One of the fundamental tests for a resist process is to determine the characteristic curve of photoresist thickness versus exposure dose, sometimes called the HD plot. Standard analysis of characteristic curve data in an HD plot yields a slope; which we term. gamma(HD) . gamma(HD) is generally regarded as an experimental determination of the process contrast or process nonlinearity, and high values of it are considered desirable. It is possible to define a more fundamental measure of the process contrast, which we term gamma(true). The present. work describes the relationship between gamma(true) and gamma(HD) using a simplified theoretical treatment that includes the effects of resist: absorption and substrate reflection. This relationship shows that increasing resist absorption tends to decrease gamma(HD), for positive resist processes. On the other hand, for negative resist processes, increasing resist absorption tends to increase \gamma(HD)\. Without accounting for the effects of absorption, \gamma(HD)\ is a poor indicator of true process nonlinearity.