Numerical aperture (NA) is a fundamental optical parameter used to characterize the performance and capabilities of an optical system, such as a microscope or a lens. It is a dimensionless number that provides information about the ability of the system to gather and resolve fine details of objects, particularly in microscopy and imaging applications.
The numerical aperture is determined by the following formula:
��=�⋅sin(�)NA=n⋅sin(θ)
Where:
- ��NA is the numerical aperture.
- �n is the refractive index of the medium between the lens or objective and the specimen (usually air or a specific immersion medium).
- �θ is the half-angle of the maximum cone of light that can enter or exit the lens or objective.
Key points about numerical aperture:
- Refractive Index (n): The refractive index of the medium affects the NA. Using immersion oil with a higher refractive index than air, for example, can increase the NA and improve resolution in microscopy.
- Half-Angle (θ): The half-angle �θ is the maximum angle at which light rays can enter or exit the optical system. A larger �θ corresponds to a larger NA, resulting in better resolution.
- Resolution: Numerical aperture is closely related to the resolution of the optical system. A higher NA allows the system to resolve finer details and distinguish objects that are closer together.
- Collection of Light: A larger NA also means that the optical system can collect more light from the specimen, resulting in brighter and clearer images.
- Objective Lenses: In microscopy, objectives with higher NAs are used for high-resolution imaging, while lower NA objectives are used for broader field-of-view observations.
- Immersion Objectives: Immersion objectives are designed to work with immersion liquids (e.g., oil or water) that have higher refractive indices than air. They can achieve higher NAs and, consequently, better resolution.
In summary, numerical aperture is a crucial parameter in optics and microscopy that quantifies the ability of an optical system to resolve fine details and collect light effectively. It is determined by factors like the refractive index of the medium and the maximum angle of light, and it plays a significant role in the quality of images produced by optical instruments.