Residual stress in GaAs-wafers was investigated on different length scales by rapid full wafer imaging and by microscopic imaging of dislocation cells, using photoelastic homodyne techniques. These non-contact and non-destructive defect and stress imaging methods will be described in detail. In connection with in situ calibration of the photoelastic SIRD™ measurement system absolute shear stress values can be extracted. Local stress fields imaged in μm-scale by the photoelastic instrument SIREX™ show the defect arrangement in cell patterns and allow to characterize the local stress enhancement. The advantages and limits of the vertically integrated photoelasticity measurement employed in both systems will be discussed.