Robert L. Richardson

Effects of Detector Nonlinearity on Spectra Measured on Three Commercial FT-IR Spectrometers

The effect of the nonlinear response of mercury-cadmium-telluride (MCT) detectors has been evaluated on three commercial Fourier transform infrared (FT-IR) spectrometers. The greater the photon flux and the photon flux density, and the smaller the area of the detector on which the source image is focused, the greater are the effects of the nonlinearity. The signal-to-noise ratio (SNR) of spectra measured with an MCT detector under conditions of high photon flux and, especially, high photon flux density is significantly less than the SNR calculated by using the manufacturers D* value. Detector nonlinearity usually leads to negative deviations in Beers law plots. An empirical correction algorithm has been applied to Beers law spectra acquired with the use of photoconductive MCT detectors and has been found to work well.

Evaluation of a Correction for Photometric Errors in FT-IR Spectrometry Introduced by a Nonlinear Detector Response

For strongly absorbing bands measured with a Fourier transform infrared (FT-IR) spectrometer, the effects of a nonlinear detector response must be eliminated before Beers law linearity can be achieved. An empirical method for greatly reducing the effect of detector nonlinearity on FT-IR Beers law spectra measured by using an FT-IR spectrometer equipped with a mercury-cadmium-telluride (MCT) detector is investigated. This first-order analytical function has been applied to correct nonlinear vapor-phase spectra and statistically evaluated for validity for spectral regions above the detector cutoff. In addition, a series of second-order functions has been evaluated to investigate the possibility that the transmittance scale is slightly nonlinear even after the first-order correction has been applied. Any improvement caused by the second-order functions was not statistically significant.

Evaluation of a System for Generating Quantitatively Accurate Vapor-Phase Infrared Reference Spectra

A static delivery system for generating infrared vapor-phase reference spectra has been constructed and validated for liquid analytes with polarities up to that of methanol. Several important factors that determine the accuracy of these mixtures have been identified. These include thorough degassing of the liquid analyte, maintenance of water content inside the system below a predetermined value, isolation of the pure analyte vapor before inlet of the fill gas to avoid back-flow of vapor, forced mixing to overcome the effect of excessively slow diffusional mixing of the analyte vapor and fill gas, and measurement within the linear Beers law range for the resolution and apodization employed. To maintain Beers law linearity, it is vital to eliminate the effect of a nonlinear response of the mercury cadmium telluride detector. In this paper, the procedure developed to ensure quantitative accuracy from statically generated gas mixtures is described. The effect of detector response linearity on photometric accuracy is discussed, along with a method of empirically correcting for this nonlinear response. The accuracy of vapor-phase samples made by this approach has been validated by comparison to the spectra of certified standards and found to be within 2% of the certified value.

Design and performance considerations of cat’s-eye retroreflectors for use in open-path Fourier-transform-infrared spectrometry

A ray-tracing analysis of cats-eye retroreflectors for use in active open-path Fourier-transform-infrared (OP/FT-IR) spectrometry and the results of testing f/0.5 and f/1.75 cats-eye retroreflectors built in our laboratory with a commercial active OP/FT-IR spectrometer are presented. The ray-tracing model is based on the optical characteristics of a commercial single-telescope monostatic OP/FT-IR spectrometer and explores trends in cats-eye behavior in practical but rigorous field conditions encountered during transportable outdoor use. All mirrors modeled are paraboloids for which the focal ratios of the primary mirror are f/0.5, f/1.75, and f/3. The effect of the focal ratio of the primary mirror, the focal length of the secondary mirror, and the off-axis alignment of the primary and the secondary mirror have been evaluated as a function of path length, including variable input-beam divergence, between the spectrometer and the cats-eye. The paraboloidal mirrors comprising the primary and secondary of the cats-eye retroreflectors tested were made in our laboratory by spin casting liquid epoxy-graphite composite mixtures followed by in situ polymerization with no postpolishing.

Generation of front-surface low-mass epoxy-composite mirrors by spin casting

Spin-casting liquid mixtures followed by in situ polymerization offers an elegant and inexpensive method of generating front surface paraboloidal mirrors of almost any focal length, aperture, and on- or off-axis character, requiring no post polishing. Mirrors have been made from a variety of substrates including epoxy-composite mixtures, with focal lengths ranging from 2 to 80 m, and apertures from 5 to 76 cm. The goal of this project was to make spin-cast mirrors for use as reflectors, telescopes, and cat’s-eye retroreflectors in open path Fourier transform infrared (OP/FT-IR) spectrometry. These mirrors were designed for portable outdoor use, and thus have been optimized for maximum ruggedness and mechanical strength, and minimal mass, coefficient of thermal expansion, and cost. After spin-casting, the only preparation prior to silvering is a thermal post-cure. Results are presented giving details of the materials, hardware, and procedures used to spin-cast paraboloidal mirrors.

Open-path atmospheric monitoring with a low-resolution FTIR spectrometer

The advantages of measuring open-path Fourier transform infrared (OP/FT-IR) spectra at low resolution are discussed both from a theoretical and experimental viewpoint. In general, the optimum combination of selectivity and sensitivity is found when the resolution is approximately equal to the average full-width at half height (FWHH) of the analytical bands. The FWHH of many bands in the vapor-phase spectra of molecules of medium size, such as chlorinated organic solvents, is approximately 20 cmMIN1, so that a resolution of 16 cmMIN1 is often found to yield the most accurate analytical results. The low baseline noise level found when spectra are measured at low resolution can allow room temperature deuterated triglycine sulfate pyroelectric bolometers to be used instead of liquid nitrogen cooled mercury cadmium telluride photodetectors for OP/FT-IR measurements.

A low-resolution spectrometer for open-path Fourier-transform infrared spectrometry

A novel open-path Fourier-transform infrared (OP-FTIR) spectrometer has been designed and constructed. The spectrometer differs from conventional OP-FTIR spectrometers in that it uses a low-resolution interferometer and a deuterated λ-alanine-doped triglycine sulfate detector. The system also incorporates separate transmitting and receiving telescopes, the mirrors of which are fabricated from spin-cast epoxy resin, and a lateral transfer hollow retroreflector. The use of these components allows for a lighter, more rugged and robust system that can easily be employed for remote sensing/continuous monitoring for both outdoor (fence line or traditional) and indoor environments. Advantages over contemporary instruments were found in weight, cost, and utility.

Reduction of Stray Light in Monostatic Open-Path FT-IR Spectrometers with a Plane Correction Mirror

Open-path Fourier transform infrared (OP/FT-IR) spectrometry is presently being used for atmospheric monitoring of infrared absorbing species in a variety of applications worldwide. Commercial off-the-shelf OP/ FT-IR systems became commercially available in 1986, and the growing use of this technique has increased quanti® cation of atmospheric species on both the local and global level. Although OP/FT-IR is used primarily for industrial monitoring, it has also become increasingly important for understanding the behavior of large-scale anthropogenic emissions. For example, OP/FT-IR has contributed signi® cantly towards understanding the role methane emissions, biomass burning, volcanoes, and other large-scale emission sources play in atmospheric chemistry, as well as the ultimate fate of these species.1± 4 The 1990 Clean Air Acts require documented reduction of air pollutants emitted by industry.5 Two con® gurations for active OP/FT-IR spectrometry are commonly employed, bistatic and monostatic.6 In the bistatic con® guration, unmodulated radiation from a broad-band infrared source is collimated by a transmitting telescope, passed through the target region, modulated by a rapidscanning interferometer, and focused onto a mercury cadmium telluride (MCT) detector operating at 77 K. In the monostatic con® guration, the infrared beam is modulated by an interferometer located immediately after the source, passed through the target region to a remotely located retrore ̄ ector, and returned to the receiving telescope and MCT detector. The latter technique, while optically more complex and less ef® cient, has the bene® ts of reducing the effects of stray light and atmospheric scintillation and requiring electrical power at only one location. In principle, stray light should be completely absent in monostatic OP/FT-IR measurements but, as noted by Russwurm and Childers,6 this has not been observed to be true in practice. In this paper, we describe a simple way of reducing the stray light in a commercial monostatic OP/FT-IR system to a negligibly low level.

Axicon as a retroreflector in open-path Fourier transform infrared spectrometry

Ray-tracing analysis of a particular reflecting axicon, a right circular cone, for use as a retroreflector in active open-path Fourier- transform IR (OP/FT-IR) spectrometry, and the results of testing a 0.305-m aperture right circular cone using a commercial active OP/FT-IR spectrometer are presented. The ray-tracing model is based on the op- tical characteristics of a commercial single-telescope monostatic OP/ FT-IR spectrometer and models the off-axis behavior normally encoun- tered under practical field conditions during field use. Of practical concern are the trends for the diameter of the beam reflected from the retroreflector as a function of the path length between spectrometer and retroreflector and misalignment of the retroreflector with respect to the transmitted beam. Construction and the results of field-testing two smaller epoxy-composite axicons replicated from the 0.305-m master are presented.

Factors inducing and correction of photometric error introduced to FT-IR spectrometers by a nonlinear detector response

For strongly absorbing bands measured with a Fourier transform infrared (FT-IR) spectrometer, the effects of a nonlinear detector response must be eliminated before Beer’s law linearity can be achieved. The effect of the non-linear response of mercury cadmium telluride (MCT) detectors has been evaluated on three commercial FT-IR spectrometers. The greater the photon flux, photon flux density, and the smaller the area of the detector on which the source image is focused, the greater are the effects of the nonlinearity. Detector nonlinearity is manifested by the generation of nonlinear Beer’s law plots. A simple correction algorithm has been applied to Beer’s law data acquired using both photoconductive MCT and pyroelectric detectors and found to work well.