To determine the individual values of the two strain components by separate principal strains, a theoretical model of strain separation by oblique incidence with a new luminescent photoelastic coating configuration is presented. Considering the non-strain related refraction effect included in the total optical response at oblique incidence, the small incidence angle technique for simplifying the post-processing complexity is given. To affirm the validity of this strain separation method based on small oblique angles, the signal-to-noise ratio for providing sufficient signal intensity has been theoretically analyzed.