Teaching Quantitative Optical Filter Choice as Part of Practical Microscopy

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Categories: GSI Online LibraryTeaching Effectiveness Award Essays

by Neil Switz, Biophysics

Teaching Effectiveness Award Essay, 2011

I approached Prof. Dan Fletcher in 2005–06 about collaborating to create a new lab course — now realized as BioE 168L: Practical Optical Microscopy, first offered in 2010 — in which students would learn the optics involved in light microscopy by building their own microscopes using standard components, and using these microscopes to gain hands-on experience with contrast methods and performance issues. My objective was to remedy the limitations of existing, theory-based classes by allowing students to learn the practical optical techniques and theory for microscopy via hands-on experience and instruction in the practical techniques used by optical engineers but not emphasized in textbooks. One example of how we designed the course to achieve that objective is provided by the materials and approach I fashioned for introducing the topic of filter specification to the class. Optimizing the filters used in fluorescence microscopy allows one to see samples more brightly, or to observe them for longer time periods before the fluorescent dyes bleach to invisibility — either of which can be the difference between success and failure in an experiment. Filter selection is also valuable in that correct choice of $500 in filters can alleviate the need for $20,000+ of other equipment. Despite this, few people, faculty or students, understand the details of specifying filters, and no textbooks cover the subject in real depth. Furthermore, students have difficulty conceptualizing the issues involved due to inexperience in thinking about light as a continuous spectrum and a lack of comfort with the meaning of the integrals required in the calculations.

I used a threefold approach to this material. First, due to the lack of substantial coverage in existing texts, I wrote detailed course notes which explained the use and choice of filters in a step-by-step fashion. Second, to reinforce understanding of the material, I created a spreadsheet-based problem set containing both a worked example the students could follow and a related problem for them to complete. The problem forces students to become familiar with the calculations in order to quantitatively compare a set of filters we specify with a set that they choose; this adds an element of fun as they try to beat the performance of our filters. They gain additional practical knowledge through being guided to find the required parts and data on manufacturers websites. Using a spreadsheet for the problem allows the students to experience the calculation as several conceptually distinct parts: because the filter, dye, and other data in the spreadsheet are listed in columns, each row incorporating data for a specific wavelength of light, students can initially ignore the potentially daunting problem of having many wavelengths involved in the calculation. As they set up the formulae for a single row, they work in the more familiar context of a single wavelength; the integrals over all wavelengths necessary to calculate the final performance data are then seen by students to be a simple summing the spreadsheet columns. An additional benefit of the spreadsheet is that it serves as a drop-in template they can use professionally in the future, since practicing scientists and engineers do the calculations in the same manner. Third and last, we used a spectrometer to acquire and display spectra of filters and light sources live during lecture, helping to bring home the immediate reality of the numeric spectral data the students had been working with.

Judging both by the solutions turned in for the problem set and by the fact that on the final exam 80% of students were able to correctly choose filters from data we provided, it is clear this combination of approaches has worked well in conveying a subject not covered in other courses or any available texts. Moreover, other components of our course have been equally successful, judging by the overwhelmingly positive end-of-semester student course evaluations and the explosive growth in demand for the class. Despite a doubling in class size, the waitlist in 2011 was overflowing, and based on student demand the course will now be offered both semesters next year; in addition, some faculty have requested a summer offering for graduate students.