Light-emitting diodes are mounted in a support block that holds the diodes in a fixed orientation. In certain embodiments, the orientation of the diodes by the support block causes the axes of the diodes to converge below the block to a common point where, in preferred embodiments, components of an excitation optical system are placed. The converging light then diverges for entry into an array of receptacles in which individual chemical reactions take place. The block contains engineered apertures that securely retain the diodes, each aperture including a shoulder for the flange at one end of each diode.