Numerical Aperture (NA)
Definition
The product of the refractive index of the medium between objective and specimen and the sine of the half-angle of the maximum light cone collected by the objective. Governs resolution, depth of field, and image brightness simultaneously.
- Controls
- Resolution, depth of field, and image brightness simultaneously
- Calculation
- Sine of the half-angle of the light cone multiplied by the refractive index of the medium
- Resolution rule
- Higher NA yields higher resolution and finer detail visibility
Common questions
What does numerical aperture actually measure?+
Numerical aperture measures how wide a light cone the microscope objective can capture. It's calculated as the sine of the half-angle of that light cone, multiplied by the refractive index of the medium between the objective and the specimen. Higher NA means the objective can gather more light and detail.
Why does NA matter in forensic microscopy?+
NA directly controls three critical image properties at once: resolution (clarity of fine detail), depth of field (how thick a specimen layer stays in focus), and brightness. A higher NA gives you sharper images, which matters when you're examining trace evidence like fibers, glass, or comparison samples.
Does the medium between the lens and specimen affect NA?+
Yes. The refractive index of the medium is built into the NA calculation. Most objectives use air (refractive index approximately 1), but oil immersion objectives use special oil (refractive index approximately 1.5), which increases NA and improves resolution without changing the physical angle of the light cone.
Related terms
- Depth of Field
- Axial thickness of the specimen that appears acceptably in focus at one time. High in stereoscopic microscopes, very low in oil-immersion compound...
- Abbe Diffraction Limit
- The minimum period of a periodic grating structure that can be resolved: d = lambda / (2 NA). Sets the hard optical...
- ASTM E766
- The ASTM International standard practice for image-scale calibration in microscopy, extended by OSAC to govern optical microscope calibration in US forensic trace-evidence...
- Comparison Microscope
- Two compound microscope bodies connected by an optical bridge that presents one half of each body's image in a single split eyepiece...
- EDX / EDXRF
- Energy-dispersive X-ray detector attached to an SEM. Identifies elements from characteristic X-rays emitted when the beam strikes the sample. Backbone of GSR...
- Empty Magnification
- Magnification beyond approximately 1000 times the objective NA; enlarges the image without adding resolvable structural information, producing blurred detail that can be...
- Fluorescence Microscope
- Uses UV or short-wavelength excitation to make fluorophores emit longer-wavelength light. Used for semen, saliva, fibres and ink discrimination.
- Infinity-Corrected Optics
- An objective design in which the back focal plane produces a collimated (afocal) beam rather than a converging beam, allowing optical elements...
- ISO 8576
- The ISO standard governing the accuracy of stage micrometers and reticle eyepieces, specifying Class I and Class II tolerance levels traceable to...
- Koehler Illumination
- The standard illumination method for research microscopy in which the light source is focused at the condenser back focal plane, producing uniform...
- Parfocality
- The property of an objective nosepiece turret such that switching objectives keeps the specimen approximately in focus without re-focusing; defined by ISO...
- Plan Objective
- An objective corrected for field curvature so that the entire image field is simultaneously in focus; required for photomicrographic evidence documentation to...
Explained in these topics
- Microscopy Fundamentals: Magnification, Resolution and NAThe product of the refractive index of the medium between objective and specimen and the sine of the half-angle of the maximum light cone collected by the obje...
- Microscopy for Forensic Science: Polarizing, Comparison, Stereoscopic, Fluorescent and ElectronSine of the half-angle of the light cone entering the objective multiplied by the refractive index of the medium. Higher NA gives higher resolution.