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228 D. Barolet Figure 1 Niels Ryberg Finsen (1860-1904). Courtesy of the Clen- dening History of Medicine Library, University of Kansas Medical Center. 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 pack- aged, 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 sur- face mounting. Through hole mounted devices are often re- ferred 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 manu- facturers take such individual chips and affix them to printed circuit boards, creating so-called “chip-on-board” LED ar- rays. 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- Figure 2 Finsen’s phototherapy. Due to expense of carbon arc light- ing, 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) Figure 3 LED technology. The red arrows indicate the flow of heat. Courtesy of Stocker Yale, Inc. 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 com- pared 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 has been deemed a non- significant risk by the Food and Drug Administration and has been approved for use in humans. Other advantages over lasers include the possibility to combine wavelengths with an array of various sizes. LED disperses over a greater surface area than lasers and can be used where large areas are tar- geted, resulting in a faster treatment time. Mechanism of Action In the same way that plants use chlorophyll to convert sunlight into plant tissue, LEDs can trigger natural intracellular photo- biochemical reactions. To have any effect on a living biological system, LED-emitted photons must be absorbed by a molecular Figure 4 A t-pack LED.PDF Image | (LEDs) in Dermatology
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