Numerical aperture (NA) is an important parameter in an optical system that describes the angular range of light that the system is capable of collecting.
How is the NA defined? What are the factors?
1 Definition: A numerical aperture is a dimensionless number that measures the angular range of light that an optical system is capable of receiving or emitting, and in turn, the system's ability to collect light.
2 Formula: The numerical aperture NA is the product of half of the refractive index (n) and aperture angle (2α) of the medium between the lens and the object to be examined. The formula is expressed as: NA = n * sin α. The aperture angle (α) is the angle formed by the object point on the optical axis of the lens and the effective diameter of the front lens of the objective.
3 Function: A higher numerical aperture can make it easier to focus the beam to a smaller size, which can provide higher resolution and focusing ability.
In the field of optics, NA describes the size of the lens light collection cone angle, which determines the light collection ability and spatial resolution of the lens. In the field of optical fibers, NA describes the size of the cone angle of light as it enters and exits the fiber.
4 Influencing factors: The size of the NA is affected by the performance and design limitations of the optical components (e.g., objective lenses, lenses) used in the optical system. Different optics have different NA ranges, and the NA also varies with wavelength.
5 Application: In microscopes, NA determines the resolution and contrast of the microscope; In optical fiber communication, the NA affects the coupling efficiency and transmission performance of the optical fiber.
Is it better to have a larger NA?
A higher value of NA does not always mean better, depending on the specific application needs.
1 Resolution: The larger the NA value, the greater the angular range of light that the optical system is able to collect, which means that the system is able to resolve smaller details, i.e., the higher the resolution. In microscopic imaging, high NA objectives typically provide higher resolution.
2 Contrast: The NA value will also affect the contrast of the image. In general, the higher the NA value, the higher the contrast of the image, and the more light is collected and delivered to the imaging plane.
3 Depth of focus: An increase in NA value usually leads to a decrease in depth of focus. Depth of focus is the extent of an object's space where the image remains sharp (located near the focal plane). At high NA values, only a small range of object planes are clear, which may limit the range of applications for imaging.
4 Brightness: Theoretically, the larger the NA value, the more light entering the system, and the image may be brighter. However, it also depends on other factors such as the brightness of the light source, the optical efficiency of the system, etc.
5 Application requirements: For some applications, such as observing thick samples or situations where a large depth of focus is required, a high NA value may not be the best choice. Conversely, for applications that require high resolution and contrast, such as cell biology research, a high NA value may be more appropriate.
6 Cost and complexity: Optical systems with high NA values are often more complex and expensive. Because higher quality lenses, more precise alignment, and more complex manufacturing processes are required.
Therefore, when selecting an optical system, it is necessary to weigh the NA value according to the specific application needs. In some cases, choosing a system with a medium NA value can strike a good balance between resolution, contrast, depth of focus and cost. For more details, please visit the official website: Fiber Optic Spectrometers - JINSP Company Limited (jinsptech.com)
Post time: Jun-04-2024