I want to image a diffuser (e.g. 1500 grit glass diffuser). Is it possible to do so using a microscope objective and normal (say aperture f/8) lenses?
When i say imaging a glass diffuser, it means imaging the whole phase function so I should not be able to see any speckles in the conjugate image plane even if illuminating the diffuser with a laser beam!
Could anyone provide me with few estimates of the numbers in terms of NA of corresponding objective and f/# of lenses to achieve this?
I was using light profilometer to get the 3D structure of diffusers. With microscope you will see 2D structure consist of scratches or/and lenses.
Thank you.
Although that works, my real intention is to make a conjugate image plane of diffuser so that I can do some sort of holography/interference based measurements on a optical bench based setup! I am just afraid that even if I try to image a diffuser in such a fashion, what I would get will be not the accurate diffuser surface/phase-only information but a mix of speckles along with. (The speckles arising because I would not be able to gather all the spatial frequencies of a diffuser)
In my own experience you will get a decent image of your diffuser, just like if you wanted to image two point sources illuminated with a laser beam. On the conjugation plane everything should be fine, but of course if you are away from it speckle will arise (that is actually one of the way to create speckle...).
Note also that your laser source itself is likely to create speckle (check if you have speckle without the diffuser!). If it does you will have to filter your laser beam or use whatever suited technique to have a clean illumination.
Hello Manish,
sadly a standard microscope is not, in general, able to collect any information about phase, only about intensity. You could try to obtain your information from some kind of phase contrast brightfield microscope (like DIC or DODT imaging).
An alternative would be doing some proper interferometry by imaging your sample through coherent light, with the objective and the lens at a distance equal to the sum of the focal lengths, and have a reference beam shining on your detector.
As a last, expensive solution, you can consider this publication:
P. Bon et al., "Quadriwave lateral shearing interferometry for quantitative phase microscopy of living cells," Opt. Express 17, pp. 13080-13094 (2009).
The authors are collaborating with a company that sells the phase measuring camera described.
I hope some of this helps
It will be challenging or impossible to image the fine grained structures with ordinary lenses if the dimensions of the grit structures are below the diffraction limit and only reflect little or no light. Using interference is limited equally if the interference pattern and particular the phase changes cannot be tracked, in which case random interference occurs as speckle pattern, as already said. Even a white-light profilometer may be limited as it relies on sufficient back-reflection of light, while diffuser typically should not have much of that but may still provide some structural info. Another suggestion in this case may be to use an AFM if the surface profile and the grit structure of the diffuser is of particular interest.
Thank you everyone for giving nice insights.
I shall most probably take the interferometry path to go ahead with this!
Maybe you can try imaging ellipsometry. Its nondestructive and got a resolution down to 1 mircon (comparable to classical microscopy). It is very sensitve and you can detect differenz in refractive index down to 0.01.
Best regards
Christian
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