© Springer Nature Switzerland AG 2021
B. G. Sidharth et al. (eds.)Fundamental Physics and Physics Education Researchhttps://doi.org/10.1007/978-3-030-52923-9_22

22. SPETTROGRAFO: A Digital Spectrometer for Educational Lab Activities

Daniele Buongiorno1  , Mario Gervasio2   and Marisa Michelini1  
(1)
Physics Education Research Unit, DMIF, University of Udine, Udine, Italy
(2)
DMIF-URDF, Università degli Studi di Udine, Udine, Italy
 
 
Daniele Buongiorno (Corresponding author)
Email: buongiorno.daniele@spes.uniud.it
 
Mario Gervasio
Email: mario.gervasio@uniud.it
 
Marisa Michelini
Email: marisa.michelini@uniud.it

Abstract

Optical spectroscopy is a significant methodological example of how physics makes use of indirect measure in order to obtain information on physical systems and validate models. Physics Education Research Unit from Udine University (IT) designed an educational path on optical spectroscopy, in which students are directly involved in experimental activities focused on interpretative plan allowing to highlight the link between energy levels in atoms and discrete light emissions.

After analyzing several commercial devices and mobile APPs allowing spectroscopic measurements, a digital spectrometer using a simple webcam implementing the various functionalities offered by the existing proposal to be connected via USB to PCs has been designed and realized. The hardware allows to use different diffraction grating, optical filters, and an optical goniometer. The software is designed to allow calibration and qualitative and quantitative measures of wavelengths and energies. Here we describe in detail the system and some experimental activities to be carried out with secondary school students and in introductory physics courses.

Keywords
Digital spectrometerOptical spectroscopyPhysics education

22.1 Introduction

Teaching modern physics in secondary school requires innovative and effective approaches since its teaching has been integrated in European school curricula since few years. This approach needs to found a scientific culture linking new theories with instruments and methods typical of physics. Optical spectroscopy representing a conceptual, methodological, and historical bridge between classical and modern physics provides a fertile experimental basis of the modern atomic theory. Its disciplinary contribution to the teaching of modern physics concerns the phenomena of quantized emission and absorption of radiation which are basilar concepts representing the main investigative tool based on light–matter interaction. Optical spectroscopy, from an epistemological point of view, is a methodological and experimental context in which the role of the energy is pivotal, a validation instrument of interpretative models through indirect measures, a way through which interpret a code, hidden in the emitted light, in order to obtain information concerning states and changes of a microscopic system, as the atom, allowing to highlight the link between light emissions and atomic energy levels. From an educational point of view, competence concerning specific inquiry modalities employed in physics can be gained during physics lectures. Existing educational proposals (Luo and Gerritsen 1993; Oupseph 2007; Scheeline 2010; Amrani 2014; Onorato et al. 2015) include simple experiments allowing qualitative and quantitative measures, but those proposals have been designed and implemented in limited contexts, since students obtain the bare measurement without focusing on the emission process or the functioning of the experimental setup. Obtaining optical spectra from luminous sources is quite easy: a CD or a cheap diffraction grating are easily available objects and the produced spectra can be collected with a digital camera or a smartphone and analyzed (quantitatively or qualitatively) with specific APPs. On the other side, commercial experimental devices are often implemented in expensive and excessively structured setups that limit students’ understanding of their principles of functioning, limiting one again to the bare measure itself. The general problem is thus that laboratorial educational proposals on optical spectroscopy in secondary school are offered in the form of sterile commercial devices, leaving teacher having the task of integrating them in a coherent educational path embedding the physics of the emission process and the importance of controlling the measuring process.

The aim of Physics Education Research Unit from Udine University (IT) is to build an educational path on optical spectroscopy allowing students to be directly involved in experimental and interpretative tasks. The pivotal laboratorial activity to effectively teach and learn physics could be supported by the opportunities offered by ICT (Information and Communication Technologies). An example has been already discussed in Gervasio and Michelini (2009) and Michelini and Stefanel (2015) in the case of an educational path on single-slit optical diffraction implementing inquiries activities based on measurements performed by students themselves searching for models explaining the observed phenomena. With this in mind, our research group analyzed the most popular commercial devices and APPs performing spectroscopic measurements, in order to design a prototype for a digital spectrometer implementing some proposals based on ICT making use of a specifically designed software allowing qualitative and quantitative analysis of a digitalized spectrum. The developing of the complete low-cost technical solution will be illustrated in the following with examples of significant measures.

22.2 Some Existing Proposals

The PASCO PS-2600 wireless spectrometer1 contains a dispersive element (a diffraction grating) and a CCD sensor (sensibility between 380 and 980 nm). Light is guided from the source to be analyzed to the dispersive element with the aid of an optical fiber. The dispersive element creates the spectrum which is projected to the CCD sensor. Thanks to a USB or Bluetooth connection, the software automatically shows the emission spectrum as a function of the wavelength (Fig. 22.1, left). Obtained spectra, with a resolution of about 3 nm, can be compared with discrete reference spectra. The device allows also the analysis of absorption and fluorescence spectra: a test cuvette with the liquid sample can be inserted inside the device, in order to obtain absorption spectra: spectra are obtained by illuminating the sample with a reference white LED light and the fluorescence spectra are obtained by illuminating the sample with light of 405 or 500 nm. The absorbance curve is automatically shown on the user interface of the software. User has only to perform automatic actions without having access to the physics characterizing the measure or data analysis, in particular no calibration phase is required.
../images/467908_1_En_22_Chapter/467908_1_En_22_Fig1_HTML.png
Fig. 22.1

PASCO PS-2600 (left) and RSPEC EXPLORER SYSTEM (right) user interfaces

Another spectrometer, whose functioning has been studied, implementing a focusing element (a lens), rather than an optical fiber, is the RSPEC EXPLORER SYSTEM digital spectrometer.2 A diffraction grating is placed in front of the webcam lens connected via USB to the PC. When the webcam is pointed towards the light source, its spectrum, in the range between 380 and 980 nm, is registered on the CCD sensor. Users limit to select the portion of the frame where the spectrum is visible. The software digitalizes it, providing a graph in which intensity (in arbitrary units) is shown as a function of the wavelength. The quality of the measures, which resolution is about 3 nm, allows to compare the recorded spectra with reference ones. The main limit of this device is represented by the fact that no calibration phase is required and the grating could not be removed, making impossible to change the dispersive power of the instrument (i.e., the pitch of the grating) and to appreciate the effect of placing a grating in front of a light source. The user limits himself to line up the zeroth order of the diffraction pattern with a reference to automatically and uniquely associate a position along the pixel array with a specific wavelength (Fig. 22.1, right).

Both the quickly described devices are quite similar to “closed black boxes,” in the sense that, despite their high quality performance, they do not allow user to approach the physics of the measure or to change the experimental conditions. Absorption measures are automatically generated by the software as well as the calibration, that is an important phase of the measuring process.

Other possibilities are available to record digitalized spectra, in a cheaper way with a Smartphone using mobile APPs, as LEARNLIGHT SPECTROSCOPY, SPECTRA UPB, LIGHT SPECTRA LITE, ASPECTRA MINI, and SPECTRUM ANALYZER. To perform those measures, mainly qualitative, which represent the main limitation of this possibilities, it is necessary to have a (self-build) spectroscope implementing a diffraction grating placed in front of the camera lens. In this way the spectrum is created and projected onto the camera sensor and it can be analyzed using the APPs. A precise calibration is possible only in few cases. Those APPs allow to obtain graphs of intensity as a function of the position along the spectrum, i.e., in different position of the digital image (Fig. 22.2).
../images/467908_1_En_22_Chapter/467908_1_En_22_Fig2_HTML.png
Fig. 22.2

User interfaces and logos of three APPs performing spectroscopic measurements. LIGHT SPECTRA LITE (top left), SPECTRA UPB (bottom left), and SPECTRUM ANALYZER (right)

22.3 The SPETTROGRAFO System

Our innovative device SPETTROGRAFO is a system implementing the simplicity of use of the existing commercial proposals and the feasibility of the analysis algorithms with new functionalities, missing in the analyzed devices, as the one to use grating with different pitches or to use colored filters to study selective absorption of light, or to have the possibility to dim the intensity of excessively luminous sources. Those little improvements allow the user to gain a better control of the measuring process, maintaining a good accuracy in the measured quantities using a low-cost instrument. The preliminary design of the prototype has been described in Buongiorno et al. (2018); here we report the final version of the system and relative specific technical characteristics with examples of employment in physics education.

22.3.1 The Hardware Components

In the SPETTROGRAFO system, a webcam is placed inside an aluminum case mounted on an adjustable tripod (Fig. 22.4). Technical data are shown in Table 22.1.
Table 22.1

Technical data of SPETTROGRAFO system

Hardware part

Data

Webcam

Focus: 15 cm—infinity

60 frame per second

Field of view is 60°

CCD

640 × 480 pix 1.2 Mpix

Case

6 cm × 6 cm × 6 cm

The dispersive element, a transmission diffraction grating could be placed in front of the lens of the webcam pointing towards the source whose light has to be analyzed. Light sources need to have small dimensions in order to produce spectra with enough separation between adjacent colors (we remind that a spectrum is the reproduction of the shape of the source at different angular positions with different colors). The device can be used also with extended light sources, making use of an external couple of black panels, working as a diaphragm with adjustable width.

Gratings with pitches of 1000 and 500 lines/mm can be used, both allowing spectral analysis in a range between 380 and 700 nm. The 1000 lines/mm grating allows to see only the first diffraction order with a resolution of 1.3 nm/pix, while with the 500 lines/mm the second order is visible, but the resolution decreases to 2.6 nm/pix. A couple of Polaroid filters can be inserted in front of the webcam to dim the light intensity of the source, avoiding the saturation of the sensor, in case it was too high. Colored filters can be also inserted in front of the webcam to study selective chromatic absorption (Fig. 22.3). The light passes through the grating and the diffraction pattern is registered on the CCD. A USB connection allows to send the digital image to the PC. A specifically designed ad-hoc software computes and analyzes the digital information.
../images/467908_1_En_22_Chapter/467908_1_En_22_Fig3_HTML.png
Fig. 22.3

The SPETTROGRAFO system (left). The horizontal support allows to place the accessories (diffraction gratings, colored and Polaroid filters) in front of the camera lens. The system could be placed on a rotating basis (right), allowing measures in “optical goniometer mode” (see below)

22.3.2 The Software Characteristics and Peculiarities

The software was developed in a Microsoft “framework.NET” environment in C# language. As shown in Fig. 22.4, it allows to visualize the image as registered by the webcam, containing both the source and its spectrum (left area of the user interface). User can select the rectangular area to be analyzed (reproduced in the upper right area of the user interface) containing the zeroth order and the spectrum of the source extending from left to right, increasing the angle. Digitalization of the image occurs when the software operates a sum of the digital information of each pixel (proportional to the incident intensity) along every column of the selected area. A graph appears in the lower right area of the user interface where the intensity in arbitrary units and proportional to the mean intensity incident on the pixels of a same column is shown as a function of the position along the spectrum identified by the column number (from 1 to 640).
../images/467908_1_En_22_Chapter/467908_1_En_22_Fig4_HTML.png
Fig. 22.4

Main user interface. Recorded image by the webcam (left) from which it is possible to select the area to be analyzed that is automatically digitalized in a graph (bottom right). In this example, the source is a hydrogen lamp observed through a 1000 lines/mm grating

Of course, at this stage, the obtained graph does not contain any physical information (i.e., wavelength) on the spectrum: it is necessary to calibrate the graph in order to obtain calibrated spectra. The software allows to calibrate the measure: it is enough to select the type of used diffraction grating (the dimension of the rating pitch fixes the pixel–wavelength relationship) or, alternatively a calibration source can also be used: fixing the position of a known wavelength allows to calibrate the measure making the hypothesis of a linear relation between position along the sensor and relative wavelength. After those operations, a calibrated graph appears in which the horizontal axis is in wavelength (nm) or in energy (eV), since the code is equipped with the energy-wavelength inverse proportionality relation: a reference spectrum appears under the graph showing an energy scale. Two movable markers allow to sign the position of the image (zeroth order) and a generic position along the spectrum of the first order, resulting in a univocal measure of wavelength (expressed in nm) or energy (expressed in eV) (Fig. 22.5, right). In this way, student appreciates that every linear position along the CCD sensor corresponds to an angular position α that can be univocally coupled to a wavelength λ with the grating formula (where d is the pitch and m the order of the spectra):
../images/467908_1_En_22_Chapter/467908_1_En_22_Fig5_HTML.png
Fig. 22.5

SPETTROGRAFO system connected to PC, pointing the source (left) and calibrated yellow LED spectra (right). The yellow marker on the left targets the zeroth order, while the red marker, moving along the spectrum, allows to measure the energy or wavelength of the corresponding position

$$ d\bullet \sin \alpha =m\bullet \lambda $$

Data can be exported in tabular form allowing further analysis with a spreadsheet. The advantage of describing a spectrum in terms of energy or wavelength allows different educational proposals: light could be seen as a wave or as a stream of photons with specific energies.

22.4 Examples of Significant Measures

In principle, every light source could be coupled with SPETTROGRAFO system: discrete spectra from gas-discharge or fluorescent lamps, continuous spectra from incandescent lamps, or band spectra from LEDs and absorption spectra could be detected and analyzed. To perform a measure, it is enough to point the webcam lens towards the source (Fig. 22.5, left). One advantage is that no optical bench is needed: operatively it is enough to assure the alignment between source, grating, and sensor in a way that the spectrum is horizontal with respect to the array of pixels. Preliminary tests showed that the distance between the grating and the source does not affect the precision of the measures.

22.4.1 Analysis of Discrete Emissions from Gas-Discharge Lamps in “Static-Mode”

Using a thin gas-discharge tube, or a thin slit placed in front of an extended lamp in order to make the shape of the source as thin as possible, the spectrum is created reproducing the shape or the source in different positions on the CCD sensor in different colors. Once digitalized, it is possible to observe and measure the position (in wavelength or energy), width, and relative intensity of the various discrete emissions (Fig. 22.6). Lines peculiar features such as intensity and width could be discussed as consequences of quantum mechanical principles. Moreover, changing the grating’s pitch, it is made clear how the resolving power of the measuring instruments changes: in particular doubling the pitch, the resolving power decreases of a factor two, and some lines can merge together and be no more angularly separated. As an example, some visible emissions in cadmium and helium spectra have been measured and they are reported in Table 22.2, compared with standard values obtained from official databases.
../images/467908_1_En_22_Chapter/467908_1_En_22_Fig6_HTML.png
Fig. 22.6

Spectra of a cadmium (left) and helium (right) gas-discharge lamp taken with the SPETTROGRAFO system

Table 22.2

Measure of the main emission lines of Cd and He performed with SPETTROGRAFO system compared with values taken from official database (https://​www.​nist.​gov/​pml/​atomic-spectra-database)

Cadmium

Helium

λmeas (nm)

λstd (nm)

Δ%

λmeas (nm)

λstd (nm)

Δ%

639

643.85

0.75

656

667.82

1.77

506

508.58

0.51

577

587.56

1.80

480

480.00

0.00

496

501.57

1.11

469

467.81

−0.25

471

471.31

0.07

–

–

–

447

447.15

0.03

As an example, in Fig. 22.7 the spectra of a common incandescent bulb is shown. The black-body shape of the spectra is quite recognizable, allowing the measure of the wavelength peak and relating it with the temperature of the incandescent body via the Stefan–Boltzmann law.
../images/467908_1_En_22_Chapter/467908_1_En_22_Fig7_HTML.png
Fig. 22.7

Spectra of an incandescent lamp taken with the SPETTROGRAFO system

22.4.2 Analysis of Discrete Emissions from Gas-Discharge Lamps in “Optical Goniometer Mode”

The LUCEGRAFO system could be placed on a rotating base (Fig. 22.3, right) making it more similar to an optical goniometer in which the dispersive element is fixed at the center of rotation of the basis (not more fixed in front of the webcam lens) and the sensor revolves circularly around it. Different portions of the spectra are thus observed if the sensor forms an angle α (the angular scale has a sensibility of 1°) with respect to the direction of symmetry (perpendicular to the grating). At various orders m, the wavelength corresponding at a specific angle could be evaluated with the grating’s formula, quoted above, reading the angle on the angular scale (a fixed vertical marker appears on the digital image as a reference to target the position). No calibration phase is required in this modality, except the operation of making the 0° angle with the position of the source (zeroth order). The advantage of having this second measurement modality available is that students can appreciate the angular symmetrical features of diffraction phenomena. In Table 22.3, measurements of the luminosity peak in the spectrum of a blue LED are shown, taken with a grating with a 500 lines/mm.
Table 22.3

Measure of the wavelength associated to the peak emission of a blue LED

m = 1

m = 2

m = −1

m = −2

Α

λ (nm)

Α

λ (nm)

α

λ (nm)

Α

λ (nm)

13°

449.9

27°

454.0

13°

449.9

27°

454.0

22.4.3 Selective Absorption of Colors and Evaluation of Transmissivity Curve

With SPETTROGRAFO system it is possible to appreciate in real time, how do colored filters modify the spectrum of a reference source containing all visible colors (for example a white LED, Fig. 22.8, left) evidencing the phenomenon of selective absorption (Fig. 22.8, right). In order to do this, it is enough to record the reference spectra and then place colored filters in front of the reference source; the resulting spectrum would be deprived of some colors, since some of them are absorbed by the filter, which is transparent to others. The percentage of absorption (or transmission) at different colors could be visualized and evaluated via the software itself, which extracts data in a tabular form (intensity vs wavelengths), that can be further analyzed with the aid spreadsheet in order to quantitatively evaluate the absorbance as a function of the color. In particular, named I0(λ) the intensity of the reference spectrum as a function of the wavelength and I(λ) the intensity of the absorption spectrum, the quantity representing the transmittance of the filter T(λ) = I(λ)/I0(λ) can be evaluated and displayed graphically (Fig. 22.9).
../images/467908_1_En_22_Chapter/467908_1_En_22_Fig8_HTML.png
Fig. 22.8

Reference spectra (left) and absorption spectrum having placed a blue filter in front of the reference source (right)

../images/467908_1_En_22_Chapter/467908_1_En_22_Fig9_HTML.png
Fig. 22.9

Elaboration with a spreadsheet: light spectrum from a white LED compared with the spectrum of the same light passing through a blue filter. Abscissa values refer to the column of pixel; on the left the peak due to the source is visible (left) and transmittance of the blue filter (right)

22.5 Conclusions

The main required features for an effective educational lab system for spectroscopic measurements, i.e., the importance of the calibration process, the insight into the physical processes, the possibility to change the accuracy, and the data acquisition modality emerged after having analyzed potentialities and limits of some available digital spectrometers and mobile APPs. A cheap but affordable digital device, SPETTROGRAFO, has been thus designed and realized in order to implement in a simple setup all the emerged needs. The original device allows analysis of optical spectra of different light sources and it consists of a USB webcam with the possibility to place in front of it different diffraction gratings, and colored filters to study selective absorption of colors. Virtual images of spectra are recorded on the CCD sensor of the webcam and observed with the aid of a specifically designed software. Calibration occurs via software itself by selecting the used diffraction grating or with the aid of a calibration source, and the tests showed that precision of the measures have uncertainties less than 5%. Recorded spectra are digitalized in a graph representing luminous intensity as a function of the wavelength, or energy. The device can be also mounted on a rotating base allowing to measure the diffraction angle and thus quantitatively evaluate the wavelength with hand-and-pencil calculations, as in the classical experiment of the optical goniometer.

SPETTROGRAFO system, prototype for a digital spectrometer, offers itself to be used both in secondary school educational labs and in university introductive physics courses, thanks to its inexpensiveness with respect to other commercial devices, to it easiness in use, and to the possibility to explore the functional role of every components of the measure setup. The main advantage of the system is that it is not presented to students as a mysterious “black box” working performing measurements without any awareness of the inner processes by students, rather, it allows students to develop a functional understanding of the measuring process, which is one of the main goal of an educational lab.

Up to now the device has been presented to a group of ten secondary school teachers and used by them in the context of “National School for Teachers on Modern Physics” within IDIFO6 project3 held in Udine University (IT) in September 2017, a teacher professional development activity. In the same IDIFO6 project, the device has been used in the educational lab for freshmen in biotechnology as a part of a wider didactical innovation project and it has been implemented in Masterclass and CLOE (Conceptual Lab of Operative Explorations) activity held in Udine University in the period January–March 2018 for secondary school students in the framework of a wider schools-university collaboration project.