High pressure mercury plasma arc-discharge lamps range between 10 and 100 times brighter than incandescent lamps (such as the tungsten-halogen) and can provide intense illumination over selected wavelength bands throughout the visible spectral region when combined with the appropriate filters. These illumination sources are highly reliable, produce very high flux densities, and have historically been widely used in fluorescence microscopy. However, compared to traditional incandescent lamps, the significant increase in brightness afforded by mercury arc lamps is accompanied by the inconvenience of critical mechanical alignment, shorter lifetime, decreased temporal and spatial homogeneity, specialized lamphouse and power supply requirements, potential explosion hazards, and higher cost. Regardless of the pitfalls, the mercury arc lamp remains a workhorse in fluorescence microscopy and is still considered one of the best illumination sources, especially for low abundance (in effect, those that have sparse targets) or weak fluorophores whose excitation maxima coincide with the spectral lines emitted by the hot mercury plasma. This interactive tutorial examines advanced mercury arc lamphouses that are capable of automatic bulb alignment and intensity control.
The tutorial initializes with a cut-away diagram of a ZEISS HBO 100 self-calibrating lamphouse displayed in the window along with the position of the pinhole projection on the calibrating photodiode and a projection of the lamp output from the collector lens system. To operate the tutorial, click on the Calibrate button to initiate automatic alignment of the arc with the lamphouse optical axis. The collector lens position can be adjusted with the Collector Lens slider and the lamp intensity can be increased or decreased with the FluoArc Intensity slider. The Power button can be used to turn the lamp off.
In a typical optical microscope configuration, the mercury lamp is positioned inside a specialized illuminator consisting of a lamp housing containing the lamp, a concave reflector mirror, an adjustable collector lens system to focus the lamp output, an electrical socket for securing and alignment of the bulb, and the external power supply (Figure 1). Depending upon the design, mercury arc lamphouses may also contain filters to block ultraviolet wavelengths and hot mirrors to block heat from entering the microscope optical train. Many lamphouses also contain external heat sinks to dissipate heat and vents that allow the dissipation of hotter air, while others also feature a large cooling fin attachment to the lamp itself (see Figure 1). In addition, the lamphouse must contain an adjustment knob for the collector lens position and provisions (either knobs or screws) for alignment of the lamp and the reflector. Of primary concern is that the lamphouse itself must not leak harmful ultraviolet wavelengths and should incorporate a switch to automatically shut the lamp down should the housing be compromised or opened during use.
Mercury arc lamps contain a precisely measured amount of metallic mercury within the envelope, and they are filled with argon or xenon, which acts as a starter gas as the mercury vaporizes. When the lamps are cold, small droplets of mercury can often be observed on the inside walls and the gas pressure inside the envelope is lower than the ambient pressure of one atmosphere. Once the lamp is ignited, the mercury vaporizes over the course of a 5 to 10 minute transition phase. During this period, the lamp is operated at higher than normal current, requiring the anode to be positioned at the bottom of the lamp to ensure proper vaporization of the mercury. For this reason, the ferrule sockets in a mercury lamphouse have different diameters (one smaller than the other) to enable correct positioning of the lamp, which itself has a larger ferrule on the anode end of the tube. Thus, mercury arc lamps are positioned vertically within the lamphouse with the anode pointing toward the bottom and the cathode pointing upward. Operating a mercury lamp at an angle exceeding 30° from the vertical position deflects the arc toward the quartz envelope resulting in uneven heating and premature darkening of the bulb. Several mercury lamp designs incorporate a reflective coating on part of the envelope to speed the vaporization transition phase and to improve thermal distribution. Because the envelope temperature influences the internal mercury pressure to a significant degree, mercury arc lamps are sensitive to airflow over the bulb and this aspect must be carefully controlled by the lamphouse.
Mercury arc lamps require a direct current (DC) power supply that is specifically designed to meet the ignition and operational requirements for each lamp design. A typical power supply must provide up to a 50 kilovolt starting pulse to ionize the gas in the arc gap, as well as an open circuit voltage three to five times the rated lamp operating voltage in order to heat the cathode to thermionic emission temperatures. Additional requirements include a maximum level of inrush current to prevent excessive thermal shock during ignition. Inrush current can be several orders of magnitude greater than the lamp circuit steady-state value and is often a contributor to ignition failure. The lamp power supply must also limit current ripple to less than 10 percent (peak to peak) to ensure long lamp life and light stability. Finally, the power supply must be able to adjust the applied current over a wide range as the voltage can significantly increase during the lamp warm-up period.
Power supplies for the HBO 100 mercury arc lamps used in optical microscopy are usually equipped with several features that enable the operator to monitor the operating conditions and lifetime. Included are an indicator light for lamp ignition, a light that signifies when the transformer has reached an internal temperature within the permissible range, a safety light to alert the operator that the safety circuit of the lamp housing is closed, and a voltage light that is enabled when the transformer is performing within the permissible voltage range. All commercial mercury lamp DC power supplies also feature a re-settable display of the total time (in hours) that the lamp has been in operation.
Lamphouses for arc lamps require continuous inspection and maintenance. The lamp socket assembly and power cord should be examined periodically for oxidized metal surfaces (socket electrodes) and the integrity of the cord. The socket electrodes are prone to oxidation and should be lightly brushed with an emery cloth (or extra-fine sandpaper) each time the lamp is changed to assure good electrical contact. The bulb, rear mirror reflector, and front collector lens should be inspected and cleaned if necessary to remove dirt, lint, and fingerprint oils. Each time the lamp is replaced, the collector lens assembly and reflector positioning mechanisms should be inspected for proper operation. The illuminator adjustment knobs or screws should be adjusted while examining the resulting motion of the collector and reflector to ensure they travel in the expected manner. The high-current power line connecting the power supply and lamphouse should not be crimped (as might occur when the line is shoved between a table and a wall) as this maneuver can stretch or loosen internal wires and lead to malfunction.
Contributing Authors
Adam M. Rainey and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.