Laser speckle patterns were used for characterization of skin topography (DermaSpec, 13N13841). A set of parameters were obtained for skin surface characterization. Preliminary data has already shown differences between skin tumor and healthy skin areas.
Scientific experiments on mineral and biological samples with Raman excitation below 300nm show a wealth of scientific information. The fluorescence, which typically decreases signal quality in the visual or near infrared wavelength regime can be avoided with deep ultraviolet excitation. This wavelength regime is therefore regarded as highly attractive for a compact high performance Raman spectrometer for in-situ planetary research. Main objective of the MIRAS II breadboard activity presented here (MIRAS: Mineral Investigation with Raman Spectroscopy) is to evaluate, design and build a compact fiber coupled deep-UV Raman system breadboard. Additionally, the Raman system is combined with an innovative scanning microscope system to allow effective auto-focusing and autonomous orientation on the sample surface for high precise positioning or high resolution Raman mapping.
The design limits of grating array spectral sensors are discussed. The limit of a grating spectrometer with respect
to the resolution is given by the diffraction limit of the grating. To approach the limit for the visible spectral region the entrance slits should reach a width of 2 μm and larger depending on wavelength and numerical aperture. The detector pixel sizes should be in the same range, which is achieved virtually by the discussed double array arrangement with a transmissive, static slit array and detector array. A number of techniques are applied for optimizing the performance as well as for miniaturization. A sub-pixel imaging including a sub-pixel analysis based on the double array arrangement virtually reduces the detector pixel sizes down to about 20%. To avoid the imaging aberrations the spectra is imaged from different entrance positions by the entrance slit array. The throughput can be increased by using a two dimensional entrance slit array, which includes a multiplex pattern or a fixed adaptive pattern. The design example of a UV-Raman spectral sensor is presented including spectral measurements.
A lightweight and compact spectrometer with spectral resolution in the order of 0.1 nm, in a spectral range from UV to
NIR, can be implemented using a spectral pre-selection technique at the spectrometer entrance, through transmission
variable filters. Such filters, based on thin-film optical coatings, have either a transmission peak or a transmission edge
that moves along one direction of the filter surface. Depending on the spectrometer design, different configurations of
the pre-selection device are possible. The operating spectral range (240-800 nm) is divided in a number of sub-ranges
and in each of them the filter transmission peak, or edge, is displaced from the minimum to the maximum wavelength
over a distance of few mm. Two cases are considered: a configuration with both a narrow-band transmission filter and a
band-pass filter having a linear spatial variation and a configuration with an edge filter having a non-linear spatial
variation. To obtain the required spatial profile of filter performance, a graded coating is deposited on a fused silica
substrate, by r.f. sputtering with a moving mask. Details on filter requirements and fabrication technology are reported.
Inline holographic microscopy is known as a technique for absolute phase retrieval. Any distortion of the needed
reference wave, e.g. from the surfaces of a test slide, causes reconstruction errors. We apply a multi plane detection
technique, which generates the same information as a hologram but does not use any reference wave. The technique was
demonstrated for microscopic resolution in case of amplitude objects [1], [2]. This paper focuses on imaging and
reconstruction of phase objects. The technique is experimentally applied to a Pleurosigma test chart and a phase test
standard. Objects with a phase step of π/3 and lateral structure sizes of 2 μm are well resolved.
A hyperspectral Hadamard transform imager is presented. It is
based on a flat field spectrometer and a spatial light modulator.
Compared to a typical dispersive pushbroom imaging spectrometer,
the signal-to-noise-ratio can be increased by 1 ... 2 orders of
magnitude for typical dimensions of the hyperspectral data cube.
Applicable spatial light modulators include individually
addressable MEMS based light modulators such as micro mirrors or
newly developed micro shutters as well as mechanical slit
positioning systems which are characterized by a fixed, movable
pattern. In contrast to the individually addressable devices, the
number of operational modes of the slit positioning system is
constrained by the mask design, but high pixel numbers and high
optical quality can be achieved by a mature and relatively simple
micro machining technology. A detailed analysis of the relative performance of hyperspectral Hadamard transform, Fourier transform and hypothetical optimal imaging spectrometers with particular respect to the dimensions of the hyperspectral data cube and the composition of the detector noise reveals a considerable signal-to-noise advantage of the Hadamard transform imaging spectrometer over the Fourier transform imaging spectrometer for domination of read noise and a large number of spectral bands. In case the noise is dominated by dark current noise, the Hadamard transform imaging spectrometer can achieve 50% of the maximum possible sensitivity which is given by the hypothetical optimal imaging spectrometer. In contrast to the Fourier transform imaging spectrometer, the Hadamard transform imaging spectrometer is also suitable for use with short
wavelengths.
We discuss grating array spectral sensors as the most promising basic architecture for future high-end micro-spectrometers. New extensions of this architecture with 2D-detector arrays are presented. They increase the spectral resolution by sub-pixel imaging. Sub-pixel architectures allow miniaturization of the spectrometers and shift the limits significantly towards the diffraction limit to the grating. To fully employ this, the slit dimensions have to be in the order of a few wavelengths, for instance down to 2 ?m in the UV region, requiring micro-machined entrance apertures. Feasible entrance apertures are transmissive MEMS, such as static slit patterns, micro shutters or mechanical slit positioning systems.
High resolution images can be extracted from a set of differently sampled low resolution images. Usually such a set of images is generated by shifting a detector array device fractions of a pixel or by moving a whole optical system in an appropriate way. Subpixel information is then encoded in the set of taken images. Another way to generate encoded subpixel information is presented in this paper. Multiple fixed images are generated in the optical part of the detector device. The latter method is the method of choice for grating diode array spectrometers. A programmable entrance slit array (MEMS device, mechanical slit positioning system), which replaces the conventional single entrance slit, generates multiple undersampled images of the same spectrum. Every slit is imaged with a different, wavelength dependent imaging scale ratio and a different imaging scale ratio and a different wavelength-aberration-dependency. The subpixel analysis has to take this into account. It is accomplished by a shift-variant superresolution algorithm, a representation of the spectrometer's optical properties and a calibration algorithm which estimates these properties from measured known gas emission spectra. The superresolution algorithm itself is nonlinear and therefore capable of recovering data lost by aberration and pixel integration. An algorithm for subpixel analysis is developed and tested. Theoretical and experimental approaches of the subpixel analysis are presented. The method is proven experimentally on a double array spectrometer. The resolution can be increased up to the factor 7 with seven entrance slits.
Multi object spectrometers measure spectra of multiple objects simultaneously. Besides others, e.g. fiber positioning systems, there is a class of multi object spectrometers which is based on a dispersing imaging optics in connection with a slit masks. Two considered approaches for reconfigurable slit masks are two-dimensional MEMS arrays, such as micro mirror or micro shutter arrays, and slit positioning devices. After an introduction to multi object spectrometry with dispersing imaging optics we calculate the effective multiplex capabilities of multi object spectrometers based on 2D MEMS and on slit positioning devices for randomly distributed objects. The observation efficiency of multi object spectrometers based on 2D MEMS is compared to integral field spectrometers and to multi object spectrometers based on slit positioning devices. We find that for typical applications the efficiency of the slit positioning approach is nearly as good as the efficiency of the 2D MEMS approach. This makes slit positioning systems a serious alternative solution to 2D MEMS devices as long as they are easier to get.
KEYWORDS: Sensors, Signal to noise ratio, Multiplexing, Optical sensors, Microelectromechanical systems, Signal detection, Detector arrays, Switching, Time metrology, Optical switching
Concepts to increase the performance of optical sensors by combination with optical MEMS are discussed. Architectures of subsystems are reviewed, which modulate or switch the amplitude of the light by scanning, multiplexing and selecting interesting signal components (multi-object-mode). Arrangements with MEMS for optical sensors and instruments can decrease the pixel size and increase their number by creating virtual pixels. A number of signal components can be detected with a smaller number of detectors (detector pixels) by scanning. If the scanning is substituted by multiplexing the best efficiency is achieved. The measurement time can be reduced by selecting interesting objects or signal components to be detected. Architectures which combine single sensors, linear sensor arrays or two dimensional detector arrays with MEMS, slit masks, and micro-mechanical devices are discussed. Such devices are micro-mirrors, micro-shutters, the slit positioning system, the fibre positioning system, and other optical switches.
Multi-object spectrometry is a special version of imaging spectrometry, where only a relatively small number of objects or interesting image points of a scene is spectrally resolved. This allows to measure nearly all interesting points of a typical astronomical scene within one or only a few measurement steps. The essential component of a multi-object spectrometer is the field selector device, which selects multiple image points for a simultaneous measurement. Reconfigurable field selectors or reconfigurable slit masks can be MEMS, such as micro mirror or micro shutter arrays. Alternative field selectors will be based on micro-mechanical devices and can mostly be referred to as slit positioning systems, since the elements which form the slits are mechanically positioned. Novel examples for such field selectors are an individual micro-mirror element positioner and bar arrays with slit structures. Two layer devices are presented, which form transmissive and reflective slits. The concepts are focused on the near infrared multi-object-spectrometer for the Next Generation Space Telescope.
We discuss principles of optical surface quality assessment. The micro topography of well polished fused silica, CaF2 and Si surfaces was examined locally and by covering large sample areas. Power Spectral Densities (PSD) were used for consistent roughness description. Subsurface damage was detected by a modified white light interferometer technique and total scattering measurement.
The surface microtopography and subsurface damage of fused silica have been examined after different stages of the manufacturing process. Results are presented of etching experiments and measurements performed by white light interferometry, atomic force microscopy and total light scattering.
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