(A) Experimental spectra of the absorption coefficient, α, for four different concentrations of resorufin in a 50 mm sodium phosphate buffer solution (pH 7.4) using a 0.2-cm path length cuvette. (B) Absorption coefficient as a function of the density of absorbing resorufin molecules (ρ) or the molar concentration of resorufin (c) at λ1=530 nm (blue circles), λ2=571 nm (red squares) and λ3=590 nm (purple triangles). This plot was obtained by taking a cross-section of the curves in panel (A) at λ1, λ2 and λ3. (C) The slope of a linear fit of curves in panel B represents the absorption cross-section of a resorufin molecule (σ) at a specified wavelength, as indicated by Eq. (12). This process was repeated for the entire range of wavelengths (450 nm<λ<700 nm) in steps of 1 nm to obtain the functional dependence of σ versus λ reported in panel C.