Glossary

Abberration

Imperfections of an imaging system. In spectrometers leading to reduction in resolution or asymmetric peak shapes.

Absorbance

Also Optical Density (OD). Quantitative measure of the reduction in light intensity after passage through a sample. Absorbance is defined as the negative logarithm of the light transmission throgh the sample. From the Lambert-Beer law this measure is proportional to the sample concentration.

Absorbance Spectroscopy

Spectroscopic technique measuring the absorption of light of a specific wavelength due to the passage through a sample.

Analog to Digital Converter

Device to translate a voltage input into a number to transfer to a computer. In a spectrometer digitizing the output of the detector.

Angle of Diffraction (AOD)

The angle at which light is diffracted from a grating. Measured from normal (perpendicular) incidence to the grating surface.

Angle of Incidence (AOI)

The angle at which light enters a grating or other optic. Measured from normal (perpendicular) incidence to the grating or optic surface.

Averaging (spectroscopy)

Recording multiple spectra and determining the arithmetic mean of the signal across the multiple spectra for each pixel separately. Reduces noise but leads to an increase in total measurement time.

Bad Pixel

Individual elements on a pixelated detector with no, too low, or too high a signal, sensitivity, or dark signal.

Bandwidth

The useful wavelength range of a grating, spectrometer, or system.

Baseline Correction

Fit of a smooth curve under the recorded spectrum (most common in Raman spectroscopy) for subsequent subtraction to eliminate any fluorescence background.

Beer’s Law

Also known as Lambert-Beer law. Relates the attenuation of light in absorbance spectroscopy to the properties of the sample, such as concentration, thickness, and the molecular-specific spectrum.

Boxcar (spectroscopy)

Type of averaging across multiple pixels for a single spectrum (in contrast to an average across multiple spectra for a single pixel). Improves noise but can leads to a reduction of optical resolution.

Calibration (spectroscopy)

The measurement of the instrument response for a collection of samples of varying concentrations, usually expressed as a regression of instrument signal as a function of concentration.

CCD

Acronym for charge-coupled device: A type of light-sensitive detector based on the ejection of electrons from a trap by light. Historically the dominant detector type for scientific equipment.

Center Wavelength (CWL)

The wavelength for which a grating is designed to operate at peak efficiency.

CMOS

Acronym for complementary metal-oxide semiconductor: A type of light-sensitive detector based on the dominant semiconductor manufacturing process, widely used in consumer cameras, bt now also replacing CCDs in scientific applications.

Dark Correction (spectroscopy)

Recording the spectrometer signal under identical measurement conditions, but without the light source to determine the detector offset to be subtracted from the sample spectrum.

Dark Current (spectroscopy)

Electrons released fomr the detector even without light due to thermal excitation. Can be reduced by cooling about two-fold for every 7 degrees temperature difference.

Dark Noise (spectroscopy)

Random variation on the signal from the detector without any light input. Consisting of counting noise on the thermal dark current, amplifier noise in the analog circuitry, and suply voltage ripple.

Deconvolution (spectroscopy)

Also called sharpening. Mathematical procedure to reduce the effects of a limited (optical) resolution on the spectrum.

Dispersion (spectroscopy)

In spectrometers, the change in the position of individual wavelengths of light on the detector (in pixels) as a function of the wavelength, expressed in pixels per nm.

Dynamic Range (spectroscopy)

Ratio of the largest and smallest signal that can be detected side-by-side. Usually the ratio of the saturation signal to the dark noise.

EEPROM

Memory in the spectrometer used to permanently save calibration parameters.

F/# or f/#

Abbreviation for f-number: Ratio of the focal length of an optical system to its clear diameter (aperture)

FC

Type of connector with tight tolerances used to deliver laser with very little coupling loss light via an optical fiber.

Firmware

Piece of software running on the microprocessor on the spectrometer to control the hardware.

Fluorescence

Light emitted from a sample after excitation to a higher level using a light source.

Fluorescence Background

Sample response to laser excitation in Raman spectroscopy stemming from fluorescence of the sample, usually manifesting itself as a broad background under the Raman spectrum.

Fluorescence Spectroscopy

Type of spectroscopy using a light source to excite a substance (fluorophore) to an excited state from which it returns to the lower state by emitting light.

f-number

Ratio of the focal length of an optical system to its clear diameter (aperture).

Focal Length

Distance between a lens and the intersection point of the rays generated from parallel ray bundle passing through the lens.

Full Width at Half Max

A measure of the width of a peak, the distance from the right to the left flank of a peak at half the height of the peak maximum.

FWHM

Acronym for Full Width at Half Maximum: A measure of the width of a peak, the distance from the right to the left flank of a peak at half the height of the peak maximum

Grating

Specifically, diffraction grating: Optical element with periodic property arranged in parallel lines called grooves leading to diffraction of light

Immersion Probe

Raman probe designed to be inserted into the sample directly, either a liquid or a powder, allowing measurements without transfer to a sample container.

Integration Time

Time over which light is allowed to generate signal on the detector, analogous to the shutter time on a camera.

Intensity Correction

Correcting for the wavelength-dependent efficiency of a specific spectrometer using a relative intensity calibration based on a standard emission source.

Irradiance

Absolute Measurement of the power of light (not quite properly called “light intensity”) received per unit area.

Lambert-Beer Law

Also known as Beer’s Law: Relates the attenuation of light in absorbance spectroscopy to the properties of the sample, such as concentration, thickness, and the molecular-specific spectrum

Laser

Intense monochromatic light source used to excite Raman emission.

Laser Clean-up Filter / Laser Line Filter

Filter inserted into the laser beam path before delivery to the sample to block any light of wavelengths shorter or longer than the laser wavelengths, for example any non-coherent broadband emission from the laser.

Laser Spot Size

Diameter of the Raman laser once focused through the front optics of the Raman probe.

Laser Wavelength

The wavelength of the laser emission used to excite Raman scatter. While Raman can be excited with any laser wavelength, trade-offs include fluorescence background, power, cost, available range, and detector sensitivity.

Library Matching

Comparison of a sample spectrum to a collection of reference spectra to find a spectrum with high similarity. Common method in Raman spectroscopy for the identification of unknown samples or the verification of known samples.

Light Leakage

Light entering the spectrometer through gaps in the enclosure.

Limit of Detection

The lowest concentration of a sample that can still be distinguished from the noise level by a specific analytical technique.

Limit of Linearity

The concentration for which an instrument response starts to deviate substantially from a linear extrapolation of the calibration curve.

Linearity (spectroscopy)

Measure of how closely the increase in signal follows the increase in light intensity on the detector, usually measured by changing the integration time and expressed as the regression coefficient R2.

Multimode

Term to specify a non-Gaussian beam profile of the laser, often close to a flat-top uniform intensity distribution. Focus is an image of the laser fiber front surface.

NA

Acronym for Numerical Aperture: Measure for the opening angle of the cone from which the optical system can accept light, expressed as the sine of the half-angle of the acceptance cone.

Noise

Unwanted random contributions to the recorded signal caused by detector counting noise (dominant at medium to larger signal levels), or electronic amplifier noise and dark signal noise (together dominant at lowest signal levels).

Numerical aperture

Measure for the opening angle of the cone from which the optical system can accept light, expressed as the sine of the half-angle of the acceptance cone.

Optical Coherence Tomography (OCT)

An imaging technique that uses light to capture micrometer-resolution images from light interference within optical scattering media. See our OCT Tutorial.

Optical Density (OD)

Also Absorbance. Quantitative measure of the reduction in light intensity after passage through a sample. Absorbance is defined as the negative logarithm of the light transmission throgh the sample. From the Lambert-Beer law this measure is proportional to the sample concentration.

Optical Fiber

Thin, flexible, drawn glass thread using total internal reflection to contain light within its core diameter, used to deliver light.

Pixel Size

Physical dimensionof a single pixel element on the detector, usually on the order of microns.

Polarization Dependent Loss (PDL)

Losses that vary, in an optical component, at different states of polarization. The difference is expressed in decibels between the maximum and minimum loss.

p-polarized Light

Light that oscillates parallel to the plane of incidence.

Quantum Efficiency

Number of electrons released from the detector on average per photon as a function of wavelength.

Raman Emission Filter

Filter inserted into the beam path in a Raman system to block unshifted laser scatter (Rayleigh scatter). Also known as a longpass filter.

Raman Probe

Optical setup used to focus a cleaned-up laser beam onto the sample, capture the scatter from the sample, separate the Raman emission from unshifted Rayleigh scatter and deliver it to the spectrometer.

Raman Shift

The change in frequency undergone by the light scattered by the sample in Raman spectroscopy, expressed in wavenumbers, or cm-1.

Raman Spectroscopy

A spectroscopic technique measuring the light scattered by a sample, used to observe vibrational, rotational, and other low-frequency modes in a system – gas, liquid, or solid.

Raman Wavenumber

Raman shifts are typically reported in wavenumbers (cm-1) which have units of inverse length as this value is directly related to excitation wavelength.

Region of Interest

Abbreviated ROI: Definition of both ends of the useable spectral range in pixels.

Relative Irradiance

Relative (in contrast to absolute) spectral irradiance measurement, relating spectral irradiance at one wavelength to spectral irradiance at another wavelength.

Reproducibility

Similarity of units in a serial production, here for example for OEM spectrometers, especially the spectral sensitivity.

Resolution

In spectrometers the width of the peak for an input consisting only of a single wavelengh for example from an atomic emission source. Often dependent on the wavelength, then usually specified as the upper limit for the average across the spectral range.

ROI

Acronym for Region of Interest: Definition of both ends of the useable spectral range in pixels.

s-polarized Light

Light that oscillates perpendicular to the plane of incidence.

Saturation (spectroscopy)

Signal level at which the spectrometer response no longer increases with light intensity. Preceeded by the onset of saturation, at which the signal to noise ratio no longer follows counting statistics.

Sensitivity (spectroscopy)

In a spectrometer, sensitivity refers to the signal from the detector for a known light intensity input to the spectrometer. It is a function of many factors, for example grating efficiency and numerical aperture. In general spectroscopy, sensitivity more often refers to the change in instrument signal with the concentration of the sample, usually expressed as the slope of the calibration curve.

SERS

Acronym for Surface Enhanced Raman Spectroscopy: A technique using nano-scale structures to amplifiy the laser light on a molecular scale, potentially increasing Raman signal by several orders of magnitude.

Sharpening (spectroscopy)

Also called Deconvolution. Mathematical procedure to reduce the effects of a limited (optical) resolution on the spectrum.

Signal to Noise Ratio

Abbreviated SNR: Ratio of the average signal level to the variation of this signal level in repeat experiments, usually expressed as a standard deviation.

Single Mode

Term to specify a Gaussian beam profile for the laser. Leads to tightest focus following Gaussian beam propagation.

Slit Size (spectroscopy)

The width of the opening of the vertical entrance slit of the spectrometer.

SMA

Type of connector used to connect an optical fiber to an instrument. Used for low-power light, not lasers.

SNR

Acronym for Signal to Noise Ratio: Ratio of the average signal level to the variation of this signal level in repeat experiments, usually expressed as a standard deviation

Software (spectroscopy)

Application running on a computer or mobile device to control the spectrometer, and to receive and display the measured spectra.

Spectra

More than one spectrum (plural of spectrum).

Spectral Irradiance

Absolute Measurement of the light power received per unit area within a narrow wavelength band.

Spectral Range

The span of wavelenghs or wavenumbers that are imaged onto the full detector.

Spectrometer

Instrument to measure the intensity of light at different wavelengths consisting of an entrance aperture, collimation optics, grating for dispersion, and optics focusing onto a detector, either single-point or a pixelated array detector.

Spectroscopy

Study of the interaction of electromagnetic raditation with matter. Techniques vary, of interest here are techniques using visible light, in particular Raman, Fluorescence, and Absorbance.

Spectrum

Result of the measurement of the light intensity at different wavelengths. For Raman and fluorescence the spectrum is the signal by itself, for absorbance the spectrum is determined as a ratio to the intensity of a reference measurement.

s-polarized Light

Light that oscillates perpendicular to the plane of incidence.

Spatial Frequency

A characteristic of any structure that is periodic across position in space such as the line spacing in surface relief grating or the fringe spacing in a volume phase grating.

Stray Light

Signal from light that followed an errant path through the spectrometer, usually of a different wavelength than the one assigned to the detector position at which it is recorded.

Surface Enhanced Raman Spectroscop

Abbreviated SERS: A technique using nano-scale structures to amplifiy the laser light on a molecular scale, potentially increasing Raman signal by several orders of magnitude.

Surface Relief Grating

A grating whose structure is machined or etched onto the surface of grating medium or substrate.

TEC cooling

Thermo-electric device that generates a temperature difference from current flow. Usually used for cooling below ambient temperature. Single stage TEC cooling can reach temperatures down to -15 °C.

Temperature control

Stabilization of the detector temperature with thermo-electric cooling to the above the dew point, avoiding condensation and not requiring encapsulation.

Thermal Wavelength Stability

Shift of a reference wavelength, for example an atomic emission line, on the detector as a function of a change in spectrometer temperature.

Transmission Optics

Optical system not using reflective elements. In spectrometers, lens systems are used for collimation and focus and a transmission grating for dispersion.

Triggering

Synchronization of an instrument by providing an external timing signal for the start of data acquisition to the instrument.

Useable Spectral Range

If filters are installed, for example in a Raman spectrometer, wavelengths at the high and low end of the spectral range could be blocked, leading to low or no signal. The useable range specifies the spectral range with significant signal.

Volume Phase Grating (VPG)

A grating whose structure is in the volume of the medium. The grating is formed by periodic differences in the index of refraction in the medium.

Volume Phase Holographic Grating (VPHG)

A volume phase grating written using holographic techniques.

Wavelength (λ)

The distance between successive crests of a wave such as an electromagnetic wave or light. Typically expressed in nanometers (nm) or microns (µm), and indicated by a lowercase Greek lambda (λ).

Wavelength Range

The spectral range of the spectrometer, expressed as the starting and ending wavelengths imaged onto the detector.

Wavenumber (ṽ)

The spatial frequency of a wave, either in cycles per unit distance or radians per unit distance. Typically expressed in inverse centimeters (1/cm or cm⁻¹). (Wikipedia)

Well Depth

In CCD detectors, the number of electrons a single pixel can be charged with prior to exposure to light. Determines the counting noise at full signal.

Working Distance

Distance between the sample and the front optics, in Raman spectroscopy the probe lens.