Abstract: A light collecting device for flow cytometry comprises an internally reflecting chamber for collecting light from a point source within the chamber over a solid angle of substantially 4 pi and directing all the collected light through an exit point at or adjacent the wall of the chamber. The chamber comprises an ellipsoidal surface extending over a solid angle of substantially 2 pi and a spherical surface extending over a solid angle of substantially 2 pi, the firsts conjugate focus of the ellipsoidal surface being coincident with the center of curvature of the spherical surface and the second conjugate focus of the ellipsoidal surface being at the center point of the spherical surface. A capillary bore passes through the first conjugate focus to accommodate a sample stream which is illuminated by laser light to produce fluorescence at the first conjugate focus.