Spectroscopy

Spectroscopy Using Compact Spectrometers

Compact spectrometers are widely used in educational applications, in field work where simplicity and compactness are crucial, and in research laboratories when a quick but accurate answer is needed. They are easily transportable, have no moving parts, and offer good spectral resolution. They are also very versatile, since they integrate easily into experimental setups with different light sources, cuvettes and other sample holders, optical fibers, lenses, and many other elements.

Axiom Optics’ compact spectrometer range now runs from the UV/Vis all the way into the mid-infrared. GoyaLab’s IndiGo instruments cover the visible and near-IR, the d-Vision from Sphere Photonics covers the visible through the short-wave infrared, and NLIR’s MIDWAVE mid-IR spectrometer covers 2.0 to 5.0 µm, where molecules have their strongest absorption bands.

Optical Principles Behind GoyaLab and NLIR Spectrometers

GoyaLab: axial transmission for the visible and near-IR

Most spectrometers on the market are based on a Czerny-Turner optical configuration, where light enters through a slit and hits a concave mirror that reflects and collimates it onto a diffraction grating. The diffracted light is then reflected and focused by a second concave mirror onto a detector. GoyaLab has developed and patented an alternative optical configuration based on axial transmission. A transmission configuration, instead of the folded reflective design required in Czerny-Turner instruments, enables significantly higher light throughput and therefore higher sensitivity.

GoyaLab spectrometers combine this axial transmission configuration, which puts more light on the detector, with a 2D CMOS detector. Vertical binning on a 2D detector inherently gives higher signal-to-noise levels than a 1D detector array.

NLIR: upconversion for the mid-infrared

Mid-infrared light is hard to detect directly. Conventional mid-IR detectors are slow, noisy at room temperature, or need cryogenic cooling. NLIR solves this with upconversion: inside the spectrometer, the incoming mid-IR light mixes with a high-power pump laser in a nonlinear crystal, and through sum-frequency generation each mid-IR photon is converted into a near-visible photon. Fast, low-noise silicon detectors, the same kind used in visible spectrometers, then read the converted spectrum.

Schematic for illustration. The pump wavelength shown is consistent with the Wavelength Conversion Module’s published mapping of 2.0 to 5.3 µm input to 695 to 886 nm output.

According to NLIR, the result is a mid-IR spectrometer with no cryogenic cooling and no moving parts, fiber-coupled and ready for both laboratory work and rugged industrial process monitoring. The same upconversion principle powers NLIR’s single-wavelength detectors and the stand-alone Wavelength Conversion Module.

Key Features of the GoyaLab IndiGo UV/Vis Spectrometer

• A visible spectrometer covering the 380 nm to 780 nm spectral region
• Optical resolution under 1.5 nm (FWHM) in the standard configuration
• Accessories for emission, absorption, fluorescence, and reflection measurements using cuvettes, optical fibers, and custom configurations
• Mobile phone app with Bluetooth connectivity for on-the-go measurements, or PC application over USB-C for more detailed analysis
• Scientific wavelength calibration with a mercury-argon lamp, or fast calibration with a compact fluorescent lamp

Key features of the NLIR MIDWAVE mid-IR spectrometer

• Full spectra from 2.0 to 5.0 µm, the region of fundamental O-H, N-H, and C-H vibrations and strong gas absorption bands
• Upconversion detection on silicon: no cryogenic cooling and no moving parts
• 6 cm⁻¹ optical resolution, which corresponds to about 2.4 nm at 2 µm and 15 nm at 5 µm
• High-speed readout of up to 400 spectra per second, with exposure times from about 11 µs to 500 ms
• Fiber input through an SMA905 connector for IR fibers with 100, 200, or 300 µm cores, and USB interface
• Companion thermal infrared light sources for transmission and reflection setups

Compare the compact spectrometer range

SpectroLab Software

The IndiGo spectrometer comes with PC software, SpectroLab, for controlling the spectrometer, acquiring data in real time, and analyzing the acquired spectra. SpectroLab is an intuitive and robust platform that also detects and controls the accessories attached to the IndiGo. Its features include:

Light source control

Turn different light sources on and off for dark and reference acquisition.

Exposure and gain

Control the detector’s exposure (from 10 ms to 23 s) and gain level.

Cyclic capture

Average spectra and adjust the time between individual spectra.

Timelapse

Acquire spectra at user-defined intervals to follow dynamic processes.

Beer-Lambert

Collect spectra at different concentrations and build a standard curve at a user-defined wavelength.

Binning

Bin pixels horizontally to smooth the spectrum and further increase signal-to-noise when high resolution is not needed.

SpectroLab also saves spectra in formats that work with other data analysis tools, including .csv, .txt, and the widely used spectroscopy .spc format.

Example Measurement: Characterization of Optical Filters

Spectral filters

A modular spectrometer is the ideal tool for detailed characterization of the transmission properties of spectral filters, or for identifying mislabeled or misplaced optical elements in a laboratory. In this example, the detailed transmittance of a bandpass filter with a central wavelength of 500 nm was measured with the IndiGo spectrometer. The setup is simple, requiring only the IndiGo spectrometer and a broadband light source.

Filter: Thorlabs FB500-40, CWL = 500 ± 8 nm, FWHM = 40 ± 8 nm, minimum transmission at CWL = 70%

The resulting transmission spectrum was compared with the filter’s Thorlabs spec sheet and with a second measurement made with an Ocean Insight Flame spectrometer and the same light source. Filter manufacturing involves a complex vacuum layer deposition process that produces normal variations in overall transmittance, central wavelength, and bandwidth, so detailed transmission characterization is essential when filters are used for specific applications. Both the IndiGo and Flame measurements, each a single acquisition (no averaging) with the same light source, reveal the expected differences from the spec sheet. Notably, the 2D detector in the IndiGo gives lower noise thanks to vertical averaging over many pixels.

Neutral density filter transmission measurement

Neutral density filters attenuate light by a nominal fraction over a wide wavelength range. Their nominal optical density (OD) indicates the magnitude of the attenuation. Percent transmission (%T) and OD are related by:

%T = 100 × 10^−OD

For example, a neutral density filter with an OD of 1.0 transmits 10% of the light, and filters with an OD above 2.0 transmit well under 1%. A simple way to characterize the full transmission profile of a neutral density filter is to use a spectrometer and a broadband light source. The spectrometer needs enough dynamic range to measure the unattenuated source and enough sensitivity to quantify the light the filter transmits.

A demonstration measurement on an OD 3.0 filter was performed with the GoLight broadband light source and the IndiGo spectrometer. In a single-shot measurement with a 250 ms integration time, the IndiGo correctly quantified a transmission of 0.1% without noise significantly affecting the result across most of its wavelength range, demonstrating the spectrometer’s sensitivity over its entire 1000:1 dynamic range.

Using the averaging feature, transmission levels of 0.01% and below near 400 nm could be quantified for the OD 3.0 filter:

Astronomy filters

Example Measurement: The Beer-Lambert Law

Equipped with the chemical analysis module, the IndiGo spectrometer performs spectrophotometric analysis by visible light absorption to determine the concentration of a solution using the Beer-Lambert law. According to the Beer-Lambert law, the absorbance of a solution at a given wavelength, A(λ), depends on the solution’s concentration c, the path length of the light through the sample l, and the extinction coefficient of the absorbing species at that wavelength, ε(λ). Within a certain concentration range the relationship between absorbance and concentration is linear, and the slope obtained from a series of dilutions of known concentration can be used to determine the unknown concentration of a solution of the same species by measuring its absorbance:

A(λ) = ε(λ) l c

In this application, a blue food colorant was diluted and used as a stock solution to prepare a series of serial dilutions for a standard curve of absorbance vs. concentration.

Each serial dilution halved the concentration, and its absorption spectrum was measured in a single shot with a 65 ms integration time. 1.0X, 2.0X, and so on refer to the dilution factor from the stock solution.

From the absorption spectra of the serial dilutions, a standard curve of absorbance vs. concentration (here, dilution factor) can be built at any wavelength. The graph below shows standard curves at three wavelengths. Strong absorbance gives more sensitive measurements, as seen in the standard curve near the absorption maximum at 630 nm, although other wavelengths can be used to avoid spectroscopic interferences at the expense of sensitivity. Over the concentration range of this experiment, all standard curves show excellent linearity, which demonstrates the IndiGo’s potential for quantitative concentration measurements.

Example Measurement: Molecular Spectroscopy of a Diatomic Molecule

In this application, the resolution and sensitivity of the IndiGo spectrometer were pushed to the limit by measuring the visible absorption spectrum of iodine vapor in a cuvette with a standard 10 mm absorption path. The IndiGo was coupled with the chemical module, which includes a high-intensity flat-spectrum white light source for absorption measurements in the visible range. A small amount of solid iodine was sealed in the cuvette and heated with a heat gun to increase the iodine vapor concentration by sublimation.

Excitation of diatomic iodine gas into its first bound electronic excited state occurs above 500 nm, and the distinct absorption features correspond to changes in vibrational quantum number. The spacing between these features reveals the separation of the vibrational energy levels of the excited electronic state. The iodine absorption spectrum was obtained in a single-shot acquisition with a 65 ms integration time. For comparison, it is shown next to the iodine spectrum from a high-resolution Agilent Cary 100 UV-Vis spectrophotometer with a 0.278 m Czerny-Turner monochromator. The Cary 100 is equipped with a 1200 lines/mm grating, a tungsten-halogen/deuterium light source, and an R928 photomultiplier tube detector, and has a wavelength accuracy better than 0.04 nm and a resolution under 0.2 nm.

The overall agreement between the IndiGo and Cary 100 spectra is very good. Some differences stand out, mainly on the left side of the spectrum. The Cary 100 has a heating block that held the sample at a constant 75 °C, while the IndiGo spectrum was taken after heating the cuvette outside the spectrometer, so absorbance values are not expected to match quantitatively. Also, the rovibronic transitions on the left of the spectrum get progressively closer together (they correspond to higher vibrational levels in the excited state potential) and need higher resolution to be resolved. Within the resolution limits of the IndiGo, the agreement is very good, and the IndiGo shows enough sensitivity for a gas-phase absorption measurement even in a single-shot acquisition.

Mid-IR Spectroscopy: Molecular Vibrations from 2 to 5 µm

The iodine example probes electronic transitions, with vibrational structure riding on top. In the mid-infrared, molecules absorb directly through their fundamental vibrations, without any electronic excitation. Each functional group and gas has its own band positions, which makes 2 to 5 µm spectra a direct way to identify and quantify species such as CO₂, CO, methane, and hydrocarbons.

NLIR’s upconversion instruments are used for:

• Combustion monitoring: following CO₂, CO, and water vapor in flames, burners, and engines
• Fast transient phenomena: capturing chemical and thermal events too fast for a scanning FTIR
• Laser pulse analysis: characterizing mid-IR laser sources with the single-wavelength detectors at up to 10 GHz
• Closed-loop process control: feeding real-time mid-IR spectra into manufacturing control systems
• Gas sensing and emissions: methane, N₂O, and CO₂ monitoring for environmental and industrial compliance

Read more in Axiom’s announcement: Axiom Optics adds the fastest and most sensitive mid-infrared spectrometers and detectors to its portfolio.

Choosing the Right Spectrometer

Frequently asked questions

What is a compact spectrometer used for?

Compact spectrometers measure how light is emitted, absorbed, transmitted, or reflected across a range of wavelengths in an instrument small enough to carry. They are used in teaching labs, field work, and research labs for tasks such as filter characterization, concentration measurements with the Beer-Lambert law, light source testing, and molecular spectroscopy.

What wavelength ranges do Axiom Optics’ compact spectrometers cover?

The IndiGo covers 380 to 780 nm, the IndiGo NIR 720 to 1050 nm, the d-Vision 500 to 1700 nm, and the MIDWAVE mid-IR spectrometer 2.0 to 5.0 µm, so together they span the visible through the mid-infrared.

How does an upconversion mid-IR spectrometer work?

The incoming mid-infrared light is mixed with a strong pump laser in a nonlinear crystal. Through sum-frequency generation, each mid-IR photon combines with a pump photon to create a near-visible photon, which fast, low-noise silicon detectors then measure. This removes the need for cryogenically cooled infrared detectors.

How does the MIDWAVE spectrometer compare with an FTIR?

An FTIR uses a scanning interferometer with moving mirrors, and a scan typically takes seconds. It offers broad spectral coverage and high resolution for detailed chemical analysis. The NLIR MIDWAVE has no moving parts and needs no cryogenic cooling, and it reads out complete 2.0 to 5.0 µm spectra at high speed, which suits fast process monitoring, combustion, and transient measurements. The two are complementary.

Which molecules can be measured between 2 and 5 µm?

The 2 to 5 µm range contains the fundamental stretching bands of O-H, N-H, and C-H bonds and strong absorption bands of gases including water vapor, methane, carbon dioxide (4.26 µm), nitrous oxide, and carbon monoxide.

Can a compact spectrometer measure neutral density filters?

Yes. In a single 250 ms acquisition, the IndiGo correctly quantified the 0.1% transmission of an OD 3.0 filter across most of its wavelength range, and with averaging it quantified transmission of 0.01% and below near 400 nm.

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