Focal Length Measurement Of Long-focus Lens Based On Talbot-Moiré Technology

The focal length is the most important optical parameter of a lens or system andis measured by various methods.But it is difficult to exmaine a long focal-lengthlenses with large aperture, which are widely applied in various technical domains suchas ICF driver, astronomical optical system, and aerial remote-sensing camera and soon. Related measurements of long focal length are mostly based on Talbot effect andMoiré fringe technology.The theories and applications of Talbot effect and Moiré fringe wereintroduced.Various methods based on Talbot-Moire technology were analysed andcompared,too.A novel method for the long focal length measurement of large aperturelens was presented and proved by some experiments of measurement of lenses withfocal lengthes of420mm and1000mm.Firstly,the second grating was set as thereference to calibrate the equivalent length of CCD pixel by comprehensive utilizationof the Canny operator, morphologic thining and sub-pixel edge detecting based ongray-scale fitting Gauss curve.Secondly,to calculate the width of Moiré fringesaccurately,the error was reduced or corrected through secondary calibration techniqueand least square method.An image processing algorithm, which includes denoisingthrough airspace, frequency-domain filter and morphologic operations,was utilized todetermine the location of centre line.Then the slope of Moiré fringes was calculatedby fitting line through least square method.Eventually the focal length of measuredlens was obtained accurately.The results indicate that the method is of excellent accuracy and repeatability. Onthe same condition, the measurement of lens with focal length of1000mm can reachhigher accuracy and better repeatability.Finally, the accuracy of focal lengthmeasurement and effective measures to reduce the error were analyzed.The method has advantages of easy principle, high accuracy and simple device, itis theoreticaly appropriate to the measurement of lens with long positive or negative focal length.Beaause this method is based on the phase variation of lens, it applies tonot only the focal length measurement but also the profile detections of lenses.