Not enough resolution? Problem could be NA
In the optical field of microscopy, lenses, optical fibers, lithography, etc., we can always see a key parameter - the numerical aperture (NA).
It is often engraved in the objective lens body and marked in the lens specification, but many beginners are confused: What is NA? What does it have to do with resolution? Today, use popular language to talk about this optical core indicator.
Numerical Aperture (NA) is a dimensionless parameter that measures the ability of the optical system to collect light and receive the angle of the light cone.
Simply put, it determines how much light the optical system can “catch” and how much detail it can transmit.
1. n: refractive index of the medium between the objective and the sample (air n ≈ 1.0, water n ≈ 1.33, cedar n ≈ 1.515);
2. α: Half angle of the aperture angle of the objective lens (the aperture angle is the object point on the optical axis of the objective lens, the maximum included angle formed with the effective diameter of the objective lens).
From the formula, it can be seen intuitively that the higher the refractive index of the medium and the larger the aperture angle, the greater the NA value.
II. Causality at the Core of Numerical Aperture and Resolution (Airy Plaque)
There is an iron law in the field of optics: the numerical aperture directly determines the upper limit of resolution.
Points are diffracted by the optical system to form an Airy patch delta.
Resolution, which means that the optical system can clearly distinguish the minimum distance between two adjacent points — the smaller the distance, the higher the resolution, and the finer the details seen.
According to Rayleigh criteria, the theoretical resolution formula for an optical system is:
This formula directly reveals the core relationship between the two
✅ NA is proportional to resolution: the larger the NA, the smaller the δ, the higher the resolution;
The shorter the ✅ wavelength λ, the higher the resolution, but NA is the more controllable and core variable.
Just enough to tell: the center of one spot coincides with the first dark ring of another spot.
Resolution formula: d = 0.61 λ/NA
The larger the NA, the shorter the lambda, and the smaller the d, the higher the resolution.
4. Numerical aperture, condenser and system resolution
Left: No condenser, only objective collects diffracted light, effective NA is low.
Right: Add a condenser to increase the light cone in both directions, increase the total NA of the system, and maximize the resolution.
V. Application of NA: From Microscopic Observation to Chip Manufacturing
Biological microscope: High NA oil microscope allows us to see the microstructure of cells, viruses and other microstructures, which is the basis of life science research;
Industrial lens: Machine vision lens relies on high NA to achieve precision detection and distinguish micron-level flaws;
Optical fiber communication: optical fiber NA determines the coupling efficiency of optical signals, affecting the transmission stability;
Chip lithography: EUV lithography breaks through the limit of the process by improving NA, so that the chip can achieve a more advanced nanoscale process.
It can be said that NA is the "performance soul" of optical systems. Without high NA, there would be no high-precision optical imaging and detection.
The next time you see "100 ×/1.4" on the objective lens, the NA parameter in the lens specification, you will know: This number directly determines how small the world you can see.
Tell us the material, the feature you need to see and the magnification range. We will recommend a configuration and quote it.
