PM- and MM-probe intensities (part a), their log-difference (part b) and -sum (part c) as a function of the specific target concentration (S/N-ratio). The curves are calculated using the Langmuir-model (Eq. (1)). The difference-plot reveals the hybridization regimes of non-specific (N), mixed (mix) and specific (S) hybridization, of saturation (sat) and the asymptotic (as) range. The parameters α and β used in Eq. (10) specify the data range spanned by the log-difference and -sum.