The accurate knowledge about the refractive index of optical materials is crucial for the production of high
performance optical components. It is known that the highest accuracy of refractive index measurements can
be achieved with goniometric measurements of prisms prepared from the optical material. The most common
approach is the method of minimum deviation of Newton-Fraunhofer. The apex angle is measured with a high
precision in reflection with an autocollimator and the angle of refraction is measured in transmission using an
additional collimator. There are also other goniometric approaches like the Abb´e method employing a purely
reflective setup with an autocollimator. In this paper we discuss and compare the two different goniometric
In the assembly of multi-component optical systems the precise positioning of every single lens element respectively lens surface is key to reach an optical performance that corresponds to the optical design. Here, in addition to lateral decentering of single elements – such as shift and tilt – the accurate positioning of the optical surfaces along the optical axis is an essential requirement. In this contribution we present the highly accurate determination of lens center thicknesses and air gaps of optical assemblies in a non-contact manner. The measurement technique is based on time-domain low coherent interferometry (also known as A-scan optical coherence tomography). Here, a low coherent interferometer signal is recorded in a Michelson-type setup as a function of a variable optical delay in the reference arm. Whenever the variable optical path length matches the path length to a lens surface, a coherence peak occurs. Thus, relative surface distances can be derived from the optical delay between two peaks. For a highly-accurate measurement a precise determination of the optical delay length with minimized Abbe-errors is required. Here, a precise long-coherent reference interferometer is superimposed to the short-coherent signal in the optical delay line. Both signals are recorded simultaneously; subsequently, the data is transferred to a PC system for the analysis. With the presented technique lens thicknesses and air spacings of up to 800 mm can be measured with a resulting accuracy significantly below 0.5 μm. The obtained results can be used for the compensation of potential deviations/errors in the manufacturing process or for quality control using a pass/fail-evaluation.
For any kind of optical compound systems the precise geometric alignment of every single element according to the
optical design is essential to obtain the desired imaging properties. In this contribution we present a measurement system
for the determination of the complete set of geometric alignment parameters in assembled systems. The deviation of
each center or curvature with respect to a reference axis is measured with an autocollimator system. These data are
further processed in order to provide the shift and tilt of an individual lens or group of lenses with respect to a defined
reference axis. Previously it was shown that such an instrument can measure the centering errors of up to 40 surfaces
within a system under test with accuracies in the range of an arc second. In addition, the relative distances of the
optical surfaces (center thicknesses of lens elements, air gaps in between) are optically determined in the same
measurement system by means of low coherent interferometry. Subsequently, the acquired results can be applied for the
compensation of the detected geometric alignment errors before the assembly is finally bonded (e.g., glued).
The presented applications mainly include measurements of miniaturized lens systems like mobile phone optics.
However, any type of objective lens from endoscope imaging systems up to very complex objective lenses used in
microlithography can be analyzed with the presented measurement system.
In dimensional nano- and micrometrology, single sensors are often combined into an array of sensors to enable faster
measurements by utilizing parallel data acquisition. If combined with appropriate scanning techniques, the use of sensor
arrays additionally facilitates the estimation and correction of systematic sensor errors and, thus, enables more accurate
measurements. To exploit these options, the arrays have to be aligned carefully with respect to the scanning direction,
and, in addition, the lateral distances between the sensors have to be determined with sufficient accuracy.
This presentation describes a method to align an optical distance sensor array parallel to the direction of a linear translation
stage, which is used to scan the specimen under test, and it describes a method to evaluate and determine the sensor
distances with high accuracy.
Alignment is a multi step procedure: The first step is to orientate a step edge profile perpendicular to the scanning direction
of the sensor using an M-array and an auxiliary CCD camera. In a second step, the line sensor array is scanned
across the edge using different rotation angles of the sensor. The positions where the different sensors cross the edge are
evaluated to obtain the sensor orientation relative to the scanning direction, the distances between the sensors, and their
We will show experimental data obtained with an optical line sensor array of three single sensors. The measurements
will be compared to simulated data carried out with a virtual experiment programmed at PTB. Relevant error sources are
assessed and the limitation of the method is discussed.
With the recently emerged large volume production of miniature aspheric lenses for a wide range of applications, a new
fast fully automatic high resolution wavefront measurement instrument has been developed.
The Shack-Hartmann based system with reproducibility better than 0.05 waves is able to measure highly aspheric optics
and allows for real time comparison with design data.
Integrated advanced analysis tools such as calculation of Zernike coefficients, 2D-Modulation Transfer Function (MTF), Point Spread Function (PSF), Strehl-Ratio and the measurement of effective focal length (EFL) as well as flange focal length (FFL) allow for the direct verification of lens properties and can be used in a development as well as in a production environment.
Aspheric lenses are of increasing importance in compact imaging systems. New developments in production
technologies have led to the so called wafer level production with several thousands of lenses on a single wafer.
This high volume production demands fast testing equipment which allows for the characterization of complete imaging
systems as well as of all of its single components. In most of the cases conventional methods cannot be used to measure
single lenses or objectives in earlier production states. Although e.g. the measurement of the modulation transfer
function is a well established method for fast and accurate quality inspection of entire objectives it has its limitation for
Due to its very large dynamic range the Shack-Hartmann sensor is able to measure a very broad range of spherical and
aspherical lenses as well as partially or fully assembled objectives. With the combination of a fast high accuracy
wavefront sensor and special positioning algorithms which allow for high throughput in mass production a new flexible
instrument has been developed.
Aspheric lenses are of increasing importance in the production of compact imaging systems. High volume productions
of such imaging systems demand fast test systems to check the quality of the lenses. The measurement of the
modulation transfer function has its limitation for aspheric lenses that are used to correct a lens system for good image
quality, but does not have good imaging capabilities as a single lens. Measuring the wavefront of aspheric lenses with a
Shack-Hartmann sensor gives a flexible tool to determine the properties of the lenses. We present measurement
principle, capabilities and different configurations of the lens testing system WaveMaster® of Trioptics GmbH.
Increasing demands for the monitoring of tolerances of small mechanical and optical precision components require improved measurement techniques. In this paper the basic concept and different optical designs of a confocal microoptical distance-sensor are presented. The sensors use the chromatic-confocal measurement principle which does not require a mechanical depth scan. Therefore, a chromatic-confocal point sensor can be designed without any moving parts. This fact is used to design a miniaturized sensor head with an outer diameter smaller than two millimetres. A special feature of the sensor head is its capability to measure sideways. This enables e.g. to measure surfaces in small drilling holes.
In this paper, the realization and characterization of a microoptical sensor using the chromatic confocal principle is presented. The sensor head is designed for distance gauging applications in high aspect ratio cavities with a diameter of about 2 mm. The first part of this paper focuses on the design and fabrication process of the hybrid optical benches, which combines refractive and diffractive micro optical components. Very tight tolerances of the optical path are required for the functionality of the sensor. Therefore the alignment structures and mounts between the different optical elements are produced from PMMA using deep X-ray lithography, the first step of the LIGA process.
In the second part of this paper the characterization of first prototypes using different light sources are described and results presented.
Depth-scanning is an established technique in macroscopic and microscopic 3-D metrology. Representative in this context are the confocal technique and the white-light interferometry. A new fast depth-scanning technique has been applied to a confocal point sensor to be used in a laser-welding application for in-process measurement. The depth measurement range can be extended to about +/-1 mm at about 1500 measurement cycles per second. The possibilities and the potential of these techniques are described. Another principle of depth-scanning is the chromatic confocal technique. In connection with a new approach, an innovative confocal setup enables the parallelization of the complete depth-scan for the complete measurement of a line cut of moved objects. In the macroscopic scale, the new measurement techniques of depth-scanning fringe projection (DSFP) was introduced recently. In the microscopic scale, it has been implemented successfully in a stereo microscope.
Increasing demands for controlling tolerances of small mechanical and optical components require improved measurement techniques. In particular, components with a complex geometry such as small holes or channels are difficult to access by classical tactile measurement systems. These systems are also limited in their measurement speed. Optical distance sensors do not have many of these disadvantages, but the sensor heads are normally too large to access e.g. small holes. Presented in this paper is a novel microoptical sensor concept using the chromatic confocal principle for distance gauging applications. This is used in high aspect ratio cavities with a diameter of about 2 mm. The distance resolution of the sensor is aimed to be in the sub-micrometer range.
The chromatic confocal approach enables the parallelization of the complete depth-scan of confocal topography measurements. Therefore, mechanical movement can be reduced or completely avoided and the measurement times shortened. Chromatic confocal point sensors are already commercially available but they need lateral scanning in x- and y-direction in order to measure surface topographies. We achieved a further parallelization in the x-direction by realizing a chromatic confocal line sensor using a line focus and a spectrometer. In a second setup, we realized an area measuring chromatic confocal microscope, which is capable of one-shot measurements without any mechanical scanning. The depth resolution of this setup can be improved by measuring in a small number of different heights. Additional information about the color distribution of the object is gained.