I took the picture below with our microscope. It's a 8mm roller bearing, lighted from below with a parallel light bundel (white). The width of the yellowish light fringe is maybe 5 micron.
Originally I was hoping to make pictures with an accuracy of like 1 micron. But I'm afraid that was way too optimistic.
-First I would like to know if this is really a diffraction pattern?
-If yes, would a blue back light make the fringes smaller? What else could I do to increase quality?
-Is there a way to calculate the distance between the fringes and the "real" object position? On this website, I found a similar question, but I don't think those formulae are valid for this case (the calculated distances are like 0.3mm between fringes).
Thanks in advance,
Edit; the first back light I used did not have a pin hole, and the light beam was not focused at all. The resulting image in the microscope looked like this; (distance per pixel is the same)
At first I was a bit disappointed when I saw all these fringes with the truly parallel light, but maybe it's a good thing to hit the limitations of the microscope (as Rod suggests below)?
Answer
Yes, definitely this is due to diffraction. But I'm surprised by the contrast and number of fringes you have with white light. Is the focus optimized? The fringes can become larger if the image plane is not on the object plane. Is the light really white? The yellow fringe is suspicious. If you can change the illumination, you can try to find a white light with flat and large spectrum, it will help to reduce the fringes. A real white light will be better than a blue light. If you use a more monochromatic light (even if not coherent as a laser), eg a LED, diffraction will increase. The best is a real white light which "washes out" the fringes (adding all the fringes of different dimensions at the different wavelengths).
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