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Figure 2 - Infrared LEDs are commonly used in remote control applications
Figure 2 - Infrared LEDs are commonly used in remote control applications

The LEDs shown in Figure 2 are all infrared types that are commonly used to send pulses in remote control applications. The perfect example of such an application is your television remote control. A television remote control will use one or more LEDs to send 40KHz modulated bursts of light to the infrared receiver module in the TV in order to control the device. There are many other uses for infrared LEDs, but because of the vast market for consumer remote controls, the availability of these LEDs makes them the perfect choice for night vision illuminator projects. Extremely high power infrared LEDs are also available, but the cost of these devices makes them less attractive than simply using an array of inexpensive infrared LEDs.

The peak wavelength of an infrared LED will determine the wavelength of the radiation. This data is important when matching the LEDs to any device that expects a narrow bandwidth. Television remote controls, for instance, use 940 nanometer infrared LEDs, so the receiver module is matched to that region of the light spectrum, often including a dark plastic filter lens that removes all but the required light radiation. Because our eyes cannot see into the infrared spectrum, some LEDs that include a plastic filter appear completely black, like the one shown in the right of Figure 2. To the imaging system, this dark plastic will appear completely transparent as the infrared radiation is picked up as normal white light.



Figure 3 - These 5mm axial lead infrared LEDs are inexpensive and easy to work with
Figure 3 - These 5mm axial lead infrared LEDs are inexpensive and easy to work with

The infrared LEDs shown in Figure 3 can be purchased from many electronics suppliers for just pennies each, especially when ordering them in bulk quantities. A night vision illuminator made with 32 infrared LEDs will be quite powerful, and may only cost $20 to build if you order your LEDs from an online electronics distributor. When you compare the cost of a 1 watt high power infrared emitter to the cost of 50 common infrared LEDs, the common LEDs will be less expensive and not require any special power supply, so they are currently the best choice. For extremely size sensitive applications, the high power emitters are better, but expect to pay a premium for both the LED base and the special driver circuit needed to power it properly.

The 5mm diameter plastic lead package shown in Figures 2, 3 and 4 are the most commonly used infrared LED, but there are also smaller packages available, including surface mount devices (SMDs). Since the goal is to output infrared radiation in a controlled manner, the 5mm body is ideal as it allows the manufacturer to include a beam shaping lens at the top of the body. SMD leds are usually non focused and for this reason not often used in infrared applications. Being able to control the focal range of the emitted radiation allows infrared LEDs to be tailored to the application. Remote control applications use a medium angle focus to allow the remote control to operate from various angles, yet output a fairly focused and intense beam to the receiver. Infrared LEDs designed for use in night vision systems may have a wider focal range in order to illuminate more of the area being seen by the camera. The focal range of the LED will be shown in the manufacturer's datasheet along with all of the other important specifications.

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