Is there a wavelength shift in the way digital cameras represent color

[jleach]

After going on a ghost tour over Halloween, several people were able to take photos of ghosts seen through an old hotel window, a few stories above our position. This got me to wonder if this was a trick that relied on the way that the digital cameras may be able to detect non-visible wavelengths, and display them in an image that we can see. I also wondered what type of invisible paint they would use to paint the ghost images that could only be seen by a digital camera. The ghost images look painted, but you can only see them after taking their picture. How was the trick done?

 

[meBigGuy]

CCD sensors in digital cameras see into the IR a bit. There is a filter in the lens system that filters this out, but it is only so-so (removing that filter is what Sony Night Vision Video is all about)

If they illuminated the pictures with stong IR (invisible to humans) it could show up in the camera but the room would appear dark to humans.

You can prove this by using your camera to look at your infrared remote control. Just look in the viewfinder while pressing the remote.

 

[sophiecentaur]

After going on a ghost tour over Halloween, several people were able to take photos of ghosts seen through an old hotel window, a few stories above our position. This got me to wonder if this was a trick that relied on the way that the digital cameras may be able to detect non-visible wavelengths, and display them in an image that we can see. I also wondered what type of invisible paint they would use to paint the ghost images that could only be seen by a digital camera. The ghost images look painted, but you can only see them after taking their picture. How was the trick done?

I think you basically have the right idea. Many solid state sensors tend to be strongly sensitive to IR. There are three sensors in a colour camera and the long wavelength sensor (people may refer to it as the Red Sensor) is sensitive over a wide spectral range, spilling well into the IR. Most cameras use a filter over the sensor to cut out most of the IR so as to bring the long wavelength sensor response to be more like that of the human eye. It will still let some IR through, though. To produce a fake ghost, all you would need would be a heater to get the surface of an object (actor in fancy dress) hotter than the background and the camera might detect a difference in the stray energy from parts of the 'dark' scene in the window. A conventional 'Infra Red' heater would probably not do the job because they tend to glow red. I believe you can purchase IR flood lamps, which have a filter which cuts out nearly all the visible red bits (for Wildlife and Security cameras).
The fact that you would only get rubbishy images would not matter in this case - even better, perhaps!
If you can get to the image sensor in your digital camera, it is possible, so they say, to remove the IR filter and you then have yourself an IR camera. It's a one way process so only do it with an old camera.

 

[meBigGuy]

No way any heater will make something visible in a consumer digital camera. You need "near IR", like what comes out of your TV remote control or IR flood lights. IR does not mean heat. There is a big difference between near (reflected) and far (thermal) IR. http://en.wikipedia.org/wiki/Infrared#Commonly_used_sub-division_scheme

You cannot remove the IR filter in a CCD camera and sense heat like in a FLIR camera.