How Does it Work?
Firstly one important point : The term "x-ray" was coined to describe the effect that filming with infrared light only has on certain types of fabric. It does not mean that this technique uses real x-rays like a hospital or dentist uses.
This filming technique takes advantage of the fact that certain camcorders, in night-vision mode are very sensitive to infrared light. Much more so than an ordinary camcorder.
Infrared light has a longer wave length than visible light which causes less of it to be reflected by certain types of material and fabric than visible light is.
The effect is that the light passes through the fabric, reflects off anything underneath and passes back out through the fabric again. This effectively renders the fabric semi-transparent - it typically takes on a "net curtain" type appearance, similar to very sheer clothing. In some cases the fabric is barely visible when viewed this way.
In order to see this effect, it is necessary to filter out all visible light entering the lens. The reason is simple:
When the visible light is reflected by the fabric it obscures any image which would be visible with only infrared.
It's surprising the number of people who fail to realise the importance of the filter.
You cannot film infrared x-rays in daylight without an infrared filter!
Imagine trying to look at someone in a dimly lit place while they are shining a torch at your eyes. You cannot see them at all, but block out the torch light and you can see the person clearly.
More information on filters.
What is Infrared Light?
Infrared light is light which has a wavelength longer than that which the human eye can see. If infrared light were visible you'd see it above the red in a rainbow. The graph below shows where infrared lies on the electromagnetic scale:
Note that this graph is simplified for use here.
You can see here that visible light lies between 350nm and 780nm from violet to red. Similarly, ultra-violet light is also invisible to the eye but lies above the range of visibility.
Infrared is often used for surveillance where it is necessary to monitor dark places without people being aware of the filming. The cameras can see the infrared light clearly lighting the scene up, but to the human eye it is totally dark.
Real x-rays, such as those a doctor would use appear well above ultra-violet and are not shown on this scale. Below infrared are microwaves and radio (also not shown).
So Why is a Night Vision camcorder so Special?
Theoretically, any camcorder or CCD (charge coupled device) based camera could be used for filming infrared light. However, all colour video cameras and some black and white units are designed to eliminate infrared light.
Normally, infrared light is deliberately blocked from reaching the CCD in a video camera by using a special infrared blocking filter. This is done because infrared light is not needed to make up the picture and would only make the colour reproduction incorrect. All light hitting the CCD creates electronic signals which are picked up by the camera's circuitry. Since these signals are used to create the video output, any infrared light appears in the final picture as a visible colour such as white, red or green (depending on the camera) - usually an undesirable effect.
The internal infrared blocking filter is typically a small piece of glass mounted in front of the CCD by various means and is not usually considered to be a removable part.
An internal infrared blocking filter
The filter itself appears tinted to the human eye. The colour may vary but it is usually a light blue or purple shade. As with any photographic filter, some unwanted light is actually passed through. Operating an infrared remote control pointing at a camcorder lens shows this clearly on screen.
With some recent models of Sony Handycam and Panasonic Movie Camera, the manufacturers took advantage of the CCD's tendency to pick up infrared light and installed a mechanism inside which shift this small filter aside allowing all infrared light through. This allows the camera to film in considerably lower light because it can "see" any infrared light along with the visible light present, greatly increasing the total amount of light actually being filmed.
To enhance the effect, they fitted high power infrared emitters in the front of the camera to allow filming in complete darkness. The camera's video processing circuits also modify the image colour, giving it a green tint.
The intended use of 0 LUX night vision
The above picture was taken at a range of about 8 feet in complete darkness. The built in infrared illuminators were used (hence the cat's glowing eyes).
When filming infrared x-ray, the emitters are usually switched off (if the camcorder supports this option).
Removal of an Infrared Blocking Filter
In my opinion this should not be attempted. The camera that above filter came from had to be completely stripped down before removal was possible. The job took several hours and involved careful de-soldering of surrounding components.
Upon reassembly, the camera's auto-focus seemed to be slightly out and the picture colours were badly effected, giving everything a red tint and lamps and flames appeared unnatural. The camera was now useless for normal filming. However, filming infrared x-ray was now possible.
The camera in question was a Sony Handycam which was produced prior to Sony inventing the Nightshot feature.
Basically, if you have a great deal of time and patience and you don't mind ruining a working camcorder then you might attempt this.
The best solution all round is to buy a Sony Handycam that has Nightshot or a Panasonic NV series Movie Camera with Nightview.