光谱仪参数入门

Devices (CCD) Arrays and Photo Diode (PD) Arrays, enabled the production of low cost scanners, CCD cameras etc. The same CCD and PDA detectors are now used in the Avantes line of spectrometers, enabling fast scanning of the spectrum, wit-hout the need of 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 spectrome-ter. The easy coupling of fibers allows a modular build-up of a system that consists of light source, sampling accessories and fiber optic spectrometer.

Advantages of fiber optic spectroscopy are the modularity and flexibility of the system. The speed of measurement allows in-line analysis, and the use of low-cost commonly used detectors enable a complete low cost Avantes spectrometer system.

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 see further the Application chapter at the end of this catalog.

A spectroscopic instrument 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.

Development of micro-electronics during the 90’s in the field of multi-element optical detectors, such as Charged Coupled

metrical Czerny-Turner design (figure 1).

Light enters the optical bench through a standard

SMA905 connector and is collimated by a spherical mirror. A plane 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.

installed 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. Sensitivity, resolution, bandwidth and stray light are further discussed in the following paragraphs.

Introduction Fiber Optic Spectroscopy

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Biomedical Technology Chemistry Colorimetry Food Technology Inline Process Control Radiometry Thinfilm Analysis

How to configure a spectrometer for your application?

For optimal UV sensitivity we recommend the back-thinned UV sensitive CCD detector, as implemented in the AvaSpec-2048x14.

For the different detector types the photometric sensitivity is given in table 4, the spectral sensitivity for each detector is depicted in figure 5.

b. Chemometric Sensitivity

To detect two absorbance values, close to each other with maximum sensitivity you need a high Signal to Noise (S/N) performance. The detector with best S/N performance is the 2048x14 pixel back-thinned CCD detector, next to the 256/1024 CMOS detector in the AvaSpec-256/1024. The S/N performance can also be enhanced by averaging over multiple spectra.

4. Timing and Speed

The data capture process is inherently fast with detector arrays and no moving parts. However there is an optimal detector for each application. For fast response applications, we recommend to use the AvaSpec- USB2 platform spectrometers. When datatransfer time is critical we recommend to select a small amount of pixels to be transferred with the UBS2 interface. Data transfer time can be enhanced by selecting the pixel range of interest to be transmitted to the PC; in general the AvaSpec-128 may be considered as the fastest spectrometer with more than 8000 scans per second.

The above parameters are the most important in choosing the right spectrometer configuration, please contact our application engi-neers to optimize and fine-tune the system to your needs. On the next page you will find a quick reference table 1 for most common applications, for a more elaborate explanation and configurations, please refer to the applications section in the back of this catalog.

In addition we have introduced in this catalog application icons, that will help you to find the right products and accessories for your applications.

In the modular AvaSpec design a number of choices have to be made on several optical components and options, depending on the application you want to use the spectrometer for.

This section should give you some guidance on how to choose the right grating, slit, detector and other options, installed in the AvaSpec.

1. Wavelength Range

In the determination for the optimal configuration of a spectrometer system the wavelength range is the first important parameter that defines the grating choice. If you are looking for a wide wavelength range, we recommend to take an A-type (300 lines/mm) or B-type (600 lines/mm) grating (see Grating selection table in the spectrometer product section). The other important component is the detector choice, Avantes offers 9 different detector types with each different sensitivity curves (see figure 5). For UV applications the new 2048x14 pixel back-thinned CCD detector, the 256/1024 pixel CMOS detectors or DUV- enhanced 2048 or 38 pixel CCD detectors may be selected. For the NIR range 3 different InGaAs detectors are available.

If you want to combine a wide range with a high resolu-tion, a multiple channel spectrometer may be the best choice.

2. Optical Resolution

If you desire a high optical resolution we recommend to pick a grating that has 1200 or more lines/mm (C,D,E or F types) in combination with a narrow slit and a detector with 2048 or 38 pixels, for example 10 µm slit for the best resolution on the AvaSpec-2048 (see Resolution table in the spectrometer product section)

3. Sensitivity

Talking about 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?) a. Photometric Sensitivity

In order to achieve the most sensitive spectrometer in for example Fluorescence or Raman applications we recommend the 2048 pixel CCD detector, as in the AvaSpec-2048. Further we recommend the use of a DCL-UV/VIS detector collection lens, a relatively wide slit (100µm or wider) or no slit and an A type grating. For an A-type grating (300 lines/mm) the light dispersion is minimal, so it has the highest sensitivity of the grating types. Optionally the Thermo-electric cooling of the CCD detector (see product section AvaSpec-2048-TEC, page 30) may be chosen to minimize noise and increase dynamic

range at long integration times (60 seconds).

Table 1 Quick reference guide for spectrometer configuration

Application AvaSpec- Grating WL range (nm) Coating Slit

FWHM DCL OSF OSC

type Resolution (nm)Biomedical 2048 NB 500-1000 - 50 1.2 - 475 -Chemometry 1024 UA 200-1100 - 50 2.0 - - OSC-UA 128 VA 360-780 - 100 6.4 X/- - -Color 256 VA 360-780 - 50 3.2 - - -

2048 BB 360-780 - 200 4.1 X/- - -Fluorescence 2048 VA 350-1100 - 200 8.0 X - OSC Fruit-sugar 128 IA 800-1100 - 50 5.4 X 600 -Gemology 2048 VA 350-1100 - 25 1.4 X - OSC High 2048 VD 600-700 - 10 0.07 - 550 -resolution 38 VD 600-700 - 10 0.05 - 550 -High UV- 2048x14

UC

200-450

-

200

2.0

-

-

-Sensitivity Irradiance 2048 UA 200-1100 DUV 50 2.8 X/- - OSC-UA Laserdiode 2048 NC 700-800 - 10 0.1 - 600 -LED 2048 VA 350-1100 - 25 1.4 X/- - OSC LIBS 2048FT UE 200-300 DUV 10 0.09 - - - 2048USB2 UE 200-300 DUV 10 0.09 - - -Raman 2048TEC NC 780-930 - 25 0.2 X 600 -Thin Films 2048 UA 200-1100 DUV - 4.1 X - OSC-UA UV/VIS/NIR 2048 UA 200-1100 DUV 25 1.4 X/- - OSC-UA 2048x14

UA

200-1100 - 25 1.4 - - OSC-UA NIR NIR256-1.7 NIRA 900-1750 - 50 5.0 - 1000 - NIR256-2.2 NIRZ 1200-2200 - 50 10.0 - 1000 -

NIR256-2.5 NIRY

1000-2500

-

50

15.0

-

1000

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For each spectrometer type, a grating selection table is shown in the Spectrometer Platforms 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 Please select Spectral range band-width from the useable Wavelength range, for example: grating UE (200-315nm)

*the spectral range depends on the starting wavelength of the grating; the higher the wave-length, the smaller the range.

For example grating UE (510-580 nm)

The order code is defined by 2 letters: the first is the Blaze (U= 250/300nm or UV for holo-graphic, B=400nm, V=500nm or VIS for holo-graphic, N=750nm, I=1000nm) and the second the nr of lines/mm (Z=150, A=300, B=600, C=1200, D=1800, E=2400, F=3600 lines/mm)

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Figure 2 Grating Efficiency Curves 300 Lines/mm Gratings

600 Lines/mm Gratings

1200 Lines/mm Gratings 1800 Lines/mm Gratings

2400 Lines/mm Gratings

3600 Lines/mm Gratings

S

p

e

c

t

r

o

m

e

t

e

r

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Figure 3 Grating Dispersion Curves

300 Lines/mm Gratings600 Lines/mm Gratings

1200 Lines/mm Gratings1800 Lines/mm Gratings

2400 Lines/mm Gratings

3600 Lines/mm Gratings

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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 2 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 instal-led fixed entrance slit in the spectrometer, or the fiber core diameter when no slit is installed.

The slits can be installed with following dimensions: 10, 25 or 50 x 1000 µm high or 100, 200 or 500 µm x 2000 µm high. Its image on the detector array for a given wavelength will cover a number of pixels. For two spectral lines to be separated, it is now necessary that they be dispersed over at least this image size plus one pixel. When large core fibers are used the resoluti-on can be improved by a slit of smaller size than the fiber core. This effectively reduces the width of the entering light beam. The influence of the chosen grating and the effective width of the light beam (fiber core or entrance slit) are shown in the tables at the product information. In Table 3 the typical reso-lution can be found for the AvaSpec-2048. Please note that for the higher lines/mm gratings the pixel dispersion varies along the wavelength range and gets better towards the lon-ger wavelengths (see also Figure 3). The best resolution can always be found for the longest wavelengths. The resolution in this table is defined as F(ull) W(idth) H(alf) M(aximum), which is defined as the width in nm of the peak at 50% of the maximum intensity (see Figure 4).

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.

In combination with a DCL-detector collection lens or thick fibers the actual FWHM value can be 10-20% higher than the value in the table. For best resolution small fibers and no DCL

Figure 4 Full Width Half Maximum

How to select optimal Optical Resolution?

Slit size (µm)

Grating (lines/mm) 10 25 50 100 200 500 300 0.8 1.4 2.4 4.3 8.0 20.0600 0.4 0.7 1.2 2.1 4.1 10.01200 0.1-0.2* 0.2-0.3* 0.4-0.6* 0.7-1.0* 1.4-2.0* 3.3-4.8*1800 0.07-0.12* 0.12-0.21* 0.2-0.36* 0.4-0.7* 0.7-1.4* 1.7-3.3*2400 0.05-0.09* 0.08-0.15* 0.14-0.25* 0.3-0.5* 0.5-0.9* 1.2-2.2*3600

0.04-0.06*

0.07-0.10*

0.11-0.16*

0.2-0.3*

0.4-0.6*

0.9-1.4*

*

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

Table 3 Resolution (FWHM in nm) for the AvaSpec-2048

Installed Slit in SMA Adapter

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The AvaSpec spectrometers can be equipped with several types of detector arrays. Presently we offer silicon-based CCD, back-thinned CCD, CMOS and Photo Diode Arrays for the 200-1100 nm range. A complete overview is given in the next sec-tion “Sensitivity” in table 4. For the NIR range (1000-2500nm) InGaAs arrays are implemented.

CCD Detectors (AvaSpec-2048/38)

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 an enormous dynamic range, only limited by the dark (thermal) current and the speed of the AD converter. The 38 pixel CCD has an integrated electronic shutter function, so an integration time of 10µsec can be achieved.

+ Advantages for the CCD detector are many pixels (2048 or 38), high sensitivity and high speed.- Main disadvantage is the lower S/N ratio.

UV enhancement

For applications below 350 nm with the AvaSpec-2048/38 a special DUV-detector coating is required. The uncoated CCD-response below 350 nm is very poor; the DUV lumo-gen coating enhances the detector response in the region 150-350nm. The DUV coating has a very fast decay time, typ. in ns range and is therefore useful for fast trigger LIBS applications.

Back-thinned CCD Detectors (AvaSpec-2048x14)

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, the new back-thinned CCD detector may be the right choice. The detector is an area detector of 2048x14 pixels, for which the vertical 14 pixels are binned (electronically added together) to have more sensiti-vity and a better S/N performance. + A dvantage of the back-thinned CCD detector is the good UV and NIR sensitivity, combined with good S/N and dynamic range

- Disadvantage is the relative high cost

Photo Diode Arrays (AvaSpec-128)

A silicon photodiode array consists of a linear array of mul-tiple 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 output an electrical signal. Most photodiode arrays have an

Detector Arrays

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.

CMOS linear image sensors (AvaSpec-256/1024)

These so called CMOS linear image sensors have a lower charge to voltage conversion efficiency than CCD array sensors and are therefore less light sensitive, but have a much better signal to noise ratio.

The CMOS detectors have a higher conversion gain than NMOS detectors and also have a clamp circuit added to the internal readout circuit to suppress noise to a low level.

+ Advantages for the CMOS detectors are good S/N ratio and good UV sensitivity.

- Disadvantages are the low readout speed, low sensitivity, and relative high cost (1024 pixels).

InGaAs linear image sensors (AvaSpec-NIR256)

The InGaAs linear image sensors deliver high sensitivity in the NIR wavelength range. The detector consists of a charge ampli-fier array with CMOS transistors, a shift register and timing generator. 3 versions of detectors are available:• 256 pixel non-cooled InGaAs detector for the 900-1750nm useable range • 256 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

Different Detector Arrays

The sensitivity of a detector pixel at a certain wavelength is defined as the detector electrical output per unit of radia-tion 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 cylindrical Detector Collection Lens (DCL) can be mounted directly on the detec-tor array. The quartz lens (DCL-UV for AvaSpec-2048/38) will increase the system sensitivity by a factor of 3-5, depen-ding on the fiber diameter used.

In Table 4 the overall sensitivity is given 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 assumed an optical bench with 600 lines/mm grating and no DCL. The entrance of the bench is an 8 µm core diameter fiber, con-nected to a standard AvaLight-HAL halogen light source. This is equivalent to ca. 1 µWatt light energy input.

In table 5 the specification is given for the NIR spectrometers, in figure 5 and figure 6 the spectral response curve for the dif-

ferent detector types are depicted.

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Table 4 Detector specifications (based on a 16-bit AD converter)

Detector TAOS 128 HAM256 HAM1024 SONY2048 TOSHIBA38 HAM2048x14

Type Photo diode array CMOS linear array CMOS linear array CCD linear array CCD linear array Back-thinned

CCD Array # Pixels, pitch 128, 63.5 µm 256, 25 µm 1024, 25 µm 2048, 14 µm 38, 8 µm 2048x14, 14 µm

pixel width x 55.5 x 63.5 25 x 500 25 x 500 14 x 56 8 x 200 14x14 (total

height (µm)height 196 µm)

pixel well depth 250,000 4,000,000 4,000,000 40,000 120,000 250,000

(electrons)

Sensitivity 100 22 22 240 160 200

V/lx.s

Sensitivity 100 440 440 40 60 50

Photons/count

@600nm

Sensitivity 4000 120 120 20,000 14,000 16,000

(AvaLight-HAL, (AvaSpec-128) (AvaSpec-256) (AvaSpec-1024) (AvaSpec-2048) (AvaSpec-38) (Avaspec 2048x14)

8 µm fiber)

in counts/µW per

ms integration time

Peak wavelength 750 nm 500 nm 500 nm 500 nm 550 nm 650 nm

Signal/Noise 500:1 2000 :1 2000 :1 200 :1 350 :1 500:1

Dark noise 60 28 60 35 35 50

(counts RMS)

Dynamic Range 1000 2500 2500 2000 2000 1300

PRNU**± 4% ± 3% ±3% ± 5% ± 5% ± 3%

Wavelength range 360-1100 200-1000 200-1000 200*-1100 200*-1100 200-1160

(nm)

Frequency 2 MHz 500 kHz 500 kHz 2 MHz 1 MHz 1.5 MHz

* DUV coated

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

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Figure 5 Detector Spectral sensitivity curves Table 5 NIR Detector Specifications

Detector

NIR256-1.7 NIR256-2.2

NIR256-2.5

Type

Linear InGaAs array Linear InGaAs array Linear InGaAs array

with 2 stage TE cooling with 2 stage TE cooling # Pixels, pitch 256, 50 µm 256, 50 µm 256, 50 µm pixel width x 50 x 500

50 x 500 50 x 500height (µm)

Pixel well depth 16,000,000 1,500,000 1,500,000(electrons)Sensitivity 350

250

200

(AvaLight-HAL, 8 µm fiber)

in counts/µW per ms integration time

Peak wavelength 1550 nm 2000 nm 2300 nm

Signal/Noise 4000:1 1200 :1 1200 :1

Dark noise 12 40 40 (counts RMS)Dynamic Range 5000 1600 1600PRNU** ± 5% ± 5% ± 5%Defective pixels 0

12 12(max)

Wavelength range 900-1750 1000-2200 1000-2500 (nm)Frequency

500 kHz

500 kHz

500 kHz

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

Figure 6 NIR Detector Sensitivity Curves

Spectrometers Stray light is radiation of the wrong wavelength 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

Encasing the spectrometer in a light tight housing eliminates

ambient stray light.

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 measure-

ments are being carried out with a laser light, shining into the

optical bench and measuring light intensity at pixels far away

from the laser projected beam. Other methods use a halogen

light source and long pass- or band pass filters.

Typical stray light performance is <0.05 % at 600 nm; <0.10

% at 435 nm; <0.10 % at 250 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 grating 2nd

order diffracted beam. The effects of these higher orders can

often be ignored, but sometimes need to be taken care of.

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 sor-

ting coating on a window in front of the detector. The order

sorting coatings on the window typically have one long pass

filter (590nm) or 2 long pass filters (350 nm and 590 nm),

depending on the type and range of the selected grating.

In Table 6 a wide range of optical filters for installation in the

optical bench can be found. The use of following long-pass

filters is recommended: OSF-475 for grating NB and NC, OSF-

515/550 for grating NB and OSF-600 for grating IB.

In addition to the order sorting coatings we implement partial

DUV coatings on Sony 2048 and Toshiba 38 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 follo-

wing grating types:

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

coated

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

coated

Stray Light and Second Order Effects

Table 6 Filters installed in the AvaSpec spectrometer series

OSF-385

Permanently installed 1 mm order sorting filter @ 371 nm

OSF-475 Permanently installed 1 mm order sorting filter @ 466 nm

OSF-515 Permanently installed 1 mm order sorting filter @ 506 nm

OSF-550 Permanently installed 1 mm order sorting filter @ 541 nm

OSF-600 Permanently installed 1 mm order sorting filter @ 591 nm

OSC Order sorting coating with 590nm long pass filter for VA, BB (>350 nm) and VB gratings

in AvaSpec-1024/2048/38/2048x14

OSC-UA Order sorting coating with 350 and 590nm longpass filter for UA gratings

in AvaSpec-1024/2048/38/2048x14

OSC-UB Order sorting coating with 350 and 590nm longpass filter for UB or BB (<350 nm) gratings

in AvaSpec-1024/2048/38/2048x14

Order Sorting Window in holder

info@avantes.com • www.avantes.comProduct name Electronics Optical bench Detector Housing AvaSpec-128 AS-161 with USB AvaBench-45, all

gratings 360-1100 nm TAOS 128

AvaSpec-128-USB2 AS-5216 with USB2

AvaSpec-256 AS-161 with USB AvaBench-45, all

gratings 200-1100 nm HAM 256

AvaSpec-256-USB2 AS-5216 with USB2

AvaSpec-1024 AS-161 with USB AvaBench-75, all

gratings 200-1100 nm HAM 1024

AvaSpec-1024-USB2 AS-5216 with USB2

AvaSpec-2048 AS-161 with USB AvaBench-75, all

gratings 200-1100 nm Sony 2048

AvaSpec-2048-USB2 AS-5216 with USB2

AvaSpec-38-USB2 AS-5216 with USB2 AvaBench-75, Toshiba 38

all gratings 200-1100 nm

AvaSpec-2048x14-USB2 AS-5216 with USB2 AvaBench-75, HAM 2048x14

all gratings 200-1160 nm

AvaSpec-NIR256-1.7 AS-5216 with USB2 AvaBench-50, HAM NIR256-1.7

grating 900-1750 nm

AvaSpec-NIR256-2.2 AS-5216 with USB2 AvaBench-50, HAM NIR256-2.2

grating 1000-2200 nm

AvaSpec-NIR256-2.5 AS-5216 with USB2 AvaBench-50, HAM NIR256-2.5

grating 1000-2500 nm

AvaSpec-xxx-2 AS-161 with USB, 2 channels AvaBench-45/75, all TAOS 128

xxx = 102/256/1024/ gratings 200-1100 nm HAM 256/1024

2048 or Sony 2048

AvaSpec Multichannel AS-161 with USB1 or AvaBench-45/75, All detectors

as Desktop AS-5216 with USB2 all gratings 200-1100 nm

or Rackmount

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