Focal spots: I. MTF separability.

Focal spot models have been based on the implicit assumption that the two dimensional MTF is mathematically separable in the form P(u, v) = P(u)P(v). Experimental results for nine foci are presented. It is concluded that the assumption of separability can be used without introducing a large error. As a consequence of this property, considerable simplification is brought to the measurement and mathematical description of the two dimensional focal distribution.