This thread should give an introduction in to the X-ray tube techniques with a strong focus on the focal spot.
Lets start with the components of an X-ray tube.


The distance between Cathode and Anode is the way for the electrons on the way to the target (red dot in the picture). 98% and more of the quants are transformed to heat, less the 2% is X-ray radiation. The heat has to be transfered out of the tube - mainly by a Cu block which is cooled by water or oil (like the engine of a car).


Most classical tube have two focal spots; this tube has two different filaments, one for the small focal spot and one for the large focal spot.


The focal spot for use can be selected in the control unit of the X-ray system. The filament is heated to make the electrons more mobile that they could fly the way to the anode.


Most filaments today are wire spirals (alternativ materials - e.g. from Scanning electron microscopes - will enter the market in 2020 with more homogeneously intensity distribution of the focal spot) with the effect, that the projected area of the filaments have two intensity maxima on the focal spot

(drawings from (c)ASTM E1165)

Let me show some examples for different applications.
1. Old „classical“ X-ray tubes for film applications with low magnification (film in contact with the test object)


2. „Modern“ X-ray tube for medium resolution and high dose (e.g. castings for automotive - also Inline-CT)


3. „Modern“ X-ray tube for high resolution and medium dose (e.g. welds, aerospace, high quality CT, AM)


4. µ-Focus X-ray tube for highest resolution (e.g. electronics, semiconductors, aerospace)

Now we look directly onto the "burning" focal spot:


1. with very low dose (less than 1/20 of max.)

2. with low dose (about 1/5th of max.) - here you can already see the shape of the focal spot

3. with medium dose and already too bright for the digital camera which took the picture

4. max dose (much too bright for 1/8000s with F22 on ISO 100)