Helmut Dietl

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.

Multispectral infrared pyrometer for temperature measurement with automatic correction of the influence of emissivity

Abstract A new method for contactless temperature measurement has been developed. (1) It is applied advantageously to highly reflecting objects of moderate temperatures (emitting no visible radiation), where conventional methods fail. A set of data is collected by radiance measurement m a number of n narrow spectral bands. These data represent numerical values for a set of analytical equations of the radiation emerging from the object. Solving the set of equations leads to u ⩽ n unknowns, which are the temperature of the object, its emissivity and the temperature of its environment. Balancing calculation enhances the accuracy. Software simulations, including the balancing calculation, the radiation transfer and the hardware of the prospected multispectral radiometer optimized the whole system prior to hardware development. The finally assembled laboratory model, a ten channel filter spectrometer, proved to function as predicted. The temperature of objects about 500 K and an emissivity as low as 0.03 has been determined with less than 1% deviation from thermocouple measurement. Measurement at lower temperatures and/or higher emissivities or higher temperatures and/or lower emissivities is possible with comparable accuracy.

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.

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.