With a single shot, this TWI interferometer measures both aspheres and free-form surfaces in a single step.

The new interferometer improves upon the existing TWI (tilted wave interferometer) by requiring only one measurement. Instead of four partial measurements, each of which contains five phase measurements only one is needed.

• The special design reduces the measurement time to a single exposure.
• Measuring times are reduced from slightly less than one minute to just a few seconds.
• Environmental influences are minimized.
• It is easy to integrate into the process chain.

The newly developed TWI is especially well-suited for measuring free-form precision optics.

The use of aspherical and free-form lenses is becoming increasingly important in designing modern optical systems. These surfaces provide additional design possibilities and improve optical imaging while reducing the number of required surfaces per design. However, producing and testing such surfaces remains challenging. The TWI currently used for quality management takes nearly a minute to achieve sufficient measurement accuracy.

Although the current TWI can operate in single-shot mode, the measurement accuracy deteriorates as a result. Currently, four partial images of the sample are taken for one measurement result, and five more for the phase evaluation. This procedure is time-consuming, and combining several consecutively generated images into one measurement makes the measuring method sensitive to environmental influences.

The Institute of Technical Optics at the University of Stuttgart has developed a new solution that aims to improve the patented TWI technology, making it possible to perform single-shot measurements while maintaining the same high measurement quality. Both the current TWI and the improved version are full-field interferometric measurement methods for aspheres and free-form surfaces, offering a high degree of flexibility. The interferometer uses a series of tilted wavefronts to compensate for deviations in the surface under test from its spherical shape. Since it is a non-zero method, costly compensation optics are unnecessary. Highly parallelized measurement data acquisition results in a short measurement time of a few seconds while achieving a high lateral resolution.

New features include the combination of a hexagonal pinhole array and three different wavelengths for the remaining three illumination steps. This allows all measurements to be carried out in one step. These features make the TWI a perfect candidate for production monitoring in the aspherical and free-form surface production process chain.