- The pulse duration for a pulsed laser
- The temporal profile of the pulse duration (the irradiance variation with time
during the pulse) - The spectral profile of the light beam (the variation in irradiance as a function of
wavelength) - The polarization state of the light
The key parameters of CW light sources include the optical power, irradiation
time, and spot size of the light beam. For pulsed lasers the key parameters are the
energy per pulse, irradiation time, spot size, pulse repetition rate (how often the
optical power delivery takes place), and number of pulses administered. For CW
light an important parameter is irradiance (given in W/cm^2 ), which is a function of
the power delivered and the laser spot size on the tissue.
The interactions of light with biological tissue can be categorized according to
the irradiance (or equivalently, the exposure rate) measured in W/cm^2 (or J/s/cm^2 )
and the exposure time, as shown in Fig.6.19. The light-tissue interaction conditions
can range from very high irradiances over extremely short times (10^10 – 1015 W/cm^2
over corresponding time periods of 10−^9 – 10 −^15 s, that is, nanoseconds to fem-
toseconds) to the situation of low irradiances over long time periods (1–
100 mW/cm^2 over corresponding time periods of hours to minutes). Thus the
irradiances span a range of nineteen orders of magnitude and the light exposure
time ranges over more than sixteen orders of magnitude. Note that the circles, ovals,
and rectangles shown in Fig.6.19merely give approximate ranges of the irradi-
ances and exposure times for the generic light-tissue interaction modes described
below.
In the various modes of interaction theradiant exposureorenergy densityranges
from approximately 1 mJ/cm^2 – 1 kJ/cm^2. The energy density sometimes is used as a
parameter to describe light doses, but this can lead to incorrect applications, because
energy is defined as power multiplied by time, that is,
fs ps ns μs ms s ks
Exposure time
Irradiance or exposure rate (W/cm
2 )
100
10 -3
1015
1012
109
106
103
Thermal: coagulation
Plasma-induced
ablation
Ablation
Photodisruption
Thermal: vaporization
Photochemical
Photobiomodulation
Fig. 6.19 Categories of
light-tissue interaction modes
and their irradiance and
exposure characteristics
172 6 Light-Tissue Interactions