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.

 

[alphy]

I can provide some insight into the question of whether there is a wavelength shift in the way digital cameras represent color. Digital cameras use sensors called charge-coupled devices (CCDs) or complementary metal-oxide-semiconductor (CMOS) sensors to capture images. These sensors are made up of tiny light-sensitive elements called pixels, which are capable of detecting light at specific wavelengths.

When light enters the camera lens, it is filtered through a series of color filters called a Bayer filter. This filter separates the light into red, green, and blue components, which are then captured by the pixels on the sensor. The camera then combines these three primary colors to create a full-color image.

In terms of detecting non-visible wavelengths, digital cameras are limited to the visible light spectrum. While some cameras may have the ability to capture infrared or ultraviolet light, this is not a standard feature in most consumer cameras. Therefore, it is unlikely that the ghost images seen in the photos were created by detecting non-visible wavelengths.

It is also important to note that digital cameras do not have the ability to "see" things that are not there. They only capture the light that is reflected off of objects in the visible spectrum. In the case of the ghost images, it is more likely that they were created using some sort of post-processing technique, such as image manipulation software, rather than relying on the camera's capabilities.

As for the question of what type of invisible paint could be used to create these ghost images, there are a few possibilities. One option could be using a type of paint that is only visible under specific lighting conditions, such as blacklight. Another possibility could be using a type of paint that is only visible when viewed from a certain angle, known as iridescent paint.

In conclusion, while digital cameras do have the ability to detect specific wavelengths of light and represent them as colors in an image, it is unlikely that the ghost images seen in the photos were created through this method. More likely, they were created using some sort of post-processing technique or special paint that is only visible under certain conditions.