Avenue for apertometer using spectral plane and Fourier correlation analysis.

Area of Science:

• Optical microscopy
• Metrology

Background:

• Accurate numerical aperture (NA) measurement is crucial for high-resolution imaging in microscopy.
• Existing NA measurement techniques can be complex or require specialized equipment.

Purpose of the Study:

• To propose a simple, practical, and cost-effective approach for measuring the numerical aperture of immersion objectives.
• To develop an automated algorithm for enhanced repeatability and reduced operational constraints.

Main Methods:

• Utilizing the Abbe sine condition with a reference generated on the spectral plane.
• Employing a coverslip on the focal plane to create a total internal reflection (TIR) ring as a reference.
• Implementing a two-dimensional (2D) Fourier correlation analysis and grayscale statistics for automated identification of TIR and clear aperture radii.

Main Results:

• Demonstrated a simple and effective method for NA measurement of immersion objectives.
• Achieved high repeatability and reduced operational complexity through an automated algorithm.
• Successfully applied the method to measure an objective lens with adjustable pupils using surface plasmon resonance as a reference.

Conclusions:

• The proposed method offers a practical and accessible solution for numerical aperture measurement.
• The automated algorithm enhances the reliability and ease of use for NA determination.
• This technique is versatile and applicable to various objective lenses, including those with adjustable pupils.