Process and apparatus for measuring the concentration of a molecule of selective spectrum in a sample substance

A method and apparatus are described for measuring the concentration of a molecule of selective spectrum in a sample substance in which errors from spurious light signals due to diffused light or from spurious fluorescent or phosphorescent light due to traces of other molecules in the sample substance are eliminated. The invention subjects the sample substance to light of a first wavelength in the absorption spectrum of the molecule to be measured and obtains a signal representative of light emission from the molecule with the spurious light signals superimposed thereon. Thereafter, the sample substance is subjected to a second excitation light having a wavelength close to the first wavelength but outside of the absorption and emission spectra of the molecule to be measured to generate a second signal representative of light emission from the molecule to be measured, which light emission results only from diffusion light and/or fluorescent or phosphorescent light due to traces of other molecules in the sample substance. A differentiator then generates a signal of the difference of the first and second signals, which is representative of the true light emission signal of the molecule to be measured.