Need help with DIY spectrophotometer. Data not what was expected. Answered
I am interested in spectrophotometry and made several colourimeters, however I am busy making my first true polychromatic spectrophotometer. I have built a decent working prototype using a incandescent bulb, a photodiode with transimpedance op-amp, a diffraction grating, and a stepper motor inside an enclosure. The arduino steps or scans the sensor across the spectrum and finishes just after the zero order light (see graph 'blank' spectral response, the small bump is the fringe pattern and the big peak is the zero order light). I am testing red, blue, and green filters with very specific wavelengths to calibrate it, but the data is not what I have expected. I even tried scanning some KMnO4 (which has a distinctive double peak) but the pattern doesn't quite match.
What I expect to see when I use each filter is three distinct and separate peaks which are clearly spaced apart from each other, however the green and the blue overlap significantly. The red seems to match the transmission spectra from the datasheet. Why do the blue and green peaks overlap? I have done some analysis and I have determined;
- The fringe separaton is scanning correctly, I replaced the photodiode with an AC726X 6-colour sensor (the grey graph) and the peaks occur at the correct points in the graph. Granted, the light is not truely polychromatic and the intensity of red wavelengths is greater than blues or violets but that should not matter if the result is a ratio beween input and output intensity.
- The measurements are repeatable and the spread of the data is about 10% of the mean values. This indicates that I am missing something in regards to the construction. Is it the angle of the light? Is the entrance slit too wide?
If anyone has and ideas please let me know. I would love to finish this project and put it on instructables. I have had an amazing learning experience with this and I would like to share it when I am done.