Contains four apertures for Heiland TRD-Z densitometer. The diameters are 0.5, 1.0, 2.0 and 3.0mm.
The smaller apertures reduce the amount of light which passes through and therefore reduce the max density which can be measured.
The aperture can be changed by pressing on the border of it.
When the aperture is changed, the zero point has to be set again.
