Some widely used optical measurement systems require a scan in wavelength or in one spatial dimension to measure the topography in all three dimensions. Novel hyperspectral sensors based on an extended Bayer pattern have a high potential to solve this issue as they can measure three dimensions in a single shot. This paper presents a detailed examination of a hyperspectral sensor including a description of the measurement setup. The evaluated sensor (Ximea MQ022HG-IM-SM5X5-NIR) offers 25 channels based on Fabry–Pérot filters. The setup illuminates the sensor with discrete wavelengths under a specified angle of incidence. This allows characterization of the spatial and angular response of every channel of each macropixel of the tested sensor on the illumination. The results of the characterization form the basis for a spectral reconstruction of the signal, which is essential to obtain an accurate spectral image. It turned out that irregularities of the signal response for the individual filters are present across the whole sensor.
Hyperspectral imaging opens a wide field of applications. It is a well established technique in agriculture, medicine, mineralogy and many other fields. Most commercial hyperspectral sensors are able to record spectral information along one spatial dimension in a single acquisition. For the second spatial dimension a scan is required. Beside those systems there is a novel technique allowing to sense a two dimensional scene and its spectral information within one shot. This increases the speed of hyperspectral imaging, which is interesting for metrology tasks under rough environmental conditions. In this article we present a detailed characterization of such a snapshot sensor for later use in a snapshot full field chromatic confocal system. The sensor (Ximea MQ022HG-IM-SM5X5-NIR) is based on the so called snapshot mosaic technique, which offers 25 bands mapped to one so called macro pixel. The different bands are realized by a spatially repeating pattern of Fabry-P´erot filters. Those filters are monolithically fabricated on the camera chip.
In manufacturing monitoring and inspection is an essential task to maintain a high product quality. Therefore a variety of
systems (e.g. tactile systems, acoustic systems, optical systems,...) is used. However there is still a lack in controlling the
product quality near the production machine. For the selection and the design of an appropriate monitoring strategy the
specification of the applied sensors is of crucial importance. Optical sensors are in general suitable to measure quality
relevant features. But they are often not robust enough, to use them in harsh environments such as the workshop floor.
However to detect as early as possible if quality runs out off specification, the high resolution of optical measurement
systems is often not needed. In these cases optical sensors can be implemented successfully even if their measurement
uncertainty is increasing due to the harsh environment. To verify this hypothesis an evaluation of environmental
influences has to be made and a comparison between the acceptable and still achieveable measurement uncertainty has to
be made.
For this reason a conceptual consideration regarding optical sensors developed for in-process monitoring is presented.
The focus will be on the investigation of the influence of the environment on the measurement result, and on strategies
how these can be estimated. Based on this an appropriate design and construction of the sensor system can be obtained.
Simulation of grating spectrometers constitutes the problem of propagating a spectrally broad light field through a
macroscopic optical system that contains a nanostructured grating surface. The interest of the simulation is to quantify
and optimize the stray light behaviour, which is the limiting factor in modern high end spectrometers. In order to
accomplish this we present a simulation scheme that combines a RCWA (rigorous coupled wave analysis) simulation of
the grating surface with a selfmade GPU (graphics processor unit) accelerated nonsequential raytracer. Using this, we are
able to represent the broad spectrum of the light field as a superposition of many monochromatic raysets and handle the
huge raynumber in reasonable time.
Optical metrology has shown to be a versatile tool for the solution of many inspection problems. The main advantages of
optical methods are the noncontact nature, the non-destructive and fieldwise working principle, the fast response, high
sensitivity, resolution and accuracy. Consequently, optical principles are increasingly being considered in all steps of the
evolution of modern products. However, the step out of the laboratory into the harsh environment of the factory floor
was and is a big challenge for optical metrology. The advantages mentioned above must be paid often with strict
requirements concerning the measurement conditions and the object under test. For instance, the request for
interferometric precision in general needs an environment where high stability is guaranteed. If this cannot be satisfied to
a great extent special measures have to be taken or compromises have to be accepted. But the rapid technological
development of the components that are used for creating modern optical measurement systems, the unrestrained growth
of the computing power and the implementation of new measurement and inspection strategies give cause for optimism
and show that the high potential of optical metrology is far from being fully utilized. In this article current challenges to
optical metrology are discussed and new technical improvements that help to overcome existing restrictions are treated.
On example of selected applications the progress in bringing optical metrology to the real world is shown.
To increase the quality of future products and decrease the manufacturing cost at the same time a systematic control of
the fabricated objects is necessary. A promising approach for inline quality control of surface and form parameters is the
use of optical measurement systems. This is due to the non-destructive nature of the optical measurement techniques. But
in the production environment there are many challenges to overcome for optical sensors. Examples are temperature
fluctuation, vibrations, fluids on the object surface and rough surfaces. Therefore, it is likely that not all optical
measurement methods are suitable for that task. Hence, a classification of the different principles is necessary with the
objective to identify the most appropriate measurement approach for a particular inspection task. In this contribution we
start with a systematic approach for a review of sensors within production systems. Then we concentrate on the most
robust class of optical sensors, the point sensors. In order to minimize the effect of mechanical vibrations it is desirable
to employ measurement techniques that are able to measure the height of an object point in a very short time. Therefore,
we focus in this work on chromatic-confocal microscopy and spectral interferometry. The aim is to compare these
measurement methods for their ability to cope with the challenges given by the production environment in general. To
this end we will develop simulation models for the mentioned techniques and compare two exemplarily sensors for their
capability to be used for process control.
Chromatic confocal spectral interferometry (CCSI) is a hybrid method for fast topography measurement, which
combines the advantages of the interferometric gain and accuracy with the robustness of confocal microscopy. The
CCSI-principle provides a single shot measurement of depth while offering a higher lateral resolution than commonly
used spectral interferometers. This contribution is focused on the modeling and simulation of a CCSI-sensor for
measuring rough surfaces, based on sequential and non-sequential ray-tracing. With the simulation, the influence of
surface roughness, surface reflectivity, and surface contamination on reliability of the sensor can be estimated.
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