Optical sign: in a crystal, the type of Birefringence.
- The Optical Sign is positive (+) when the variable Extraordinary Ray ε has Refractive Index n greater than that of the constant Ordinary Ray ω (nε > nω);
- The Optical Sign is negative (-) when the constant Ordinary Ray ω has Refractive Index n greater than that of the variable Extraordinary Ray (nω > nε).
- The Optical Sign is positive (+) when the Refractive Index n of the β Ray (intermediate) is closer to the Refractive Index n of the α Ray (numerically lower), that is, when it results in: (γ – β) > (β – α), where γ is the numerically higher Refractive Index;
- The Optical Sign is negative (-) when the Refractive Index n of Beam β (intermediate) is closer to the Refractive Index n of Beam γ (numerically higher), or when it results in: (γ – β) < (β – α), where α is the numerically lower Refractive Index.
