Lights, Camera, Spectroscope! The Basics of Spectroscopy Disclosed

Communication Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Lights, Camera, Spectroscope! The Basics of Spectroscopy Disclosed Using a Computer Screen José J. Garrido-González,† María Trillo-Alcalá,‡ and Antonio J. Sánchez-Arroyo*,§ †

Department of Organic Chemistry, Faculty of Chemical Sciences, University of Salamanca, E-37008 Salamanca, Spain Department of Science, Virgen de Atocha High School, E-28007 Madrid, Spain § Department of Organic Chemistry, Faculty of Chemical Sciences, Complutense University of Madrid, E-28040 Madrid, Spain ‡

S Supporting Information *

ABSTRACT: The generation of secondary colors in digital devices by means of the additive red, green, and blue color model (RGB) can be a valuable way to introduce students to the basics of spectroscopy. This work has been focused on the spectral separation of secondary colors of light emitted by a computer screen into red, green, and blue bands, and how the intensity of these bands can be modulated if the portions of each primary color are modified in the RGB coordinates. The option found in the PowerPoint program for defining RGB values in the background of slides has been used in order to tune the color of the analyzed light. On the other hand, a CD-ROM based spectroscope has been found to provide enough resolution for this kind of analysis and an accessible way to perform it. These studies can be carried out qualitatively, comparing the different spectra observed through the spectroscope, as well as quantitatively, if these spectra are photographed using a digital camera and they are plotted after the analysis of the images using ImageJ, an open source program. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Analytical Chemistry, Physical Chemistry, Hands-On Learning/Manipulatives, Spectroscopy

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INTRODUCTION Spectroscopy constitutes one of the fundamental parts in the curriculum of introductory chemistry courses. Its relevance lies in the ability of absorption and emission spectroscopic techniques in order not only to study the structure and properties of atomic and molecular systems but also to quantify the amount of analyte found in a sample, bridging the gap between basic knowledge and applied chemistry. As a consequence, spectroscopy can be highlighted among the most powerful tools used by chemists in a wide range of multidisciplinary fields like astronomy,1 art,2 or medicine.3 It has been previously pointed out in this Journal that many undergraduate students show a lack of understanding of fundamental and experimental concepts about spectroscopy due to, at least, two main reasons: the unaffordable price of most spectrophotometers and the fact that these instruments can be seen as black-box systems.4 In this sense, these limitations have been overcome with the development of spectrophotometers built from common and inexpensive materials, where a CD-ROM5 or a DVD6 was used as the diffraction grating, obtaining remarkable results. Most examples found in the literature using this kind of spectroscope have been devoted to absorption spectroscopy,7,8 although emission studies have also been performed.9 Despite the educational value of these proposals, we suggest a new variation where the light of different colors produced by a computer © XXXX American Chemical Society and Division of Chemical Education, Inc.

screen is diffracted, compared, and analyzed using a CD-ROM spectroscope coupled to a digital camera. For this aim, we take advantage of the additive color model RGB used in computer graphics, which rules the generation of a wide pattern of secondary colors mixing the three additive primary colors of light: red, green, and blue. As a consequence, the light coming from a computer screen will always be the sum of bands located in the red, green, and blue regions of the spectra, more or less relevant depending on the RGB values, which can fluctuate between 0 and 255.10 These coordinates which summarize the portion of each primary color can be easily defined for the slide background in a widely used program such as PowerPoint. As a result, changes in the RGB coordinates open the door to observe qualitatively the appearance or disappearance of different bands through the CD-ROM spectroscope. Moreover, the comparison between different spectra can be carried out quantitatively if a digital camera is used as detector and the recorded spectra are plotted using an open source program such as ImageJ.11,12 With this approach, students will use spectroscopy as a tool not only for understanding how colors are produced in digital screens but also for viewing how changes in the RGB values Received: October 5, 2017 Revised: March 6, 2018

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DOI: 10.1021/acs.jchemed.7b00763 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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correlate with a modification in the intensity of the bands for each primary color. As a consequence, it is possible to make an analogy between the RGB values for primary colors and an analyte which emits more or less light depending on its concentration, visualizing and making clear the fundamentals of RGB analysis as an analytical tool.13 In this sense, the aim of this work is to show the educational possibilities of an economical emission spectrophotometer setup which many chemical educators can use in order to teach the basics of spectroscopy, especially those teachers who face a lack of funds which prevents the acquisition of such instruments and classrooms where the large number of students is a relevant limitation for experimental work.

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EXPERIMENTAL SETUP AND RESULTS

Detailed information about the CD-ROM based spectroscope used in this work as well as the procedures followed to record the spectra with a digital camera and analyze them with ImageJ software can be found in the Supporting Information. Unlike the spectroscope described previously in this Journal, which is based on a reflective use of the CD-ROM grating,5 we found that a transmission spectroscope built from a cardboard paper towel tube was more suitable for these experiments.14 In this kind of spectroscope, one end of the tube is closed with an opaque screen where a narrow slit is cut and the CD-ROM grating is placed in the other end as a window (see Procedure 1 and Figures S1−S6 in the Supporting Information). Thereafter, a series of slides with different color backgrounds were created using PowerPoint (see Procedure 2 and Figure S7 in the Supporting Information). The first series of slides (Table S1 in the Supporting Information) was devoted to detect how the light of secondary colors such as magenta, yellow, and cyan are made by mixing two pure primary colors (red and blue yield magenta light, red and green afford yellow light, blue and green lead to cyan light). The same was done for white light (tertiary color), made by mixing all the pure primary colors. In the second series of slides (Tables S2−S4 in the Supporting Information), R and B coordinates were modified in order to detect the intensity changes of the correspondent band. In all the cases, the analysis can be done qualitatively, just looking through the spectroscope, or quantitatively if the spectra recorded with a digital camera15 are plotted using ImageJ (see Procedure 3 and Figures S8−S12 in the Supporting Information). Regarding the computer used as light source, a TFT-LCD screen (thin film transistor-liquid crystal display) based on LED technology has been employed. A schematic view of the experimental setup used

Figure 1. Chromatic palette of additive primary colors for the RGB model (red, green, blue), the secondary colors (magenta, yellow, cyan), and tertiary color (white) with the correspondent spectrum photographed for each case.

Figure 2. Spectra obtained from the analysis of images shown in Figure 1 using ImageJ. Additive primary colors are represented in all the spectra with a red solid line (255, 0, 0), green solid line (0, 255, 0), and blue solid line (0, 0, 255). Secondary/tertiary colors are depicted in dashed lines: (spectrum a) white (255, 255, 255), (spectrum b) magenta (255, 0, 255), (spectrum c) cyan (0, 255, 255), and (spectrum d) yellow (255, 255, 0). B



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for recording the images with the digital camera can be seen in Figure S13 in the Supporting Information. Spectroscopic Analysis of Secondary Colors of Light

The ability of spectroscopy to distinguish the different components found in light can be well-understood if the spectra from additive primary and secondary colors are compared. Figure 1 shows a chromatic circle where the primary colors red, green, and blue are separated by the secondary color which emerges by mixing both pure primary colors. The recorded spectrum obtained with the CD-ROM based spectroscope for each color is placed in the external part of the circle, while the center shows the spectrum for the white light. From a qualitative point of view, it is possible to detect mainly a single band for each primary color. However, the presence of residual bands different from the pure primary color (especially for blue light, where a slight green emission is detected) has to be pointed out. This fact can be attributed to the quality of the luminescent phosphors used by the computer screen. On the other hand, the presence of two main bands for each secondary color, the same ones mixed in order to get the secondary color, can be clearly seen: blue and red for magenta, red and green for yellow, blue and green for cyan, and all the primary colors for white. The RGB coordinates used can be found in Table S1 in Supporting Information. Graphical spectra like those shown in Figure 2 can be plotted if the previous images are analyzed using ImageJ. They cannot be considered as archetypal spectra because the intensity is depicted as a function of the distance of the light path due to diffraction measured in pixels, instead of the wavelength. However, as stated in the Supporting Information (second step of Procedure 3), pixels can be considered as a wavelength model if all the images are analyzed in the same way. As well, intensity values obtained with ImageJ correspond to the amount of light found for each pixel in a grayscale. These spectra constitute a more visual way to confirm the additive generation of secondary colors in the RGB model and the evidence deduced from images in Figure 1 for white light (Figure 2a), magenta light (Figure 2b), cyan light (Figure 2c), and yellow light (Figure 2d). It has to be noted that the intensity of the spectra of secondary colors shown in Figure 2 has been divided by 2 in order to avoid the overlap between bands of primary and secondary/tertiary colors. Moreover, the residual green emission detected in the pure blue light analysis corresponds to the band located around pixel 480 (Figure 2a−c), but this is not the same as the pure green emission band shown in Figure 2a−d. This fact reinforces the idea that this residual emission is due to the quality of the phosphors used by the computer screen.

Figure 3. (a) Series of spectra obtained when R = G = 0 and B coordinate is gradually increased: 1 (B = 0), 2 (B = 62), 3 (B = 125), 4 (B = 177), and 5 (B = 255). (b) Series of spectra obtained when R = 255, G = 0 and B coordinate is gradually increased: 1 (B = 0), 2 (B = 62), 3 (B = 125), 4 (B = 177), and 5 (B = 255). (c) Series of spectra obtained when G = 0, B = 255, and R coordinate is gradually increased: 1 (R = 0), 2 (R = 62), 3 (R = 125), 4 (R = 177), and 5 (R = 255).

for mixtures of two additive primary colors like blue and red. In order to reinforce the analogy between analyte concentration and RGB values, it is interesting to increase the value of the coordinate for one color while the other one remains fixed. If the red coordinate is constant and the portion of blue is changed, the series of spectra will be similar to Figure 3b. However, if the value for the blue coordinate is constant and the red component is increased, the series of spectra will change in the manner of Figure 3c. Again, students will observe qualitatively how the changes introduced in the RGB values (concentration) are

Analysis of the Correlation between RGB Values and Changes in Emission Intensity

The potential of spectroscopic techniques to quantify the analyte concentration in a sample can also be studied with this approach, using the RGB values as an analogy of concentration. Taking into account that the computer screen used in these experiments afforded wide emission bands associated with each primary color, the study has been focused only on changes in blue color and mixtures of blue and red colors. Figure 3a represents the series of images obtained from the spectroscope when the portion of blue color in the RGB coordinates is gradually increased (with R = G = 0). From a qualitative point of view, it is easy to see that the greater the portion of blue is, the brighter the line is corresponding to this color in the image of the spectrum. On the other hand, the same procedure can be performed

Figure 4. Spectra obtained from the analysis of the images shown in Figure 3a, where R = G = 0 and the B coordinate is modified: 1, B = 0; 2, B = 62; 3, B = 125; 4, B = 177; 5, B = 255. C



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associated with a modification of the spectra. The RGB coordinates for these experiments can be found in Tables S2−S4 in Supporting Information. On the other hand, the graphical spectra for the images shown in Figure 3 can also be plotted using ImageJ. Figure 4, built from the images shown in Figure 3a, represents how the band corresponding to the blue component grows in intensity when only the portion of blue color is increased. In addition, Figures 5 and 6 plot the changes observed in the images found

Figure 7. Maximum intensity for blue and red emission bands represented as a function of the R and B coordinate values, respectively.

Moreover, by means of a combination of Figures 4−6, students would be able to understand how spectroscopy can be used not only for the analysis of samples where only one analyte (color) is of interest (Figure 4), but also to perform multicomponent analysis, where more than one compound shows an emission band in the sample (Figures 5 and 6).

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CONCLUSIONS In summary, as is shown in Figure 8, the approach presented in this work is thought to achieve the active involvement of

Figure 5. Spectra obtained from the analysis of the images shown in Figure 3b, where R = 255, G = 0 and the B coordinate is modified: 1, B = 0; 2, B = 62; 3, B = 125; 4, B= 177; 5, B = 255.

Figure 6. Spectra obtained from the analysis of the images shown in Figure 3c, where G = 0, B = 255, and the R coordinate is modified: 1, R = 0; 2, R = 62; 3, R = 125; 4, R = 177; 5, R = 255.

in Figure 3b,c, where the red or blue coordinates remain constant, respectively. It can be easily seen from Figures 4 and 5 that saturation at the blue band is reached when the value in the B coordinate is higher than 177. Moreover, this trend can be observed if the maximum intensity of the band is represented as a function of the value introduced in the RGB coordinate (Figure 7). Interestingly, the intensity of the band grows in a linear regime when the value for the coordinate is lower than 177. This kind of behavior can be educationally valuable in order to introduce students to the concepts of linear calibration and instrumental saturation. On the other hand, Figure 7 also shows the changes of intensity for the red band when the R coordinate is modified. In this case, the linear distribution is only fulfilled when R is higher than 125. Below this value, the digital camera was not able to fully detect the increment of intensity in the red component (which is quite lower than the maximum intensity for the blue band). However, it has been considered that this observation could be useful to clarify the idea of detection limit.

Figure 8. Concept map summarizing the instructional goals.

students in all aspects of spectroscopic techniques, from the construction of the spectrophotometer to the analysis of the recorded spectra. As a result, they get a global view of the fundamentals of spectroscopy (both instrumental and procedural), going beyond modern black-box spectrophotometers, where students interact basically with the software.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00763. Details regarding the construction of the transmission CD-ROM spectroscope (Procedure 1), the way to set D



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camera to the light. If the automatic focus is used, the camera will adapt its sensitivity for each case, so the analysis where the RGB values are gradually modified cannot be done.

RGB coordinates using PowerPoint (Procedure 2), the graphical plot of the photographed spectra using ImageJ (Procedure 3), and the schematic representation of the experimental setup (PDF, DOC)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Antonio J. Sánchez-Arroyo: 0000-0002-4093-9915 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank M. J. Mancheño and J. Osío from Complutense University of Madrid, S. del Mazo from University of Salamanca, as well as R. López and L. Belardo from Virgen de Atocha High School for helpful comments and discussions.

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REFERENCES

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Lights, Camera, Spectroscope! The Basics of Spectroscopy Disclosed

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