In this contribution we introduce a compact version of a broadband static Fourier transform spectrometer (bs- FTS) for the mid-infrared spectral range. The bsFTS covers a spectral range from about 4.5 µm to 14 µm, respectively 2220 cm−1 to 700 cm−1 at a spectral resolution of 8 cm−1. As, in contrast to scanning Fourier trans- form spectrometers, the interferogram is modulated not over time but in the spatial domain, the measurement speed is only limited by the detector. This allows for infrared spectroscopy at 25 Hz to 200 Hz using uncooled microbolometer arrays. Besides liquid measurements in attenuated total reflection (ATR), demonstrating the accuracy and linearity of the bsFTS, we show time-resolved analyses of 1,1,1,2-Tetrafluoroethane (R134a) and carbon monoxide test gases to prove the suitability of the system for high-speed spectroscopy.
Laser triangulation is an optical method for measuring distances to an object. A laser beam is directed towards a measurement surface, and the diffusely reflected light is collected by an imaging system onto a detector. The absolute distance can be obtained by using known geometric relations of the system and the position of the laser spot on the detector. Therefore, a change in the measurement distance results in a corresponding movement of the imaged laser spot, defining the sensitivity of the system in pixels per millimeter. This value depends on the geometrical and optical design of the laser triangulation setup, especially the base distance between the laser and the imaging lens, as well as its focal length. As those parameters also influence the geometric dimensions and the possible measurement range of the device, the sensitivity cannot be increased arbitrarily. Thus, the sensitivity of a standard laser triangulation system is limited to a certain value. In this contribution, structured optical surfaces are applied onto the measurement surface to further increase the sensitivity. Through the spatial modulation of the imaged laser spot intensity distribution, the calculated laser spot displacement is larger than its actual geometrical displacement. This effect is examined through simulations with a bar structure, which leads to an improvement of the sensitivity by a factor of up to 5.7 at a distance of 1 m and a measurement range of 2 mm. Eventually, the concept is proven in measurements and feasible implementations of such a structure are considered.
In this contribution we present a broadband static Fourier transform spectrometer (bsFTS) based on a single- mirror interferometer containing only off-the-shelf optical components and an uncooled microbolometer detector
array. The system uses concave mirrors instead of lenses and therefore covers a wide spectral range from 3.6 μm to 17 μm at a spectral resolution of 12 cm-1. Furthermore, dispersion effects can be minimized and the system can thus be designed with increased temperature stability. We demonstrate the optical and mechanical design of
the current laboratory prototype and compare the instrument to a scanning Fourier transform infrared (FTIR) spectrometer. Additionally, we present a technique for simultaneously acquiring the sample spectrum and the background spectrum. Thereby, a variation of the background over time can be compensated continuously and hence the bsFTS presented in this contribution offers significant potential with regard to long-term stability.
Laser spot detection is an important step of laser triangulation and limits its accuracy. Common methods to determine the center position include circle fitting, the Hough transformation, the gray centroid method or Gaussian fitting. As these algorithms were often tested in different set-ups and under various conditions, they generally lead to diverse results. The aim of this contribution is to investigate the algorithms in a more objective and realistic approach. After a short introduction to laser triangulation, basic information about laser spot center detection is given. Fundamental limitations of the laser spot detection are then considered and analyzed. The investigations are followed by evaluations of measurements in a real laser triangulation setup. Through this approach we could show that the influence of the spatial quantization and the quantization due to the limited bit depth of the detector can be in a similar range as the deviation due to speckle noise in the image. By using adapted image processing steps, the performance of the laser spot center determination could be improved significantly.
Hyperspectral imaging is an established technique for process analysis capturing a two-dimensional spatial image and the spectral information for each pixel simultaneously. When moderate spectral resolution is sufficient, static imaging Fourier transform spectrometers (sIFTS) can offer a viable alternative to their scanning counterparts in the mid-infrared spectral range. Therefore, in this paper we present a sIFTS concept based on a single-mirror interferometer which shows no internal light losses and still works with extended light sources, achieving sufficient signal-to-noise ratios. The interferometer consists of a beam splitter, a plane mirror and a lens, which makes it both inexpensive and relatively easy to adjust. For a proof of principle we present a transmission measurement setup including a light source module, imaging optics and a single-mirror interferometer. The system achieves a spectral resolution of 12 cm−1 in a spectral range from 2700 cm−1 to 800 cm−1 , respectively from 3.7 μm to 13 μm. The spatial resolution amounts to about 10.10 lp/mm, the results for a sample containing different polymers show good agreement with a laboratory FTIR spectrometer.