Download Deflectometry: 3D-Metrology from Nanometer to Meter

Deflectometry: 3D-Metrology from Nanometer to Meter
G. Häusler1,2, M. C. Knauer1, C. Faber1, C. Richter1, S. Peterhänsel1, C. Kranitzky1, K. Veit2
1
Institute of Optics, Information and Photonics, University of Erlangen-Nuremberg, Staudtstr. 7/B2, 91058 Erlangen, Germany
2
3D-Shape GmbH, Henkestr. 91, 91052 Erlangen, Germany
Author e-mail address: [email protected]
Suggested classification: (2) Application Enhanced Technologies in Optical Metrology
We will discuss deflectometry as an imaging principle with a wide spectrum of new applications, and we will
demonstrate some novel deflectometric sensors. The intrinsic features of deflectometry—incoherence, source
encoding, high dynamical range, simplicity, and scalability—enable new sensors and unexpected applications. The
local slope of specularly reflecting surfaces can be measured for objects from µm-size to meter-size, in reflection as
in transmission.
The deflectometric principle has been used inherently for a long time to detect small shape irregularities at smooth
surfaces by observing a glass surface which at the same time displays a mirrored image of a fluorescent tube. The
irregularities of the surface cause a deformation of the mirrored image of the lamp.
To acquire the local slope (gradient) of specular surfaces quantitavely, the lamp is replaced by a big diffusing screen
with a well-defined grating. Normally, we focus onto the surface and acquire a blurred image of the mirrored
pattern. The optimal pattern is a sinusoidal grating because there is no phase perturbation caused by the defocusing.
Moreover, the sinusoidal pattern enables the detection of small deformations by phase shifting techniques – hence
the name “Phase-measuring deflectometry”.
Because of the low spatial coherence and the information theoretic advantage that deflectometry measures the local
slope instead of the height, we can detect small height variations δh of less than 1 nm within the lateral resolution
cell of the system! It is possible to show that the minimum detectable height variation δh does not depend on the
lateral resolution. Thus, we can adapt and scale deflectometry for a wide spectrum of applications.
The standard setup is suited for the measurement of the surfaces of eyeglass lenses, mirrors or reflectors. Besides the
measurement of surfaces in reflection, it is also possible to measure in transmission. One application is the
measurement of the local refractive power of car glass windows. Here, the local refraction can be measured with an
error better than 1 mD.
In macroscopic systems the free working distance is big; hence the depth of field allows a simple design of the
system. For microscopic applications we use a beam splitter to use the same micro-objective for illumination and
observation. By that means, the diffusing screen is replaced by an aerial image of the sinusoidal pattern. The
resulting images display intriguing features: the slope encoding of the intensity image makes the images quite
attractive. And there are further options: we can expand the depth of field, in order to achieve all the features that
make SEM images so intriguing, with deflectometry as well: high lateral resolution, slope encoding and big depth of
field. Transmission microdeflectometry acquires local variations of the optical path length within the specimen. The
generated image is similar to differential interference contrast or phase contrast. However, microdeflectometry is
quantitative and sensitive to some object details not visible with PC.
To sum up: The physical and information theoretical properties of deflectometry are by far not yet exploited.