Page 50 - Fundamentals of Light Microscopy and Electronic Imaging

ILLUMINATORS AND THEIR SPECTRA 33

Metal halide lamps, which have a spectral output similar to mercury, are becoming
popular because they are bright (150 W), have a long bulb life (1000 hr), and have a
large electrode gap (5 mm). Nikon Corporation promoted their use in combination with
a liquid fiber bundle for delivering bright, homogeneous illumination for video
microscopy.

Demonstration: Spectra of Common Light Sources

Please note: Never look directly at unattenuated mercury or xenon beams,
because they are, respectively, extremely UV- and IR-rich and potentially damag-
ing to the eye!
It is useful to become familiar with the spectra of common illuminators by
indirect inspection of their light with a spectroscope or a diffraction grating.
There are several ways to do this:

• For display and discussion in a group, set up the I-beam/optical bench and
project the diffraction pattern of a diffraction grating on a projection screen
as shown in Figure 2-10.
• For individual study, wrap a piece of aluminum foil containing a narrow slit
aperture over the mouth of the illuminator and examine the slit at a distance
of several feet with a handheld diffraction grating held close to the eye. A
transparent holographic grating works best. To make a slit, cut a 1 cm long
slit in the foil using a razor blade, while holding the foil placed against a
sheet of stiff cardboard.
• An inexpensive handheld spectroscope based on a sine-wave (holographic)
diffraction grating is available from Learning Technologies, Cambridge,
Massachusetts. Direct the unfiltered beam of the illuminator onto a white
card or projection screen positioned several feet away and follow the instruc-
tions for using the spectroscope. The advantage of the spectroscope is that it
permits you to determine the wavelengths of colors and emission lines from
a built-in wavelength scale. You should perform these observations in a dark-
ened room.

Examine the spectrum of a tungsten lamp or quartz halogen lamp first. The
continuous, smooth nature of the spectrum and the relative falloff in brightness at
the blue end of the spectrum are characteristic. Examine the spectrum with the
power supply adjusted at the minimum and maximum permissible settings to see
the shift in the peak spectral output. As power increases, the intensity of shorter
bluer wavelengths increases. (The peak emission wavelength in the infrared also
decreases, but this cannot be seen visually.)
Next, examine the spectrum of the xenon arc and notice the uniform intensity
across the entire visible range. Careful inspection will show that the spectrum is
not perfectly smooth, but rather has weak emission lines in the visible range near
470 nm (blue) and also at the red end of the spectrum near 680 nm. Fifty percent
of the output of this lamp is in the IR, where prominent, though invisible, emis-
sion lines occur at 800 nm.