This work describes a method for determination of a paraxial focal length of optical systems using measurements of the Strehl definition. The influence of spherical aberration on the value of the measured effective focal length of an optical system is analyzed, and a method is proposed for an elimination of this influence and determination of the paraxial focus position and the paraxial focal length of a lens from the measurement of the effective focal length and the Strehl definition.

Paraxial focal length calculation using Strehl definition measurement

This work describes a method for an experimental determination of a paraxial back focus position and a paraxial focal length of optical systems. It is analyzed an influence of spherical aberration on the value of the measured effective focal length of an optical system and the method is proposed for an elimination of this influence and the determination of the paraxial back focus position and the paraxial focal length of a lens from its effective focal length and the Strehl definition measurements.

Determination of paraxial focal length of lens using Strehl definition measurement

Several methods for measurement of focal length of a positive lens involve the measurement of distances of the object and image planes from principal planes, which are mathematical planes; the positions of which are difficult to figure out accurately. The Talbot method for focal length measurement is a method, which does not require the distances from the principal planes. Further Talbot interferometry can be used to measure focal length variation over the surface of a progressive power lens. A technique for the measurement of focal length using Talbot interferometry and moire effect has been described. The Talbot image of a Ronchi grating placed just after a test lens is superimposed on the second grating to give us the moire fringes. The angle of tilt of the moire fringes can be used to find out the focal length. The effect of unequal pitches of the gratings on moire formation is also investigated. The investigation is also carried out using binary gratings having equidistant circular rings (circular gratings). The theory and experimental results obtained with both types of gratings have been discussed.

Rahul K. Choudhary

Papers

Talbot Interferometry for Focal Length Measurement Using Linear and Circular Gratings