Self-reversed line shapes have been observed in the low temperature phosphorescence spectrum of 1,4-dibromonaphthalene crystals when the crystals are illuminated by a collimated laser beam. Self-reversal is a consequence of reabsorption of the phosphorescence emission. The precise shape of the self-reversed lines is a very sensitive measure of small changes in the unreabsorbed emission spectrum with respect to the absorption spectrum and energy shifts as small as 1% of the linewidth can be measured. For dibromonaphthalene we attribute these changes to macroscopic variations in strain within the crystal. Numerical calculations of the line shapes based on a simple but realistic model are compared with the experimental line shapes.