Key Takeaways
- Numerical aperture (NA) measures how much light an optical system can gather, impacting brightness and resolution.
- However, a higher NA can also reduce the depth of focus and cause non-uniform illumination. Imaging systems with high NA face diffraction limits and potential issues with vector light effects.
- To avoid overestimating resolution, it’s crucial to consider these limitations.
- Avantier specializes in designing custom optical systems, ensuring optimal performance and avoiding common pitfalls through expert solutions and precision manufacturing.
Overestimation of the Numerical Aperture
The numerical aperture (NA) is an important parameter for microscopy and other optical applications. Many people assume that increasing numerical aperture will increase the quality of their imaging, but it turns out that’s not always the case. Sometimes, a higher NA may lead to reduced depth of focus and a lack of uniform illumination.
Just how does numerical aperture relate to performance, and is there a way to determine the ideal aperture for a given application? What exactly is the numerical aperture? We’ll look at the answers to these questions here, and give you the information you need to avoid this optical pitfall.
This article is part of a multi-part series on optical pitfalls in the optics industry Each article will look at a topic that is often confused, and provide a brief, easy-to-understand explanation of the key foundational concepts involved.
What is the Numerical Aperture?
The numerical aperture of an optical system tells us how much light can enter the system. There are two principal factors that determine the numerical aperture of a system— the refractive index of the medium between the optic and sample being imaged, and the maximum ray angle of light that can enter the system, as measured by a half-cone from the optical axis. More specifically, the numerical aperture can be calculated by
NA = n × sin(θ)
When n is the index of refraction and θ the maximum angle of light that enters the system.
For a microscope objective, the numerical aperture is directly related to both the index of refraction of the medium and the size of the objective lens aperture. As more light is able to enter the system, the numerical aperture increases.
How Does the Numerical Aperture Relate to Performance?
When your optical system is able to gather light from a wider range of angles, the overall lighting is likely to increase. You’ll get brighter images, and these images will often have higher resolution. This extra resolution is also related to the increased light; with more light, your imaging system is able to resolve even finer details.
That’s why it’s easy to confuse higher numerical aperture with better imaging, and overestimate the benefits an objective or lens with high numerical aperture can give your system. But there are two important things to remember. For one, the maximum resolution of every imaging system is limited by what we call the diffraction limit.
What’s more, when we increase the NA, not all the changes in imaging are positive ones. As the numerical aperture increases, the depth of focus also becomes shallower. This makes the system sensitive to changes in focus; and the larger the numerical aperture gets, the more this instability can become a problem.
A high numerical aperture can also cause issues with illumination. Essentially, you’ve pulled back the curtains, letting in more light; but that also may mean that the illumination is less uniform.
Those are not the only issues you may come across when working with high numerical aperture. Imaging with NA higher than 0.55 must also take into account the vector nature of light. When working with linear polarized light, you can expect to see astigmatic effects in the image.
How to Avoid Overestimation of the Numerical Aperture
How can you avoid overestimating the resolution of an optical system based on the numerical aperture? Take the time to look at what is happening within your optical system as a whole, and take into account image degradation that may occur from a shallow depth of field, nonuniform illumination, and vector effects.
If your numerical aperture is relatively small, many of these effects may be insignificant. The larger the aperture becomes, though, the more complex the optical situation is likely to become.
Optical Design at Avantier
Sometimes the most foolproof way of ensuring you don’t fall into any of these optical pitfalls is making sure you have experience and expertise on your side. At Avantier, we specialize in working with clients worldwide to design and manufacture custom optical systems to precise specifications. Fifty years of experience designing and producing cutting-edge optical systems gives our teams a unique advantage: we know what can go wrong, and we know how to avoid it. State of the art manufacturing equipment enables us to produce many optical components in-house, and we can also source materials from suppliers all over the world.
Contact us today if you’d like to set up an initial consult or chat about how to make the Avantier advantage yours.
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