Digital sensors are sensitive to infrared light as well as the visible spectrum. Usually this is a bad thing for normal photos, so most cameras have an infrared blocking filter in front of the sensor which stops most (but not all) infrared to prevent it having an effect under most conditions.
To take photos in infrared we need to reduce the amount of visible light hitting the sensor and swamping the tiny amount of infrared that gets through the filter. The easiest way to do this without modifying the camera is to make another filter which allows infrared to pass but blocks most or all of the visible light. Because this method allows very little light to reach the sensor exposure times are usually fairly long, typically about 5 to 30 seconds in daylight. Unless the camera can meter using it’s sensor (point and shoots and DSLRs in liveview mode) you’ll usually need to shoot in manual mode when using the filter since the separate metering sensors aren’t designed for infrared.
Making the Filter
The filter material needs to be transparent to infrared light but opaque to visible light. Some things that work well are unexposed (black) slide film, overexposed negative film and lighting gels (if you combine 2 or 3 layers of different colours so no visible light gets through). Whatever you use you’ll need several pieces big enough to cover the camera’s lens.
You’ll need to experiment with the number of layers depending on how effective the camera’s infrared blocking filter is and what kind of effect you want.
There are two ways to do this, the single exposure method and the double exposure method:
Double Exposure Method
The double exposure method consists of taking two separate exposures, one infrared (using the filter) and one visible, then combining the two images afterwards so the infrared image (converted to monochrome) replaces one of the colour channels.
In this example the IR channel replaces red, the red channel replaces green and the green channel replaces blue, giving the same effect as colour infrared film.
This method can give better looking images, but care needs to be taken to make sure the IR channel isn’t under or over exposed compared to the visible image (not too hard to fix but you might need to experiment a bit) and the two images are exactly alligned.
Using the infrared image as a saturation or value layer can also have some interesting effects.
Single Exposure Method
The single exposure technique relies on a bit of visible light getting through the filter and some colours of pixels on the sensor being more sensitive to infrared than others. For this to work you’ll probably need to experiment with the filters a bit so they let just enough visible through, try different numbers of layers of material and/or combinations of lighting gels.
When it’s working you should be able to see a difference between infrared and visible light (clear sky sould be a different colour to green plants). This is the image straight from the camera with levels adjusted (false colour images are often fairly low contrast)
In this example IR looks purplish and visible looks greenish here, but it varies between cameras and filters. For best results set your camera’s white balance to custom, and use something that reflects more or less equal amounts of visible and infrared as the white point, Clouds and tarmac work well for this.
Colours will vary depending on the camera and filters used, For best results experiment with Photoshop or GIMP’s channel mixer tool to re-arrange channels.