Light Emitting Stones

Again something from Piet Lassche. Years ago he found in his neigbour's garden typical stones. He was told that these stones were used by the Germans during the second world war. It were light emitting stones and used as lines on airports. I tested them with a radiationmeter, but nothing happened. The question now is, who knows something about these stones??????

The top of the stones.

The layer on the top was emitting the light.

The bottom of the stone, why this typical hole??.

The typical surface.

Who knows this logo??

Type-number ???

Did it look like this ???

Well, the answer I found during a visit in september 2015 on the former military airbase "Twenthe" in the eastern part of the Netherlands.
I had an explanation of the former German airfield during WW2. It appeared that the runways of the field were not provided with lights, the enemy could find them, but with stones. They had a surface of white enamel that reflected the light of the moon or the headlights of the airplanes. So there was no active radiating surface. The stones can still be seen on Twente Airport near Enschede, Holland. See picture .