• Absorption (optical)

    How an optical component captures (some of a) light ray's energy, transforming electromagnetic energy into internal energy. Results in a reduction of a system's optical throughput.

  • Accelerating voltage

    A microscope parameter that describes the energy of the incident electron (beam).
    Usually, it is presented in Voltage (kV) or Energy (keV).

  • Angle-resolved image

    A 2D data set (image) where each pixel describes a unique emission direction (see Emission Patterns).

  • Array detector

    A pixelated detector that is useful to acquire wavelength-resolved spectra and/or an angle-resolved image in a single frame.


  • Band-to-band (radiative) recombination

    The process of electrons jumping down from the conduction band to the valence band in a radiative manner.

  • Bandgap

    An energy range in a solid where no electron states can exist. Bandgaps can be found in insulators and semiconductors. In graphs of the electronic band structure of solids, the bandgap is the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band.

  • Bandpass

    Describes a range of wavelengths (i.e., a 50 nm bandpass corresponds to λ ± 25 nm).

  • Binning

    A method of reading out the information captured in a pixelated detector, e.g., a CCD in which a group of pixels is read out together, reduces the readout noise.

  • Blaze wavelength

    The wavelength that a diffraction grating is designed to operate at with the smallest losses.


  • Carrier generation

    A mechanism by which solids may absorb energy from the incident electron beam in which free carriers are generated as electrons exit from the valence band to the conduction band.

  • Cathodoluminescence (CL)

    The emission of photons from a material when struck by a high-energy electron beam.

  • Central wavelength

    The wavelength that passes through an infinitesimally small exit slit. Also used to indicate the middle of the range wavelength in a wavelength-resolved spectrum.

  • Charge-coupled device (CCD)

    An imaging device where the charge is moved to an area within the same device where it can be digitized and is used for acquiring spectroscopic data in parallel.

  • Chromatic aberration

    The tendency of a lens to focus light of different wavelengths to different points.

  • Collection mirror

    Optical component used to collect photons emitted from a sample.

  • Collimated beam

    A beam of light in which the light travels in parallel lines.

  • Color center

    An impurity or defect center that gives rise to visible luminescence from insulators via a mid-gap energy state.


  • Dark current

    Thermally generated charges in the silicon sensor of a camera.

  • Dark noise

    Statistical variation in the number of electrons thermally generated within the pixel in a photon-independent fashion; the electron equivalent of photon shot noise. Dark noise = √(dark current * integration time).

  • Diffraction grating

    An optical element used in a spectrometer to disperse light by wavelength.

  • Dispersion

    The separation of light according to wavelength.


  • Electroluminescence (EL)

    The transformation of electrical energy into electromagnetic radiation.

  • Electromagnetic spectrum

    The electromagnetic spectrum is the range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies.

  • Electron probe microanalyzer (EPMA)

    An analytical tool used to determine the chemical composition of small volumes of solid materials; works similar to a scanning electron microscope.

  • Electron-hole pair

    The free carriers created when an electron in the valence band is promoted to the conduction band (leaving behind a hole).

  • Emission pattern

    Representation of the directional (angular) dependence of the luminescence, typically presented as a 2D plot in polar coordinates.

  • Energy

    Property of the emitted light. Related to wavelength by E = hc/λ, where h is Planck's constant and c the speed of light.

  • Entrance slit

    Component of a spectrometer that determines the spectral resolution and throughput (see Exit slit).

  • Exciton

    An exciton is a bound state of an electron and an electron-hole which are attracted to each other by the electrostatic Coulomb force.

  • Exit slit

    Component of a spectrometer that determines the spectral resolution and throughput (see Entrance slit).


  • Field of view

    The observable area via an optical device; sometimes described by spatial extent (meters) or angular range.

  • Filter housing

    A component used to insert an optical filter.

  • Free carriers

    Electrical charge carriers (electrons and/or holes) which are free to move within the conduction energy band.

  • Free-space coupling

    An optical system based on open beams and reflective surfaces rather than image conduits such as optical fibers.


  • Generation rate

    The rate at which free carriers are generated (#/s).

  • Generation volume

    The region in a sample in which the signals being monitored in an SEM are formed.

  • Gruen range

    The stopping range of incident electrons within the material; described by a mathematical model.


  • Hot spot

    The region from which maximum optical collection efficiency is achieved when the specimen is at the focal point of the collection mirror.

  • Hyperspectral image

    See Spectrum image.


  • Image contrast

    The difference in the color and/or brightness of an object compared to the background within a field of view.

  • Infrared

    A part of the electromagnetic spectrum at longer wavelengths than visible by the human eye.
    Wavelengths ~700 nm – 1 mm


  • Lateral focal range

    Describes the geometric extent of the object image that can be viewed by an optical system without vignetting.

  • Longpass filter

    An optical filter that transmits wavelengths longer than a defined wavelength.

  • Luminescence

    The transformation of a diverse range of energies into light (electromagnetic radiation).


  • Majority carrier

    The more abundant charge carriers in a semiconductor primarily responsible for current transport.

  • Minority carrier

    The less abundant charge carriers in a semiconductor.

  • Minority carrier lifetime

    The average time it takes for a minority carrier to recombine.

  • Monochromatic

    A single wavelength (color) of light.

  • Monochromator

    See Spectrometer.

  • Multiple linear least squares (MLLS)

    MLLS fitting is a method to fit a number of reference spectra and/or models to a spectrum.


  • Non-linear least squared (NLLS)

    NLLS is fitting models to spectral features to quantify the spectral peak properties.

  • Non-radiative recombination

    When an electron in the conduction band recombines with a hole in the valence band and the excess energy is emitted in the form of a phonons rather than a photon.

  • Numerical aperture (NA)

    A dimensionless number that characterizes the range of angles over which an optical system can accept (or emit) light.


  • Optical axis

    An imaginary line that defines the path along which light propagates through an optical system.

  • Optical cathodoluminescence

    An attachment for an optical microscope that employs a hot cathode flood gun to generate an electron source for cathodoluminescence investigation.

  • Optical fiber

    A thin flexible fiber with a glass core through which light signals can be sent with very little loss of strength.

  • Optical filter

    An optical element that selectively transmits light of different wavelengths.


  • Panchromatic

    See Unfiltered.

  • Parabolic mirror

    An optical component designed to collimate light emitted from a Lambertian source at the focal point of the parabola.

  • ParaCL

    Description of a spectrum acquired using an array detector.

  • Photoluminescence (PL)

    Light emission from any form of matter after the absorption of photons (electromagnetic radiation).

  • Photomultiplier tube (PMT)

    Device used to detect light by converting an incident photon into a pulse of electrons.

  • Photon

    The basic unit of light.

  • Photon counting

    Signal processing method in which individual photons are counted (photons/s).

  • Polarization

    A property of a light wave that describes the relationship between the directions of oscillation of the electric and magnetic components of an electromagnetic wave.

  • Polychromatic

    See Unfiltered.

  • Principle component analysis (PCA)

    A statistical method to summarize information in a data set containing individuals/observations described by multiple inter-correlated quantitative variables.

  • Prism

    A transparent optical element that refracts light; can be used as an alternative dispersive element to a diffraction grating.


  • Radiative recombination

    When an electron in the conduction band recombines with a hole in the valence band and the excess energy is emitted in the form of a photon.

  • Read out noise

    A combination of noise sources which originate from the process of amplifying and converting the photoelectrons in a camera.


  • Scanning electron microscope (SEM)

    A type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons.

  • Scanning transmission electron microscope (STEM)

    A type of transmission electron microscope in which the electron beam is focused to a fine spot (with the typical spot size 0.05 – 0.2 nm).

  • Serial

    Description of a spectrum acquired using a single element detector (PMT or diode).

  • Shortpass filter

    An optical filter that transmits wavelengths shorter than a defined wavelength.

  • Shot noise

    Variance in the recorded signal level that arises as a result of the statistical nature of quantum particles such as photons or electrons. Shot noise = √(signal).

  • Spectral resolution

    A value that defines the smallest separation of wavelengths that can be reliably measured by an optical spectrometer.

  • Spectrograph

    An instrument designed to separate light by its constituent wavelengths.

  • Spectrometer

    An instrument designed to separate light by its constituent wavelengths.

  • Spectrum image

    A 3-dimensional data set containing spatial (x and y) and spectral (λ) information.

  • Spherical aberration

    A failure of a lens to focus light to the same point.

  • Stokes vector

    The Stokes parameters are a set of values that describe the polarization state of electromagnetic radiation.

  • Sweet spot

    The position at which a specimen should be located for optimum performance of a mirror-based CL detector.


  • Transmission electron microscope (TEM)

    A type of electron microscope that produces an image by analyzing electrons transmitted through a specimen.

  • Turret

    A component of an optical spectrometer that holds one or more diffraction gratings and controls the selection and angle.


  • Ultraviolet (wavelengths)

    A band of the electromagnetic spectrum with a wavelength from 10 nm to 400 nm but, in CL, typically >150 - 400 nm due to limitations in detection hardware.

  • Unfiltered (wavelengths)

    Without spectral discrimination i.e. all wavelengths of light.


  • Vignetting

    Optical effect that leads to a reduction in an image's brightness toward the periphery (compared to the center) due to light loss.

  • Visible (wavelengths)

    The portion of the electromagnetic spectrum that is visible to the human eye, typically wavelengths from 380 – 740 nm.


  • Wavelength

    A property of the electromagnetic wave of light that describes the spatial period of the wave.

  • Wavelength- and angle-resolved emission patterns

    A CL data set that contains angular and wavelength information.

  • Wavelength-filtered

    A CL signal where the wavelength information is limited by the wavelength range of interest; wavelength filtering may be performed by an optical filter or using the wavelength selecting slit (exit slit) of a spectrometer.

  • Wavelength-filtered spectrum

    The CL signal dispersed by wavelength where the wavelength domain is recorded in a stepwise manner as the wavelength domain is changed iteratively.

  • Wavelength-resolved

    A CL signal dispersed by wavelength.

  • Wavelength-resolved spectrum

    The CL signal dispersed by wavelength where the wavelength domain is recorded in a single measurement.