Finsen’s phototherapy. Due to expense of carbon arc lighting, single lamp directed light through four water-cooled focusing lenses, allowing several patients to be treated simultaneously. Each patient had nurse attendant to focus light to single small region for up to 1 hour. (Reprinted from Bie V: Finsen’s phototherapy. BMJ 1899;2:825) 

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... therapy is one of the oldest therapeutic modalities used to treat various health conditions. Sunlight benefits in treating skin diseases have been exploited for more than thousands of years in ancient Egypt, India, and China. Solar therapy was later rediscovered by Niels Ryberg Finsen ( Fig. 1, Fig. 2), 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 vul- garis. Phototherapy involving the use of an artificial irradiation source was born. 1 It was only many years later that light therapeutic benefits were uncovered again using other segments of the electro- magnetic 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 his surprise, the laser did not cause cancer but improved hair growth that was shaved off the animal’s back for the purpose of the experiment. This was the first demonstration of “photobiostimulation” with low-level laser therapy (LLLT), thereby opening a new avenue for medical science. This casual observation prompted 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. The use of LLLT and LEDs is now applied to many thousands of people worldwide each day for various medical conditions. LED photobiomodulation is the newest category of nonthermal light therapies to find its way to the dermatologic armamentarium and will be the focus of this review. Initial work in this area was mainly developed by National Aero- nautics 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 in- vestigated 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). LEDs are complex semiconductors that convert electrical current into incoherent narrow spectrum light. LEDs have been around since the 1960s but have mostly been relegated to showing the time on an alarm clock or the battery level of a video camera. They have not until recently been used as sources of illumination because, for a long time, they could not produce white light— only red, green, and yellow. Nichia Chemical of Japan changed that in 1993 when it started pro- ducing blue LEDs which, combined with red and green, produce white light, opening up a whole new field for the technology. The industry has been quick to exploit it. LEDs are based on semiconductor technology, just like computer processors, and are increasing in brightness, energy efficiency, and longevity at a pace reminiscent of the evolution of computer processors. Emitted light are now available at wavelengths ranging from ultraviolet (UV) to visible to near infrared (NIR) bandwidth (247 to 1300 nm). LED arrays are built using diverse methods each hinging on the manner in which the chips themselves are packaged by the LED semiconductor manufacturer. Examples of packaged, lensed LEDs are t-pack LED and surface mount LEDs (Figs 3-5). These packages can be affixed to a heat-sinking substrate by using either a “through hole” mounting or surface mounting. Through hole mounted devices are often referred to as t-pack LEDs. Importantly, it is also possible to procure wafers of bare, unpackaged chips, also called “dice.” By using automated pick-and-place equipment, some manufacturers take such individual chips and affix them to printed circuit boards, creating so-called “chip-on-board” LED arrays. LED array is thus assembled on a printed circuit board. The pins or pads or actual surfaces of the LED chips are attached to conductive tracks on the PCB (printed circuit board). Assemblies built from t-pack LEDs are often unsatis- factory in that they do not always provide sufficiently uni- form lighting, are not well heat-sinked, and they are bulky due to the size (several millimeters) of each t-pack device. Nonetheless, for certain applications, t-packs prove to be the most appropriate, cost-effective solution. However, when t- packs cannot provide the required performance, however, chip-on-board emerges as the answer. A significant difference between lasers and LEDs is the way the light energy is delivered [optical power output (OPD)]. The peak power output of LEDs is measured in milliwatts, whereas that of lasers is measured in watts. LEDs provide a much gentler delivery of the same wavelengths of light compared to lasers and at a substantially lower energy output. LEDs do not deliver enough power to damage tissues and do not have the same risk of accidental eye damage that lasers do. Visible/NIR-LED light therapy ...