Optical spectroscopy is a technique for measuring light intensity in the UV-, VIS-, NIR- and IR-region. Spectroscopic measurements are being used in many different applications, such as color measurement, concentration determination of chemical components or electromagnetic radiation analysis. For more elaborate application information and setups, please click on the Application link on our website.
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 also an exit slit, 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 90’s in the field of multi-element optical detectors, such as Charged Coupled Devices (CCD) arrays and Photo-Diode (PD) arrays, enabled the production of low cost scanners, CCD cameras, etc. 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 fiber-optics in the communication technology, low absorption silica fibers have been developed. Similar fibers can be used as measurement fibers to transport light from the sample to the optical bench of the spectrometer. The easy coupling of fibers allows a modular build-up of a system that consists of light source, sampling accessories and fiber-optic spectrometer. Furthermore fiber-optic enable the introduction of sampling into harsh and difficult to access environments.
The low cost, modularity, flexibility and speed of measurement made possible by fiber-optic spectrometers have resulted in wide adoption of this technology in a variety of industries.
The heart of most AvaSpec fiber-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 SMA-905 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 projected onto a 1-dimensional linear detector array.
Avantes AvaSpec-HS1024x58/122 high-sensitivity spectrometers have a revolutionary new optical bench design with multiple toroid mirrors which ensure that the full numerical aperture of the fiber entrance will be projected on the backthinned CCD array.
All of our optical benches have a number of components installed inside, allowing a wide variety of different configurations, depending on the intended application. The choice of these components such as the diffraction grating, entrance slit, order-sorting filter, and detector coating have a strong influence on system specifications such as sensitivity, resolution, bandwidth and stray-light. Each of these specification is discussed in detail in the following paragraphs.
The modular AvaSpec line of instruments provides you with a number of configuration options to optimize the optical and spectroscopic performance of your instrument for your application.
This section provides you some guidance on how to choose the right grating, slit, detector and other configuration options, to be installed in your AvaSpec.
- Wavelength Range
In the determination of the optimal configuration of a spectrometer system the wavelength range is 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 lesser range (approximately 500 nm) 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 an Avantes Sales Engineer or representative for a recommended grating configuration.
- 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. Avantes offers 15 different detector types with each different 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 CCDs and thermo-electrically cooled CCDs UV/VIS instruments. The instruments are particularly better in the UV and NIR relative to standard CCD detectors. The AvaSpec NIRLine is comprised of instruments with InGaAs arrays for longer wavelength measurements from 900-2500 nm.
For high-speed applications, the 2048 pixel CCD detectors in the AvaSpec-ULS2048 and AvaSpec-ULS2048L from the StarLine are normally the best options. For VIS-only applications where high-resolution is not needed but speed and signal to noise are important, the 128 pixel PDA detector in the AvaSpec-128-USB2 may be the best option. For low-light level applications such as fluorescence and Raman, the SensLine instruments may 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 broad wavelength range. For example, an AvaSpec StarLine (UV/VIS) spectrometer can be combined with a NIRLine spectrometer to enable measurements from 200-2500 nm 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 optimized 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 maximize throughput into the optical bench.
New is the AvaSpec-RS with replaceable slit that makes your spectrometer a versatile instrument for both high-resolution and high-sensitivity measurements.
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 optimized 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 user of a detector collection lens (DCL-UV/VIS or DCL-UV/VIS-200), which is a cylindrical lens with focuses light from larger core fiber-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 minimize light dispersion are available. Some more demanding applications also require thermo-electric cooling of the CCD detector (see product
section AvaSpec-ULS2048LTEC and AvaSpec-ULS3648TEC) to minimize 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 spectrometers 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 our application engineers to optimize 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.
For selection guide for spectrometer configuration for your application and more details about choosing spectrometer components such as grating, please click here.