Protective glasses are used to protect camera modules and optical sensors from mechanical stress or environmental influences. To ensure consistent quality and avoid image distortion caused by these optics, the transmitted wavefront needs to be controlled and defects like inclusions need to be detected. This way, identification of defective parts and monitoring of the manufacturing process becomes possible. In this paper we discuss transmitted light test setups for wavefront testing of optical windows and the accuracy and stability that can be achieved. Additionally, we show procedures for wavefront-guided alignment of optical systems. The second part of the paper deals with the testing of fast steering mirrors, which are often used in free-space communication or fast scanning applications. The reflected wavefront carries information about the quality of the 3D surface shape. We demonstrate how to assess the mirror quality using a reflected light test setup.
Traceable, highly accurate calibration of wavefront sensors such as Shack–Hartmann sensors is a challenging task and an active field of research. We have developed a measurement system for the traceable calibration of wavefront sensors employing spherical wavefronts and a point light source in combination with a three-axis linear stage. We obtain an absolute calibration of sensor errors, including the reference spot position and wavefront gradient-dependent errors for each microlens individually. We discuss the main error influences and present an initial measurement uncertainty budget for the calibration. The calibration can be performed with an expanded measurement uncertainty (95% coverage interval) of better than 5 μrad for the wavefront gradient deviation.
Testing micro optics, i.e. lenses with dimensions down to 0.1mm and less, with high precision requires a dedicated design of the testing device, taking into account propagation and wave-optical effects. In this paper, we discuss testing methods based on Shack-Hartmann wavefront technology for functional testing in transmission and for the measurement of surface shape in reflection. As a first example of more conventional optics testing, i.e. optics in the millimeter range, we present the measurement of binoculars in transmission, and discuss the measured wave aberrations and imaging quality. By repeating the measurement at different wavelengths, information on chromatic effects is retrieved. A task that is often tackled using Shack-Hartman wavefront sensors is the alignment of collimation optics in front of a light source. In case of a micro-optical collimation unit with a 1/e² beam diameter of ca. 1mm, we need adapted relay optics for suitable beam expansion and well-defined imaging conditions. In this example, we will discuss the alignment process and effects of the relay optics magnification, as well as typical performance data. Oftentimes, micro optics are fabricated not as single pieces, but as mass optics, e.g. by lithographic processes. Thus, in order to reduce tooling and alignment time, an automated test procedure is necessary. We present an approach for the automated testing of wafer- or tray-based micro optics, and discuss transmission and reflection measurement capabilities. Exemplary performance data is shown for a sample type with 30 microns in diameter, where typical repeatabilities of a few nanometers (rms) are reached.
An increasing part of the optic industry’s added value consists of micro optical components. This increases the demand for effective test methods for micro optics. When conventional test methods are transferred to micro optics, special attention should to be paid to diffraction effects, wave front propagation and precise sample imaging. We show how a well-established tool – the Shack-Hartmann wave front sensor (SHWFS) – which uses a micro lens array as a key element, can be used for testing micro lenses. Different measurement configurations for transmitted light and reflected light testing are discussed. A system which takes advantage of a combination of these test configurations to retrieve information on surface quality, wave front performance, focal length, and defects is presented. Measurement results are shown to demonstrate the system performance.
A commercial Shack-Hartman wavefront sensor is being used in a test setup installed at the Wendelstein 40 cm
telescope to test methods for telescope alignment based on reverse optimization. Measured low-order Zernike
wave-fronts are being used to determine the misalignment of the telescopes optical elements. Then a procedure
to optimize wave-front performance by aligning the telescope secondary mirror is applied.
The setup contains a collimating optical system, the Shack-Hartman sensor and a guiding and acquisition
camera.
ESA commissioned a Tiger Team to review the discrepancy between the prediction and measurement of the telescope
back focal length. A team of 16 engineers and scientists collocated at ESA's Estec facility to review the test results in
the context of the mission requirements and predictions for behavior of the telescope. Extensive analysis was performed
on the random and systematic errors in the test results. Both room temperature and cryogenic test data was scrutinized.
Error budgets, test results, and conclusions from the Tiger Team will be discussed.
A technique for testing aspherical surfaces without the use of a null-corrector is described. The wave-front sensor for the detection of the aspherical wave-front is a Shack- Hartmann sensor which has a significantly expanded range of measurable wave-front slopes. The optical testing set-up, the used algorithm for the dynamic range expansion, and experimental results are presented in this paper.
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