As defined on Wikipedia, Spirit photography is a type of photography whose primary attempt is to capture images of ghosts and other spiritual entities, especially in ghost hunting. It was first used by William H. Mumler in the 1860s. Mumler discovered the technique by accident, after he discovered a second person in a photograph he took of himself, which he found was actually a double exposure. Seeing there was a market for it, Mumler started working as a medium, taking people's pictures and doctoring the negatives to add lost loved ones into them (mostly using other photographs as basis). Mumler's fraud was discovered after he put identifiable living Boston residents in the photos as spirits.
Let's think about this for a moment. The idea is taking a photograph of something we, as humans, cannot see with the naked eye. What special property do cameras posses that our eyes lack?
Let's briefly explore how a camera works .
A camera allows light to enter the lens by opening the shutter, the light refracts when entering the lens and converges at a focal point. The photo emlusive film reacts to the light focused on it, creating an image on the film. This is essentially how our eyes work as well. Just replace the film with our retinas.
For this to apply to ghosts, they would need to be able to either emit light or reflect natural light. Meaning, we would see them! Some have claimed that they exist outside of the visual spectrum.
First, we can throw out the idea that they're emitting this energy, because they would have to somehow collect or ingest energy first to transfer radiant energy.
Now, let's begin at the low energy end and work up. Cameras would be very useless if they could "see" radio or microwaves, being that the air is full of them. Not only from space, but radio stations, cellphone towers, satellite transmissions.
Infrared, or IR, is detectable by specific types of cameras. It's essentially heat energy. If heat were being reflected off of a ghost, the ghost in turn would be heated, you could imagine. I can point out that this is contrary to the descriptions made by ghost hunters, claiming ghost create cold spots.
After the IR comes the visible spectrum followed by Ultraviolet. Most ultraviolet rays are blocked by ozone, and those that aren't can be dangerous to humans. Typically, you don't hear of ghost hunters getting melanoma from haunted houses.
Next are X-rays. Prolonged exposure to x-rays can be very dangerous and even more so in the case of Gamma rays, which can destroy cells. These harmful rays are blocked by Earth's atmosphere anyway.
Where do ghost fit in? They don't. Photographic evidence of ghosts will never exist because it simply is impossible.
"But, wait!", you say. I have seen pictures of something that was not visible at the time, but appeared on film.
ORBS, or sometimes "spirit lights", have been a trend in the paranormal community since the mid-eighties. Thankfully, it seems that most people are catching on to the reality behind this phenomenon, but some still cling to the paranormal explanation.
To put this as simply as possible, orbs are particles near the the camera, but out of the focus region, that are illuminated by the camera's flash. The end.
Some paranormalists believe that there are true orbs among the misidentified ones (typically, there own orb photos) and that they can distinguish the two easily. Others say that orbs are not spirits at all, but energy being transfered from batteries and power-lines to the ghost, allowing it to manifest. This idea is easily shot down by the fact that energy travels in the path of least resistance. Traveling down a conductive wire and then exiting through an insulated coating then traveling through the air in a globular form truly breaks the laws of physics. Sorry.
So, having established the fact that spirit photography is impossible, how do I reconcile all of the photographs of ghosts being taken everyday?
There's an app for that.