Focal length measurement based on Fresnel diffraction from a phase plate

Area of Science:

• Optics and Photonics
• Optical Metrology
• Diffraction Physics

Background:

• Accurate measurement of optical system parameters like effective focal length (EFL) and back focal length (BFL) is crucial for imaging system design and performance evaluation.
• Traditional methods for focal length determination can be complex, require specialized equipment, or have limitations in measuring a wide range of focal lengths.

Purpose of the Study:

• To introduce and validate a novel, simplified method for measuring the EFL and BFL of optical imaging systems.
• To demonstrate the applicability of Fresnel diffraction at a phase step for precise optical metrology.
• To provide an easy-to-use technique suitable for a broad spectrum of optical components.

Main Methods:

• Utilizing Fresnel diffraction patterns generated by the phase step at the boundary of a transparent plate illuminated by monochromatic light.
• Analyzing the periodic intensity variations along the central fringe of the diffraction pattern by varying the incident angle of a convergent or divergent beam.
• Measuring the extrema positions of the intensity distribution to accurately calculate EFL and BFL.

Main Results:

• The proposed method accurately determines both EFL and BFL for optical systems.
• Experimental validation using five different lenses showed results consistent with manufacturer specifications.
• The technique demonstrated capability in measuring both positive and negative focal lengths.

Conclusions:

• The Fresnel diffraction-based method offers a simple, accurate, and versatile approach for measuring EFL and BFL.
• The compact and low-noise measuring setup makes it practical for various optical applications.
• This technique provides a valuable alternative for optical system characterization.