It looks very similar to something I have used in the past. When looking through the eyepiece a loop of wire is in focus and when a current is applied it glows red and the more current will turn it to orange then all the way to white hot. When placed in front or next to a heat source which in my case was a small furnace the wire would appear lighter or darker than the heat source. Then when a current was adjusted you could match the colour perfectly and it would tell you the exact temp of the inside of a hydrogen furnace. This device may have worked in a similar fashion.