44 PART II A Deeper Look into a Light Microscope
Xenon Arc (XBO) Lamps
Xenon arc lamps feature a largely continuous and uniform spectrum across the
entire visible spectral region. The luminous arc burns in an atmosphere of pure
xenon gas at high pressure. It produces a bright white light that closely mimics
natural
sunlight. XBO is suited to stringent applications requiring the simultaneous
excitation of multiple fluorophores over a wide wavelength range still
sometimes
asked for analytical fluorescence microscopy. They are now very
difficult
to obtain.
Metal Halide (HXP) Lamps
Metal halide illumination sources are rapidly emerging as a serious challenger to
the application of mercury and xenon arc lamps for investigations in fluorescence
microscopy. These light sources feature a high-performance arc discharge lamp
housed in an elliptical reflector that focuses the output into a liquid light guide
for delivery to the microscope. The lamps are designed to generate a concent-
rated spot of light at a specified distance. One advantage of pre-assembled reflector
lamps is that each time the lamp is replaced, the reflector is also replaced into
a fixed position within the lamphouse, thus eliminating the stringent and cumbersome
alignment requirements of mercury and xenon arc lamps. The reflectors
on metal halide lamps are coated with multiple dichromatic interference filter
layers that enable much of the thermal radiation to pass through the reflector
while the ultraviolet and visible wavelengths are concentrated at the focused spot.