1. Thermal Oil Immersion & Match Point ($T_m$)
In automated Glass Refractive Index Measurement (GRIM3), freshly crushed glass fragments ($100 - 300\text{ }\mu\text{m}$) are immersed in calibrated silicone oil B whose refractive index $n_{\text{oil}}(T)$ decreases linearly with increasing temperature $T$ on a Mettler hot stage ($dn/dT \approx -4.0 \times 10^{-4}\text{ }^\circ\text{C}^{-1}$):
$$n_D^{20} = a_0 + a_1 \cdot T_{\text{match}}$$
At the match temperature $T_{\text{match}}$ (monitored at $\lambda = 589.3\text{ nm}$, Sodium D line), phase contrast between the glass edge and silicone oil reaches a minimum (Becke line disappearance).
2. Pre- and Post-Annealing $\Delta RI$ Discrimination
When questioned ($Q$) and known ($K$) float glass fragments exhibit indistinguishable initial refractive indices ($n_i$), laboratory muffle-furnace annealing ($550^\circ\text{C}$ for 30 min, cooled at $2^\circ\text{C/min}$) relieves compressive thermal tempering stress:
$$\Delta RI = n_{\text{annealed}} - n_{\text{initial}}$$
- Tempered automotive side/rear glass: Exhibits large positive $\Delta RI \ge +0.00100$ ($+100 \times 10^{-5}$).
- Annealed architectural float glass: Exhibits minimal shift $\Delta RI \le +0.00030$.