Plasma Chamber
The observation port into a plasma chamber shows a well-defined interference pattern. The contour fringes, observed in white light, are due to a very small particle stuck between two glass sheets creating a wedge effect. The 15 fringes observed in the green light portion of the spectrum (550 nm) suggest the particle height to be about 4 μm.
—Catalin Florea, Avo Photonics