Sky lanterns and diffraction gratting

It has become quite common to receive from witnesses testimonies of unidentified nocturnal lights which after investigation turned to be simple sky lanterns.
These sky lanterns (aka “Thai lanterns”) are in fact simple hot air balloons using the Montgolfier brother’s principle. The air inside the lantern is heated by a burner, which makes the lantern rise in the air. Once the burner is off, the lantern slowly comes down to the ground.

Using a diffracting grating allows detecting them in few seconds.
By simply placing the diffraction grating in front of the lens, the typical incandescent source spectrum can be easily observed.

This is significant time saving for investigators, but also for the witness who will not have to spend time trying to understand the origin of the observed object.

Thanks to Audrey S. for forwarding us these pictures through the V.I.G.I.E Project website, as well as for the short video showing the Thai lanterns spectrum acquisition.