What is Vitrinite reflectance, time–temperature index?

Vitrinite reflectance, $R_o$, is the percentage of incident light reflected by polished grains of vitrinite, an organic component of coal and of kerogen, measured under a microscope; it rises irreversibly with heating and records the highest thermal maturity a rock has reached. The time–temperature index (TTI) of Lopatin is the sum $\sum_n \Delta t_n\,2^n$ over the intervals of $10\,{}^{\circ}\mathrm{C}$ that a rock has passed through, $\Delta t_n$ being the time in millions of years spent in the interval numbered $n$, with $n = 0$ for 100 to $110\,{}^{\circ}\mathrm{C}$, $n = 1$ for 110 to $120\,{}^{\circ}\mathrm{C}$, $n = -1$ for 90 to $100\,{}^{\circ}\mathrm{C}$, and so on.