We define a set of pupils, named Fresnel spirals (FS), consisting of v
ariable step spirals with radial transmittances corresponding to Fresn
el zone plates. We calculate the diffraction pattern at near and far f
ields for those pupils. The intensity distribution in those regions wa
s experimentally verified. As for Fresnel zone (FZ) plates, focusing a
long the optical axis is found. In the transverse directions, the diff
raction pattern intensity shows also spiral geometry and the turning s
ense of the spiral reverses after focusing. (C) 1998 Published by Else
vier Science B.V. All rights reserved.