In short
- Interferometers measure flatness by comparing a test surface with a very flat reference surface using coherent laser light.
- Each fringe is half a wavelength of height. In a reflective set-up such as a Fizeau interferometer, one fringe of deviation equals λ/2, about 316 nm at 632.8 nm.
- The method is non-contact, fast and nanometre-resolved, which is why it is the standard for lenses, mirrors, wafers and precision parts.
- Results depend on the set-up: control temperature, vibration and air currents, calibrate against a known flat and analyse the interferogram with software.
Interferometers are key instruments in metrology, especially for measuring surface flatness with very high precision. This article covers the physics behind interferometers, how a flatness interferometer tester works, and why the method is so important for precision manufacturing. For related measurement topics, see Integrating Interferometry and Quantum Interferometry and Essential Laser Measurements.
Physics fundamentals of interferometry
A coherent wavefront, usually from a laser, is split into two or more parts that travel different paths.
The light must be coherent, its waves in a fixed phase relationship, to form clear interference patterns. Lasers provide both spatial and temporal coherence.
When the split beams recombine, the difference in their optical path lengths decides whether they interfere constructively or destructively.
A path difference of zero or a whole number of wavelengths gives a bright fringe. A difference of half a wavelength (or an odd multiple) gives a dark fringe.
The fringe pattern, the interferogram, is used to calculate flatness. Any deviation of the surface changes the path length of one beam and bends or shifts the fringes.
One beam reflects from a highly flat reference surface and the other from the test surface. The fringes map the difference between the two.
In reflection, light travels to the surface and back, so a height step h changes the optical path by 2h. Moving from one fringe to the next therefore corresponds to a height change of half a wavelength:
Δh = N · λ / 2
where N is the number (or fraction) of fringes of deviation and λ is the wavelength. With a 632.8 nm laser, one fringe is 316.4 nm and a quarter-fringe bow is about 79 nm.
The flatness interferometer tester
- Set-up: a typical tester includes a stable light source (such as a helium-neon laser at 632.8 nm), a beam splitter, the reference and test surfaces, and a camera or detector that captures the interference pattern.
- Fizeau interferometer: a common type for flatness testing. The test and reference surfaces are placed close together and the gap between them is illuminated by a collimated beam.
- Analysis: the interferogram is analysed, often with dedicated software, to determine the flatness of the test surface: peak-to-valley (PV), RMS and the shape of the deviation.
- Environmental control: interferometry is very sensitive, so temperature, vibration and air currents must be controlled for accurate results.
- Calibration: the tester must be calibrated against surfaces of known flatness, so that the reference error can be separated from the test-surface error.
- Non-contact method: the measurement does not touch, damage or alter the surface.
Flatness interferometer testers are vital in precision engineering, semiconductor manufacturing, optics production and every field where tight flatness tolerances matter. Their precise, non-contact measurements make them essential tools of modern metrology.
Interferometers vary in complexity and design, but they all rest on the same optical principles. By analysing interference patterns, they can measure length, changes in refractive index, surface irregularities and the shapes of optical surfaces with remarkable precision.
Fringes to flatness calculator
Read the fringe bow (how far a fringe deviates from a straight line, as a fraction of the fringe spacing) or count the fringes of deviation. The tool converts it to surface height for a reflective (double-pass) test such as a Fizeau interferometer. Add the number of tilt fringes and the aperture to get the wedge angle.
Why interferometric flatness measurement matters
The importance of interferometer measurements for flatness comes down to several key points:
High precisionfractions of a wavelength
Interferometers measure flatness to a fraction of the wavelength of light. This is crucial in industries where tiny deviations matter, such as optics, semiconductor wafers and precision-machined surfaces.
Non-contact measurementno damage, no contact error
The measurement needs no physical contact, so delicate or finely finished surfaces are not damaged or altered, and contact-induced errors are avoided.
Speed and efficiencylarge areas, high throughput
Measurements are fast, so large areas or many components can be inspected quickly. This supports manufacturing throughput.
High resolutionnanometre scale
Interferometers resolve features at the nanometre scale and reveal small deviations that could affect the performance of the final product.
Versatilitymaterials and production stages
Interferometers work in many environments, on many materials and at every stage of manufacturing, from initial production to final inspection.
Quality controllenses, mirrors, flats
Where flatness is a quality benchmark, as for lenses and mirrors, interferometry gives a reliable reference for verifying that parts meet specification.
Data analysissurface maps and reports
Modern interferometers work with software that maps flatness across the surface and highlights the areas that need attention.
Standardisationconsistent and repeatable
Interferometric measurements are standardised, consistent and repeatable, and they are recognised internationally, which makes results comparable between supplier and customer.
Interferometer software and systems from IZAK Scientific
IZAK Scientific develops interferometer analysis software with advanced image processing. It supports your quality assurance and generates detailed reports for every part.
We can build a customised analysis tool that fits into your existing system, or create one from scratch. We also offer consultation to design an interferometer for flatness inspection, or a complete ready-to-use system. Learn more about our electro-optics design and development services.
The laser source matters too: a clean, well-collimated beam gives straight, high-contrast fringes. Our laser beam profiler verifies beam size and shape, and the Gaussian Beam Calculator helps plan the collimation.
When you work with IZAK Scientific, you gain a partner committed to accuracy and efficiency. Get in touch to discuss your flatness measurement needs.
Frequently asked questions
QHow do you calculate flatness from interference fringes?
QWhat is a Fizeau interferometer?
QWhat affects the accuracy of interferometric flatness measurement?
Tzachi Sabati
CEO, IZAK Scientific
Physicist specializing in photonics and quantum technologies, with deep expertise in quantum sensors and advanced optical systems. Leads the Advanced Quantum Lab course at the Technion, bridging academic excellence with industry innovation. At IZAK Scientific, provides cutting-edge photonics-based solutions, developing customized inspection and sensing systems for R&D and production. Passionate about advancing quantum sensing applications and integrating novel technologies to meet industry needs.
