Before revision: Many biochemical reactions involve a stream of chemical reactants (ligand molecules) flowing over a surface to which other reactants (receptors) are confined. Scientists measure rate constants associated with these reactions  in an optical biosensor: an instrument in which ligand molecules are convected through a flow cell, over a surface on which receptors are immobilized.
After revision: Optical biosensors are often used to measure kinetic rate constants associated with these reactions in an optical biosensor: an instrument in which ligand molecules are convected through a flow cell, over a surface on which receptors are immobilized.