Abstract
A novel method for long focal length measurement is presented, which is accomplished by obtaining the angle of moire fringe formed by Talbot effect of Ronchi grating. When the Ronchi grating was illuminated by spherical wave (plane wave pass through a lens), the period of the Talbot image of the grating, which has a relationship with the radius of the spherical wave (focal length of the lens), will change. When put another Ronchi grating at the place of the Talbot image, moire fringe formed. Correspondingly, the angle of the moire has a relationship with the focal length. So the focal length can be measured by calculating the angle of the moire fringe. The formula of calculating the long focal length from the angle of moire fringe is derived. Error analysis demonstrates that this method can be applied to the real-time testing with high accuracy.
