for the piezospectroscopic coefficient was used here, which is sufficient for imaging and following the spatial and temporal changes in stress. Stress fluctuations in homogeneous areas yield an estimated uncertainty on relative values of stress of 40 MPa from pixel to pixel, and less than 15 MPa for 3 × 3 pixel squares, i.e. over about 15 microns for low resolution images, and 6 or 3 microns for higher resolution images.