Aerial-image modulation

Aerial-image modulation measures the variation of optical intensity in the image delivered to a photoresist before its chemical and development response. A common contrast measure is $M=(I_{\max }-I_{\min })/(I_{\max }+I_{\min })$, for nonnegative intensities with nonzero denominator. Equal bright and dark intensities give $M=0$; a dark minimum with a positive maximum gives $M=1$.

For example, intensities $9$ and $1$ give $M=0.8$. Multiplying both by the same exposure factor leaves $M$ unchanged, although it changes the deposited dose and can change the printed pattern. Image modulation and photoresist contrast therefore describe different stages of photolithography: the optical signal and the material's conversion of that signal into a developed profile.^[ref]