Biomedical Engineering Reference
In-Depth Information
Figure 1.3
Ruby rod attached to high-power voltage. Flashlamp assembly is
adjacent to rod. Normally the lamp is attached atop the rod assembly. Mirror
allows the rod to be excited circumferentially.
Figure 1.1
Example of compression technique. The plastic piece is used to
“remove” blood so that lentigo can be treated with 1.5-ms pulsed dye laser
without purpura.
Incident
laser light
Regular
reflectance
Epidermal
remittance
Dermal
remittance
Stratum
corneum
(~10
μ
m)
θ
i
Internal
reflection
Absorption
Epidermis
(~100
μ
m)
Scattering
Basal layer
Dermis
(~4 mm)
Transmission
Figure 1.4
Diode laser where the diode light is coupled into fi ber that is
attached to the handpiece.
Figure 1.2
The behavior of light at skin surface.
The greatest advantages of laser light are the intensity and
the monochromaticity—this allows a degree of precision that
is hard to reproduce with nonlaser sources. The beam of the
laser is easy to manipulate. The beam can, for example, be
expanded, or focused, quite easily. On the other hand, with
nonlaser sources, such as fl ashlamps, one cannot exceed the
brightness of the source lamp. It follows that the intensity of
the beam can only be attenuated once emitted from the lamp
surface. Lasers can create very high intensity light because of
the property of stimulated emission. With laser, a lamp similar
to the intense pulsed light (IPL) fl ashlamp pumps the laser
cavity. The amplifi cation of the light within the cavity sets
laser light apart from other sources. Laser is really a fancy way
to convert lamplight to a more powerful monochromatic form
(8). The high power of the light (especially peak power with
pulsed lasers) is not attainable outside of laser sources. The
ability to focus the laser beam is an important contributor to
the peak power density of the laser. Very small beam angles can
be obtained that are not possible with intrinsically divergent
nonlaser light sources (i.e., IPL) (9).
With respect to lasing media, there are diode lasers, solid-
state lasers, and gas lasers. An example of a solid-state laser is
the neodymium (Nd) laser. These lasers have a rod that is
pumped by a fl ashlamp (Fig. 1.3). Miniaturized diode lasers
have become more powerful and popular. Some diode lasers
are housed separate from the handpiece (Fig. 1.4). Others are
confi gured with the laser diodes in the handpiece (Fig. 1.5).
Modern diode lasers are capable of much higher powers than
in the past years, but their peak powers are still limited com-
pared with most pulsed solid-state lasers (10).
IPL devices are becoming increasingly popular (11-17).
Because the absorption spectra of skin chromophores are not
monochromatic, a broadband light source is a logical approach
for cosmetic applications. Rather than using a lamp to pump a
laser, these devices use the lamp directly. Appropriate fi ltration
creates the optimal output spectrum for a particular applica-
tion. Much like a slide projector with a specifi c color slide,
these bright lamps produce spectra that can mirror the absorp-
tion spectrum of melanin, hemoglobin, and even water. The
advantages of these devices are their fl exibility, but one disad-
vantage is that one must hold the lamp, cooling, and the high-
voltage source in the hand. This creates a larger “umbilical
cord” compared with the fi bers and articulated arms used by
most laser devices. Also, the beam diverges quickly, and non-
laser beams are not optically as easily manipulated as true laser
beams. Very short pulses (e.g., Q-switched nanosecond (ns)
