OSA key terms

A dimensionless measure of the range of angles over which an optical system accepts or emits light, equal to n·sin θ for refractive index n and half-angle θ. It sets resolution and light-gathering ability.

The fundamental bound on the smallest resolvable detail imposed by the wave nature of light, roughly λ/(2·NA) for a lens. No geometric perfection can beat it without changing wavelength or aperture.

The bright central spot of the diffraction pattern formed by a circular aperture, surrounded by faint rings. Its radius, about 1.22·λ·f/#, defines the practical spot size of a focused beam.

The image formed by an optical system of a point source, describing how the system spreads energy spatially. It is the basis for convolution models of imaging and for judging image sharpness.

The Fourier transform of the point spread function, describing how well the system transfers spatial frequencies from object to image. Its magnitude is the modulation transfer function.

The magnitude of the optical transfer function, plotted as contrast versus spatial frequency. It quantifies how much sinusoidal contrast survives imaging and is a standard lens specification.

The ratio of effective focal length to entrance pupil diameter. Smaller values mean larger apertures, more light, shallower depth of field, and finer diffraction-limited spots.

The range of object distances over which an image remains acceptably sharp for a given blur criterion. It shrinks as aperture and magnification increase.

The relation n₁·sin θ₁ = n₂·sin θ₂ governing refraction at an interface between media of different refractive index. It underlies ray tracing, prism dispersion, and total internal reflection.

Complete reflection of light at an interface when it strikes from the denser medium beyond the critical angle. It enables optical fibers, prisms, and light guides.

The ratio of light speed in vacuum to its speed in a medium, describing how strongly the material bends light. It varies with wavelength, producing dispersion.

A figure of merit for a glass's dispersion, defined as (n_d − 1)/(n_F − n_C). High values indicate low dispersion; pairing high and low values enables achromatic designs.

Image blur or color fringing caused by wavelength-dependent focal length. Axial color shifts focus along the axis; lateral color shifts magnification across the field.

A lens doublet, typically of two glasses with differing dispersion, that brings two wavelengths to a common focus. It reduces but does not eliminate chromatic aberration.

A monochromatic axial defect in which rays from the aperture edge focus at a different distance than paraxial rays. It blurs the spot and grows rapidly with aperture.

An off-axis aberration producing a comet-shaped blur that worsens with field height and aperture. It arises when magnification varies across the pupil.

An off-axis aberration in which tangential and sagittal ray fans focus at different distances, smearing point images into lines. It increases with the square of field angle.

An aberration in which the sharp image surface is curved rather than flat, so a planar detector cannot be sharp everywhere. Field flatteners or curved sensors compensate.

The sum of powers divided by refractive index over all optical elements, whose inverse approximates the field-curvature radius. Designers balance positive and negative elements to flatten the field.

A lens surface whose profile departs from a sphere, described by conic and polynomial terms. It corrects spherical aberration and distortion with fewer elements.

An aspheric surface profile using only even powers of radial distance, keeping the surface rotationally symmetric. It is the common form for molded glass and plastic lenses.

The product n·y·u (or n·h·sin θ) that remains constant through an optical system, linking object height, ray angle, and index. It constrains field, aperture, and magnification trade-offs.

A conserved geometric measure of light spread, equal to area times solid angle times refractive index squared. It limits how much flux can be concentrated or transferred through a system.

The ray from an off-axis object point that passes through the center of the aperture stop. It defines field height and pupil locations used in aberration analysis.

The ray from the axial object point that grazes the edge of the aperture stop. It defines the system's aperture and is used to compute focal length and aberration contributions.

The physical aperture that limits the cone of light accepted from an axial object point. Its image in object space is the entrance pupil.

The image of the aperture stop as seen from object space. It determines the apparent aperture and is the reference for f-number and numerical aperture.

A laser beam whose transverse profile is a Gaussian with a flat phase front at its waist. It propagates with a hyperbolic width and a characteristic Rayleigh range.

The location along a Gaussian beam where the spot radius is smallest and the wavefront is planar. It sets divergence and the depth over which intensity stays high.

The distance from a Gaussian beam waist over which the beam area doubles, equal to π·w₀²/λ. It indicates how collimated the beam stays near focus.

A non-equilibrium condition in which more atoms occupy an excited state than a lower laser level. It enables net stimulated emission and optical gain.

Emission of a photon that is coherent with an incident photon and in the same mode. It provides amplification and is the physical basis of laser action.

The fractional increase in optical intensity per unit length in an amplifying medium. It depends on inversion and cross-section and must exceed losses for lasing.

The point at which round-trip gain equals round-trip loss, allowing sustained oscillation. Below it the device acts as an amplifier or spontaneous emitter.

A field distribution that reproduces itself after one round trip in a resonator, satisfying phase and boundary conditions. Longitudinal modes differ in frequency; transverse modes differ in spatial shape.

The propagation distance over which a source's phase remains predictable, inversely related to spectral bandwidth. It governs interference visibility and holographic recording.

The degree to which a light field's phase at one point correlates with its phase at a later time. It is high for narrow-linewidth sources such as stabilized lasers.

An instrument that superposes light from two or more paths to measure phase differences. Fringe shifts reveal displacement, surface shape, or refractive index changes with sub-wavelength sensitivity.

A common-path interferometer that compares a test surface with a reference flat or sphere across an air gap. It is the workhorse for measuring optical surface figure.

The difference between the highest and lowest deviations of a surface or wavefront from its reference. It is sensitive to isolated defects and often paired with RMS roughness.

The standard deviation of wavefront deviation from its mean, expressed in waves. It correlates better than PV with image quality through the Marechal criterion.

The ratio of a system's peak intensity at focus to that of an aberration-free system. Values above about 0.8 are conventionally treated as diffraction-limited.

The image profile produced by a narrow line object, from which MTF is derived by Fourier transform. It is measured directly with a knife-edge or slit target.

The focus distance beyond which everything from half that distance to infinity is acceptably sharp at a given aperture. It maximizes depth of field for fixed-focus systems.

The laser exposure level, in energy or power per area, below which injury is considered unlikely. It depends on wavelength, duration, and pulse characteristics.

The space around a laser within which the MPE could be exceeded under normal operation. It defines the boundary for engineering and administrative controls.

A hazard classification (1, 2, 3R, 3B, 4) assigned by wavelength and output, indicating risk and required controls. Class 1 is safe under normal use; Class 4 is hazardous even to scattered light.

A logarithmic measure of attenuation, OD = −log₁₀(T), where T is transmittance. Each unit reduces transmitted intensity tenfold; eyewear and filters are rated this way.