Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 316

Metal halide (HXP) lamps have a similar emission spectrum to mercury lamps.
In addition to higher radiation levels in the continuous regions between lines,
HXP lamps have an emission output featuring pressure-broadened versions of the
prominent mercury arc spectral lines.
One advantage of arc lamps is that one light source can be used for either
broadband illumination for reflectance imaging or narrowband illumination for
fluorescence and reflectance imaging. The disadvantages are that they are bulky,
and one or more filters are required to select the spectrum for optimal fluorescence
excitation.
With recent advances in efficiency and thermal management, light-emitting
diodes (LEDs) are finding more and more applications in biomedical imaging.
LEDs produce an output with a spectrum bandwidth wider than the laser but much
narrower than the arc lamp. The diverse spectra afforded by LEDs make it possible
to select an individual LED to supply the optimum illumination wavelength for a
specific application. Although LEDs are regarded as narrowband light sources, there
are still tails outside of the LED spectrum. These tails must be blocked when LEDs
are used as the excitation light source in fluorescence imaging system.
LEDs have several advantages over the other noncoherent light sources, includ-
ing compact size, low power consumption, low heat generation, fast switching, high
emission stability, and extremely long life span. LEDs can instantly illuminate at full
intensity as soon as electrical current is applied. The controllable stability of LED
intensity is a key benefit for biomedical imaging when consistent and repeatable
measurement is required for quantitative analysis.
A laser, the only coherent light source, is monochromatic. Lasers produce
a highly coherent light, which can be collimated, expanded, and focused to a
diffraction-limited spot. The laser beam can be easily coupled into an optical fiber
and delivered to a remote site. Many fluorescence-scanning devices for biomedical
applications use lasers as their excitation light sources, given that the combination
of focused energy and a narrow spectrum width contributes to excellent sensitivity
and resolution.
There are a number of commonly used lasers in fluorescence imaging. Argon
lasers generate a variety of wavelengths from UV to visible light, for example,
364, 457, 488, and 514 nm. They are often used as the excitation source for
many common fluorophores, such as fluorescein isothiocyanate (FITC) and Cy2.
Neodymium:yttrium aluminum garnet (Nd:YAG) solid-state lasers can generate a
strong line at 532 nm using a frequency-doubling method and can be used to excite
Cy3. The helium-neon (HeNe) laser at 633 nm is usually used to excite Cy5. HeNe
lasers with other wavelengths have also become available.

9.3.2.2 Illumination Path

The basic requirement of the illumination system for fluorescence imaging is to
closely match the excitation peak of the fluorophore to excite the fluorescence
effectively. Other considerations include illumination efficiency, intensity, and