Light therapy is one of
the oldest therapeutic modalities used to treat various health conditions
in history. Sunlight benefits in treating some skin diseases have been
exploited for more than thousands of years in ancient Egypt, India, and
China. Solar radiation therapy was later rediscovered by Niels Ryberg Finsen, a Danish physician and scientist who won in 1903
the Nobel Prize in Physiology or Medicine in recognition of
his contribution to the treatment of diseases, notably lupus vulgaris and
open skin tuberculosis. Phototherapy involving the use of an artificial
irradiation source was born.
It was only many years later that light
therapeutic benefits were uncovered again using other segments of the
electromagnetic spectrum (EMS) with visible and
near-infrared wavelengths. In the late 1960s, Endre Mester, a
Hungarian physician, began a series of experiments on the
carcinogenic potential of lasers by using a low-powered ruby laser (694 nm)
on mice. To everybody surprise, the laser did not cause any cancer but
improved the hair growth. This was the first demonstration of
“photobiostimulation” with low-level laser therapy (LLLT), thereby opening
a new applications for medical science. This lead him to conduct other
studies provided support for the efficacy of red light on wound healing.
Since then, medical treatment with coherent light sources (lasers) and
noncoherent light (light-emitting diodes, LEDs) has expanded.
LED photobiomodulation is the latest
category of nonthermal light therapies to find its way to the
dermatologic armamentarium. Initial work in this area was mainly
developed by National Aeronautics and Space Administration (NASA). NASA
research came about as a result of the effects noted when light of a specific
wavelength was shown to accelerate plant growth. Because of the deficient
level of wound healing experienced by astronauts in zero-gravity space
conditions and Navy Seals in submarines under high atmospheric pressure,
NASA investigated the use of LED therapy in wound healing and obtained
positive results. This research has continued and innovative and powerful
LEDs are now used for a variety of conditions ranging from cosmetic
indications to skin cancer treatment (as a photodynamic therapy light
source).
THE TECHNOLOGY
A light-emitting diode (LED) is complex semiconductor light source that convert
electrical current into incoherent narrow spectrum light. LEDs are used as
indicator lamps in many devices and are increasingly used for other lighting.
Appearing as practical electronic components in 1962, early LEDs emitted
low-intensity red light, but modern versions are available across the visible,
ultraviolet and infrared wavelengths, with very high
brightness.
When a light-emitting diode is switched on, electrons are
able to recombine with electron holes within the device, releasing energy in
the form of phatons. This effect is called electroluminescence
and the color of
the light (corresponding to the energy of the photon) is determined by the
energy gap of the semiconductor. An LED is often small in area (less than
1 mm2), and integrated optical components may be used to shape
its raduiation patern. LEDs present many
advantages over incandescent light sources including lower energy consumption,
longer lifetime, improved physical robustness, smaller size, and faster
switching.
To understand why a LED emmit a certain wavelight it is
necessary to analyse it’s components: The LED consists of a chip of
semiconducting material doped with
impurities to create a p-n junction. As in other diodes, current flows easily
from the p-side, or andode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrones and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a
lower energy level, and releases energy in the
form of a photon.
The wavelength of the light emitted, and thus its color depends on the band gap energy of the materials forming the p-n junction. Infrared and red devices are made with gallenium arsenide. Advances in
materials have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.
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