Figure 30-33 Absorption spectra of some gases and materials important to anesthesia. Note that absorption is not constant over wavelengths. Therefore, it is important to choose the proper wavelength to measure. In addition, when multiple substances are present, it is still possible to measure their concentrations, provided that enough wavelengths are available. (The solution becomes one of multiple equations with multiple unknowns.) (From Gravenstein JS, Paulus DA, Hayes TJ: Capnography in Clinical Practice. Boston, Butterworths, 1989.)