Measuring Aspheres: Selecting the Best Technique.

The benefits of aspheric lenses?are numerous: They allow for a reduction in spherical aberrations and are ideal for focusing or collimating light, as they can achieve a low ?-number. Aspheres also allow the same or better performance using fewer lenses, which often translates to a reduction in both size and weight in a system.

New applications in imaging increasingly require the use of aspheres, due to performance and size constraints. Manufacturing these surfaces requires equally innovative metrology methods. It is important to assess the strengths and weaknesses of the most common metrology methods that are used to quantify the surface accuracy of aspheres, and identify the criteria for selecting the most appropriate technique to measure specific classes of optical surfaces.

The interferometry that is typically used to measure the surface accuracy of optical components requires a planar reference wave to measure planar parts, or a spherical reference wave to measure spherical parts. Neither of these two approaches is ideal for measuring aspheres. The aspheric departure, or distance between the best fit radius of the asphere and its actual aspheric surface, creates very densely spaced interference fringes that are challenging to resolve. Because a spherical reference wave that matches the best fit radius of the asphere in question cannot always provide acceptable measurements, alternate methods to measure aspheric surfaces must be used.

To compare the performance of several asphere metrology methods, as well as the practical aspects of implementing their use in a production environment, consider the following methods:

• Contact profilometry, which records the position of a stylus as it traces a 2D path across the surface of a lens.
• Stitching interferometry, which combines multiple subaperture interferograms to create a complete, highly accurate map of the optic.
• Computer-generated holography, which performs standard interferometry on an aspheric component by using a computer-generated diffractive optical element to convert a spherical wavefront to an aspheric reference wavefront that matches the desired part profile.
• Chromatic confocal sensing, which illuminates a surface with a white light point source and senses the wavelength reflected from a surface to perform noncontact profilometry.
• Multiwavelength interferometry, which performs distance measurements using several discrete wavelengths to enhance the accuracy of surface reconstruction.

The above metrology techniques are all capable of measuring the peak-to-valley (PV) and root mean square (rms) accuracy of an optic compared to the nominal design. The key things to consider when selecting a measurement method are precision, speed of measurement, cost, and whether the part in question is within the geometrical space that can be adequately characterized by a given technology.

Stylus method suited for ground optics

In contact profilometry, a stylus is physically dragged across the surface of an optic. The other end of the stylus features a diffraction grating. Using the deflection of a laser directed toward the grating, the displacement of the stylus is recorded in a 2D trace. Contact profilometry is ideal for ground optics that do not reflect enough light to be measured with standard interferometry. Because the stylus is in contact with the optic, there are very few limitations to what shapes can be measured in terms of aspheric departure and points of inflection, although very steep slopes may not be measurable.

The use of aspheres continues to grow in applications that have requirements including aberration minimization, high-resolution imaging with fewer optical components, and elements with low ?-numbers. Two-dimensional contact profilometry has long been the standard for measuring the surface form of aspheric optics, but as the demand for higher precision aspheres continues, so does the need for better asphere metrology that provides full 3D surface data.

With increasingly tight requirements for asphere surface accuracy, it is important to consider what method of asphere metrology is best-suited to the application at hand.

Reference: photonics.com/Articles