Christian Faber

Deflectometry challenges interferometry: the competition gets tougher!

Deflectometric methods that are capable of providing full-field topography data for specular freeform surfaces have been around for more than a decade. They have proven successful in various fields of application, such as the measurement of progressive power eyeglasses, painted car body panels, or windshields. However, up to now deflectometry has not been considered as a viable competitor to interferometry, especially for the qualification of optical components. The reason is that, despite the unparalleled local sensitivity provided by deflectometric methods, the global height accuracy attainable with this measurement technique used to be limited to several microns over a field of 100 mm. Moreover, spurious reflections at the rear surface of transparent objects could easily mess up the measured signal completely. Due to new calibration and evaluation procedures, this situation has changed lately. We will give a comparative assessment of the strengths and – now partly revised – weaknesses of both measurement principles from the current perspective. By presenting recent developments and measurement examples from different applications, we will show that deflectometry is now heading to become a serious competitor to interferometry.

Deflectometry vs. Interferometry

Quantitative deflectometry is a new tool to measure specular surfaces. The spectrum of measurable surfaces ranges from flat to freeform surfaces with steep slopes, with a size ranging from millimeters to several meters. We illustrate this by several applications: eye glass measurements, measurements of big mirrors, and in-line measurements in ultra-precision manufacturing without unclamping of the sample. We describe important properties of deflectometry and compare its potentials and limitations with interferometry. We discuss which method is superior for which application and how the potential of deflectometry may be developing in the future.

Quantitative Deflectometry Challenges Interferometry

“Phase-Measuring Deflectometry” (PMD) has been developed at Erlangen to measure spatially resolved slope data of specular surfaces [1], [2] and [3]. The principle is simple: A camera observes the image of a sinusoidal grating via reflection at the surface under test. The local slope of the sample is encoded in the deformation of the observed pattern [4]. Meanwhile, PMD is well established to control the manufacturing of aspheric eyeglasses. PMD even has the potential to challenge interferometry. This specifically, because in contrast to interferometry, PMD does not display retrace errors. We will discuss the objectives and potentials of PMD on the road to sub-micrometer accuracy, as well as new applications including machine-integrated measurements.

Machine Integrated Measurement of Ultra Precision Machined Specular Non-Rotational Symmetrical Surfaces

In this paper, current results of a research project combining ultra precision machining and optical measurement are presented. The goal is to improve the quality of specular freeform surfaces manufactured by ultra precision slow slide servo turning by running appropriate correction cycles on the basis of machine integrated measurements. These measurements are conducted using the principle of Phase Measuring Deflectometry (PMD) in order to optically acquire full-field 3D-height data. For this purpose, a special setup the so called Mini PMD that can be operated within the limited installation space of an ultra precision machine tool has been designed and implemented. Results of machine integrated measurements of a specular non-rotational symmetrical surface are presented. Furthermore, using Mini PMD and a rotationally symmetric test surface, a complete correction cycle is demonstrated without the necessity of taking the workpiece off the machine for measurement.

SIM and Deflectometry: New Tools to Acquire Beautiful, SEM-like 3D Images

Structured-illumination microscopy and microdeflectometry acquire the shape of microscopic objects with a noise level down to 1 nanometer, a depth of field 100 times larger than the Rayleigh depth, and slope angles up to 80°.

Deflektometrie macht der Interferometrie KonkurrenzDeflectometry Rivals Interferometry

Zusammenfassung Die Deflektometrie ist eine in den letzten Jahren neu entwickelte Methode zur Vermessung glatter Oberflächen. Sie gestattet es, auch auf Freiformflächen Höhenvariationen im Nanometerbereich zu vermessen. Um die positiven Eigenschaften der Deflektometrie zu verstehen, betrachten wir die physikalischen Grenzen sowie informationstheoretische Aspekte. Abschließend wird die Vielseitigkeit der Deflektometrie anhand von Anwendungsbeispielen demonstriert.