Page 53 - Fundamentals of Light Microscopy and Electronic Imaging

36 ILLUMINATORS, FILTERS, AND THE ISOLATION OF SPECIFIC WAVELENGTHS

alignment. If the arc’s image does not expand symmetrically, you must
make an additional round of adjustments. Sometimes it helps to expand the
image slightly with the collector lens before making additional adjustments.
• While looking in the microscope at a focused fluorescent specimen, adjust the
collector lens of the illuminator until the image is bright and uniform from
one edge of the field to the other and across all diameters of the field. At this
position the arc is focused on the objective’s back aperture. In epi-illumination,
the objective lens functions as both a condenser and an objective.

For incandescent filament lamps the procedure is easier. After remounting the bulb,
turn on the power supply and examine the lamp filament in the microscope using a tel-
escope eyepiece or Bertrand lens. Focus the telescope to view the edge of the condenser
diaphragm at the front aperture of the condenser. In the absence of a diffuser screen, the
filament should be sharply focused; if not, adjust the lamp’s collector lens. Notice the
pattern of vertical lines representing the filament. Center the image of the filament, then
center the image of the reflection so that the vertical lines of the primary and reflection
images are interdigitated (see Fig. 3-4). Some illuminators do not contain adjustable
collector lenses for low-power lamps. If the filament image seems off-center, try
remounting the bulb.

“FIRST ON—LAST OFF”: ESSENTIAL RULE
FOR ARC LAMP POWER SUPPLIES

It is very important to understand the potential hazard of turning on and off an arc lamp
power supply located near functioning electronic equipment. Arc lamps should be
turned on and allowed to stabilize for a minute or two before turning on the other pieces
of nearby electronic equipment. Although the power supply and cable are generally well
shielded, a momentary 20,000–50,000 V surge passing between the DC power supply
and the arc lamp generates magnetic fields that are strong enough to damage sensitive
integrated circuits in nearby VCRs, electronic cameras, and computers. Upon turning

Figure 3-4
Alignment of a filament lamp. In the absence of a diffuser screen, the image of the lamp can
be seen in the back aperture of the objective lens using an eyepiece telescope or Bertrand
lens. The reflection image of the filament is adjusted to interdigitate with the filament loops
seen in the direct image.