Using interference fringes to check optical flatness
Measuring flatness without an interferometer
Typically to measure the flatness of an optic, scientists use a device called an interferometer (a Twyman-Green or Fizeau interferometer to be specific). Flatness is the measurement of the maximum peak-to-valley distance, from the highest point on the optic’s surface, to the lowest point.
However, rather than using this expensive technology there is a more visual method that you can use to find out if an optic is flat.
A simple setup: What you need
It only requires 2 optics and a light.
- one of the optics should be known to be flat (aptly called an optical flat)
- and the other optic is the one you would like to measure the flatness of
By layering the optic of unknown flatness on top of the optical flat and applying a single colour light (ideally of a known wavelength) you can see the interference pattern and roughly calculate the flatness!
How it works: Interference fringes
When the light is bounced from near 0° incidence (perpendicular to the optic), you can see what are called “interference fringes”.
These are visualisations of constructive and destructive interference:
- Constructive interference occurs when wavelengths at the same part of their cycle interact and their energy combines – resulting in higher peaks and troughs for the wave, leading to the lighter bands in the pattern.
- The opposite is destructive interference, where wavelengths interact when at opposite parts of their cycle, flattening the peaks and troughs. This cancels out the energy to create the dark bands.
You will always see a fringe pattern due to the thin wedge of air between the optics. It leads to a double reflection, which means that the waves will cross and interact with each other.
Interpreting the pattern
If the bands you can see are straight, the test optic is as flat as the optical flat. With a perfectly flat optic, the phase shift will not change along the fringes, so they will be straight. However, if the interference fringes are showing up curved, the optic is not flat.
This is because the optic is resting on the highest peak so the air wedge will be larger across certain portions of the optic. If you know the wavelength of the light you are using, you can even measure the curvature of the bands to determine the level of flatness!
Curious about TWE?
If you are curious about the difference between flatness and TWE – transmitted wavefront error – you can find more information here.
