We present a simple method for measuring the effective focal length without determining the location of principle plane of the lens. The method is based on the measurement of confocal backreflection axial responses from the front and back surfaces of a reference plate with known refractive index and thickness. We proved the concept by measuring the effective focal lengths of thin singlet lenses and complex microscope objectives. The theoretical limit of measurement precision varies depending on the numerical aperture of the lens. This method can provide an alternative focal length measurement method for complex lenses or lenses that are permanently attached to other structures. Measurement errors were analyzed theoretically and improvements in measurement accuracy were discussed.
© 2011 Optical Society of America
Original Manuscript: April 29, 2011
Revised Manuscript: July 5, 2011
Manuscript Accepted: July 6, 2011
Published: September 9, 2011
Do-Hyun Kim, Dexiu Shi, and Ilko K. Ilev, "Alternative method for measuring effective focal length of lenses using the front and back surface reflections from a reference plate," Appl. Opt. 50, 5163-5168 (2011)