The geometric principle of a swing-arm profilometer as a novel method to measure aspherics are introduced. The surface error of an asphere is obtained by testing its relative departure from some reference sphere

and different reference spheres are realized by adjustment of the length of the swing-arm and the angle between rotary axis and optical axis. A measuring set-up is built based on this principle

which consists of an ultra-precise 2-axes rotating-table

a high stiffness

lightweighted measurement arm

a 4-DOF stage and a scanning sensor

the testing software and data-processing software are programmed with the VC++6.0 and Matlab platform

respectively. The profilometer has advantages in shorter measuring range for a sensor and simple rotary measuring motion

and the disadvantages in complicated calibration and large DOF adjustment. A mathematic model is established to gain the error of conic aspheric parameters and surface error based on the nonlinear optimization of measured data and nominal figure

and the full-apearture of a 3D asphere is tested by measuring several transversals. Finally

a grinded paraboloid (

D

=200 mm

R

=1 400 mm) is tested

results indicate that the system has a good repeatability (

<

0.8 μm) and accuracy (

<

1 μm).