Optical versus thermal focal spot size

X-ray tubes with reflection target are benefiting from the so called "Goetze - principle". The optical length of the focal spot will be reduced with smaller target angles. In this way higher load can be positioned at the same focal spot. The termal focal spot can be calculated with the following table:
The optical spot varies with different directions looking at the target. The table below shows the calculated optical focal spot size at different angles.

DIN EN 12543-2

The measurement according to
DIN EN 12543-2 gives a direct view on the focal spot by means of a pinehole camera. It is taken for focal spots larger than 100µm.

DIN EN 12543-5

The method according to
DIN EN 12543-5 is taken for small focal spots below 100µm. The focal spot size is calculated from the unsharpness of e.g.
a tungsten ball.