光學傳感器應用筆記：點源發射器

　　LED（發光二極管），半導體光源，已經引進和發展了幾十年。LED被應用在許多設備中，如指示器和一般照明產品，例如照明元件。作為一種綠色光源，LED可以為許多應用提供長壽命和高效率的光源。然而，對于一些特殊的應用，標準的LED并不總是完美的選擇。點源排放（PSE）提供需要精確的光束，如編碼器應用的一個不錯的選擇，機器視覺和醫用纖維。

　　PSE是一種半導體二極管結構類似于一個標準的LED，但是，光是通過一個明確界定的圓形區域，通常是25μm - 200μm直徑發射。產生的光是“光斑”，輸出光產生非常窄的幾乎平行的視角。與激光二極管相比，這兩個特性非常適合于需要接近平行光源和更低功率的應用

　　Figure 1 shows side view structure of standard LED and PSE. The first difference in these two structures is emitting light direction. Standard LED output light is directed to the side. In order to refocus the light direction， standard LEDs normally need a reflective cavity to force the light from the side to the top. This can cause light output loss， power dissipation， and variations in final output light beam and viewing angle. However， PSEs emit light to the upper surface through an aperture / window on top of the structure. The second difference in these two structures is the position of the cathode contact. The cathode contact pad of a standard LED is typically located in the center of the structure， which can obstruct light output due to the top wire bond. SEs can easily solve this problem by locating the cathode contact wire bond to the side of the aperture window， eliminating any obstructions and dark spots （represented in Figure 2）。 The light emitted from the standard LED （left in Figure 2） has several dark spots due to the bonding pad， obstruction from the wire bond as well as the reflector cup （shown in Figure 3）。 A PSE （right in Figure 2） has a much more narrow， defined， and precise beam with no dark spots.