V. Components

Last updated
Save as PDF

Page ID: 60641

Contributor
Analytical Sciences Digital Library

$ \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } $ $ \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} $$\newcommand{\id}{\mathrm{id}}$ $ \newcommand{\Span}{\mathrm{span}}$ $ \newcommand{\kernel}{\mathrm{null}\,}$ $ \newcommand{\range}{\mathrm{range}\,}$ $ \newcommand{\RealPart}{\mathrm{Re}}$ $ \newcommand{\ImaginaryPart}{\mathrm{Im}}$ $ \newcommand{\Argument}{\mathrm{Arg}}$ $ \newcommand{\norm}[1]{\| #1 \|}$ $ \newcommand{\inner}[2]{\langle #1, #2 \rangle}$ $ \newcommand{\Span}{\mathrm{span}}$ $\newcommand{\id}{\mathrm{id}}$ $ \newcommand{\Span}{\mathrm{span}}$ $ \newcommand{\kernel}{\mathrm{null}\,}$ $ \newcommand{\range}{\mathrm{range}\,}$ $ \newcommand{\RealPart}{\mathrm{Re}}$ $ \newcommand{\ImaginaryPart}{\mathrm{Im}}$ $ \newcommand{\Argument}{\mathrm{Arg}}$ $ \newcommand{\norm}[1]{\| #1 \|}$ $ \newcommand{\inner}[2]{\langle #1, #2 \rangle}$ $ \newcommand{\Span}{\mathrm{span}}$$\newcommand{\AA}{\unicode[.8,0]{x212B}}$

Any camera that outputs a 24 bit JPEG file is adequate as a detector. Let the students bring whatever they have to lab. Their familiarity with their own cameras and cell phones obviates the need for instructors to document anything about the detectors, or even to know how to use them. When, in prior practice, has it been feasible to use instruments where the students were presumed to be technically more proficient than the instructors?

Every spectrometer needs a baseplate. A PDF file of a simple design is included in this paper. What it lacks in rigidity and sophistication is made up for by low cost. Any photocopier or printer can provide adequate replicas. Using heavy paper or thin cardstock has a slight advantage. The original drawing was made in Autocad 2004, and is linked to this paper.

The light source is a blue-LED-pumped fluorescent light source, dubbed a white LED (not to be confused with LEDS that are three independent emitters, one red, one green, and one blue). We have used an RL-5-W5020 device, the specifications for which can be found at http://www.superbrightleds.com/specs/w2_specs.htm. The output divergence angle, ±9^o, is narrow enough to provide high intensity, while being narrow enough to confine most light to the area of the grating. Approximate output as claimed by the vendor is replotted in Figure 2. Output power is specified for a current of 20 mA which requires a drive voltage of 3.8 V. For simplicity, we instead use a CR2032 3V battery, commonly employed in remote keyless entry transmitters. The LED/battery combination results in a current of barely 2 mA, so output is only about 1/10 of the full power rating (see Figure 3). While the batteries are available off-the-shelf world-wide, we have found Digikey (www.Digikey.com) is a particularly inexpensive vendor.

Figure 2. LED Emission Spectrum for 20 mA drive current. (replotted from Superbright LED website)

Figure 3. LED and Battery. LED diameter is 5 mm.

Plastic 1 cm square, 3.5 mL cuvettes are entirely adequate; they are opaque below 300 nm, but the light source only emits above 425 nm. Commonly available from most lab supply houses, 100 unit quantities are available at approximately $0.15 each from Vernier Software and Technology (www.vernier.com/accessories/a...ate=basic.html).

Finally, one must obtain transmission diffraction gratings. Edmund Scientific Optics http://www.edmundoptics.com/onlineca...productID=1490 is a convenient reseller. Edmund's stock number NT54‐512 is a 500 line/mm grating available for $0.78 each in quantities of 80. Apparently, Edmund purchases the gratings from Rainbow Symphony http://www.rainbowsymphonystore.com/scienanded1.html. Rainbow's price for 500 line per mm gratings starts at $0.40 in quantities of 50. An important reason that the spectrometer design and software are being shared through a Creative Commons license is in the hope that without the possibility of license fees or aggregator's markup, costs will remain low, allowing wide use. The assembled spectrometer (sans detector) is shown in Figure 4.

Figure 4. Assembled Spectrometer. LED, perched 1" above desk, shines through cuvette, aimed directly at grating, which in turn is braced with transparent tape. Spectrum and LED glare are unretouched (laboratory background removed for clarity). Picture obtained with Nikon D50 camera.