In traditional confocal microscopy, there is a trade-off between spatial resolution and field of view due to the limitations of objectives. To solve this problem, diffractive optical elements (DOEs) with overlapping apertures are used to generate high-NA illumination spots in a large area. However, currently such DOEs can only be used as illuminators which are not suitable for 3D surface measurements. In this work, the idea of superposition is utilized to expand the scope of application of the DOEs. These DOEs are designed by simulation and tested in the experiments. The results show that the proposed DOEs can be used in 3D surface measurements and have the potential to solve the problem of high-NA objectives.
Chromatic confocal spectral interferometry combines the benefits of scanning free acquisition of the axial dimension with interferometrically increased depth accuracy. However, so far it has been difficult to separate the confocal signal from the interferometric signal. It is, of course, possible to apply the established CCM evaluation methods. In that case, the available phase information, that offers a decreased measurement uncertainty and to some degree the removal of disturbing artifacts at steep surface inclinations, is not taken into account. In fact, it is not straight forward to interpret the signal. In comparison to white light interference microscopy, the signal suffers from a chirp. This means that it cannot be associated with a single beating frequency, which corresponds to the interferometrically encoded z-value. However, a modified lock-in technique has in the past successfully been applied, demonstrating a significant advantage in comparison to the conventional CCM procedures. Here, we will introduce the concept of k-space phase equality, which enables the separation of the confocal and the interferometric signal and furthermore offers an extended measurement range. The principle is based on signal modification in the z-space, which corresponds to the Fourier domain of the recorded spectral signal. The evaluation is then performed in the spectral domain, where the phase signals for all z-positions with respect to the corresponding wavelength are evaluated. As a result, a phase signal with reduced aberration terms, similar to an interferometric signal, is obtained, which can hence be evaluated using established techniques.
A model describing the signal generation in chromatic confocal imaging is presented here. It can be used to understand the signal development process accounting for wave-optical phenomena using scalar wave theory. The influence of the optics in terms of aberrations, the specimen in terms of roughness and further parameters on the signal generation process will be investigated. Moreover, the possibility to adapt the model to investigate other spectral imaging systems, such as chromatic confocal spectral interferometry will also be shown.
Traditional spectral unmixing involves intense signal processing applied on multispectral or hyperspectral data captured from an imaging device, which is highly time-consuming. In this article, a novel method, namely "optical unmixing", is proposed to alleviate the post processing effort by replacing the heavy computation with a spectrally tunable light source. By choosing spectral features of the light source intelligently, the abundance map of each material can be retrieved with minimum computation from gray value images captured by a normal camera. For n unknown endmembers, 3n + 1 measurements are required to retrieve the abundance maps with proposed algorithms.
A technical realization of a multispectral camera is proposed, by multiplexing a light source with six different spectra. A monochrome line scan camera with six pixel rows is used as detector. The special feature of this acquisition approach is its high speed capability. The scan speed is as high as the frame rate of the line scan camera and not affected by the multiplexing. As application a chromatic confocal microscope was build up. From a data acquisition perspective up to 284 million 3D points per second can be measured. A real time signal processing is proposed, too.
The probably best known chromatic sensor is the chromatic confocal point sensor, which is an optical
displacement sensor (depicted in Fig. 1). It uses different wavelengths to encode the distance and has one
measurement spot. Beside this prominent example, there are plenty of other realizations. E.g. Lee lists fiber
optical sensors which measure temperature, displacement, current, strain and more. A variant of the chromatic
confocal point sensor is used within this paper as example to apply the proposed method, referred to as CCT
(chromatic confocal triangulation) sensor. In contrast to the point sensor the CCT sensor has many measurement
spots next to each other (typically 2000 measurement spots in a row).