Infrared (or infra-red) LEDs emit light at a wavelength invisible to the naked eye. This makes it difficult to tell whether the LED is working or not – TV remote controls with suspect battery or bad connections being a prime example. Fortunately most digital cameras are sensitive to IR and can be used to test infrared LEDs.
View the LED through the digital camera. When the LED is lit most cameras will display a purple-tinted image of the LED.
The purple hue is interesting.
We can see that the blue sensors respond well in the blue region of the spectrum, and the green and red to theirs. The surprise is that they all respond well in the IR part of the spectrum. The colour rendered by the camera looking at the IR LED will depend on the relative strengths of the colours at the frequency of interest. If we examined a 900 nm IR LED we could expect a blue tinge on the image as the blue sensor is more efficient at this wavelength.
