A novel method for testing the collimation of coherent optical beam is presented. Two prisms are used to define a thin air
wedge with a wedge angle. A wide range of beam sizes can be tested down to a few millimeters in diameter. By
redirecting the transmitted beam back to the air wedge so that it experiences a shear in the reversed direction, a double-shearing
interferometry can be established. The gap separation and wedge angle can be varied to maintain high
sensitivity (twice that of the wedge plate design) over the wide beam size range.