The principe of the diffracting gauge is based on the farfield diffraction spectrum of a slit. In extensometry this gauge permits to measure easely and cheaply strain components with great sensibility and fidelity in time.
Jean P. L. Ebbeni,
"The Diffracting Gauge In Extensometry", Proc. SPIE 0136, 1st European Conf on Optics Applied to Metrology, (18 April 1978); doi: 10.1117/12.956168; https://doi.org/10.1117/12.956168