The activation behavior of nitinol actuators is typically examined in test rigs equipped with sensors for measuring mechanical and electrical parameters. However, these test rigs allow for measurements only under laboratory conditions; assessing the actuator behavior in real-world applications is not feasible. Therefore, a non-invasive, optical measurement method for relative strain detection of shape memory actuators is presented in this work. It is based on the diffraction of a laser beam by an optical grating applied to the surface of the actuator. While the grating period changes with the strain state of the actuator during activation, a camera captures the interference pattern to detect the diffraction angle of the maxima spots.