The cell phone spectrometer has been used three times as of this writing. First, a group of instrumental analysis students at the Faculty of Chemistry, Hanoi University of Science worked with the components. By fluke, two‐dimensional "double axis" diffraction gratings (diffraction/dispersion in two directions simultaneously, see http://www.rainbowsymphonystore.com/scienanded1.html accessed 6/30/09) were obtained, giving more complicated visual patterns that could be used easily since the software had no predetermined, assumed diffraction pattern to fit. Second, a group of high school teachers from Illinois were given the components and allowed to explore how a spectrometer so designed might fit into their classrooms. Finally, 26 high school students (mostly seniors, but at least 2 sophomores) in groups of three, attending a summer outreach program at Clark Atlanta University were given the components and some guidance in their use. In all cases, while there was a sequence of questions that could be followed, the students/participants were encouraged to ignore the writeup and proceed until they could obtain reproducible exposures at controlled dispersions so that absorbance experiments could be performed. At that point, "primary color" samples (Kool Aid for the high school teachers, CuSO4 for students) were provided.
In all three cases, the classroom dynamics were the same. After a few minutes of confusion and intense questioning of the instructor, the idea of "playing in the sandbox" to optimize throughput, dispersion, exposure, alignment, stability, and so on "clicked." In place of a frenzied instructor, there was intense student‐student interaction, with an occasional, "look at this!" as the photographic spectra started rolling forth. A few students never tried to engage the project for reasons unclear; those who stayed focused for at least 5 minutes generally completed assembly and were able to obtain pedagogically‐significant data i.e. data that led them to understand one or more of the ideas previously listed.
For illustrative purposes, here are data obtained by this paper's authors. A Nikon D50 camera, operating without flash, set for closeup focus, and aimed at the spectrum transmitted by water or 20 μM Methylene Blue, obtained f/4, 1/30 s exposures as shown:
Figure 5. Spectral Data for 20 μM Methylene Blue. Left spectrum: I0. Right spectrum: I. Insets combined using Windows Paint. Line near the left end of the green part of the spectrum due to dirt on the grating. Note change in yellow part of spectrum due to MB absorption.
A screen grab of the spectra as processed by the software, showing raw intensity data, is in Figure 6. The green line across the spectra shows the region plotted, and the dimmer lines above and below the green central line show the range of pixels summed. Note that near reported wavelengths of 450 nm and 625 nm, the I0 spectrum can be seen to saturate. Wavelength calibration can be seen to be terrible; methylene blue absorption is centered at 655 nm in a well‐calibrated measurement, but turns up here at 590 nm (Figure 7). The reason for non‐calibration is clear from the way the wavelength calibration is set; the user simply guesses which pixels correspond to the extreme wavelengths emitted by the LED and detected by the camera, with no knowledge of the red or blue cutoff of the sensor. If this were a "real" measurement, such arbitrariness would be unacceptable. Here, it helps make the case for careful calibration, for showing the effect of the spatial extent of the light source on dispersion and resolution. Saturation at blue wavelengths in both spectra and in red wavelengths for the reference spectrum illustrates dynamic range limitations. Because these spectra were taken in a darkened room, stray light is minimal. In a brightly‐lit room as shown in Figure 4, stray light is also obvious.
Figure 6. Screen Grab of Raw Intensity Data for 20 μM Methylene Blue.
The artifactual "absorbance" at blue wavelengths (<512 nm on the graph abscissa in Figure 7) makes the students question when Beer's Law fails due to the instrument as well as when chemistry may be involved. Is there a way to distinguish the two? Inexpensive cameras adjust exposure to avoid detector saturation; getting reproducible exposure is easiest when there is much stray light to fool the camera electronics, but of course this generates dynamic range and background subtraction difficulties. Students can explore the noise reduction due to signal averaging by varying the number of rows of pixels average to produce the raw spectrum. The lack of correspondence between I(λ) for any color sensor and the purported output of the LED naturally leads to discussion of quantum efficiency, throughput, and the fraction of generated light that actually reaches the detector. A mercury penlamp could be used to show wavelength range, resolution, and wavelength calibration (warning: such lamps typically produce substantial ultraviolet light; while brief exposure is unlikely to cause sunburn, eye protection is essential). The range of discussion that may come from this instrument is yet to be fully explored; in no case as yet have all students had convenient ways to offload their spectra to their laptops in real time. Emailing spectra to the instructor served as a proxy for full, real‐time participation.
Figure 7. Absorbance Spectrum Corresponding to Figure 6.