The out-of-plane displacement field at the crack tip region of a compact tension specimen is measured by stereo vision and used to generate a shadow spot on the virtual screen placed behind the specimen by using the mapping equation of reflected caustics. The J values are then computed using the maximum transverse diameter of the generated shadow spot. In the experiment, the in-plane displacement data are also recorded to obtain J measurements along contours surrounding the crack tip. Results indicate that for crack extensions up to 0.275 mm, the J values evaluated using the generated shadow spot are in good agreement with the measurements from the in-plane displacement data and the Merkle-Corten formula. However, the J values obtained using in-plane displacement data are still close to the Merkle-Corten results, even when the crack growth of Δa≈4.0 mm is reached.