Page 47 - Fundamentals of Light Microscopy and Electronic Imaging

30 ILLUMINATORS, FILTERS, AND THE ISOLATION OF SPECIFIC WAVELENGTHS

10000 10000

1000 1000
Hg100
Hg200
Xe75
Xe150 100
Relative intensity
100

10 10
Halogen 12 V 100 W

1 1
Tungsten 6 V 30 W

0.1 0.1

400 500 600 700 800
Wavelength (nm)

Figure 3-1
The spectra of various illuminators. Tungsten filament lamps give continuous emission, but
their output is reduced at shorter wavelengths; mercury and xenon arc lamps are brighter,
although mercury contains prominent emission lines in the visible range. Notice that over
much of its range, the intensity of a 75 W xenon bulb (XBO) is several times greater than that
of a 100 W mercury lamp (HBO). Although higher-wattage arc lamps generate a lower
luminous flux (lumens/mm /s), they cover a much larger area and their total luminous output
2
is considerably greater.

inexpensive, easy to replace, and provide bright, even illumination when used together
with a ground glass filter; hence their popularity in nearly all forms of light microscopy.
These lamps produce a continuous spectrum of light across the visual range, with peak
output occurring in the red and infrared (IR) range and blue and ultraviolet (UV) output
being the weakest. Excitation of the filament is regulated by a continuously variable
power supply. As voltage and excitation are increased, brightness increases and the
spectrum shifts to increasingly higher-energy photons with shorter wavelengths. There-
fore, color balance of the light from an incandescent lamp varies depending on the volt-
age applied to the lamp. When producing color micrographs, a specific voltage is
selected in order to obtain a consistent and predictable spectrum of wavelengths. Spe-