A method and an apparatus for optical measurement of a liquid sample placed in a sample well where the sample in the sample well is exposed to excitation light at first wavelength from an excitation light source, which excitation light generates a collection of singlet state oxygen molecules from donor molecules in the liquid sample, said singlet state oxygen molecules reacting with acceptor molecules in the liquid sample causing said acceptor molecules to emit chemiluminescence emission light at second wavelength, which second wavelength is shorter than said first wavelength, and where the emission light produced by the excitation light is measured with a detector, characterized in that the excitation light source is a LED.