isosbestic

Denoting the wavelength of light at which two related compounds have identical extinction coefficients; for example, the wavelength at which the absorption spectra of hemoglobin and oxyhemoglobin cross is their isosbestic point. Spectrophotometry at that wavelength measures total concentration of hemoglobin, regardless of the extent to which it might be oxygenated.

[Ger. isosbestisch, fr. G. isos, equal, + sbestos, extinguished]