Greetings. Please bear with me as this is my first Instructable. However, I have built this effect for this last Halloween and have had some pretty good results with it. So I thought I'd share.
Videos and pics of my test setup for this effect, as well as my entire Halloween setup for 2014 are at the following link: https://goo.gl/photos/aEG3HtFUVeJmWEsz9
UPDATE: This is also the project that was mentioned in the October 2015 issue of Popular Science (page 67).
The video above demonstrates the effect. The basic setup uses a projector, a PIR motion detector, raspberry pi, and a piece of plexiglass with a thin spray coating of reflective paint. A loud, startling transparent image appears when a person walks in front of it. I will go over the steps and shell script code used in creating this effect.
The following steps will outline the creation of each of the components of the effect and some possible alternatives.
Plexiglass or Lexan (as large as you need). May be purchased at a place like Home Depot or Lowes.
Reflective spray paint (such as Reflect-All, Rust-Oleum 214944 Reflective Finish, or Night Brite). These are a little harder to find in most of the brick and mortar stores and you may have to go online. Alternatively, you can try a very thin/sparse coat of glass frosting spray paint, which is easier to find.
Wood pieces for frame. I happened to have some left over baluster pieces from a deck project.
Screen - Lay the clear screen down on a tarp and spray a very thin layer of paint over one side of the screen. You do not need much paint at all. I essentially tried to mist on the paint from a good height in order to get a nice sparse, relatively uniform layer. Try practicing first on some cardboard or scrap pieces to get the technique down before spraying onto the screen. Ideally, your screen should remain transparent and nearly clear.
Frame - A large piece of plexiglass is likely too floppy to stand well without a support frame. Lexan (polycarbonate) is stiffer but may be more brittle (and is typically more expensive). You can either build a standing frame or hang the screen from an overhead support. I had to build a standing frame, since I was using this effect outdoors.
I bolted together some leftover deck rail balusters that I had laying around, as shown in the picture above. The screen is sandwiched between pairs of supports at the bottom and side(s) and tightened with bolts drilled through the support pairs. The support legs at the bottom are also drilled and bolted to one of the bottom supports, as shown. Right angle brackets have been screwed into the side and bottom supports.
Theatrical scrim - or any type of thin semitransparent gauzy material hung from a doorway or other support. In the right light (ie. backlight), this should be pretty transparent from the front but still act as a good medium to project a ghost onto. This is used to good effect in this YouTube video. More detailed video instructions are here.
TransScreen - Used in movies and advertising. This is the type of screen my project is trying to simulate at a much cheaper price.
Fog Screen - If you are really ambitious, this is a great effects screen and several people have posted their own builds: