A mercury arc lamp comprises a glass envelope including a center portion. A cathode and an anode are sealed within the envelope. The internal ends of the cathode and the anode lie in close proximity to each other within the center portion of the envelope. Electrically conductive positive and negative contacts are connected to the anode and the cathode, respectively. The interior of the envelope contains mercury and cadmium to be vaporized during operation of the lamp. A pair of reflective coatings is around the envelope. Each coating is located near an internal end of the cathode and the anode, respectively, with a space between the coatings so that the arc between the cathode and the anode is adapted to emit radiation through the space. A flow cytometry apparatus which uses a mercury arc lamp such as described above, is also within the purview of the present invention.