Thunder Optics SMA Spectrometer & Spectragryph

June 11, 2021 English Posts, Light 9,859 Views

Abstract: in this post we describe the Thunder Optics SMA Spectrometer used with the Spectragryph software. We plan to configure and calibrate the instrument and use it for the acquisition and study of the spectra of some light sources. For the measurements we will also use some accessories such as optical fibers, collimators and calibration light sources.

Introduction

Light spectroscopy is a fascinating technique that allows us to study the spectrum of electromagnetic radiation emitted by light sources. The analysis of the radiation spectrum is the basis for understanding the physical phenomena of light-matter interaction that occur at the atomic and molecular level. The spectroscopy technique, in its many variations, is widely used in many sectors in the study of physical, chemical and biochemical processes.
The instrument that enable the analysis of light is called spectrometer and is a high precision optical instrument, often having a very high cost. In this blog we have extensively dealt with the construction and applications of a DIY spectrometer: DIY Diffraction grating Spectrometer. We now want to present a new commercial spectrometer characterized by low cost and respectable performance. The tool we are going to describe promises to fill the existing gap between self-built tools and research grade tools: in this sense it is an ideal tool for all applications, for example the educational ones, which do not require the highest performance levels.

The SMA Spectrometer

The instrument is the SMA-E model from Thunder Optics. It comes in a sturdy black painted aluminum container with small dimensions. On the front there is the SMA 905 connector for the optical fiber, on the back there are the main data of the instrument and there is the output point of the connection USB cable (USB 2.0) (Fig. 1).

Fig. 1 – Thunder Optics Spectrometer

The instrument is based on a 1800×1200 px Sony IMX CMOS sensor. The dispersion of the light spectrum is achieved with a transmission diffraction grating of 1000 lines/mm and the entrance slit, by default of 100 μm, is metallic of high quality. The wavelength range covered is very wide and goes from 350 nm to about 900 nm. The adoption of a very “bright” sensor with a high number of pixels allows the use of a narrow slit thanks to which an excellent resolution of about 1.5 nm is obtained. The optics of the instrument are obviously pre-aligned and the user only has to connect it to the PC and proceed with the configuration/calibration, as described in the following paragraphs.

Together with the spectrometer there is a wide range of accessory instrumentation which includes light sources for calibration, optical fibers, collimators and cuvette holders for absorption and fluorescence measurements (Fig. 2).

Fig. 2 – Accessories for the Spectrometer: light sources, optical fibers and cuvette holder with collimators

Link to Thunder Optics products:

Spectragryph software

For the recording and processing of the spectra we have adopted the Spectragryph ver. 1.2.15 software. Spectragryph is a complete and modern software. It is extremely flexible and can handle many different devices, including webcam-type CMOS sensor-based spectrometers like the one from Thunder Optics. This makes it a valuable tool for all spectrophotometry applications: from educational to research applications. For non-commercial use, for example by enthusiasts and students, there is also a free license.

Once the software is started, the application GUI is displayed, shown in the image (Fig. 3), where the main sections and functions available are indicated.

Fig. 3 – Spectragryph GUI

After connecting the spectrometer to the USB port of the PC, you can proceed with the recognition of the device: select the “Acquire” tab (Fig. 4) and then the “Device Type” dropdown list.

Fig. 4 – Acquire section

Currently the list includes various devices, the item we are interested in is the “USB webcam” (Fig. 5-a). After making this selection in the dropdown list of devices, the “ThunderOptics” device must then be selected and then click the “Connect” button.

Fig. 5 – (a) Dropdown list for device selection (b) Window for device configuration

As soon as the connection with the spectrometer is activated, a new window will be shown in which the image acquired by the instrument’s CMOS sensor is displayed (Fig. 6). At this point, a light source must be connected to the instrument with the optical fiber, or the light source, for example a fluorescent lamp, must be placed in front of the SMA 905 input connector. The spectrum of the light source should appear in the window. The ROI (Region of Interest) should be adjusted to fit the displayed spectrum, as shown in Fig. 6-a. During the acquisition the ROI can be controlled from the Transform tab, by clicking on the “Spectrum from Image” button, as shown in Fig. 6-b.

Fig. 6 – (a) Image acquired by the CMOS sensor

Fig. 6 – (b) ROI checking

At this point the instrument is ready for the spectrum recording. There are many configuration parameters that can be adjusted in order to optimize the acquisition process (Fig. 5-b), among these we mention the exposure time which can be modified from a maximum of 0.5 s to lower values ​​according to powers of 2: 0.5 – 0.25 – 0.125 – etc. The recording itself can take place in different ways, for example: single shot, continuous, additive. The spectrum obtained can also be averaged over several samples in order to improve the signal/noise ratio. The device configurations (together with the calibration coefficients) can be saved on files in sgas format. For all the details on the use of the Spectragryph application and the configuration of the Thunder Optics spectrometer, please refer to the instrument documentation and to the manual software.

Link to Spectragryph:

Instrument calibration

The configuration/calibration file for the specific instrument is supplied together with the spectrometer. The calibration translates the spectrum expressed in “pixels” into a spectrum expressed in wavelength (nm). The instrument therefore arrives already calibrated and ready for use and does not require further operations. In case of need (it is however advisable to contact Thunder Optics support) you can also repeat the calibration operation. This is done by recording the spectrum of a light source and comparing it with its known emission spectrum. For example, a fluorescent lamp (Fig. 7-a) or the Mercury-Argon lamp supplied by Thunder Optics (Fig. 7-b) can be used.

Fig. 7 – (a) Calibration with optical fiber, collimator and fluorescent lamp (b) Calibration with optical fiber, collimator and mercury-argon lamp

Once the reference spectrum has been recorded the calibration function can be activated (caliper button in the Acquire tab), a window will be displayed (Fig. 8) showing the peaks of the spectrum automatically identified by the application. For each of these the correct wavelength will be entered, with these data the software calculates the coefficients of the interpolation polynomial. At the end of the calibration phase, the spectrum can be displayed with the X axis expressed in nm.

Fig. 8 – Calibration with fluorescent lamp spectrum

Acquisition of Light Sources Spectra

After configuring and calibrating the instrument we can proceed with measurement of the spectra of our interest. The spectrometer is equipped with an SMA 905 connector for the optical fiber therefore the normal use of the instrument involves the fiber, with a collimator (Fig. 7-a) or directly connected to the light source, with fiber and connector SMA 905 (Fig. 7-b). We recommend using an optical fiber with a fairly large core, for example 400 μm or 600 μm, in order to collect an adequate amount of light. If an optical fiber is not available, the spectrometer can be used by exposing the SMA input connector directly to the light source, as shown in the following image (Fig. 9-a).

Fig. 9 – (a) Recording spectrum with direct coupling (b) Recording DPSS Laser spectrum with fiber optic and cuvette holder

For the spectra recording of laser sources the optical fiber with the collimator and the holder can be conveniently used, as shown in Fig. 9-b and in the image below (Fig. 10).

Fig. 10 – (a) Recording laser He-Ne spectrum (b) Recording DPSS green laser spectrum

To record the spectrum of the Sun we used the fiber with the collimator pointed towards a sheet of white paper illuminated by sunlight, as shown in the image (Fig. 11).

Fig. 11 – Recording Sun spectrum with fiber and collimator

Fluorescent Lamp for Calibration

The fluorescent lamp (it contains Mercury vapors) is a “classic” in the study of light spectra because it provides an easily available example of a source with a discrete line emission. The emission of this lamp consists of the set of Mercury emission lines to which the fluorescent emissions of the “phosphors” deposited on the glass of the lamp (consisting of salts of Europium) are added. When switched on, the emission in the infrared region is also significant, which however is considerably reduced in steady state operation. The spectrum of a fluorescent lamp, shown in the following image (Fig. 12), can also be conveniently used for the calibration of the spectrometer.

Fig. 12 – Spectrum of a fluorescent lamp

The main emission lines that can be identified in the spectrum of a fluorescent lamp and that can be used for the calibration of the instrument are the following (Table I):

365 nm -> Mercury 588 nm -> Europium phosphors
404,6 nm -> Mercury 611 nm -> Europium phosphors
435,8 nm -> Mercury 631,1 nm -> Europium phosphors
546 nm -> Mercury 709 nm -> Europium phosphors
580 nm -> Mercury

Table I

Mercury-Argon Lamp for Calibration

The lamp that uses a mixture of low pressure vapors of Mercury and Argon is ideal for calibrating the instrument, this because it has a set of narrow emission lines that extend from near ultraviolet to infrared. The part up to 580 nm is “covered” by the emissions of Mercury, while the Argon has numerous emission lines in the near infrared. In the following image (Fig. 13-a) we report the spectrum of the emission of the Hg-Ar light source provided by Thunder Optics.

Fig. 13 – (a) Spectrum of a reference Mercury-Argon light source

Fig. 13 – (b) Spectrum detail

In the figure above (Fig. 13-b) we report an enlarged part of the spectrum to show the separation of the two mercury emission lines at approximately 577 and 579 nm, separated by only 2 nm that the instrument allows to resolve easily.

The main emission lines that can be identified in the spectrum of a Mercury-Argon source and that can be used for the calibration of the instrument are the following (Table II):

365 nm -> Mercury 696,5 nm -> Argon 763,5 nm -> Argon 826,4 nm -> Argon
404,6 nm -> Mercury 706,7 nm -> Argon 772,4 nm -> Argon 842,4 nm -> Argon
435,8 nm -> Mercury 727,3 nm -> Argon 794,8 nm -> Argon 852,1 nm -> Argon
546 nm -> Mercury 738,4 nm -> Argon 800,6 nm -> Argon 912,3 nm -> Argon
580 nm -> Mercury 750,3 nm -> Argon 811,5 nm -> Argon

Table II

Mini Light Source

The Thunder Optics “suite” of products also includes an incandescent lamp-based light source. This type of source emits continuously over the entire visible spectrum, with increasing intensity towards the infrared wavelengths. This source, whose spectrum is shown in the image below (Fig. 14), can be conveniently used as a reference source for optical absorption measurements (transmittance and absorbance).

Fig. 14 – Spectrum of “mini light source”

Sodium Vapor Lamp

The sodium vapor lamp, to be clear, the one used in public lighting before the advent of LED lighting, is an interesting light source because when turned on it behaves like a low pressure source, while in steady state, when the gas is heated, it behaves like a high pressure gas source. In the first phase the lamp has narrow emission lines, in particular the Sodium line at 589 nm, while in steady state the emission lines are very broadened due to the effect of the collisions between the atoms and take on a classic Lorenzian shape. In particular, the line at 589 nm becomes progressively wider and wider, especially towards the outer wings and in the center, while at 589 nm the self-absorption line caused by the external and colder layers of sodium vapor begins to become evident. The spectra acquired with our spectrometer are shown in Fig. 15.

Fig. 15 – (a) Spectrum of sodium vapor lamp just ignited (b) Spectrum of sodium vapor lamp in steady state


Fig. 15 – (c) Spectrum detail at sodium “detail”

In the figure above (Fig. 15-c) we report an enlarged part of the spectrum to show the separation of the two emission lines of the sodium “doublet” at 589 and 589.6 nm, separated only by 1 nm that the instrument allows to resolve quite clearly.

One of the Spectragryph options is the spectrum acquisition mode: for example, it can be acquired one-shot or continuous, replacing the last spectrum in the diagram. For light sources associated with phenomena that change over time, different acquisition modes can also be very useful, for example the additive one that allows you to view on the diagram all the spectra that are gradually acquired. By applying this acquisition mode to the sodium lamp during the transient, it is possible to appreciate how the shape and intensity of the radiation emitted by the lamp varies over time: the result is shown in the following image (Fig. 16).

Fig. 16 – Variation of the spectrum of a sodium vapor lamp from ignition to steady state

Spectral Tubes

Spectral discharge lamps are based on the light emission by an ionized gas at very low pressure. The ionization of the gas is obtained by means of a high voltage discharge which makes free electrons and positive ions migrate to the different ends of the lamp (where the electrodes are present). During the discharge, the gas atoms are excited by the current flow and emit the excitation energy in the form of light radiation having a characteristic spectrum, usually with discrete lines or bands.
In our laboratory we tested the spectrometer with three spectral lamps containing Hydrogen, Carbon Dioxide, and Nitrogen: the spectra are shown in the following diagrams (Fig. 17, 18, 19). In the spectrum of hydrogen its main emission lines can be seen: Hα (656 nm), Hβ (486 nm) and Hδ (434 nm). In the spectra of carbon dioxide and, especially nitrogen, the emission lines overlap with continuous bands of radiation that correspond to the energy bands of the molecules. In all the spectra there are also two emission lines at 777 nm and 845 nm produced by the oxygen that has contaminated the gases present in the tubes.
In Fig. 23, we also report the emission spectrum of a Neon lamp, in which emissions in the red and near infrared range are predominant.
To check and verify the emission lines of the various elements, you can refer to the NIST site which allows you to search both by element and by wavelength.

Fig. 17 – Hydrogen H2 emission spectrum

Fig. 18 – Carbon dioxide CO2 emission spectrum

Fig. 19 – Nitrogen N2 emission spectrum

Fig. 23 – Gas Neon Emission spectrum

He-Ne Laser

For the verification of the spectrometer and its calibration, in addition to the various reference light sources, lasers can also be used, and in particular the Helium-Neon laser (He-Ne), characterized by a stable and well-defined emission at wavelength of 632.8 nm. The following diagram (Fig. 20) shows the spectrum acquired with the spectrometer connected to the optical fiber and to the collimator.

Fig. 20 – He-Ne Laser emission spectrum

DPSS Laser

Other interesting sources to examine with the spectrometer are the DPSS lasers (diode-pumped solid-state laser). These are the classic diode lasers readily available on the online market. These lasers have inside them a laser diode that emits in the near infrared, this radiation passes through one or two crystals which, thanks to a physical phenomenon known as the second harmonic generation, give rise to a radiation with wavelength halved which therefore falls within the range of green, blue or near ultraviolet. With the instrument we acquired the spectra of a UV laser and a blue laser, shown in the following image (Fig. 21). From the spectra it can be seen that, together with the main emission, the pumping infrared emission that escapes through the laser optics is always present.

Fig. 21 – (a) DPSS Laser UV (b) DPSS Laser blue

Sun Spectrum

As a last example of a spectrum we report the spectrum of the sun (Fig. 22). The spectrum of the sun was acquired by pointing the optical fiber equipped with the collimator towards a sheet of white paper illuminated by the sun (Fig. 11). The sunlight reflected off the white surface is sufficient to produce a detailed spectrum in which many interesting features can be found. From the spectrum we can see how the emission starts from wavelengths that fall in the UV range to continue with a long tail towards the infrared. The emission is strongly distorted by atmospheric absorption but it is clear that it can be approximated by the emission spectrum of a black body at a temperature of about 5500 °K (temperature of the solar photosphere). Superimposed on the continuous emission there are numerous absorption lines and bands (Fraunhofer lines) which correspond to the absorption of the gases present both in the solar atmosphere and in the earth’s atmosphere (Table III).

Fig. 22 – Sun spectrum with Fraunhofer absorption lines

Wavelength [nm] Designation – Element
393,4 nm Line K – Calcium
430,7 nm Line G – Calcium
438,4 nm Line e – Iron
486,1 nm Line F – Hydrogen Hβ
517,2 nm Line b – _Magnesium
527,0 nm Line E2 – Iron
588,9 nm Line D – Sodium
656,3 nm Line  C – Hydrogen Hα
686,7 nm Line B – Molecular oxygen
759,4 nm Line A – Molecular oxygen

Table III

Conclusions

The Thunder Optics SMA Spectrometer we used proved to be an excellent tool and its use with the Spectragryph software enhances its potential. The wavelength range is very wide and allows to acquire from the near UV up to 900 nm, the resolution and accuracy of the spectra shows an excellent value of about 1-2 nm. The SMA 905 professional connector allows the use of a whole series of accessories such as optical fibers, collimators, cuvette-holder and reference light sources, enabling the instrument for advanced applications such as absorption, fluorescence and reflection spectroscopy.

Note

The author of the Post is in no way involved in the Thunder Optics business.

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