Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 3

1.2.1 Light Sources

Sunlight is the cheapest light source. Spectroscopic system based on sunlight source
has been used in remote sensing, weather forecast, and forestry monitoring. For
biomedical spectroscopic system, dedicated light sources are often required. The
light source must be stable, predictable, measurable, and reliable. The most often
used light sources for biomedical spectroscopy include broadband lamps with
wavelength spanning from ultraviolet (UV) to visible to near-infrared (NIR), light-
emitting diodes (LEDs), and gas or solid-state lasers. The recent development of
programmable light sources, fiber lasers, and supercontinuum light source can also
be used for biomedical spectroscopy. Choice of these light sources depends on the
application requirement on wavelength, intensity, beam size, and beam quality of
the light source.

1.2.1.1 Broadband Light Source (High-Pressure Arc Lamp, Incandescent

Lamp, Programmable Light Source, Supercontinuum
Light Source)

Common broadband light sources include high-pressure arc lamp and incandescent
lamp. They can be obtained from suppliers such as Newport and Ocean Optics
(see Appendix for more details). High-pressure arc lamp generates intense and
broadband emission from UV to NIR (200-1,000 nm). The commonly used high-
pressure arc lamps include xenon (Xe) lamp, mercury (Hg) lamp, or mercury-xenon
(Hg-Xe) lamp. These types of lamps consist of two tungsten electrodes in a quartz
bulb filled with high-pressure gases. Example emission spectra of these types of
lampsareshowninFig. 1.1 . Because there are so many spikes (peaks), these types
of lamps are superior for fluorescence excitation. They are also frequently used
for reflectance spectroscopy. Care must be exercised when handling these lamps
because the high-pressure bulb is explosive. Xe lamp also has strong UV output;
protective headgear, safety eyewear, and leather gloves are highly recommended.
Incandescent lamp is the simplest light source. The commonly used one is the
quartz tungsten halogen (QTH) lamp. The spectra of QTH lamp are also shown
in Fig. 1.1 . The spectra are very smooth and covering the visible range and the
NIR range. This type of lamp is particularly useful for transmission and absorption
measurement. Calibrated QTH lamp can also be used for intensity calibration of a
spectrometer (see Sect. 1.2.6 ).
Based on the broadband lamp, a digital programmable light source was devel-
oped recently [ 14 ]. It is not a stand-alone lamp, but a system that can modulate
the intensity and spectral shape of the light source. It consists of a broadband
lamp, a dispersive grating, and a digital micromirror device (DMD). The DMD is a
micromirror array typically consists of 1;280 1;024 pixels with each pixel being
12:6 m 12:6 um. Each pixel is a micromirror which can be turned on and off and
controlled by the computer by tilting the direction of the mirror toward or away from