EXFO manufacture a wide variety of products for the optical communications industry but also market products for lifesciences research. One of their lifesciences products is the X-Cite120 illumination system for fluorescence microscopes. High quality fluorescence microscopy and particularly quantitative imaging are dependent upon stable and intense illumination sources. Most laboratory fluorescence microscopes rely upon mercury or xenon arc lamps for illumination with more specialized, and consequently expensive, microscopes using lasers for illumination. These sources have undoubtedly been utilized extensively in research, however, they are not without their limitations. Mercury arcs have limited power for green fluorophores, xenon sources emit a great deal of heat and arc lamps in general have short lifetimes and require alignment; lasers are relatively expensive, require scanning mirrors and can only be optimally used with certain fluorophores.
The X-Cite120 from EXFO is designed to improve and simplify microscope illumination and consequently obviate many of the problems associated with existing illumination sources. The heart of the system is a pre-aligned 120 W proprietary metal halide lamp. This lamp simply snaps into position in the lamp module and as it is pre-aligned the system is immediately ready to use. Precise alignment of lamps can greatly alter fluorescence intensity and spatial homogeneity which is exceptionally important for quantitative imaging. Starting experiments with reliable alignment and avoiding any initial alignment is certainly beneficial.
The X-Cite120 is rated for 1500 hours, much longer than the 300 hour rating for mercury arc lamps. The lamp contains halide gas to prevent deposits from the tungsten electrode from forming on the inner surface of the quartz lamp. In normal lamps such deposits absorb light to decrease emission intensity and create hot spots that lead to lamp overheating and failure. Lamp lifetime is also extended by continual monitoring of lamp temperature with the proprietary Intelli-Lamp system. This feature prevents re-ignition of the arc before a safe lamp temperature is reached. The emission spectrum of the metal halide lamp is ideally suited to commonly applied green and red fluorophores. Emission intensity is substantially increased for green fluorophores and equal for red fluorophores when compared to a 100 W mercury arc. The warm-up time of the lamp is relatively short at 90 seconds. Further, the voltage across the electrodes is increased by Electronic Control Gear (ECG) during the life of the lamp to maintain power output. Lamp power stability is excellent although regular calibration is recommended for quantitative experiments. ECG also monitors lamp life and eventually ‘shuts down’ a lamp at about 60 % maximum power as a safety feature.
The lamp module itself is 13.5” x 5.5” x 6.5” and weighs 7 pounds, as such this unit can easily be stored behind a microscope. A lightguide is used to optically connect the lamp module to a specific coupler that is compatible with a particular microscope. The coupler is a collimating lens that optimizes the light path from the light guide into the specific microscope. Presently X-Cite120 compatible microscopes include many Nikon, Zeiss, Olympus, Leica and Atto Biosciences models with a complete list of microscopes available on the EXFO website. The lightguide used with this system is liquid filled and, like all such fibers, this guide is relatively fragile. The suggested lifetime is ~3 years, but this is dependent upon many factors – for instance it is especially important not to over-bend the guide. The lamp module clearly displays lamp lifetime and other settings and error messages. The lamp module can also be equipped with an iris to modulate output intensity (100 %, 75 %, 50 %, 25 %, 12 % and 0 %). Finally, PC control of the iris, lamp and shutter is possible through a software interface and an RS232 port on the X-Cite120 PC, the 120’s big brother.
Easy installation and day-to-day use and the intensity and stability of illumination from this system make it superior to existing lamp set-ups. The initial cost of the X-Cite120 and the cost of replacement lamps are undoubtedly higher than standard systems. However, when the extended lifetime of the metal halide lamp is considered, the X-Cite120 actually becomes cheaper than competing systems. We currently have three X-Cite120 systems in the lab and all users seem very happy with the performance of these illumination systems.
Peter Haggie, Ph.D.
Post-Doctoral Fellow
University of California, San Francisco
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