Optically stabilized mercury short-arc lamp as UV light source for microscopy

For certain applications in microscopy, mercury vapor short arc lamps are utilized as UV-light sources because of their high intensity and their road spectrum. Unfortunately, they are also very unstable. Especially for single wavelength fluorescence image cytometry, there is a need for a stable, high intensity light source. Substantially improved stability was achieved using optical feedback and fiberoptic scrambling. The system uses a photodiode to monitor the light intensity, and feeds the readout back to a controller. The controller compares this readout to a preset reference voltage and adjusts the lamp supply current accordingly. The optical fiber scrambles the light to correct the effects of arc wander. Preliminary results of performance tests of this system show a coefficient of variation (CV) of less than 0.1 percent over 20 hours at a sample frequency of 30 Hz. This CV is a factor of 30 better than a conventional current stabilized mercy vapor short arc lamp. Scrambled optical feedback is a necessary addition for systems with mercury short arc lamps, especially for image fluorometry applications.