Biomedical Engineering Reference
In-Depth Information
100
80
60
40
20
0
-0.5
0
0.5
1
1.5
2
2.5
(
A
)
Time (ms)
120
100
80
60
40
20
0
-20
-0.5
0
0.5
1
1.5
2
2.5
(
B
)
Time (ms)
100
80
60
40
20
0
-20
-2
0
2
4
6
8
10
(
C
)
Time (ms)
100
80
60
40
20
0
-20
-10
0
10
20
30
40
50
(
D
)
Time (ms)
Figure 1.13
Various pulse profi les for pulsed dye laser. (
A)
Scleropus 1.5 ms; (
B
) V-Beam 1.5 ms; (
C
) V-Beam 6 ms; (
D
) V-Beam 40 ms.
Abbreviation
:
a.u., arbitrary units.
Laser light
and is not normally associated with purpura. This suggests
that there is a critical difference in purpura threshold for this
wavelength range (500-600 nm) between 125- and 2000-
Remitted
Air
Tissue
r
s
micropulse durations, regardless of the total macropulse
length.
μ
Totally
reflected
molecular basis of lti
Most devices
developed
for cosmetic rejuvenation are based on
photothermal or “electrothermal” mechanisms, that is, the
conversion of light or electrical energy to heat. Two fundamen-
tal processes govern all interactions of light with matter:
absorption and scattering. Besides scattering and absorption,
photons can be totally internally refl ected at a tissue-air inter-
face, or they can be remitted out of the tissue (Fig. 1.14) (4,18).
Absorption and excitation are necessary for all photobiologic
effects and laser-tissue interactions. Plank's law describes how
much energy is invested in one photon as a function of wave-
length. Energy is proportional to frequency and inversely pro-
portional to wavelength. Thus, a 532-nm photon (532 nm is
the distance between two of the transverse waves in a stream of
Target
melanocyte
Absorbed
Z
Absorbed
Figure 1.14
The behavior of light at and just below the skin surface.
light) is twice as energetic as a 1064-nm photon (5,24,25).
When absorption occurs, the photon loses its energy to an
atom or molecule, known as a chromophore. Upon absorption,
the photon ceases to exist, and the chromophore becomes
excited; it may undergo photochemistry or may dissipate the
energy as either heat or reemission of light (e.g., fl uorescence).
