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1. INTRODUCTION & BACKGROUND
Billions of visible LEDs are produced each year, and the emergence ofhigh brightness AlGaAs and AlInGaP devices has given rise to many new markets. Thesurprising growth of activity in, relatively old, LED technology has been spurred by theintroduction of AlInGaP devices. Recently developed AlGaInN materials have led to theimprovements in the performance of bluish-green LEDs, which have luminous efficacypeaks much higher than those for incandescent lamps. This advancement has led to theproduction of large-area full-color outdoors LED displays with diverse industrialapplications.The novel idea of this article is to modulate light waves from visible LEDsfor communication purposes. This concurrent use of visible LEDs for simultaneoussignaling and communication, called iLight, leads to many new and interestingapplications and is based on the idea of fast switching of LEDs and the modulationvisible-light waves for free-space communications. The feasibility of such approach hasbeen examined and hardware has been implemented with experimental results. Theimplementation of an optical page link has been carried out using an LED traffic-signal headas a transmitter. The LED traffic light (fig 1 below) can be used for either audio or datatransmission. Audio messages can be sent using the LED transmitter, and the receiverlocated at a distance around 20 m away can play back the messages with the speaker.Another prototype that resembles a circular speed-limit sign with a 2-ft diameter wasbuilt. The audio signal can be received in open air over a distance of 59.3 m or 194.5 ft.For data transmission, digital data can be sent using the same LED transmitter, and theexperiments were setup to send a speed limit or location ID information.The work reported in this article differs from the use of infrared (IR)radiation as a medium for short-range wireless communications. Currently, IR links andlocal-area networks available. IR transceivers for use as IR data links are widely availablein the markets. Some systems are comprised of IR transmitters that convey speechmessages to small receivers carried by persons with severe visual impairments. TheTalking Signs system is one such IR remote signage system developed at the Smith-Kettlewell Rehabilitation Engineering Research center. It can provide a repeating,directionally selective voice message that originates at a sign. However, there has beenvery little work on the use of visible light as a communication medium.The availability of high brightness LEDs make the visible-light mediumeven more feasible for communications. All products with visible-LED components (likean LED traffic signal head) can be turned into an information beacon. This iLighttechnology has many characteristics that are different from IR. The iLight transceiversmake use of the direct line-of-sight (LOS) property of visible light, which is ideal inapplications for providing directional guidance to persons with visual impairments. Onthe other hand, IR has the property of bouncing back and forth in a confined environment.Another advantage of iLight is that the transmitter provides easy targets for LOSreception by the receiver. This is because the LEDs, being on at all times, are alsoindicators of the location of the transmitter. A user searching for information has only tolook for lights from an iLight transmitter. Very often, the device is concurrently used forillumination, display, or visual signage. Hence, there is no need to implement anadditional transmitter for information broadcasting. Compared with an IR transmitter, aniLight transmitter has to be concerned with even brightness. There should be no apparentdifference to a user on the visible light that emits from an iLight device.It has long been realized that visible light has the potential to be modulatedand used as a communication channel with entropy. The application has to make use ofthe directional nature of the communication medium because the receiver requires a LOSto the audio system or transmitter. The locations of the audio signal broadcasting systemand the receiver are relatively stationary. Since the relative speed between the receiverand the source are much less than the speed of light, the Doppler frequency shift observedby the receiver can be safely neglected. The transmitter can broadcast with viewing angleclose to 180. The frequency of an ON period followed by an OFF period to transmitinformation is short enough to be humanly unperceivable; so that it does not affect trafficcontrol. This article aims to present an application of high-brightness visible LEDs forestablishing optical free-space links.