# What type of interferometry set up do I need to image a small structure at a distance?

Please forgive me, optics isn't my forte.

I'm trying to work out how to image the surface structure of an object at a distance.

Considerations are:

• Monochromatic light- it must be imaged in the infrared.
• Object size is around 1cm square, distance to the object up to 3m.
• Preferable to not have to focus on the object.
• Needs to happen quickly (i.e. an exposure as opposed to scanning).

Hypothetically this could be done with a suitably high resolution IR camera but I want to explore the possibility of using an interferometric set up as this may produce better results.

What type of interferometer can I use, and what considerations do I need to be aware of?

## 1 Answer

In principle, a Fizeau interferometer with a typical Nd:YAG NPRO (wavelength of $\sim 1\ \mu m$) will work.

Imaging an object with horizontal dimensions (with respect to the optical axis) of 1 cm won't be a problem with that setup, but if the vertical dimensions are larger than 10's of microns, then the image analysis needed to reconstruct the object can be difficult. You will need to roughly match the input mirror reflectivity to the reflectivity of your object. In addition, an interferometer of this type will only provide you with a height map (similar to a topographic map) of the object, but I'm assuming that is what you mean by an image.

If you truly want to image the object in 3 dimensions, then you could set up a holographic interferometer. One of the advantages of holography is that it uses the scattered light from an object whereas interferometry typically needs the object to have a reasonable reflectivity to work. Setting something like this up is a rather serious undertaking though; mostly because the image processing techniques needed are rather involved and are still the subject of active research.