Which factor influences the effective focal spot size most directly?

• A. Filament size
• B. Anode angle
• C. Glass envelope thickness
• D. Collimation settings

Answer: (B)

The apparent size of the focal spot on the image is determined by the line-focus principle, which ties the image sharpness directly to the geometry of the anode. The anode is tilted, so the actual focal spot on the target is projected onto the image receptor. A smaller anode angle makes this projection smaller, yielding a smaller effective focal spot and crisper detail; a larger angle increases the projected size and reduces sharpness. Filament size sets the size of the actual focal spot at the target, but the factor that most directly changes what you see on the radiograph is the anode angle. Glass envelope thickness and collimation influence other aspects like beam containment and field size, not the apparent focal spot size.