Troubleshooting a remote control with a digital camera
Modern remote controls use infrared LEDs to transmit signals to the electronics they control, whether this is a television, DVD player, cable box... or recently, the Nintendo Wii controller. For this reason, unlike omni-directional radio transmitting controllers such as a garage door opener, they must be pointed in the general direction of the television's IR sensor to operate.
The advantage, and disadvantage, of using IR LEDs for this purpose is that they use light that is invisible to the human eye. This makes it unlikely for the sensor to accidentally pick up ambient light as a command signal, and also removes the sensor's light as an unnecessary distraction. However it also makes it impossible to visually inspect the remote control for problems.
One day, messing around with my friend's digital camcorder (with remote control), we discovered that digital cameras are sensitive to the IR wavelengths used by remotes. They show up as a bright white light. This is true of all digital cameras, including camera phones and digital security cameras (nearly all modern security cameras are digital).
As it turns out, my brother worked for Circuit City for a while, and part of their standard troubleshooting routine for customers with remote control complaints was to test out the IR LED with a camera phone. If the LED shows up on the camera phone's display when any button is pushed, then the remote is probably working properly and the fault lies with the sensor. Otherwise, if the front of the remote control remains dark, the remote is probably at fault. This is usually correctable by replacing the batteries or cleaning the battery contacts.
Note: I have heard reports of at least one digital camera that did not display the LED's light. This was a Nikon D70, owned by a serious amateur photographer, so it may have had some feature to filter the IR. This is unlikely to be an issue with camera phones.
This interesting property of digital cameras has a few other applications. For example, the Wii sensor bar uses IR LEDs to give position information to the wireless remote, and a digital camera will identify these easily. The sensor bar uses 5 LEDs on each side, probably for brightness and redundancy in case one burns out. Two candles can actually be used in place of the Wii sensor bar, because the heat from the flame produces enough infrared radiation to be detectable by the Wii remote.
Recently, the proliferation of security cameras monitoring every square inch of London has given birth to a DIY field of counter-measures designed to defeat, or at least annoy, the constant, intrusive, public surveillance of innocent citizens. Once again, IR LEDs can be used to discretely transmit light that is detectable by the camera but not by the unaided human eye. A cluster of high-intensity LEDs such as the I-R.A.S.C. (for "infra-redlight against surveillance camera") device can be used to wash out the image around the light source. This is similar to shining a flashlight directly into someone's face in an otherwise dark area — the area immediately around the light is obscured due to the extremely high contrast. Although the I-R.A.S.C. is designed to be worn on a hat, IR light can penetrate thin layers of cloth so it is possible to hide them discreetly without attracting too much attention.