Theoretical background: Spectrometers

Introduction Fiber Optic Spectroscopy

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.

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 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.

Optical Bench Design

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.
optical bench
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.

How to configure a spectrometer for your application

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.
  • 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 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.

Table 1 Quick reference guide for spectrometer configuration

Application

AvaSpec-type

Grating

WL range (nm)

Coating

Slit

FWHM Resolution (nm)

DCL

>OSF

OSC

Biomedical

ULS2048

NB

500-1000

-

50

1.2

-

475

-

Chemometry

ULS2048

UA

200-1100

-

50

2.0

-

-

OSC-UA

Color

128

VA

360-780

-

100

6.4

X/-

-

-

ULS2048

BB

360-780

-

200

4.1

X/-

-

-


Fluorescence

ULS2048XL

V, VB, UB

350-1100, 300-800

-

200

8.0

X

305

OSC

 HS1024x122TEC  HS-500-0.33 200-1160 - 200 10.0 - - OSC

Fruit-sugar

128

IA

800-1100

-

50

5.4

X

600

-

Gemology

ULS2048

VA

350-1100

-

25

1.4

X

-

OSC

High resolution

ULS2048

VD

600-700

-

10

0.07

-

550

-

ULS3648

VD

600-700

-

10

0.05

-

550

-

High UV/NIR-Sensitivity

HS1024x122TEC

HS-500-0.33

200-1160

-

200

10.0

-

-

OSC

Irradiance

ULS2048

UA

200-1100

DUV

50

2.8

X/-

-

OSC-UA

Laserdiode

ULS2048

NC

700-800

-

10

0.1

-

600

-

LED

ULS2048

VA

350-1100

-

25

1.4

X/-

-

OSC

LIBS

ULS2048

D, E, F

200-900

DUV

10

0.09

-

-

-

Raman

ULS2048LTEC

NC

780-930

-

25

0.2

X

600

-

Thin Films

ULS2048

UA

200-1100

DUV

-

4.1

X

-

OSC-UA

UV/VIS/NIR

ULS2048

UA

200-1100

DUV

25

1.4

X/-

-

OSC-UA

ULS2048XL

UA

200-1100

-

25

1.4

-

-

OSC-UA

NIR

 

NIR512-1.7TEC

NIR200-1.5

1000-1750

-

25

5.0

-

1000

-

NIR256-2.0TEC

NIR150-2.0

1000-2000

-

50

10.0

-

1000

-

NIR256-2.5TEC

NIR100-2.5

1000-2500

-

50

15.0

-

1000

OSC-NIR

How to choose the right grating?

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

gratings.jpg

The way in which the grooves are formed separates gratings in two types, holo-graphic 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.

Avantes AvaSpec spectrometers come with a permanently installed grating that must be specified by the user. Additionally the user needs to indicate what wavelength range 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 multi-channel 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 multi-channel 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 selection

Grating Efficiency Curves
300 lines/mm gratings
Grating 300
600 lines/mm gratings
Grating 600
1200 lines/mm gratings
Grating 1200
1800 lines/mm gratings
Grating 1800
2400 lines/mm gratings
Grating 2400
3600 lines/mm gratings
Gratings-Overview pag12
HS 500 lines/mm gratings
HS500
HS 830-1000 lines/mm gratings
HS830-1000
HS 1200 lines/mm gratings
HS 1200
NIR 100-200 lines/mm gratings
NIR 100-200
NIR 200-300 lines/mm gratings
NIR 200-300
NIR 300-400 lines/mm gratings
Grating overview NIR pag 13

 

Grating Dispersion Curves
300 lines/mm gratings
Avabench75 2048-pag14
600 lines/mm gratings
disp 600
1200 lines/mm gratings
disp 1200
1800 lines/mm gratings
disp 1800
2400 lines/mm gratings
disp 2400
3600 lines/mm gratings
disp 3600

How to select optimal 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 fiber core when no slit is installed.

For AvaSpec spectrometers the available slit widths are 10, 25, 50, 100, or 200 µm wide x 1000 µm high, or 500 µm wide x 2000 µm 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 fibers are used the resolution can be improved by a slit of smaller size than the fiber 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 (fiber core or entrance slit) are shown in the tables provided for each AvaSpec spectrometer instrument.

In the table below the typical resolution can be found for the AvaSpec-ULS2048. Please note that for the higher lines/mm gratings the pixel dispersion varies along the wavelength range and gets better towards the longer wavelengths.

The resolution in this table is 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 as well, 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 fibers (>200 µm) 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 fibers are recommended.

All data in the resolution tables are based on averages of actual measured data (with 200 µm fibers) of our 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 10 µm 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.

New is the 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 (25 µm slit) or higher throughput (200 µm slit) or somewhere in between (50 or 100 µm slits).

 

Resolution (FWHM in nm) for the AvaSpec-ULS2048

Slit size (µm)

Grating (lines/mm)

10

25

50

100

200

500

300

0.80-0.90*

1.10-1.20*

2.30

4.60

9.00

22.0

600

0.40-0.50*

0.3.

1.15

2.31

4.50

11.0

830

0.28

0.40

0.80

1.60

3.20

8.0

1200

0.18-0.22*

0.29

0.61

1.18

2.20

5.5

1800

0.10-0.16*

0.19

0.35-0.42*

0.80

1.60

4.0

2400

0.08-0.11*

0.10-0.15*

0.28

0.55

1.10

2.8

3600

0.05-0.08*

0.10

0.18

0.38

0.75

1.9

*depends on the starting wavelength of the grating; the higher the wavelength, the bigger the dispersion and the higher the resolution

Detector arrays

The AvaSpec line of spectrometers can be equipped with several types of detector arrays. Presently we offer silicon-based CCDs, back-thinned CCDs, and Photo-Diode Arrays for the 200-1100 nm range. A complete overview of each is given in the next section “ Sensitivity “. For the NIR range (1000-2500 nm) InGaAs arrays are implemented.
All detectors are tested in incoming goods inspection, before they are used in our instruments. Avantes offers full traceability on following detector specifications:

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

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

The Charged Coupled Device (CCD) detector stores the charge, dissipated 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 being 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 350 nm with the AvaSpec-ULS2048/2048L/3648 a special DUV-detector coating is required. The uncoated CCD-response below 350 nm is very poor; the DUV lumogen coating enhances the detector response in the region 150-350 nm. The DUV coating has a very fast decay time, typ. in ns range and is therefore useful for fast-trigger LIBS applications.

pagina 16 Starline detectoren vrijstaand

 

Photo Diode Arrays (AvaSpec-128)

A silicon photodiode array consists of a linear array of multiple photo-diode elements, for 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.

pagina 16 Senseline detectoren vrijstaand

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

For applications requiring high quantum efficiency in the UV (200-350 nm) and NIR (900-1160 nm) 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 14 µm and a height of 500 µm, 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 1.4 mm or nearly 3.0 mm

+  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 relatively higher cost.

pagina 16 Nirline detectoren vrijstaand

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 1.75 µm no cooling is required and these detectors are available in both 256 and 512 pixels. Detectors for the extended range 2.0-2.5 µm 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).

7 versions of detectors are available:

  • 256 pixel non-cooled InGaAs detector for the 900-1750 nm range
  • 256/512 pixel cooled InGaAs detector for the 900-1750 nm range
  • 256 pixel 2-stage cooled Extended InGaAs detector for the 1000-2000 nm range
  • 256/512 pixel 2-stage cooled Extended InGaAs detector for the 1000-2200 nm range
  • 256 pixel 2-stage cooled Extended InGaAs detector for the 1000-2500 nm 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 fiber or slit, the mirror performance and the use of a Detector Collection Lens are the main parameters. With a given set-up it is possible to do measurements over about 6-7 decades of irradiance levels. Some standard detector specifications can be found in Table 4 detector specifications. Optionally a DCL (Cylindrical Detector Collection) lens can be mounted directly on the detector array. The quartz lens (DCL-UV/VIS for AvaSpec-ULS2048/3648) will increase the system sensitivity by a factor of 3-5, depending on the fiber 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 fiber diameters larger than 200 µm and round- to-linear assemblies.
The SensLine has the most sensitive detectors in Avantes’ instrument line, 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 @ 600 nm
The number of Photons of 600 nm 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 @ 600 nm.

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 UA 300 lines/mm grating covering 200-1100 nm (AvaSpec-128 with grating VZ 350-1100 nm), DCL if applicable, and 50 µm slit. The measurement setup for 350-1100 nm has a 600 µm fiber connected to an AvaSpere-50-LS-HAL, equivalent to an optical power of 1.14 µW.For the UV/VIS measurement at 220-1100 nm we connected the 600 µm fiber to an AvaLight-DHS 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.

 

Detector Specifications (based on a 16-bit AD converter)

StarLine

Detector

TAOS 128

SONY2048

SONY2048L

TOS3648

Type

Photo diode array

CCD linear array

# Pixels, pitch

128, 63.5µm

2048, 14 µm

3648, 8 µm

Pixel width x height (µm)

55.5 x 63.5

14 x 56

14 x 200

8 x 200

Pixel well depth (electrons)

250,000

40,000

90,000

120,000

Sensitivity Photons/count @600nm

10

2 4

5

Sensitivity

in counts/µW per ms integration time

430,000 (AvaSpec-128)

310,000 (AvaSpec-ULS2048)

470,000 (AvaSpec-ULS2048L)

160,000

(AvaSpec-ULS3648)

Peak wavelength

750 nm

550 nm

450 nm

550 nm

QE (%) at peak 40%

Signal/Noise

500:1

200 :1

300 :1

350 :1

Dark noise (counts RMS)

15

33

20

34

Dynamic Range

4380

2000

3300

1900

PRNU**

± 4%

± 5%

Wavelength range (nm)

360-1100

200*-1100

Frequency

2 MHz

1 MHz

 

SensLine
 Detector

HAM2048x16

HAM2048x64

HAM2048XL

HAM1024x58 HAM1024x122
 Type

Back-thinned CCD array

 Back-thinned CCD array  Back-thinned CCD array  Cooled Back-Thinned CCD array Cooled Back-Thinned CCD array 
 # Pixels, pitch  2048x14, 14µm  2048x64, 14µm  2048, 14µm  1024 x 58, 24 µm 1024 x 122, 24 µm
 Pixel width x height (µm)  14 x 14  14 x 500  24 x 24 (total height 1.4 mm)  24 x 24 (total height 2.9 mm)
 Pixel well depth (electrons)  200,000  1,000,000
 Sensitivity Photons/count @600nm  4 16
Sensitivity

in counts/µW per ms integration time

 200,000 (AvaSpec-ULS2048x16)  600,000 (AvaSpec-ULS2048x64)  460,000 (AvaSpec-ULS2048XL)  850,000 (AvaSpec-HS1024x58) 1,270,000 (AvaSpec-HS1024x122)
 Peak wavelength  600 nm   650 nm
 QE (%) at peak  78%  92%
 Signal/Noise  500:1  500:1  450 :1  1000 :1  1000:1
 Dark noise (counts RMS)  17 8
 Dynamic Range   3800 8,000 
 PRNU**

±3%

 Wavelength range (nm)    200-1160  
 Frequency 1.33 MHz   1 MHz  250kHz  

 

* DUV coated

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

Figure 5 Detector Spectral Sensitivity Curves

Sensitivity UV-VIS-NIR-2

sensitivity_uv-vis-nir.jpg



In the next table the specification is given for the NIR spectrometers, followed by the spectral response curve for the different detector types are depicted.

Sensitivity
For NIR detectors 2 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.

NIR Detector Specifications

Detector NIR256-1.7 NIR256-1.7TEC NIR512-1.7TEC NIR256-2.0TEC NIR256-2.2TEC NIR512-2.2TEC NIR256-2.5TEC-HSC
Type Linear InGaAs array Linear InGaAs array with 2 stage TE cooling
# Pixels, pitch 256, 50 µm 512, 25 µm 256, 50 µm 512, 25 µm 256, 50 µm
Pixel width x height (µm) 50 x 500 25 x 500 50 x 250 50 x 500 25 x 500 50 x 250
Sensitivity HS
in counts/µW per ms
1,300,000 (integral 1000-1750 nm) 2,770,000 (integral 1000-1750 nm) 2,770,000 (integral 1000-1750 nm) 70,000 (integral 1000-2000 nm) 77,000 (integral 1200-2200 nm) 38,500 (integral 1200-2200 nm) 145,000 (integral 1000-2500 nm)
Signal/
Noise (HS)
2000:1 1700:1 1500:1 1200:1 1400 :1
Dark noise HS (counts RMS) 14 13 21 12 16
Dynamic Range HS

 

4000 5000 3300 4800 2800
Sensitivity LN
in counts/µW per ms
74,000 (integral 1000-1750 nm) 96,000 (integral 1000-1750 nm) 96,000 (integral 1000-1750 nm) 4,000 (integral 1000-2000 nm) 2,750 (integral 1200-2200 nm) 1,375 (integral 1200-2200 nm) 84,000 (integral 1000-2500 nm)
Signal/Noise (LN) 6000:1 3600:1 4000:1 4100:1 3685:1
Dark noise LN (counts RMS) 8 16 8 12
Dynamic Range LN 8000 4000 8000 3600
Peak wavelength 1550 nm 1500 nm 1850 nm 2000 nm 2300 nm
QE (%) @ peak 90% 70% 80% 60% 65%
PNRU** ± 5% 10% ± 5% 10% ±5%
Defective pixels (max) 0 12 5 10 12
Wavelength range (nm) 900-1750 1000-2000 1000-2200 1000-2500
Frequency 500 kHz 2.4 MHz 500 kHz 2.4 MHz 500 kHz

** Photo-Response Non-Uniformity

 

NIR Detector Sensitivity Curves

sensitivityCatIX NIR

Stray Light and second order effects

IMG 9831

Order Sorting Window in holder

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 favor stray-light rejection relative to crossed designs. Additionally, Avantes Ultra-Low Stray-light (AvaSpec-ULS) spectrometers have a number of internal measures to reduce stray-light from zero order and backscattering.

When working at the detection limit of the spectrometer system, the stray-light 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 stray-light. Stray-light measurements are conducted using a halogen light source and long-pass or band-pass filters.

Typical stray-light performance for the AvaSpec-ULS and a B-type grating is <0.04% at 250-500 nm. 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 long-pass optical filter 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 have one long-pass filter (600 nm) or 2 long-pass filters (350 nm and 600 nm), 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 3 mm thick give much better 2nd order reduction than the 1 mm filters. The use of following long-pass filters is recommended: OSF-475-3 for grating NB and NC, OSF-515-3/550-3 for grating NB and 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 300 nm 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 and to enhance sensitivity and decrease noise in the visible range.

This partial DUV coating is done automatically 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

 Filters installed in AvaSpec spectrometer series

OSF-305-3

Permanently installed 3 mm order sorting filter @ 305 nm

OSF-385-3

Permanently installed 3 mm order sorting filter @ 385 nm

OSF-475-3

Permanently installed 3 mm order sorting filter @ 475 nm

OSF-515-3

Permanently installed 3 mm order sorting filter @ 515 nm

OSF-550-3

Permanently installed 3 mm order sorting filter @ 550 nm

OSF-600-3

Permanently installed 3 mm order sorting filter @ 600 nm

OSF-850-3

Permanently installed 3 mm order sorting filter @ 850 nm

OSC

Order sorting coating with 600 nm long pass filter for VA, BB (>350nm) and VB gratings in AvaSpec-ULS2048(L)/3648/2048x16/64/XL

OSC-UA

Order sorting coating with 350 and 600 nm long pass filter for UA gratings in AvaSpec-ULS2048(L)/3648/2048x16/64/XL

OSC-UB

Order sorting coating with 350 and 600 nm long pass filter for UB or BB (<350nm) gratings in AvaSpec-ULS2048(L)/3648/2048x16/64/XL

OSC-HS500

Order-sorting coating with 350 and 600 nm long-pass filter for HS500 gratings in AvaSpec-HS

OSC-HS900

Order-sorting coating with 600 nm long-pass filter for HS900 gratings in AvaSpec-HS

OSC-HS1000

Order-sorting coating with 350 nm long-pass filter for HS1000 gratings in AvaSpec-HS

OSC-NIR Order-sorting coating with 1400 nm long-pass filter for NIR100-2.5 and NIR150-2.0 gratings in AvaSpec-NIR256/512-2.2/2.5TEC

Thermal Stability

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 our comprehensive Quality Control procedure and therefore closely monitored during the production and assembly process. All of our 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 test 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 this catalog for the different 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 below values 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 reasons. First the CCD detector itself has a temperature dependency, for most detectors there are black pixels that are read out and are 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 most spectrometers the average intensity increase/decrease is within ±4% for ± 10°C thermal cycling.

In the figure a typical test result for a thermal cycling can be seen.

Thermal-stability

Spectometer platforms

AvaSpec-2048L
Av
aSpec 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 200-1100 nm. The AvaSpec-FAST series of instruments is specially 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, color measurement
  • Irradiance and emission measurements for environmental, light characterization, and optical emission spectroscopy
  • High-speed measurements for process control, LIBS or laser/pulsed source characterization
  • Absorbance chemistry


AvaSpec StarLine instruments are fully integrated with Avantes’ modular platform, allowing them to function stand-alone, or as multi-channel instruments. These products are 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 system.

The StarLine instruments are available with our standard AvaBench-45 optical bench (45 mm focal length) or the Ultra-Low Stray-light (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-2048xl-2
AvaSpec SensLine

The AvaSpec SensLine family of products is Avantes’ response to customers who require higher performance for deman-ding 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 back-thinned CCD detectors featured in the AvaSpec SensLine product family are high quantum efficiency detectors with excellent response in the UV, VIS and NIR from 200-1160 nm.

AvaSpec SensLine instruments are fully integrated with Avantes’ modular platform, allowing them to function standalone, or as multi-channel 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 system.Avantes’ innovative ultra-low stray-light (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-NIR-1
AvaSpec NIRLine

The AvaSpec NIRLine instruments are high-performance, near-infrared spectrometers that are optimized for the demands of measuring long 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 thermo-electrically cooled and un-cooled instruments. AvaSpec-NIR256-1.7 features an uncooled 256 pixel InGaAs detector.  All other instruments in the NIRLine have thermo-electric, 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. Avantes’ 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 characterization and material identification
  • Irradiance measurements, such as solar monitoring
  • Qualitative measurements of feed and food