It consists of two rotating disks with holes in them. The holes are arranged on each disk at equal angular intervals and a constant distance from the centre, but the holes in one disk do not align with the holes in the other. A sample of phosphorescent material is placed in between the two disks. Light coming in through a hole in one of the discs excites the phosphorescent material which then emits light for a short amount of time. The disks are then rotated and by changing their speed, the length of time the material glows can be determined.