our webcam has a sensor behind the lens which is sensitive to most colours of light and IR (and also ultra violet). It has a infrared lens on the front which stops the IR, and then a series of filters on the sensor chip itself to only allow one colour through to each sensor on the chip so they can detect different colours.
Camera working normally
Our camera
In a webcam the IR is stopped by the infrared lens, and then there are coloured filters in front of each pixel so the camera can determine colour.
If we put a visible light filter on the camera only the IR can get through which goes through all the coloured filters.
When we just let IR into the camera by putting our infrared lens on the front it will pass through all the coloured filters to slightly varying degrees, as these infrared lens are not designed to stop it.
Why is my webcam sensitive to IR anyway?
Webcams detect light by having lots of small sensors on a piece of silicon. Each of these sensors is a small diode – a one way valve for electricity, and the electronics is trying to push electricity the wrong way through this diode.
This normally doesn’t work because a diode is designed so that there are no free electrons to carry an electric current when you try and push electricity the wrong way. But if a photon of light with enough energy hits the middle of the diode it will knock an electron off a silicon atom which can now move and carry electric current. The electronics then measures this current and so how much light has hit the sensor.
A sensor
Light on Photodiode
Most webcams use photodiodes as sensors, these have an area in the centre with no fee electrons to carry current.
If light hits the photodiode it knocks an electron off an atom, which can now move carrying electric current
Any colour of light above the mid infra red (so near IR, visible, UV etc) will have enough energy to do this, so your camera is sensitive to infra red light. More expensive CCD chips found in real digital cameras work slightly differently but on the same principle, so they are also sensitive to IR.
This article comes from thenakedscientists edit released