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VIS-NIR-DR spectrometer | SIMTRUM Photonics Store

Products / Light Analysis /Spectrometers /FT Infrared Spectrometer(900-16000nm) /VIS-NIR-DR spectrometer(360-2500nm) /
VIS-NIR-DR spectrometer(360-2500nm)
Overview Specifications Working Principle Software Q&A Related Products Order Now！

SIMTRUM'S VIS-NIR-DR spectrometer is a broadband instrument designed for diffuse reflectance measurements in the visible and near-infrared. VIS-NIR DR combines two spectrometers: a standard multichannel grating spectrometer that covers the visible spectral range (360nm-950nm) and NIR scanning Fourier-transform spectrometer with an extended range InGaAs detector (900 to 2500 nm). Both spectrometers are connected to a 5 cm, internally illuminated integrating sphere which ensures illumination of the sample by perfectly diffuse light.

The VIS-NIR DR allows you to measure the reflectivity of materials and paints over the complete solar spectral range - from 360 to 2500nm. This is important for the painting, building or solar energy industries, because it provides crucial information about the solar energy absorbed and transformed into heat by those materials or paints.

Designed for ease of use, our dedicated software merges the information delivered by the two instruments and outputs a single spectrum covering the 350-2500nm interval. This software also features a direct calculation of the total solar reflectance (TSR) value for reference measurements.

Features

  • Large wavelength range（360-2500nm）
  • Grating spectrometer & FT-NIR combined
  • Internally illuminated integrating sphere
  • High resolution < 5nm
  • Fast acquisition time (5 s)
  • Direct calculation of TSR value

Principle

The VIS-NIR DR spectrometer combines a multichannel grating spectrometer, covering 360nm-950nm, with a scanning Fourier-transform spectrometer with an extended range InGaAs detector, which is sensitive in the NIR range from 900 to 2500 nm. Both spectrometers are connected to the internally illuminated integrating sphere via a bifurcated optical fiber.

The use of an Fourier transform spectrometer for the NIR and a grating spectrometer for the VIS is the optimal solution for diffuse reflection measurements:

For the VIS range it is preferable to have high sentivity because of the decreasing intensity of the halogen lamp in particualr for wavelengths below 450nm. So a grating spectrometer is the most adequate choice for this spectral range.

For the NIR spectral range, the Fourier-transform scanning spectrometer is superior to gratings spectrometer. Indeed, multichannel grating spectrometers are based on diode arrays with defective pixels and other problems. These problems are particulary important in the long range NIR (up to 2.5 microns) where InGaAs arrays suffer from severe technical limitations.

 

Applications

Total solar reflectance (TSR)

Lowering our need for energy and reducing our carbon footprint are crucial objectives for our future. Houses must be as energetically autonomouns as possible, which has to be achieved with a well engineered energy balance, and good thermal insulation. Solar radiation participates to this energy balance. In some situations, such as in cold countries, one may want the solar radiation to be absorbed as much as possible to help heating the building. In warm countries, converesely, a highly reflective surface is preferable.

However, is the reflectivity measurement in the visible range of the solar spectrum sufficient to asses the overall reflectivity of a surface?

As illustrated in the graph below, more than half of the enegy of the solar spectrum lies in the near-infrared (NIR) between 700 and 2500nm. This radiation participates to the heating, and must therefore be taken into account.

In order to give a standard comparison between the reflectivity of different surfaces, that takes into account the whole solar spectrum, a standard parameter called Total Solar Reflectance (TSR) has been introduced. Essentially, the TSR is a reflectivity weighted by a standardized solar spectrum (see ASTM ® norms G173 and E903):

The following figure illustrates the VIS-NIR reflectivity spectrum of two different paints. Their reflectivity of both paints is the same in the visible range, but it is considerably different in the NIR range. Hence, although these two paints would look identical to the human eye, one of them will absorb more solar radiation than the other.

The VIS-NIR DR SPECTROMETER was expressely developed to measure the TSR. It does cover the whole spectral range from 360 to 2500nm, and it is a fast, reliable, easy to use and comparatively low-priced instrument.


General Specifications

Model LA-AR01-VIS-NIR-DR
  Spectral range [nm] 350-2500nm
  Resolution [nm] ＜5
  Measurement geometry Diffuse illumination, 8°viewing angle
  Integrating sphere diameter [mm] 50
  Integrating sphere port diameter [mm] 10
  Illumination source Halogen, 5W
  Single acquisition scanning time [s] ＜5
  SNR ＞1000:1
  Inter-instrument repeatability ±1% at 50% R
  Repeatability on white reference ＜0.2% variation at 50% R

 

Other Specifications

  Software interface Windows 7/10/11
  Operating power 20 W @ 12 VDC
  Communication interface USB 2.0
  Dimensions [mm] 380×250×320
  Weight [kg] 8
  Operating temperature [℃] 10-40

 


FT-IR spectroscopy: high resolution measurements

Introduction

Fourier Transform infrared (FT-IR) spectrometers have proven to be efficient and reliable tools for a large variety of applications targeting the near infrared (NIR) and mid-infrared (MIR) regions of the electromagnetic spectrum. One of the most critical specification of FT-IR spectrometers is their spectral resolution , as this defines the scale of the features to be distinguished e.g. in the absorbance spectrum of a gas. It is thus quite natural to wish for the highest achievable resolution when purchasing an FT-IR spectrometer. Due to their operational mode, achieving high resolution measurements using FT-IR spectrometers is however not entirely straightforward. This technical note explains the main limits of high resolution FT-IR measurements.

 

Fig .1 Simplified FT-IR setup

 

A simplified FT-IR is depicted in Fig.1. It consists of a beamsplitter, a fixed mirror and a movable mirror. Light from a purely monochromatic light source is split in two beams, assumedly in equal parts (50% beamsplitter). Each beam bounces back on either mirror (fixed or movable) before being recombined and focused onto the detector. When the movable mirror is at the same distance from the beamsplitter as the fixed mirror, both beams travel the same distance and recombine in-phase, yielding constructive interference. By displacing the moving mirror by a distance ℓ, one introduces an optical path difference (OPD) between the two beams Δ given by:Δ=2ℓ.

Upon recombination at the detector, the optical intensity as a function of OPD is given by: I(0)=0.5I0[1+cos(2πν0∆)]=IDC+IAC(∆).

Where I0 is the source intensity at wavenumber ν0 (the wavenumber is the reciprocal of the wavelength). The AC part of the interference record is labelled IAC(Δ) and is called the interferogram. For a purely monochromatic source (as considered here), the interferogram is a pure cosine function.

 

Fig 2. Interferogram of a purely monochromatic light source

 

Here the wavenumber (or equivalently the wavelength) of the source as well as its intensity can be retrieved from a direct observation of the interferogram (amplitude and period of the cosine function). For a broadband source, the interferogram IAC(Δ) and spectrum I0(ν) of the source are related via a Fourier transform operation:

I0(v)=∫∆maxIAC(∆)cos(2πν0∆)d∆

Obviously, the OPD cannot be made arbitrarily large and has to reach a value Δmax that is defined by technological design. Given the nature of the relationship between the interferogram and the spectrum (Fourier transform), it turns out that Δmax also defines the achievable spectral resolution. To a first approximation, the spectral resolution Δν of an FT-IR is given by:

∆ν=(∆max)-1

and is often expressed in cm-1. So why not simply increase the maximum OPD to enhance the spatial resolution of an instrument ? While this is true, special care has to be considered when operating at high resolution. As explained hereafter, the mirror maximum displacement is limited by the divergence of the system and the dimensions of the detector.

 

 

HR measurement: effect of beam divergence

We consider the exact same setup as in Fig.1. The beam divergence is accounted for by observing the behavior of the so called "extreme ray", which makes an angle α with respect to the "central ray" discussed previously.

 

 

Fig 3. FT-IR setup with a divergent source

 

 

The two extreme rays (reflected from either the fixed or the moving mirror) hit the lens with an angle α, unlike the central ray which hits the lens at normal incidence. They are thus focused on another point on the detector. Moreover, their OPD is shorter than for the central ray:

∆ext=2ℓcos(α)

The larger the angle α, the greater the difference with the central ray OPD Δcen=2ℓ. Consider now the case where the difference in OPD between the central ray and the extreme ray is equal to one half of the source wavelength, that is:

∆cen-∆ext=2ℓ[1-cos(α)]=λ/2

In this scenario, when the central rays are in phase, then the extreme rays are out of phase (and vice versa). Consequently, the intensity over the detector surface follows the profile shown in Fig. 4.

 

Fig 4. Intensity over the detector surface due to a highly diverging beam

 

 

Since the detector yields a single value that corresponds to the average intensity received on its surface, the signal detected in this case corresponds to the average optical intensity only, and all information regarding the interference signal vanishes. Practically speaking, the interferogram will start losing contrast as the moving mirror is scanned as shown in Fig. 5.

 

 

Fig 5. Loss in interferogram contrast due to a diverging beam

 

 

This effect is naturally existing in all interferometers based instruments (such as FT-IR) and cannot be avoided. It can however be properly managed by appropriately trading-off the parameters involved in equation (6), namely :

  • Δν: the effect is more pronounced for high resolution measurements due to the larger OPD required.
  • α: the effect is more pronounced for a highly diverging beam.
  • λ: the effect is more pronounced at short wavelengths (large wavenumbers).

 

For most applications in solids and liquids, the size of the observable features is typically broader than 2cm-1, and high resolution (HR) measurements performed at 0.5cm-1 are usually not required. In addition, these would prove challenging due to the added contrast loss described in this document. HR measurements might still be a viable option for specific applications, such as e.g. laser characterization, where the highly collimated laser beam prevents the dramatic loss of the interferogram contrast.


We fully appreciate and value the multiple benefits that a dedicated, performant and reliable software can bring to your application. Automatic data collection, parameters changes, status diagnosis and many other essential tasks should be implemented as simply and as efficiently as possible in order to get the most out of your spectrometer. This philosophy led to the development of a multi-threading, cross-platform and versatile software application, the digital acquisition system or AoDAQ.

The AoDAQ simultaneously takes care of:

1. Handling communication with the FT-IR via USB

2. Processing raw signals to deliver a spectrum

3. Running a TCP Ethernet server

The AoDAQ can be installed on all sorts of computers, from desktop machines to embedded, low-power single board computers. Thanks to the hosting of a TCP server, the instrument data and parameters can be accessed locally and/or remotely. All communication with the instrument eventually reduces to a set of TCP/IP commands that allow to quickly acquire data, adjust parameters, monitor the instrument status etc. using the programming environment of your choice. 

 


1.What resolution should I use for my application?

In FTIR, resolution is traded-off with two other experimental metrics that are acquisition time and signal-to-noise ratio (SNR). Increasing resolution, meaning reducing the resolution parameter number, will result in longer acquisition time, and poorer SNR. In general, it is recommended to work at the "worst" possible resolution, that is the limit resolution that allows to distinguish the features of the sample or substance that you are characterizing. Liquids and solids present broader features than gases or gas mixtures and can generally be probed with standard resolution instruments (down to 2cm-1). Gas analysis or light sources characterization (typically lasers) usually benefit from a sharper resolution of 0.5cm-1.

 

2.How is the equivalent wavelength resolution calculated?

Due to its working principle, FT-IR provides uniformly sampled spectra in the form of wavenumber (ν) within a given spectral range, with the unit of cm-1. Wavenumber is simply defined as the reciprocal of wavelength (λ). The resolution of a FT-IR is a constant wavenumber (Δν), but varies with the wavelength (Δ λ) due to the inverse relationship between these two units. The conversion between wavenumbers resolution and wavelengths resolution is Δλ=λ2 · Δν, as shown in the following figure:


 

3.What is the acquisition rate of this spectrometer?

The acquisition rate varies with the resolution. At the standard resolution of (4cm-1), the scanning rate is ~5Hz, and at the high resolution of (0.5cm-1), the scanning rate is ~1Hz, that is, the scanning rate is inversely proportional to the resolution.

 

 

4.What are the differences between DLADTGS detectors and MCT detectors?

DLADTGS is based on the pyroelectric effect. When exposed to infrared radiation, its temperature will change, causing the polarization inside the crystal to change. MCT is a bandgap type photoconductive detector. DLADTGS has a wider spectral response range (up to 18-20 μm), and MCT detectors utilize thermoelectric cooling to achieve higher sensitivity, which can suppress dark current and improve their signal-to-noise ratio.

 

 

5.How much better is the performance of LN4 cooled detectors than that of MCT cooled detectors?

The SNR of the LN4 cooling system is approximately ten times that of the MCT cooling system.

 

 

6.Why is the performance of the Fourier transform spectrometer superior to that of the dispersive grating spectrometer?

The performance of a grating spectrometer mainly depends on the slit and detector. Due to the different dark noise of each pixel point, additional noise will be introduced during measurement. FTIR spectrometers are generally superior to dispersive grating spectrometers due to their higher light throughput, better signal-to-noise ratio, higher spectral resolution and wider wavelength range. These advantages make FTIR spectrometers the preferred choice in many applications, especially in places where high sensitivity and accuracy are required.

 

 

7.Can an FTIR spectrometer be used to measure pulsed laser?

Sure, but the power of the laser must be limited to avoid irreversible damage to the detector. The average power shall not exceed 25mW, and the peak power of pulses shorter than 1µs shall not exceed 100W. It is recommended to use a set of fixed or variable attenuators to adjust the optical power to avoid detector saturation. Secondly, the repetition frequency of the laser must exceed 25khz to avoid numerical artifacts (aliasing) in the measured spectrum.

 

 

8.Can the concentration of the substance being measured be obtained directly?

No. A spectrometer can only provide the measured spectrum. Users must obtain the concentration of substances in the sample through specific algorithms and calibration data.

 

 

9.Can this spectrometer be used for mineral identification?

The FT-NIR spectrometer is highly suitable for mineral identification. FT-NIR is capable of generating high-quality and high-resolution (better than 1nm) reflection spectra within the spectral range of 900-2550nm (SWIR) within a few seconds.


Related products

Model Wavelength Range Resolution Detector  
  FT-NIR spectrometer 900-2500 nm 2/4/8 cm-1(user selectable) Extended type InGaAs PIN photodiode, 2TE cooled
  FT-MIR spectrometer 200-16000 nm 0.5/2/4/8cm-1(user selectable) MCT (4-TE cooled)
MCT (LN2 cooled)
DLATGS
  Fiber coupled FT-IR spectrometer 200-16000 nm 2/4/ 8cm-1 (user selectable) InGaAs (2-TE cooled)
MCT (4-TE cooled)
MCT (LN2 cooled)
  VIS-NIR-FIB spectrometer 350-2500 nm <1.5 nm silicon array detector (3648 pixels) 16-bit ADC.extended range InGaAs photodiode, 2TE cooled, 24-bit ADC.
  VIS-NIR-DR spectrometer 360-2500 nm 5 nm Extended range InGaAs detector

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LA-AR01-VIS-NIR-DR
Spectral range：350-2500nm,Resolution：＜5nm,SNR：＞1000:1,Single acquisition scanning time：＜5s 		4-6week $35200.00

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