Springer-Verlag London 2014
Keyvan Nouri (ed.) Handbook of Lasers in Dermatology 10.1007/978-1-4471-5322-1_1
1. Basic Principles of Lasers: Interactions Between Lasers and Tissue
Salma Pothiawala 1
(1)
Department of Dermatology and Cutaneous Surgery, University of South Florida, 13330 USF Laurel Drive, Tampa, FL 33612, USA
(2)
Department of Dermatology, Laser and Skin Surgery Center of Northern California, University of California Davis, School of Medicine, 3835J Street, Sacramento, CA 95816, USA
(3)
Wellman Center for Photomedicine, Massachusetts General Hospital, 999 Summer Street, Suite 205, Stamford, CT 06905, USA
(4)
Connecticut Skin Institute, 999 Summer Street, Suite 205, Stamford, CT 06905, USA
Omar A. Ibrahimi (Corresponding author)
Email:
Abstract
Lasers have become extremely important treatment devices in the field of dermatology. They have a variety of applications, ranging from the treatment of vitiligo, cutaneous T-cell lymphoma, hair removal, and skin resurfacing, among others. It is therefore fundamental for the clinician to have an understanding of laser-tissue interactions.
Keywords
Dermatology Lasers Physical properties Tissues Thermal properties Optical properties
Introduction
Lasers have become extremely important treatment devices in the field of dermatology. They have a variety of applications, ranging from the treatment of vitiligo, cutaneous T-cell lymphoma, hair removal, and skin resurfacing, among others. It is therefore fundamental for the clinician to have an understanding of laser-tissue interactions.
Spontaneous and Stimulated Emission
LASER is an acronym for l ight a mplification by s timulated e mission of r adiation [].
Spontaneous emission is the process by which an excited atom spontaneously emits a photon. Electrons go from excited to a resting state when a photon of energy is released [].
A laser contains a laser chamber, a lasing medium (solid, liquid, or gas) and an external source of energy. Stimulated emission occurs when the external source of energy causes electrons to be excited in the lasing medium. A cascade reaction is generated when these excited electrons release photons, which then collide with other excited electrons in the lasing medium and cause a release of many identical photons at the same time. Laser light continues to be generated as long as the above cascade perpetuates [].
Laser Light Properties
Monochromacity
As opposed to light from the sun, laser light is monochromatic and emits a well-defined wavelength of light (Table ].
In terms of clinical significance, this monochromatic property of laser light allows it to target specific chromophores, such as water, hemoglobin, and melanin, and allows for specific clinical applications [].
Collimation
Laser beams are parallel to each other, and therefore exhibit collimation. A collimated beam is created in the laser chamber when light is reflected between two mirrors and only the exit of parallel waves is allowed [].
In practice, a lens on a laser focuses the parallel light beam down to the smallest possible spot size, or the diffraction-limited spot, to allow the light to focus on the clinical target [].
Radiometry
The four main concepts in understanding laser light and skin interactions are energy, power, fluence, and irradiance [].
The amount of light emitted from a laser can be quantified by both energy and power. Energy represents work (measured in joules), while power (measured in watts or joules per second) is the rate at which energy is expended [].
The intensity of the laser beam on the skin is a function of the area of skin over which it is spread (i.e., the spot size) [].
Fluence (measured in joules per square centimeter) is the energy density of a laser beam.
Fluence=wattsseconds/cm2=joules/cm2=laser outputpulse duration/spot size
Irradiance (measured in watts per square centimeter) refers to the power density of a continuous wave laser beam, and it is inversely proportional to the square root of the radius of the spot size [].
Exposure time, fluence, and irradiance of a laser can be altered depending on the particular clinical use desired by the clinician [].
Table 1.1
Lasers used in dermatology []
Laser wavelength (nm) | Chromophore |
---|
Excimer 308 | DNA/RNA |
KTP 532 | Hemoglobin |
Pulsed dye 585595 | Hemoglobin |
Q-switched ruby 694 | Blue, black tattoo pigment |
Long-pulsed ruby 694 | Melanin |
Q-switched Alexandrite 755 | Blue, black, green tattoo pigment |
Long-pulsed Alexandrite 755 | Melanin |
Diode 810 | Melanin |
Q-switched Nd:YAG 1,064 | Tattoo pigment |
Long-pulsed Nd:YAG 1,064 | Melanin |
Long-pulsed Nd:YAG 1,320 | Water |
Diode 1,450 | Water |
Er:glass 1,540 | Water |
Er:YAG 2,940 | Water |
Carbon dioxide 10,600 | Water |
Tissue Interactions
Skin Optics
Laser interacts with skin in four possible ways: reflection, absorption, scattering, or transmission [].
Transmission is the passage of light through a tissue without altering either the tissue or the light itself [].
Reflection occurs when light bounces off the surface of this tissue without entry into tissue. Four percent to seven percent of light is reflected off the skin secondary to the difference in the refractive index between stratum corneum and air [].
Scattering refers to the fragmentation of light after it has entered the skin, and it results from the interaction of light with varied elements that makeup tissue. It mainly results from interaction of light with dermal collagen [].
Effects on tissue are only achieved if light is absorbed as this results in the release of photons [].
Thermal Interactions and Selective Photothermolysis
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