Glossary of Terms
actinometric quantity
A physical quantity that describes radiative transfer in terms of the number of photons, rather than the total mount of energy.
blackbody, ideal
A theoretical surface that absorbs all radiation that is incident upon it, regardless of wavelength or surface temperature.
blackbody radiation
Diffuse emission of radiation from the surface of an ideal blackbody.
coherence
The degree to which two rays interfere with each other, or to which a ray may interfere with itself when subjected to a time delay.
cross grating
A type of diffraction grating with periodicity in two directions.
diffraction grating
A periodic structure from which transmitted and reflected radiation can only propagate in specific directions governed by the frequency of radiation and the size of the unit cell.
diffraction order
In a diffraction grating, an integer indicating the number of wavelengths between waves emanating from adjacent unit cells before they constructively interfere.
diffuse emission
Emission of radiation from a surface in a random direction, following a probability distribution based on Lambert’s cosine law.
diffuse reflection
Reflection at a surface in a random direction, following a probability distribution based on Lambert’s cosine law.
emissivity, spectral
The ratio of the amount of radiation per unit surface area, per unit wavelength or frequency, released by an object, compared to an ideal blackbody radiation source of the same size and shape.
Fresnel equations
A set of equations for computing the reflection coefficients and transmission coefficients for rays as they cross a boundary between two media with different refractive indices.
Gaussian beam
A beam of electromagnetic radiation in which the electric field amplitude follows a Gaussian distribution. In the ideal case, this is the TEM00 mode solution to the electromagnetic wave equation in free space.
geometrical optics
The analysis of electromagnetic wave propagation through electrically large domains by treating the electromagnetic waves as rays.
grating
See diffraction grating and cross grating.
graybody radiation
Diffuse emission from a surface at which the spectral emissivity is constant, independent of wavelength or temperature. Blackbody radiation is the special case for which this constant value of the spectral emissivity is unity.
intensity
A measure of the energy density of an electromagnetic wave; for a plane polarized wave, the intensity is equal to the magnitude of the time-averaged Poynting vector.
internal transmittance
A material property indicating the fraction of electromagnetic energy that is transmitted through a material sample of a given thickness, neglecting Fresnel losses at the surfaces of the sample. Can be used as an alternative means of specifying the imaginary part of the refractive index.
marginal ray
The ray that is released from the center of the object at the maximum aperture. This ray will pass through the edge of the aperture stop.
monochromatic light
Light of uniform frequency or wavelength.
Mueller matrix
A 4-by-4 matrix which can be multiplied by the Stokes vector to apply the effects of various optical components to a ray.
optical dispersion model
A function giving the refractive index as a function of wavelength. The coefficients for any given model will usually be determined empirically.
optical axis
For simply rotationally symmetric optics, this is the axis of rotational symmetry. The optical axis will pass through the center of curvature.
optical path length
The integral of refractive index along the ray trajectory. Over a given time interval, the optical path length of a ray is equal to the distance the ray would have propagated in a vacuum.
part
IA subsequence of geometry instructions, usually comprising several geometry primitives, that can be added repeatedly to a COMSOL geometry sequence to avoid repetitive steps in geometry setup.
phase
The quantity that indicates the variation of the instantaneous electric field with respect to time and position.
plane of incidence
For nonnormal incidence of a ray at a surface, the unique plane containing the surface normal and the incident ray direction vector. For normal incidence, any plane containing the surface normal may be selected.
photon
The quantum of the electromagnetic field.
polarization
A description of the orientation of the electric field for a polarized or partially polarized ray, and for the way in which this electric field varies as the ray propagates.
polarization, circular
The state of polarization in which the electric field amplitude has two orthogonal components of equal magnitude, with a phase shift of π/2 between them, such that the electric field vector traces a helical pattern as the ray propagates.
polarization, linear
The state of polarization in which the electric field only oscillates within a single plane.
power
The energy transferred per unit time.
Poynting vector
The vector representing the directional energy flux of an electromagnetic field.
primary ray
A ray that is released into a model independently of the trajectories of other rays. Primary rays are created by release features.
principal curvature direction
A unit vector which, together with the normal to a surface, defines a plane in which one of the principal radii of curvature of a surface is defined.
polychromatic light
Light with a distribution of frequencies or wavelengths.
principal curvature
The reciprocal of one of the principal radii of curvature of a surface.
principal radii of curvature
The maximum and minimum values of the radius of curvature of curves that can be obtained via the intersection of a surface with a plane.
quasi-monochromatic light
Light with a distribution of frequencies in which the changes in free-space wavelength are much smaller than the average free-space wavelength.
radiant exitance, spectral
The amount of electromagnetic energy emitted from a surface per unit time, per unit surface area, per unit wavelength or frequency.
radiant exitance, total
The amount of electromagnetic energy emitted from a surface per unit time, per unit surface area.
radiometric quantity
A physical quantity that describes radiative transfer in terms of the amount of energy, including such units as watts, watts per square meter, watts per steradian, and watts per square meter per unit wavelength.
reflection coefficient
A dimensionless coefficient that is used to compute the intensity of reflected rays. At boundaries between media, reflection coefficients are computed using the Fresnel equations.
refractive index
The ratio of the speed of light in a vacuum to the speed of light in a medium. This is also known as the absolute refractive index.
refractive index, relative
The ratio of the absolute refractive index to the refractive index of air.
release feature
A feature that releases a predetermined number of rays and assigns initial values to dependent variables associated with the rays.
secondary ray
A ray that is produced by the interaction of existing rays with a domain or boundary feature. The degrees of freedom for all secondary rays are preallocated when beginning to run a study.
Snell’s law
The set of equations that indicates the directions in which reflected and refracted rays will propagate when a ray interacts with a boundary between two media.
specular reflection
Idealized reflection at a smooth surface in which the reflected ray lies within the plane of incidence and the angles of incidence and reflection are equal.
spot diagram
A type of plot for visualizing the focusing capability of an optical system. It displays the intersection points of rays with a surface, which is often the image plane of the optical system.
Stokes parameters
The four Stokes parameters completely characterize the intensity and polarization of a fully polarized, partially polarized, or unpolarized ray. The Stokes parameters can be interpreted as indicators of the ray intensity that would be measured when sending a ray through various arrangements of polarizers and wave retarders.
thermo-optic dispersion model
A function giving the change in refractive index as a function of wavelength and temperature.
transmission coefficient
A dimensionless coefficient that is used to compute the intensity of refracted rays at boundaries between media. Transmission coefficients are computed using the Fresnel equations.
wave vector
A vector in the direction of ray propagation, which indicates the spatial dependence of the instantaneous electric field for coherent radiation.
wave number
The magnitude of the wave vector, equal to ω/c, where ω is the angular frequency and c is the speed of light in the medium.
wave retarder
An optical component that applies a phase shift to radiation of a specific polarization with respect to radiation of an orthogonal polarization.
Zernike polynomial
A type of polynomial basis that is orthonormal on the unit circle in 2D. Each term comprises a radial component and an azimuthal component, which are multiplied together. The Zernike polynomial basis is used to report optical path differences when interpreting monochromatic optical aberrations.