Near-field diffraction-based focal length determination technique. (May 2017)

• Record Type: Journal Article
• Authors: Torcal-Milla, Francisco Jose; Sanchez-Brea, Luis Miguel
• Is Part Of: Optics and lasers in engineering. Volume 92(2017)
• Page Start: 105
• Page End: 109
• Publication Date: 2017-05
• Subjects: Focal length -- Metrology -- Self-imaging; Lasers in engineering -- Periodicals; Optical measurements -- Periodicals; Optics -- Periodicals; Lasers en ingénierie -- Périodiques; Mesures optiques -- Périodiques; Optique -- Périodiques; 621.36605
• DOI: 10.1016/j.optlaseng.2016.09.008
• Languages: English
• ISSNs: 0143-8166

Abstract

Abstract: An accurate and simple technique for determining the focal length of a lens is presented. It consists of measuring the period of the fringes produced by a diffraction grating at the near field when it is illuminated with a beam focused by the unknown lens. In paraxial approximation, the period of the fringes varies linearly with the distance. After some calculations, a simple extrapolation of data is performed to obtain the locations of the principal plane and the focal plane of the lens. Thus, the focal length is obtained as the distance between the two mentioned planes. The accuracy of the method is limited by the collimation degree of the incident beam and by the algorithm used to obtain the period of the fringes. We have checked the technique with two commercial lenses, one convergent and one divergent, with nominal focal lengths (+100±1) mm and (−100±1) mm respectively. We have experimentally obtained the focal lengths resulting into the interval given by the manufacturer but with an uncertainty of 0.1%, one order of magnitude lesser than the uncertainty given by the manufacturer. Abstract : Highlights: A simple and accurate focal length determination technique is proposed. It is based on self-imaging phenomenon and measures the period of the focused/defocused self-images at different distances from the lens. We achieve an accuracy of 0.1% in the focal length, one order of magnitude lesser than the accuracy given by the manufacturer.