A DIY hologram project based on AIRR ("Aerial Imaging by Retro‐Reflection") developed by Prof. Yamamoto. Using two optical component, if you count the observers eyes too it's one more :D This optical system refocuses the light-path in mid air, hence you get the impression of a floating image in mid air.
The main optical components are a half mirror (aka magic mirror) and a retroreflector. A retroreflector is like a mirror, but it reflects always the photons back to the light sources. For example, all the street signs utilises retroreflectors as a foil to get higher visibility. A similar foil is used for this project, but with a better optical quality and reflectivity value.
The half mirror is self made. The glas is cut from a photo picture frame, and a sun shield / privacy foil was applied.
The following animation shows, from the side, the light-path for the upper half of the half mirror. The blue lines are the retroreflectors, the green line is the half mirror.
http://www.k2.t.u-tokyo.ac.jp/perception/AIRR_Tablet/index-e.html
https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2168-0159.2013.tb06364.x