The target of the X-ray tube is angulated to produce all the following effects, EXCEPT
To decrease the effective focal spot
To increase the image sharpness
To energize the photons
To dissipate heat
The target is inclined about 20 degrees to the central ray of the x-ray beam; this causes the effective focal spot to be approximately 1 mm × 1 mm, as opposed to the actual focal spot, which is about 1 mm × 3 mm. 
This smaller effective focal spot results in a small apparent source of x rays and an increase in the sharpness of the image, with a larger actual focal spot size to improve heat dissipation.
Ref: Oral Radiology, Principles and Interpretation / Stuart C. White, Michael J. Pharoah - 7th ed - pg - 6