Abstract: Detecting light in a capillary electrophoresis optical system includes input and output windows in the outside surface of a capillary tube. The width of the windows is not substantially greater than the bore of the tube. A fiber optic input is spaced from the capillary tube so that the light is directed to the tube and is matched to the window width. The light output from the tube is matched to be received by a fiber optic output spaced from the capillary tube. The optical light envelop from the fiber input through the tube and to the fiber output is governed by the LaGrange Invariant. The matching is effected in accordance with the Numerical Aperture of the fiber optics.