Spectral Measurements Labortory #6 (First Half) 6.163 Strobe Project Lab Spring 1996 George Madrid Friday p.m. Partners: Wilfredo Santos Ernest Mirales ---------------------------------------------------------------------- ABSTRACT In this lab, we explore the uses of a spectroscope for analysis of certain characteristics which are distinguished by spectral emissions in the range of visible light. We determine the correction curve or our spectroscope and use this to isolate the fundamental frequencies of two light sources. We examine the absorption and transmission spectra of some common items and observe that the equipment is much more reliable in the midrange of the visible spectrum. ---------------------------------------------------------------------- INTRODUCTION This lab concerns the use of the spectroscope as an analytical tool in the laboratory. The spectroscope is a device for measuring the intensity of light at many discrete wavelengths. Using the spectroscope, we get measure either the absorption or transmission spectra of a light source. The absorption spectrum of an item measures the amount of light absorbed by that item. This is done by comparing the light given off by that item, to the light which illuminates it. The transmission spectrum of something indicates the amount of light which is allowed to pass through that item from the light source. By comparing the spectra of things, we can visually distinguish them from one another. In fact, this is exactly what we do when we see, although we are not usually aware of the process involved. By understanding this process, we can enable computers to perform it. Spectra are also used to determine the elemental makeup of objects which can only be observed by their electromagnetic emissions. Every chemical element has certain spectral characteristics which can be looked for in distant phenomena. For example, the makeup of the sun and of distant stars and comets is often determined via spectral analysis. This lab is concerned only with those wavelengths that lie in the visible spectrum, which is to say wavelengths from about 400-800nm; although, wavelengths outside this range can also be used. ---------------------------------------------------------------------- EQUIPMENT Spectralon disk Desklamp Strobotac Strobolume Spectrometer (and computer) Tangerine Sweater Apple (with bruise) Neutral density filter Red filter Matlab 4.2 ---------------------------------------------------------------------- PROCEDURES For all procedures, the object being measured was placed six to eight inches from the light source, and the spectroscope sensor was placed a few centimeters away from the object at an angle of about 45 degrees. Part One Position the spectralon disk and the lamp and sensor. Turn on the lamp, turn off the other room lights. Adjust the clock rate so that the reading is strong but not saturated. Take the reference spectrum. Turn off all light sources and take a Dark Reading. View the spectrum on the computer and write down the name of the resulting file. Use the formula for black body radiation to determine the ratios between the curve and the actual values. This is the correction curve. Part Two Position as for part one, except use Strobotac instead of incandescent lamp. Adjust the clock rate, and take a new dark reading. Record the spectrum and note the name of the file. Apply the correction curve to determine the actual spectral characteristics of the strobotac. Repeat all steps for strobolume. Part Three Position as for part one, placing neutral density filter over the sensor. Take a new dark reading. Take a new reference spectrum with the filter removed. Restore the filter and take a transmission spectrum. Record the file name. Repeat last step with red filter. Part Four Position as for part one, with apple instead of disk. Adjust the clock rate. Take a new dark reading. Take a new reference spectrum using the spectralon disk. Measure an absorption spectrum for a red portion of the apple. Record the file name. Measure an absorption spectrum for a green portion of the apple. Record the file name. Measure an absorption spectrum for a bruised portion of the apple. Record the file name. Part Five Position as for part one. If necessary, adjust clock rate, take a new dark reading, then take a new reference curve. Record an absorption spectrum for the film lid, the tangerine, and for the sweater. Part Six Run matlab and play with the rgb program. ---------------------------------------------------------------------- RESULTS Part One Clock rate set at 18kHz. Filename "1fam.pc". (Graph "Spectral measurement of 2800K blackbody".) After computing the correction curve, graph it. (Graph "Spectral sensitivity correction curve".) Part Two Clock rate set at 10kHz. Filename "2fam.pc". (Graph "Spectral Distribution of Strobotac (10 kHz)".) After elementwise multiplication by correction curve, graph it. (Graph "Corrected Spectral Distribution of Strobotac".) Filename "5fam.pc". (Graphs "Spectral Distribution of Strobolume (10 kHz)".) After elementwise multiplication by correction curve, graph it. (Graph "Corrected Spectral Distribution of Strobolume".) Part Three Clock rate set at 10kHz. Filename "4fam.trm". (Graph "Transmittance of neutral density filter.") Filename "5fam.trm". (Graph "Transmittance of red filter.") Part Four Clock rate set at 25kHz. Filename "7fam.trm". (Graph "Reflectance of red part of apple.") Filename "8fam.trm". (Graph "Reflectance of green part of apple.") Filename "9fam.trm". (Graph "Reflectance of bruised of apple.") Part Five Clock rate set at 25kHz. Filename "10fam.trm". (Graph "Reflectance of a film lid.") Filename "11fam.trm". (Graph "Reflectance of a tangerine.") Filename "12fam.trm". (Graph "Reflectance of a sweater.") Part Six This didn't work. See the transcript. ---------------------------------------------------------------------- DISCUSSION Part One This was pretty straight-forward. The correction factors get rather high at the two extremes of measurement. This is noteworthy. Since the instrument is less sensitive at these areas, it is more prone to error. Since the correction factors are so high, these errors will be multiplied. Part Two This was cool. We were able to see the peaks produced by the gases in the bulbs. Xenon was present as evidenced by the peaks. I'm assuming that the other peaks were produced by the filament or some other physical component of the bulb. Part Three Looking at the data, we can see that the neutral density filter is about 25 percent. The ends of the spectra are very scraggly, though. This is due to the high error amplification that was mentioned earlier. The red filter comes in at about 600nm. It is not necessary to apply the correction curve to this since the filter produces the same factor of difference whether it is scaled (corrected) or not. Mathematically, this is a result of the associative property of multiplication. Part Four Analysis of the data shows a far greater reflectance by the bruise in the range of 500-600nm. This fact could be used to produce the desired automated system by setting up an apparatus that measured the reflectance of the apples in this range, rejecting those for which the reflectance was too high. ---------------------------------------------------------------------- CONCLUSION & RECOMMENDATIONS Overall, we can see that the spectroscope has a wide range of uses. When to apply the various corrections can be a little confusing, and so it requires a bit of thought. As usual, we must pay close attention to the characteristics of our equipment. For example, the data for the neutral pass filter shows a lot of noise less than about 400nm and more than 800nm. Between those two values, the result is a pretty steady 25%.