Peak optical depth of the 3 μm ice absorption feature (${\tau }_{3.0}^{max}$) as a function of the ice color excess Λ(W₁ − I₁) for the calibration sample. The various symbols represent the different literature sources listed in Table 1. Each point represents a literature measurement with propagated uncertainties. The black dashed line shows the best-fit relation, while thin solid lines represent 500 random samples from the fit parameter space. Gray dashed lines mark ${\tau }_{3.0}^{max}$ = 0 and Λ(W₁ − I₁) = 0.