= Solution
Reading the straight-line extrapolation in the printed <photoresist contrast> curve gives approximately $D_0=20\ \mathrm{mJ\,cm^{-2}}$ at unit remaining thickness and $D_c=100\ \mathrm{mJ\,cm^{-2}}$ at clearing. On the logarithmic dose axis,
$$
\boxed{\gamma=\frac1{\log_{10}(D_c/D_0)}
=\frac1{\log_{10}5}\simeq1.43.}
$$
The critical image modulation follows by requiring one exposure time to give $D_{\max}\geq D_c$ in a bright region and $D_{\min}\leq D_0$ in a dark region:
$$
\frac{I_{\max}}{I_{\min}}\geq\frac{D_c}{D_0}.
$$
For unit object modulation, this means
$$
\boxed{M_c=\frac{D_c-D_0}{D_c+D_0}=\frac{100-20}{100+20}\simeq0.667.}
$$
The doses are graphical estimates, so extra significant figures would not represent greater measurement accuracy.
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