresses induced by hygrothermal loads on composite laminated plates. The method consists in measuring by the fringe projection technique the full displacement field induced by temperature or moisture absorption on plates with tension-bending coupling, 0/90 unsymmetric plates for instance: internal stresses, whatever the plate typology, can be inferred by such measurements by using a classical theory of plates. When 0/90 plates are very thin, the behaviour can be rather complex, as deformations occur to behave nonlinearly with temperature or absorbed moisture, due to the presence of large displacements. We show the ability of the method to catch experimentally such a nonlinear behaviour and we compare results with a model which takes into account the effect of large displacements.