Numerical Aperture is a measure of the acceptance angle of an optical system, which is the maximum angle at which light can enter or exit the system. It is calculated as the product of the refractive index of the medium and the sine of the half-angle of the cone of light entering or exiting the system.
A higher Numerical Aperture means a larger cone of light can be collected, resulting in a higher light collection efficiency. This is because a larger acceptance angle allows for more light to enter the system, increasing the amount of light that can be used for imaging or detection.
A Numerical Aperture of 0.95 or higher is generally considered high. However, the value of a high Numerical Aperture varies depending on the specific application and type of optical system being used. For example, a high NA for a microscope objective may be different from a high NA for a fiber optic cable.
Using a high Numerical Aperture can improve the resolution and sensitivity of an optical system. This is because a larger cone of light allows for finer details to be captured and more light to be collected, resulting in a clearer and brighter image. High NA also allows for better control of the depth of field and the ability to image deeper into a sample.
One limitation of using a high Numerical Aperture is that it requires careful alignment and maintenance of the optical system. The angle of the cone of light must be precisely controlled, which can be challenging and may lead to reduced stability and increased complexity of the system. Additionally, high NA may result in a smaller field of view and decreased working distance, making it more difficult to image larger or thicker samples.