Volker Tank

Calibration of a Fourier transform spectrometer using three blackbody sources

A procedure to calibrate a Fourier transform spectrometer is presented. Blackbody sources of three different temperatures are used to eliminate errors in the calibration that result from the limited accuracy of the temperature measurement of the calibration sources. With three spectra of blackbodies it is possible to assume that the temperatures are unknown variables as are the parameters of the functions that describe the spectrometer. A nonlinear Gaussian balancing calculation is used to determine these unknown variables and to minimize the influence of noise. A comparison between the results obtained with this method and a conventional calibration procedure is presented.

Optimization of a Michelson interferometer with a rotating retroreflector in optical design, spectral resolution, and optical throughput

A newly designed Michelson interferometer for Fourier spectroscopy [ J. Opt. Soc. Am. A8, 1991 ( 1991)] utilizes a nutating retroreflector (cube corner mirror) to generate alterations in geometrical and optical paths. The practical optomechanical design of a Fourier-transform spectrometer incorporating a rotating retroreflector for path-length alteration is considered. [The instrument has been given the name MIROR, for Michelson Interferometer with a Rotating Retroreflector.] Two parameters of the instrument are essential: the maximum optical path difference, which yields the spectral resolution of the instrument, and the diameter of the transmitted beam, which determines the throughput and hence the achievable signal-to-noise ratio. The maximum allowable beam diameter is calculated as a function of the geometry and the orientation of the rotating retroreflector and the other optical components. The geometrical configuration and the orientation of all the optical components with respect to one another are also optimized for the maximum transmitted beam diameter when the required path difference is given. A principal investigation of different possible configurations of the optical components is presented. Then a quantitative optimization for an interferometer employing a retroreflector having a 5-in. (12.7-cm) aperture diameter requiring an optical path difference of more than 10 cm (spectral resolution better than 0.1 cm−1) is performed. Finally a simplified but enhanced design is described.

INFRARED SENSOR FOR THE DETECTION AND PROTECTION OF WILDLIFE

A new application of IR sensors for the detection of game is presented. The device detects wildlife (mammals, birds) by using the difference in IR radiation of wildlife and its.

The spectral emissivity of natural surfaces measured with a Fourier transform infrared spectrometer

Abstract The spectral emissivity of materials which are commonly encountered in remote sensing, i.e. concrete, tar, two types of soil and sand, were measured by means of a Michelson type Fourier transform spectrometer (FTS). The measurements were performed in the infrared spectral region, namely within the atmospheric windows (3.4–4.2 μm, 4.5–4.6 μm and 8.8–12.5 μm), with the samples being heated to approximately 348 K. In this article the basic equations that radiometrically describe the experiments are presented. A brief section is devoted to an explanation of the principle of the operation of a Fourier transform spectrometer and its advantages for these types of measurement. A more detailed discussion of the evaluation process including instrument calibration and evaluation of the measured spectral radiances follows. The presentation of the achieved results shows that the procedure is—with restrictions—applicable to highly reflecting material (aluminium) as well.

Ray tracing through an eccentrically rotating retroreflector used for path-length alteration in a new Michelson interferometer

A newly developed Michelson interferometer for Fourier spectroscopy is described. It utilizes a nutating retroreflector (cube-corner mirror) to generate alterations in geometrical and optical paths. The nutation is achieved by the rotation of the retroreflector eccentrically, and it is tilted with reference to the optical axis of the interferometer. The forward–backward stop-and-go movement of a reflecting element of conventional Michelson interferometers is thus replaced by a continuous rotation. The design aims at a fast, simple, rugged and service-free, reliable spectrometer for field or airborne atmospheric monitoring. For Fourier spectroscopy, the instantaneous and the maximum difference between the two optical paths of the interferometer is of substantial importance. Performing the Fourier transform requires knowledge of the instantaneous difference; the maximum difference determines the spectral resolution of the device. The mathematical deduction of the path of a beam of radiation traversing the retroreflector is performed. First ray tracing is calculated for a fixed retroreflector, dependent on the angle of incidence. Then the path of a ray is deduced for the rotating retroreflector as a function of angle of incidence, eccentricity, and angle of rotation. It is shown that the path difference changes sinusoidally with the angle of rotation. The maximum path difference is expressed as a function of eccentricity and angle of incidence. Numerical results are presented.

Michelson interferometer with a rotating retroreflector: Investigations on special features

Abstract A new type of Michelson interferometer is under development. It has been given the name MIROR which stands for: Michelson Interferometer with a ROtating Retroreflector. Optical path alterations are achieved by continuously rotating a retroreflector in one or both arms of the interferometer. First investigations on the features of the laboratory model have been performed. The spectral resolution, which is limited by the dimensions of the retroreflector, has been optimized. The possible influence of the retroreflectors mirror boundaries on the interferences of the laser and IR radiation has been considered and found to be negligible. The speed of measurement, i.e. time resolution has been adapted to the capability of the signal processing electronics (analog to digital converter). It has been proved, that spectral and time resolution for this design are independent of each other, i.e. the limiting parameters are those of the electronics (conversion respectively transfer rates). The influence of the inherent sinusoidally modulation of the path alteration velocity on the resulting spectra has been investigated. For this purpose the spectra resulting from single and double sided interferograms of different length have been compared.

Calibration of a Fourier transform spectrometer using three black body sources

A procedure to calibrate a Fourier transform spectrometer is presented. Blackbody sources of three different temperatures are used to eliminate errors in the calibration arising from the limited accuracy of the temperature measurement of the calibration sources. With three spectra of blackbodies it is possible to assume that the temperatures are unknown variables, as are the parameters of the functions describing the spectrometer. Nonlinear Gaussian balancing calculation is applied to determine these unknown variables and to minimize the influence of noise. A comparison between results obtained with this method and a conventional calibration procedure is presented.

The temperature dependence of the time constants of thermoelectrical infrared detectors derived from their digitized step function response

Abstract The temperature dependence of the time constants of two different pyroelectrical (LiTaO 3 and PVIDF) and one thermopile detectors has been investigated. The step function response of the detectors is measured to determine their time constant(s) τ (n) using a balancing calculation. A radiation step is applied to the detectors by means of a black body source and a fast camera shutter. The step response function is digitized and the resulting data set is used in the calculation algorithm to determine iteratively the time constants τ (n) and the height k of the signal. Due to the balancing effect of the method, the results are of very high accuracy. The procedure is performed at different controlled detector temperatures, leading to the temperature dependence of the detector. It has been found that the time constants drop with increasing detector temperature, where the quantitative agreement with the theoretical ideal behaviour depends on the design of the specific detector. Since the temperature dependence can be quite expressed, consideration of this effect is necessary when designing a radiometer. Care must be taken when temperature stabilizing an instrument above the ambient temperature. This may considerably shift the time constant(s). With the knowledge of the black body temperature the data are used to calculate the responsivity R of the detector and the additional determination of the detector noise leads to the detectivity D ∗.

Michelson interferometer with a rotating retroreflector : a laboratory model for environmental monitoring

Designed for atmospheric pollution monitoring, a breadboard model of a new Michelson interferometer has been developed. It utilizes a nutating retroreflector to generate alterations in the geometrical and optical paths. The forward-backward stop-and-go movement of a reflecting element of conventional Michelson interferometers is thus replaced by a continuous rotation. At this state the instrument employs a 6.3-cm (2.5-in.) diam rotating retroreflector, a ZnSe beam splitter, and a HgCdTe detector at liquid nitrogen temperature, sensitive in the 8-14-microm band. It allows spectral resolutions of up to 2 cm(-1). The device is linked via an analog digital interface to a desktop computer which performs interferometer control, data acquisition, Fourier transform, and display of the spectra.

Spektroskopische Fernmessung von Luftschadstoffen unter Einsatz eines Michelson Interferometers mit rotierenden Retroreflektoren

Vorgestellt wird ein neuartiges Michelson Interferometer fur die Fourierspektroskopie, das mit rotierenden Retroreflektoren anstelle eines translatorisch bewegten Planspiegels arbeitet. Die Eignung des Gerats speziell fur den mobilen Einsatz auf dem Gebiet der Umweltmestechnik wird anhand der Forschungsarbeiten beschrieben.