Numerical aperture is a single number that summarizes the light-gathering and light-delivering capability of a UV light guide — yet it is routinely overlooked in lamp system selection until it becomes the explanation for why an installed curing system delivers less UV power than expected. Understanding what numerical aperture means, how it is determined, and how it interacts with the rest of the optical system turns an abstract specification into a practical selection tool.
Defining Numerical Aperture
Numerical aperture (NA) is a dimensionless parameter that describes the range of angles over which a light guide can accept or emit light. It is defined mathematically as:
NA = n × sin(θ)
where n is the refractive index of the medium surrounding the guide’s entrance or exit face (typically air, with n ≈ 1.0), and θ is the half-angle of the acceptance or emission cone.
In physical terms: a light guide with NA = 0.39 can accept light entering within a half-angle of approximately 23° from the guide’s optical axis. Light entering at steeper angles — more oblique to the axis — does not undergo total internal reflection efficiently and is lost as heat in the guide walls rather than transmitted to the output.
At the output end, the same NA defines the divergence of the exiting beam: light exits in a cone with half-angle equal to arcsin(NA), spreading from the guide face as it propagates toward the cure surface.
How NA Is Determined by Guide Construction
For a fiber optic light guide, NA is determined by the refractive indices of the fiber core (n_core) and cladding (n_cladding):
NA = √(n_core² − n_cladding²)
A higher refractive index differential between core and cladding produces a higher NA — the guide accepts a wider cone of input light. Fused silica fiber light guides used in UV curing applications typically have NA values in the range of 0.22 to 0.39.
For liquid light guides, the NA is determined by the refractive index of the optical fluid and the surrounding jacket material. High-quality liquid guides can achieve NAs up to approximately 0.59, enabling them to accept a wider cone of input light and extract more of the LED array’s output than a lower-NA fiber guide of the same diameter.
NA and Coupling Efficiency
The coupling efficiency between an LED array and a light guide — the fraction of the lamp’s UV output that actually enters and propagates through the guide to the output face — depends critically on how well the LED’s emission cone is matched to the guide’s acceptance cone.
An LED emitting in a Lambertian pattern produces output across a wide angular range. A guide with a low NA accepts only the central portion of this emission; a guide with a higher NA accepts a larger cone and therefore captures a higher fraction of the LED’s output.
The coupling optics between the LED array and the guide’s proximal face shape the LED’s emission cone to match the guide’s acceptance angle as closely as possible. A well-designed coupling system that matches the LED’s emission half-angle to the guide’s NA maximizes the fraction of LED output that enters the guide. Mismatch — where the coupling optic directs light at angles that exceed the guide’s NA — means a portion of the LED output strikes the guide face but is not efficiently guided, producing heating at the input end and reduced throughput.
NA and Spot Size at the Cure Surface
At the output end of the guide, the NA determines the divergence of the exiting beam. A guide with NA = 0.39 exits light in a cone with half-angle of approximately 23°. At 10 mm working distance from the guide face, the beam has spread to a diameter of approximately 2 × 10 × tan(23°) ≈ 8.5 mm (assuming the guide face is 1 mm diameter and ignoring face diameter contribution at this working distance).
A guide with NA = 0.22 exits at approximately 12.7° half-angle, producing a smaller spot at the same working distance and higher irradiance per unit area — provided the same total UV power exits the guide.
This means that NA is not simply “higher is better.” A higher-NA guide captures more LED output and delivers it to the cure surface, but also produces a more divergent beam that spreads to a larger spot and lower irradiance at a given working distance. A lower-NA guide delivers a narrower beam with higher irradiance at the cure surface but may accept less of the LED’s total output.
The right NA for a given system depends on the target spot size, the required irradiance, and the working distance — all three of which must be balanced against the guide’s coupling efficiency with the lamp system.
NA and Cure Head Optics
When a focusing or collimating lens is added to the cure head exit, it reshapes the diverging output into a smaller, more intense spot or a collimated beam. The performance of these exit optics also depends on NA: the lens must be designed to handle the full NA of the guide’s output cone without vignetting (cutting off the outer portions of the beam) or aberrating edge rays that arrive at large angles.
A lens matched to the guide’s output NA captures the entire beam and reshapes it efficiently. A lens designed for a lower-NA guide, used with a higher-NA output, loses the outer annulus of the beam, reducing throughput. This lens-guide matching is one of the reasons that cure head optic accessories are typically specified for use with particular light guide NA values.
Measuring the Effective NA
The effective NA of a light guide as installed in a system can be measured by imaging the output beam at a defined distance and measuring the beam diameter, then calculating the half-angle and deriving NA from the geometry. This measurement should be performed with the guide connected to the lamp and operating under normal conditions — not measured on the guide alone — because the coupling optics at the proximal end affect what angular distribution of light actually enters the guide.
If you are comparing UV LED spot lamp systems with different light guide types and need guidance on NA selection for a specific working distance and spot size requirement, Email Us and an Incure engineer will assist with the optical analysis.
NA in System Specification Documents
When reviewing UV LED spot lamp specifications, the NA of the light guide is sometimes listed and sometimes omitted. When it is not listed, the irradiance at a specified working distance and spot size provides equivalent practical information — but knowing the NA allows an engineer to calculate expected performance at different working distances and evaluate the system’s sensitivity to working distance variation in production.
For precision UV curing applications where the working distance may vary within a tolerance band, understanding how NA translates to spot size variation and irradiance change across that tolerance band is essential for process capability analysis.
Contact Our Team to discuss numerical aperture selection and UV spot lamp optical configuration for your curing application.
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