of the skin. Light photons
that interact
with skin can be either scattered
or
absorbed. Since
the scattering coefficient is typically
10 to 100 times
larger than the absorption coefficient
the photons
quickly lose their original direction of
propagation and
penetrate through the skin in a
diffusion-like
penetration process. Photons are
eliminated only
after they undergo an absorption
event in which
all of the energy of the photon is
absorbed in a
chromophore of the skin (such as
melanin,
hemoglobin or oxy-hemoglobin) and all of
its energy is
transformed into heat. If enough heat is
generated in the
target tissue (hair follicle, shaft or
bulge in the case
of hair removal applications), and
the target
reaches a high enough temperature, it can
be permanently
damaged, thus enabling hair removal.
The
understanding of light interaction with tissue is
quite difficult
and our ability to model it is somewhat
limited for two
main reasons:
1. The exact
optical and thermodynamic properties
of human
skin are not well understood and it is
quite complex to
consider them in a realistic
mathematical
model.
2. Even if a
good optical and thermodynamic model
exists, one has
to take into account the variation
in optical and
other properties from one patient
to another and
even variations of properties from
one area of the
skin to another on the same
patient.
This situation
becomes quite a challenge for the
physicians
treating patients for hair removal by using
light. This is
because they are in a very narrow
operational
regime that on the one hand ensures
patient safety,
and on the other hand uses a high
enough energy to
achieve a long clinical effect.
The only real
form of energy that is being used is
thermal, and the
most important parameter for both
safety and
efficacy is temperature. Hence, the key
question is:is
their any reliable way by which we can
measure
temperature in real time (during the pulse)?
Monitoring
skin surface temperature with infrared
sensor appears to
be the most logical way to measure
skin heating.. This method is based on measurements
of thermal
infrared radiation from the skin surface in
the range of
4µ to 12µ. Because of strong
absorption
of this radiation by tissue, the depth of skin
temperature
monitoring is limited by 10-20µ. This
upper part of
skin usually consists of a layer of dead
cells, called
stratum corneum. The thermal and
optical
properties of this layer differ from the living
tissue and its
temperature does not provide enough
information for
skin safety. An additional problem is
that using
skin moistening for effective thermal and
optical coupling
decreases the reliability of measured
data.
Therefore, a new
effective method of skin heating
monitoring
should be developed.
In spite of the
disadvantages of the method, it helps
to understand
the influence of different treatment
parameters on
skin heating.