Raman Imaging and Raman Mapping - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 101

5.2.9 Thermal Damage

Thermal damage from a tightly focused laser beam limits the amount of power
that can impinge on a sample, and therefore may determine the time required
to collect a Raman spectrum. More power can be used if the light is distributed
over a greater area on the specimen. An illumination geometry that distributes
heat effectively and so reduces thermal damage is said to have a power distri-
bution advantage [27]. Although there is still no consensus definition of this
parameter, the thermal damage threshold can be measured experimentally for
any material and the power distribution advantage can be defined relative to a
standard condition, such as point focus of a laser of a specified wavelength. It
is also possible to estimate for different illumination geometries the maximum
power that can impinge on a sample without causing thermal damage.
Using classical heat transport theory, Zhang et al. have evaluated several
illumination geometries for effective heat dissipation [28]. Under steady-state
conditions, for a fixed illumination area and laser power the best geometries
are as follows: array of spots > line (high aspect rectangle) > rectangle

≈
circle. These results are understandable because heat generated at a point can
be conducted away in all directions in which the heat source does not extend. If
global illumination is used, then except at the edges of the illuminated region,
heat can be dissipated in only one dimension (axial), while for an array of
spots heat can be dissipated in all three dimensions. What is surprising is
how few points are required for an array of tightly focused spots to match
the heat dissipation of a focused high aspect rectangle. If the total areas are
equal, four spots have the same power distribution advantage as a rectangle
of aspect ratio 35:1, while only nine spots are required to match the power
distribution advantage of a rectangle of aspect ratio 120:1.
Laser excitation power is a limiting factor in the application of non-invasive
in vivo Raman imaging and tomography. Several guidelines for laser exposure
limits exist, including guidelines for safe use, as well as maximum permissi-
ble exposure limits for eye and skin. Organizations providing guidelines for
laser exposure include the American National Standards Institute (ANSI), the
Food and Drug Administration (FDA), the Occupational Health and Safety
Administration in the United States (OSHA), and numerous other agencies
worldwide. In addition to exposure limits, factors such as pigmentation and
scarring will alter the risk associated with laser irradiation on skin. A variety
of skin type classification systems exist which may help in guiding exposure
based on skin pigmentation, including the Fitzpatrick and Roberts skin type
classification systems.

5.3 Selected Applications

Several recent studies that illustrate the scope of Raman imaging to fields of
study not otherwise covered in this volume are presented here.