does anyone help?

nobody helps me ?

Dear Oguz,

just divide the area of (perpendicular) illumination by the square of the distance (to x-ray focus) .

Solid angle omega= area/distance².

The type of 'area' depends on what you are looking for:

for example:

detector area or

cross-section of x-ray window or

cross-section of slits or

other type of diaphragms

or.....?