Our church has a multimedia system that usually works well. But sometimes the remote control for the ceiling-mounted projector won鈥檛 work unless the operator stands directly below the projector. This always happens when the sun shines through a window and across the front of the projector. Do the sun鈥檚 rays block the infrared signal in some way, and if so, how does this happen?
Along with visible light, sunlight contains infrared radiation, which the remote control uses, and which passes straight through glass. Its intensity is vastly greater than that from the remote control, so the receiver on the projector is swamped, an effect called ambient light saturation.
The infrared signal received will be more intense than the projector was designed for, and at all frequencies 鈥� as if all the buttons on hundreds of remote controls were being pressed at the same time. Consequently, the projector cannot recognise the remote鈥檚 signal, and so does nothing.
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When standing directly below the projector you are closer to it, so I suspect that the signal from the remote control is stronger and can be differentiated from the background radiation.
Adam J. Hewitt, Guildford, Surrey, UK
Infrared receiver chips use an automatic gain control to pass on the signal at the correct level for decoding. In the presence of a strong infrared source, such as sunlight, the automatic gain control will go to minimum setting. The remote transmitter will then fail to activate the limiter/decoder section of the receiver chip unless the infrared signal from the remote can be increased. The way to do this is for the operator to move close to the receiver. Alternatively, find the location of the receiver sensor on the projector and position a screen to block out the sun.
Brian Powell, Newcastle-under-Lyme, Staffordshire, UK
Infrared signals are subject to 鈥渁mbient light saturation鈥�, and so can be lost amid bright sunlight in the way that torchlight becomes invisible on a sunny day. Large plasma TV screens with their bright displays are notorious for dampening home-cinema infrared remotes. Contrarily, I once owned a TV with such a sensitive IR receiver that it would turn itself off when the room鈥檚 fluorescent light was switched on.
Philip Broughton-Mills, Derby, UK