Spectrometer Theory | angliainstruments

Theoretical background, operating theory and configuration information

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What is fibre optic spectroscopy?

Optical spectroscopy is a technique for measuring light intensity in the UV, VIS, NIR and IR region, and a spectrometer is frequently the device used to do this. Spectroscopic measurements are being used in many different applications, such as colour measurement, concentration determination of chemical components or electromagnetic radiation analysis to name just a few. Traditionally, it was typically necessary to take a sample of what ever medium you wanted to measure, and then measure it with a spectrometer in a laboratory. These days, progress in optical fibre technologies has revolutonised the spectroscopy business, and their use now allow us to go right to the object being measured. Devices that make use of this technology are correctly classed as spectrometers, but more specifically, they're fibre optic spectrometers. Using this technique enables a truly unlimited number of applications in both the lab and field, with new applications being discovered everyday.

How does a spectrometer work?

A spectroscopic instrument or spectrometer generally consists of entrance slit, collimator, a dispersive element, such as a grating or prism, focusing optics and detector. In a monochromator system, there is normally an exit slit too, and only a narrow portion of the spectrum is projected on a one element detector. In monochromators, the entrance and exit slits are in a fixed position and can be changed in width. Rotating the grating scans the spectrum.

The development of micro electronics during the 1990’s in the field of multielement optical detectors, such as Charged Coupled Devices (CCD) arrays and Photo Diode (PD) arrays, enabled the production of low cost scanners, CCD cameras, and other similar devices. These same CCD and PDA detectors are now used in the Avantes AvaSpec line of spectrometers, enabling fast scanning of the spectrum, without the need for a moving grating.

Thanks to the need for fibre optics in the communication technology, low absorption silica fibres have been developed. Similar fibres can be used as measurement fibres to transport light from the sample to the optical bench of the spectrometer. The easy coupling of fibres allows a modular build up of a system that typically consists of a light source, sampling accessories and fibre optic spectrometer. Furthermore, fibre optics enable the introduction of sampling into harsh and difficult to access environments.

The low cost, modularity, flexibility and speed of measurement made possible by fibre optic spectrometers has resulted in a great many industries embracing the technology.

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Optical bench design

The heart of most AvaSpec fibre optic spectrometers is an optical bench with 37.5, 45, 50 or 75 mm focal length, developed in a symmetrical Czerny-Turner design. Light enters the optical bench through a standard SMA905 connector and is collimated by a spherical mirror. A plain grating diffracts the collimated light, a second spherical mirror focuses the resulting diffracted light. An image of the spectrum is then projected onto a 1 dimensional linear detector array.

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How to configure a spectrometer for your application

The modular AvaSpec line of instruments provides you with a number of configuration options to optimise the optical and spectroscopic performance of your instrument for your application.

This section provides you with some guidance on how to choose the right grating, slit, detector and other configuration options, to be installed in your AvaSpec spectrometer.

Wavelength Range

In the determination of the optimal configuration of a spectrometer system, the wavelength range is a key parameter that defines the appropriate grating choice. If you are looking for a wide (broadband) wavelength range, we recommend the use of a 300 lines/mm grating known as an “A” type grating in Avantes product line. For a lesser range (approximately 500nm) but higher resolution, you might consider a 600 lines/mm or “B” type grating. Higher lines/mm gratings (1200 – C type, 1800 – D type, 2400 – E type, 3600 – F type) provide higher resolution for applications that require this. Broadband gratings provide the greatest flexibility but may not provide the best performance for specific application. Contact us for a recommendation on a suitable grating for your task.

Detector choice

The choice of your wavelength range along with the demands of your measurement speed and accuracy often suggests the appropriate detector for your application. The AvaSpec platform offers 15 different detector types, each with their own unique sensitivity curves. The AvaSpec instrument line is divided into three groups based upon general requirements.

The AvaSpec-Starline is comprised of general purpose UV/VIS instruments with low cost CCD and PDA detectors.

The AvaSpec Sensline is comprised of higher performance back-thinned CCD's and thermoelectrically cooled CCD UV/VIS instruments. These instruments offer superior performance, particularly in the UV and NIR, especially when compared to standard CCD detectors.

The AvaSpec NIRLine is comprised of instruments with InGaAs arrays for longer wavelength measurements from 900-2500nm.

For high speed applications, the 2048 pixel CCD detectors in the AvaSpec-ULS2048 and AvaSpec-ULS2048L from the StarLine are frequently the best options. For VIS only applications where high resolution is not required, but speed and signal to noise are important, the 128 pixel PDA detector in the AvaSpec-128-USB2 can be an excellent choice. For low light level applications such as fluorescence and Raman, the SensLine instruments are likely to be the most appropriate. The AvaSpec-NIRLine features 7 different InGaAs detectors for various applications.

The modularity and inter-compatibility of the AvaSpec line also make it possible to combine two or more detectors in a single instrument enclosure to provide optimal performance over a very broad wavelength range. For example, an AvaSpec-StarLine (UV/VIS) spectrometer can be combined with a NIRLine spectrometer to enable measurements from 200-2500nm in a single instrument.

Optical Resolution & Slit Size
If high optical resolution is required, you may want to consider a grating with higher lines/mm (1200- C type, 1800 – D type, 2400 – E type, 3600 – F type), thus limiting the range of the instrument to a more narrow range. Additionally, it is advisable to consider a detector with 2048 or 3648 pixels and a small slit (10 or 25 µm). For the best resolution with all other criteria of lesser importance, the AvaSpec-ULS3648 with a 10 micron slit is optimal. Slit size is a key factor in determining both resolution and throughput into the optical bench. It is important to balance your need for resolution with the need for sensitivity and throughput into the optical bench. If resolution is optimised without considering the need for throughput, you may not have adequate light to get a stable measurement. As previously mentioned, for optimal resolution our smallest slit (10 microns) is recommended. If your application does not require the highest possible resolution and is not one that has an excess of light (laser measurement for example), we recommend that you consider as large a slit as possible to maximise throughput into the optical bench.

A new option is the AvaSpec-RS with replaceable slit that makes your spectrometer a versatile instrument so one can swap the slit size easily to continually optimised for changing demands of sensitivity and resolution.

Sensitivity

When considering sensitivity, it is very important to distinguish between photometric sensitivity (how much light do I need for a detectable signal?) and chemometric sensitivity (what absorbance difference level can still be detected?)

Photometric Sensitivity

For the best photometric sensitivity, a combination of a high throughput optical bench and a high quantum efficiency (QE) detector is recommended. The instruments in the AvaSpec-SensLine are specifically optimised for photometric sensitivity.

For example, fluorescence applications require high photometric sensitivity and Avantes AvaSpec-HS1024x122-TEC-USB2 is the highest performance instrument we offer for this application. For Raman applications where the combination of resolution and sensitivity is required, we recommend our AvaSpec-ULS2048L-USB2 spectrometer. To further enhance photometric sensitivity, we recommend the use of a detector collection lens (DCL-UV/VIS or DCL-UV/VIS-200), which is a cylindrical lens with focuses light from larger core fibre optics and bundles down onto the smaller detector pixels.

For additional photometric sensitivity, a larger slit or no slit and a 300 line/mm A-type grating to minimise light dispersion are available. Some more demanding applications also require thermo-electric cooling of the CCD detector to minimise noise and increase dynamic range at long integration times (up to 60 seconds).

Chemometric Sensitivity

To detect drastically different absorbance values, close to each other with maximum sensitivity, you need high Signal to Noise (S/N) performance. The detectors with best S/N performance are again in the AvaSpec-SensLine series with the AvaSpec-HS1024x122-TEC at the top of the line. The S/N performance can also be enhanced by averaging multiple spectra. The square root of the number of averages translates to the improvement in signal to noise.

Timing and Speed
The data capture process is inherently faster with linear detector arrays and no moving parts as compared with a monochromator design, however, there are optimal detectors for each application. For high speed applications such as measurements involving pulsed lasers and light sources, we recommend the AvaSpec-128-USB2, AvaSpec-ULS2048-USB2, AvaSpec-ULS2048L-USB2 or the AvaSpec-FAST spectrometers.

Each of these instruments supports high speed data acquisition with the capability of starting an acquisition within 1.3 microseconds of receiving an external trigger. The AvaSpec-FAST spectrometers can support integration times as low as 0.5 milliseconds, the AvaSpec-128-USB2 supports 0.06 milliseconds and the AvaSpec-ULS2048 and ULS2048L support 1.1 millisecond integration times. Since data transfer time is critical for these applications, Avantes unique Store-to-RAM mode enables on board storage of up to 5000 spectra to the instrument RAM buffer.

The above parameters are the most important in choosing the right spectrometer configuration. Please contact us to optimise and fine tune the system to your needs. The table on this page provides a quick reference guide for spectrometer selection for many common applications. The system recommendations in this table are for simple configurations of mostly single channel spectrometers.

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Choosing the right grating

A diffraction grating is an optical element that separates incident polychromatic radiation into its constituent wavelengths. A grating consists of a series of equally spaced parallel grooves formed in a reflective coating deposited on a suitable substrate.

The way in which the grooves are formed separates gratings in two types - holographic and ruled. The ruled gratings are physically formed onto a reflective surface with a diamond on a ruling machine. Gratings produced from laser constructed interference patterns and a photolithographic process are known as holographic gratings.

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AvaSpec spectrometers are supplied with a permanently installed grating that must be specified by the user. In addition, the user needs to indicate the wavelength range that needs to reach the detector. Sometimes the specified usable range of a grating is larger than the range that can be projected on the detector. In order to cover a broader range, a dual or multichannel spectrometer can be chosen. In this configuration each channel may have different gratings covering a segment of the range of interest. In addition to broader range, a dual or multichannel spectrometer also affords higher resolution for each channel.

For each spectrometer type, a grating selection table is shown in the spectrometer platform section. Table 2 illustrates how to read the grating selection table. The spectral range to select in Table 2 depends on the starting wavelength of the grating and the number of lines/mm; the higher the wavelength, the bigger the dispersion and the smaller the range to select.

Below the grating efficiency curves are shown. When looking at the grating efficiency curves, please realize that the total system efficiency will be a combination of fiber transmission, grating and mirror efficiency, detector quantum efficiency and coating sensitivities. The all new dual-blazed grating is a 300 lines/mm broadband grating (covering 200-1100 nm) that has optimized efficiency in both UV and NIR. On the bottom the grating dispersion curves are shown for the AvaSpec-ULS2048.

Grating Efficiency Curves for AvaSpec-Starline/Sensline

Gratings Efficiency Curve 300l/mm

Gratings Efficiency Curve 1800l/mm

Gratings Efficiency Curve 600l/mm

Gratings Efficiency Curve 2400l/mm

Gratings Efficiency Curve 1200l/mm

Grating Efficiency Curve 3600l/mm

Pixel Dispersion Curves for AvaSpec-Starline/Sensline

Pixel Dispersion Curve 300l/mm

Pixel Dispersion Curve 1800l/mm

Pixel Dispersion Curve 600l/mm

Pixel Dispersion Curve 2400l/mm

Grating Efficiency Curves for AvaSpec-HS (high sensitivity optical bench)

Gratings Efficiency Curve 500l/mm

Gratings Efficiency Curve 830l/mm

Pixel Dispersion Curve 1200l/mm

Pixel Dispersion Curve 3600l/mm

Gratings Efficiency Curve 1200l/mm

Grating Efficiency Curves for AvaSpec-NIRLine

Gratings Efficiency Curve 100 & 200l/mm

Gratings Efficiency Curve 200 & 300l/mm

Gratings Efficiency Curve 400 & 600l/mm

How to select optimum optical resolution

The optical resolution is defined as the minimum difference in wavelength that can be separated by the spectrometer. For separation of two spectral lines it is necessary to image them at least two array pixels apart.

Because the grating determines how far different wavelengths are separated (dispersed) at the detector array, it is an important variable for the resolution. The other important parameter is the width of the light beam entering the spectrometer. This is basically the installed fixed entrance slit in the spectrometer, or the fibre core diameter when no slit is installed.

For AvaSpec spectrometers, the available slit widths are 10, 25, 50, 100, or 200µm wide x 1000µm high, or 500µm wide x 2000µm high. The slit image on the detector array for a given wavelength will cover a number of pixels. For two spectral lines to be separated, it is necessary that they be dispersed over at least this image size plus one pixel. When large core fibres are used, the resolution can be improved by a slit of smaller size than the fibre core. This effectively reduces the width of the light beam entering the spectrometer optical bench. The influence of the chosen grating and the effective width of the light beam (fibre core or entrance slit) are shown in the tables provided for each AvaSpec spectrometer instrument.

The resolution is alawys defined as Full Width Half Maximum (FWHM), which is defined as the width in nm of the peak at 50% of the maximum intensity.

Graphs with information about the pixel dispersion can be found in the gratings section above, so you can optimally determine the right grating and resolution for your specific application.

For larger pixel height detectors (3648, 2048L, 2048XL) in combination with thick fibres (>200µm) and a larger grating angle, the actual FWHM value can be 10-20% higher than the value in the table. For best resolution small core diameter fibres are recommended.

All data in the resolution tables are based on averages of actual measured data (with 200µm fibers) of the Avantes Quality Control System during the production process. A typical standard deviation of 10-25%, depending on the slit diameter and the grating should be taken into account. For 10µm slits, the typical standard deviation is somewhat higher, which is inherent to the laws of physics. The peak may fall exactly within one pixel, but may cover 2 pixels causing lower measured resolution.

A new option is the new replaceable slit feature, available on all ULS spectrometers and the uncooled NIR 1.7 spectrometer. The spectrometers come with one installed slit and a slit kit which includes all four slit sizes, so you can opt for higher resolution (25µm slit) or higher throughput (200µm slit) or somewhere in between (50 or 100µm slits).

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Detector arrays

The AvaSpec line of spectrometers can be equipped with several types of detector arrays. At present, we offer silicon based CCD's, back thinned CCD's, and Photo Diode Arrays for the 200-1100nm range. A complete overview of each is given in the following paragraphs on 'Sensitivity'. For the NIR range (1000-2500nm) InGaAs arrays are used.

All detectors are tested in incoming goods inspection, well before they are used in our instruments. Avantes offers full traceability on following detector specifications such as:

• Dark noise
• Signal to noise
• Photo Response Non-Uniformity
• Hot pixels

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StarLine CCD Detectors (AvaSpec-ULS2048/2048L/3648)

The Charged Coupled Device (CCD) detector stores a charge, which dissipates as photons strike the photoactive surface. At the end of a controlled time-interval (integration time), the remaining charge is transferred to a buffer and then this signal is transferred to the AD converter. CCD detectors are naturally integrating and therefore have enormous dynamic range, only limited by the dark (thermal) current and the speed of the AD converter. The 3648-pixel CCD has an integrated electronic shutter function, so an integration time of 10µs can be achieved.

Advantages for the CCD detectors are large numbers of pixels (2048 or 3648), high-sensitivity and high-speed.

Main disadvantage is the lower S/N ratio relative to other detector types.

UV enhancement

For applications below 350nm with the AvaSpec-ULS2048/2048L/3648, a special DUV detector coating is required. The uncoated CCD response below 350 nm is very poor, so a DUV lumogen coating enhances the detector response in the region 150-350nm. The DUV coating has a very fast decay time, typycally in the ns range and is therefore useful for fast trigger LIBS applications.

Photo Diode Arrays (AvaSpec-128)

A silicon photodiode array consists of a linear array of multiple photo diode elements, and in the case of the AvaSpec-128, this is 128 pixels. Each pixel consists of a P/N junction with a positively doped P region and a negatively doped N region. When light enters the photodiode, electrons will become excited and generate an electrical signal. Most photodiode arrays have an integrated signal processing circuit with readout/integration amplifier on the same chip.

Advantages for the Photodiode detector are high NIR sensitivity and high speed.

Disadvantages are limited amount of pixels and no UV response.

SensLine Back-thinned CCD Detectors (AvaSpec-ULS2048x16/x64/XL/HS1024x58/122)

For applications requiring high quantum efficiency in the UV (200-350nm) and NIR (900-1160nm) range, combined with good S/N and a wide dynamic range, back-thinned CCD detectors are the right choice.

Both uncooled and cooled backthinned CCD detectors are offered, the uncooled backthinned CCD detector has 2048 pixels with a pixel pitch of 14µm and a height of 500µm, to have more sensitivity and a better S/N performance. For even better sensitivity and S/N, the cooled backthinned CCD detector is the best choice, it has 1024 pixels, each of them with 58 or 122 vertically binned pixels, giving an effective detector height of 1.4mm or nearly 3.0mm

Advantage of the back-thinned CCD detector is the good UV and NIR sensitivity, combined with good S/N and dynamic range.

Disadvantage is the higher cost in comparison to CCd's or PDA's.

InGaAs linear image sensors (AvaSpec-NIR256/512)

The InGaAs linear image sensors deliver high sensitivity in the NIR wavelength range. The detector consists of a charge amplifier array with CMOS transistors, a shift register and timing generator. For InGaAs detectors the dynamic range is limited by the dark noise.

For ranges up to 1750nm, no cooling is required and these detectors are available in both 256 and 512 pixels. Detectors for the extended range of 2.0-2.5µm all have 2- stage TE-cooling to reduce dark noise and are available in 256 and 512 pixel versions (1.7 and 2.2 detectors only).

Seven versions of detectors are available:

256 pixel non-cooled InGaAs detector for the 900-1750nm range
256/512 pixel cooled InGaAs detector for the 900-1750nm range
256 pixel 2-stage cooled Extended InGaAs detector for the 1000-2000nm range
256/512 pixel 2-stage cooled Extended InGaAs detector for the 1000-2200nm range
256 pixel 2-stage cooled Extended InGaAs detector for the 1000-2500nm range

Sensitivity

The sensitivity of a detector pixel at a certain wavelength is defined as the detector electrical output per unit of radiation energy (photons) incident to that pixel. With a given A/D converter this can be expressed as the number of counts per mJ of incident radiation.

The relation between light energy entering the optical bench and the amount hitting a single detector pixel depends on the optical bench configuration. The efficiency curve of the grating used, the size of the input fibre or slit, the mirror performance and the use of a Detector Collection Lens are the main parameters.

With a given setup, it's possible to conduct measurements over about 6-7 decades of irradiance levels. Some standard detector specifications can be found in the table below on detector specifications. As an option, a DCL (Cylindrical Detector Collection lens) can be mounted directly on the detector array. This quartz lens (DCL-UV/VIS for AvaSpec-ULS2048/3648) will increase the system sensitivity by a factor of 3-5, depending on the fibre diameter used. The DCL-UV/VIS-200 can be used for the AvaSpec-ULS2048L/3648/2048XL to have a better vertical distribution of light focusing on the detector and is primarily for fibre diameters larger than 200µm and round to linear assemblies.

The SensLine has the most sensitive detectors in AvaSpec instrument line, with three backthinned detectors and two cooled CCD detectors.

In the tables below, the UV/VIS detectors are depicted with their specifications - please find below some additional information on how those specifications are determined.

Pixel Well Depth (electrons)
This value is specified by the detector supplier and defines how many electrons can fit in a pixel well before it is saturated, this value determines the best reachable Signal to Noise (=√(Pixel well depth)).

Sensitivity in Photons/count @ 600nm
The number of Photons of 600nm that are needed to generate one count of signal on a 16-bit AD converter, the lower this number is, the better is the sensitivity of the detector. The calculation of the number of Photons/count is (Pixel Well depth in electrons)/16-bit AD/Quantum Efficiency @ 600nm.

Sensitivity in counts/µW per ms integration time
Sensitivity here is for the detector types currently used in the UV/VIS AvaSpec spectrometers as output in counts per ms integration time for a 16-bit AD converter. To compare the different detector arrays, we have them all built up with an optical bench with grating UA, 300 lines/mm covering 200-1100nm (AvaSpec-128 with grating VZ 350-1100 nm), DCL if applicable, and 50µm slit. The measurement setup for 350-1100nm has a 600µm fibre connected to an AvaSpere-50-LS-HAL, equivalent to an optical power of 1.14 µW. For the UV/VIS measurement at 220-1100nm, we connected the 600µm fibre to an AvaLight-DH-S through a CC-VIS/NIR diffuser, equivalent to 2.7 µW power.

Peak wavelength and QE @ peak
The peak wavelength is provided by the detector supplier as well as the Quantum Efficiency, defined as the number of electrons generated by one photon.

Signal/Noise
Signal/Noise is measured for every detector at Avantes Quality Control Inspection and defined as the illuminated maximum Signal/Noise in Root Mean Square for the shortest integration time. The RMS is calculated over 100 scans.

Dark Noise
Dark noise is measured for every detector at Avantes Quality Control Inspection and defined as the non illuminated noise in Root Mean Square for the shortest integration time. The RMS is calculated over 100 scans.

Dynamic Range
The dynamic range is defined as the (maximum signal level-baseline dark level)/dark noise RMS.

Photo Response Non-Uniformity
Photo Response Non-Uniformity is defined as the max difference between output of pixels when uniformly illuminated, divided by average signal of those pixels. PRNU is measured for every detector at Avantes Quality Control Inspection.

Frequency
The frequency is the clock frequency at which the data pixels are clocked out through the AD-converter.

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Detector specifications for the AvaSpec-Starline

* DUV coated. ** Photo Response Non-Uniformity = max difference between output of pixels when uniformly illuminated, divided by average signal

Detector specifications for the AvaSpec-SensLine

* DUV coated. ** Photo Response Non-Uniformity = max difference between output of pixels when uniformly illuminated, divided by average signal

UV/VIS/NIR Detector response curves

NIR Detector response curves

For NIR detectors, two different modes are available.

The default setting is for high sensitivity mode (HS) - this means more signal at a shorter integration time. The other mode of operation is low-noise (LN), this means a better S/N performance.

Sensitivity, S/N, dark noise and Dynamic Range are given as HS and LN values.

Detector specifications for the AvaSpec-NIRLine

** Photo Response Non-Uniformity

Stray light

Stray-light is radiation of undesired wavelengths that activates a signal at a detector element. Sources of stray-light can be:

Ambient light
Scattering light from imperfect optical components, or reflections of non-optical components
Order overlap

Avantes symmetrical Czerny-Turner optical bench designs favour stray-light rejection when compared to crossed designs.In addition, Avantes Ultra-Low Straylight (AvaSpec-ULS) spectrometers have a number of internal measures to reduce straylight from zero order and backscattering.

When working at the detection limit of the spectrometer system, the straylight level from the optical bench, grating and focusing mirrors will determine the ultimate limit of detection. Most gratings used are holographic gratings, known for their low level of straylight. Straylight measurements are conducted using a halogen light source and longpass or bandpass filters.

Typical straylight performance for the AvaSpec-ULS and a B-type grating is <0.04% at 250-500nm. Second order effects, which can play an important role for gratings with low groove frequency and therefore a wide wavelength range, are usually caused by the 2nd order diffracted beam of the grating. The effects of these higher orders can often be ignored, but sometimes need to be addressed using filtering. The strategy is to limit the light to the region of the spectra, where order overlap is not possible.

Second order effects can be filtered out, using a permanently installed longpass optical filters in the SMA entrance connector or an order sorting coating on a window in front of the detector. The order sorting coatings on the window typically has one longpass filter (600nm) or two longpass filters (350nm and 600nm), depending on the type and range of the selected grating.

In the table below, a wide range of optical filters for installation in the optical bench can be found. The filter types that are 3mm thick give much better 2nd order reduction than the 1mm filters.

The use of following longpass filters is recommended:

OSF-475-3 for grating NB and NC

OSF-515-3/550-3 for grating NB

OSF-600-3 for grating IB.

For backthinned detectors, such as the 2048XL and 1024x58/122, we recommend an OSF-305 filter when the starting wavelength is 300nm and higher.

In addition to the order sorting coatings, we apply partial DUV coatings on the Sony 2048 detectors to avoid second order effects from UV response, to enhance sensitivity and decrease noise in the visible range.

This partial DUV coating is automatically included for the following grating types:

UA for 200-1100 nm, DUV400, only first 400 pixels coated

UB for 200-700 nm, DUV800, only first 800 pixels coated

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Filter options for the AvaSpec Series

Thermal stability

All AvaSpec spectrometers have no moving parts inside and are in nature extremely robust and stable. The thermal stability of our spectrometers is part of the comprehensive Quality Control procedure and therefore closely monitored during the production and assembly process.

All spectrometers undergo overnight thermal cycling, during which wavelength shift, intensity drop and spectral tilt are registered and checked against our QC acceptance norm. More specifically, the following tests are being carried out during the thermal cycling from 15°C to 25°C to 35°C back to 25°C:

Full Width Half Maximum

During the thermal cycling, the average FWHM value is measured and has to fit with a certain standard deviation within the QC acceptance norm as can be found in the catalogue for the various configurations.

Peakshift

During thermal cycling, the shift of peaks is monitored and depicted as shift in pixels per °C. Depending on the grating angle, the maximum allowed peakshift is defined. For most gratings, the values below are the QC acceptance norm. For gratings with many lines/mm starting at high wavelengths (VD, VE), the peak shift can double.

The max allowed peakshift =± 0.1 pixel per °C for an AvaSpec-ULS2048 with a pixel pitch of 14μm. Average peakshift is ± 0.04 pixel per °C for an AvaSpec-ULS2048. For an AvaSpec-ULS3648 with a pixel pitch of 8μm the max allowed peakshift is ± 0.17 pixel per °C. For the AvaSpec-128 and for the AvaSpec-NIR256 with relative large pixels of 50μm, the peakshift is limited to ± 0.03 pixel per °C. For backthinned and NIR detectors with a 25μm pitch as in the AvaSpec-HS1024x58/122 and AvaSpec-NIR512, the peakshift is limited to ± 0.06 pixel per °C.

Intensity stability and Spectral tilt

Temperature sensitivity on the intensity axis can have a number of causes. Firstly, the CCD detector itself has a temperature dependency. For most detectors, there are black pixels that are read out and subtracted from the rest of the data pixels, the so-called Correct for Dynamic dark (CDD). However, CDD will not correct for spectral tilt, which is partially also a detector property. The aluminum optical bench and the optical components are engineered in such a way that the thermal expansion does not lead to large increase in tilt or sensitivity.

For the majority of spectrometers, the average intensity increase/decrease is within ±4% for ± 10°C thermal cycling. In the figure to the right, a typical test result for a thermal cycling can be seen.

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Spectrometer Platforms

AvaSpec StarLine
The AvaSpec-StarLine family of instruments is compromised of high performance spectrometers which exceed the demands of most general spectroscopy applications. The StarLine includes high speed instruments for process control (AvaSpec-128 and AvaSpec-FAST-series), high resolution instruments for demanding measurements like atomic emission (AvaSpec-ULS3648) and versatile instruments for common applications such as irradiance and absorbance chemistry (AvaSpec-ULS2048 & AvaSpec-ULS2048L). This instrument line offers an array of solutions for varied uses, while providing excellent price to performance ratios.

The AvaSpec-ULS2048/2048L and AvaSpec-3648 are based on front illuminated linear CCD arrays and thanks to Avantes DUV coating, can measure wavelengths from 200-1100nm. The AvaSpec-FAST series of instruments is specifically designed for high speed acquisitions such as pulsed light source and laser measurements. The AvaSpec-128 is an ultrafast photo-diode array-based instrument for visible and near-infrared applications.

Instruments in the AvaSpec StarLine family are designed to perform in a variety of applications such as:

Reflection and transmission measurements for optics, coatings, colour measurement
Irradiance and emission measurements for environmental, light characterisation, and optical emission spectroscopy
High speed measurements for process control, LIBS or laser/pulsed source characterisation
Absorbance chemistry

AvaSpec StarLine instruments are fully integrated with Avantes modular platform, allowing them to function stand alone, or as multichannel instruments.

These products are also fully compatible with other AvaSpec instruments in our AvaSpec SensLine and NIRLine. The entire AvaSpec StarLine is available as an individual lab instrument or an OEM module for integration into a customers existing systems.

The StarLine instruments are available with our standard AvaBench-45 optical bench (45 mm focal length) or the UltraLow Straylight (ULS) optical bench (75 mm focal length). The AvaSpec StarLine instruments are also available with a number of premium options such as irradiance/intensity calibration and non-linearity calibration.

AvaSpec SensLine
The AvaSpec-SensLine family of products is our response to customers who require higher performance for demanding spectroscopy applications such as fluorescence, luminescence and Raman.

The AvaSpec-SensLine product line includes five high sensitivity, low noise spectrometers. Three of the instruments are based on back thinned detector technology, of which two feature high performance thermoelectrically cooled detectors. The other two models are based on standard CCDs, upgraded to high performing instruments as a result of Avantes unique and recently improved detector cooling technology. The backthinned CCD detectors featured in the AvaSpec SensLine product family are high quantum efficiency detectors with excellent response in the UV, VIS and NIR from 200-1160nm.

AvaSpec-SensLine instruments are fully integrated with Avantes modular platform, allowing them to function standalone, or as multichannel instruments. These products are fully compatible with other AvaSpec instruments in our AvaSpec-StarLine and AvaSpec-NIRLine product families. The entire AvaSpec-SensLine is available as a lab instrument or an OEM module for integration into a customers existing systems. Avantes innovative ultralow straylight (ULS) and revolutionary new High Sensitivity (HS) optical benches are the core optical technologies in the AvaSpec-SensLine. These highly stable optical benches combined with our high performance AS5216-USB2 electronics board deliver high performance instruments at affordable prices.

All members of the AvaSpec SensLine are designed to provide performance features such as:

High stability
High sensitivity
High speed acquisition
Low noise

AvaSpec NIRLine
The AvaSpec-NIRLine instruments are high performance, near infrared spectrometers that are optimised for the demands of measuring longer wavelengths. This line provides leading edge performance for dispersive NIR instruments with toroidal focusing mirrors and dynamic dark correction for enhanced stability.

The NIRLine is comprised of both thermoelectrically cooled and non cooled instruments. AvaSpec-NIR256-1.7 features an non cooled 256 pixel InGaAs detector. All other instruments in the NIRLine have thermoelectric, peltier cooled InGaAs detectors which support cooling down to -25°C against ambient.

AvaSpec-NIRLine instruments are fully compatible with our AvaSpec-StarLine and SensLine spectrometers. AvaSpec-NIRLine instruments are available as laboratory instruments or OEM modules. AvaSpec-NIRLine instruments are available with a number of premium options such as irradiance/intensity calibration and non-linearity calibration.

The AvaSpec-NIRLine of instruments are designed to perform in a variety of applications such as:

Moisture content measurement of liquids, solids and powders for inline and quality control purposes
Quantitative and qualitative measurement of volatile organics such as ethanol, and methanol
Plastic characterisation and material identification
Irradiance measurements, such as solar monitoring
Qualitative measurements of feed and food