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We measure the effective optical depth in the Lyman-alpha (Ly$\alpha$) forestusing 40,035 quasar spectra from the Twelfth Data Release (DR12) of the BaryonOscillation Spectroscopic Survey (BOSS) of SDSS-IV. A rigorous selection basedon spectral index and the equivalent width of the C IV emission line is appliedto choose seven uniform samples with minimal intrinsic variations acrossredshifts. Modeling the redshift evolution of the effective optical depth witha power-law, $\tau_\mathrm{eff}=\tau_0 (1 + z)^\gamma$ produces $\tau_0=(5.54\pm 0.64)\times 10^{-3}$ and $\gamma=3.182 \pm 0.074$. The 2.2% precisionestimate on $\gamma$ is dominated by systematic errors, likely arising from thebias and uncertainties in spectral index estimates. Even after incorporatingthe systematic errors, this work provides the most precise estimates of opticaldepth parameters to date. Finally, using the reconstructed Ly$\alpha$ forestcontinuum to directly measure the transmitted flux ratio as a function ofredshift, we find deviations of less than 2.5% from the predictions from theglobal model and no convincing evidence for signal associated with He IIreionization.

Measurements of effective optical depth in the Ly$\alpha$ forest from the BOSS DR12 Quasar sample