Biophotonics_Concepts_to_Applications

• 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