# Do I understand the Fizeau interferometer? How to read interferogram?

1. Apr 24, 2014

Hi,

I'm just wondering if someone could help me better understand how the Fizeau interferometer works...

From my understanding, the source shines light through a collimating lens and the light goes through the beam splitter. After the light goes through the beam splitter, in goes through a partially-reflective reference flat. The light that goes through the reference continues on and then bounces off of the test surface. As this light passes upward through the reference flat, it interferes with the light that is initially reflected off of the reference flat. This creates the interference waves. This is then reflected off of the beamsplitter and then passes through another lens. Finally you can analyze the interferogram. Is this understanding correct? I'm mainly confused about where the light waves really interfere. Does it interfere as the light passes through the reference or does it interfere where the two lines in the above diagram are merged? Or both?

Also, What would the interferogram look like if I had a perfectly horizontal reference, and a slightly incline test flat? Could you tell what the angle of the test flat is based on this interferogram?

2. May 4, 2014

### Greg Bernhardt

I'm sorry you are not generating any responses at the moment. Is there any additional information you can share with us? Any new findings?

3. May 5, 2014

### Bandit127

I am far from an expert on interferometry and I wasn't going to post on this question for that reason. But you haven't had much response so I can reply from a user's point of view.

I don't know if your interferometer will work, but it looks feasible enough that I would probably build it and see.

The light source should produce coherent light and it is typically a helium/neon (he/ne) or diode laser with a wavelength around 650 nm.

I expect you already know that it will work by producing images of interference of light waves. This interference is (in turn) additive and destructive at the wavelength of the light coming from the source.

So if your optical system is perfect and your plate perfectly flat and perpendicular to the beam you wouldn't see any interference and your image would have even illumination.

If your plate was tilted you would see interference relative to the amount of tilt. The interference pattern produces "fringes". You can count these and multiply by the wavelength to get a measurement of the tilt.

If you know the beam diameter you could convert the measurement to an angle.

Have a look at the tutorials on the following site for more information.
http://www.grahamoptical.com/phase.html
Figure 2 on this page shows tilt being removed. Notice how the amount of fringes decreases towards zero.

As above, if it was me and I had the resources I would learn by building it.

Last edited: May 5, 2014
4. May 8, 2014