Measuring the lens focal length by laser confocal technique

A new laser confocal focal-length measurement (LCFM) is proposed for the high-accuracy measurement of lens back focal length. LCFM uses the peak point of confocal response curve to precisely identify the lens focus, and uses the elastic contact between reflector R and the lens last surface to determine the test lens last vertex. The distance between the reflectors on these two positions is then measured by a laser distance interferometer, and then the accurate back focal-length of test lens is obtained. Compared with existing methods, LCFM significantly improves the measurement accuracy and simplifies the system structure by laser confocal technique, and reduces the product development cost. LCFM is especially suitable for the back focal-length measurement of large aperture lens.