Hyperspectral imaging is gaining importance in many areas of research, industry and medicine. It makes it possible to visualize information almost in real time. In order to realize a measuring station for inverse hyperspectral imaging, a fiber-coupled light source was developed to increase the spectral power density with 40 LEDs with 17 wavelengths in the range from 388 nm to 805 nm. An automated measuring station was developed in which both illumination types, bright field and dark field, as well as the imaging unit were integrated. The automated control of the components of the measuring station makes it possible to record the spectral information of the sample within 15 seconds. A liquid lens is used for this purpose. It enables correction of the focus difference of all wavelengths for a resolution of up to 3.5 μm. Furthermore, the use of a highly sensitive industrial camera without color filters maximizes the spatial resolution. To evaluate the system, a standardized sample and prepared skin, muscle, tendon and bone tissue were examined. With the realized measuring station for inverse hyperspectral imaging and the numerical processing of the resulting image data, cellular structures and features of the biological tissue can be made visible and thus differentiated.
In today's world, more and more emphasis is placed on non-invasive, label-free diagnostic types in order to avoid the destruction of tissue structures. One example is Flow cytometry, which allows the differentiation of single cells. In order to realize a spectrally and angularly resolved scattered light measurement setup, which allows both the differentiation of cell clusters and provides information about the cell state, a special multispectral light source in the visible/near infrared wavelength range was developed. For this purpose, single-mode fiber-coupled laser diodes of defined wavelengths are coupled into a polarization-maintaining fiber using a developed wavelength-selective coupler and an optical switch. The desired polarization is set by a polarization-maintaining fiber using paddles. A developed electronical circuit with integrated temperature control enables the selection of the wavelengths as well as the control of the laser diodes. In addition to that, the light source achieves the required modulated operation in the nanosecond range to generate short pulses of 600 ns with a peak pulse power of about 3 mW for time-resolved data acquisition. The fiber-based system can be flexibly integrated into a scattered light measurement setup, and principal component analysis was used to differentiate between the tissues of pig heart, pig liver, pig stomach, and sheep tendon based on the scattered light.
For interferometric measurement methods such as optical white light interferometry, the coherence length of the implemented broadband light source is of central importance in order to realize a high axial resolution as well as a large dynamic range of the measurement system. In the case the broadband light sources are spectrally resolved by a spectral dispersive element, the spatial coherence and emission duration define primarily the coherence of the light source when no stimulated emission occurs. Broadband light sources with a high brilliance, such as supercontinuum and laser-driven plasma light sources, are particularly interesting for interferometric measurement methods because of their high spectral width and beam quality with enormous spectral power density. Especially for interferometric measurements, the coherence of the light source is essential. In the scope of the investigations, the coherence length and spectral power density of highpower supercontinuum light source based on a Yb-doped photonic crystal fiber in a nonlinear fiber amplifier setup and a commercially available laser-driven plasma light source were comparatively investigated with conventional white light sources. It was shown that the investigated high-power supercontinuum light source can be used very well for interferometric investigations. Furthermore, the achieved spectral pulse peak power densities exceed the laser-driven plasma light sources by a factor of one million.
The scattering properties of biological tissue are highly dependent on the structure size, refractive index, and wavelength of the incident light. Furthermore, these scattering characteristics are strongly influenced by movements of the scattering objects. A method is developed to determine the angular- and spectral-resolved scattering properties that enabled the characterization of biological nano- and microscaled cell structures. Nanosecond pulses from a spectrally filtered supercontinuum light source are captured and time-resolved to depress background noise and minimize disruptive effects of the biological cells. The scattering characteristics of a monolayer of mouse fibroblast L929 cells are measured at defined wavelengths in a standard cell culture plate. Because of the size and distribution of the scattering structures, a Fourier transform-based Mie scattering scheme is used to analyze the data. The system is tested to detect structural changes of mouse fibroblast L929 cells before and after poisoning with Triton X100. The final result is the development of a contamination-free method to study pathological changes in cell cultures, necrosis, or other cell-damaging effects.
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