Jérôme Genest

Radiometry in line-shape modeling of Fourier-transform spectrometers

A radiometric model of the instrument line shape (ILS) of Fourier-transform spectrometers is presented. We show first that common line-shape models are based on distribution of the radiant intensity in the interferometer. The complete steps between the source and the ILS are exposed as the core of the model. Relationships between the ILS, the spectrum as measured by the instrument, and the spectrum as emitted by the scene are demonstrated from the ILS model. Then the formal radiometric modeling of the ILS is derived, including the contribution of the aperture of the optical system. The particular case of a centered circular aperture with a uniform Lambertian radiance in the field of view is discussed. Conditions are deduced to ensure that the only spectral variation of the ILS is a scaling with wave number, as is usually assumed in current line-shape models. The ILS dependence on the scene is also discussed, and the effect of taking into account the radiometry on the ILS is estimated for the case of an ideal thin lens used as a collimator.

Impact of the optical aberrations on the line shape of Fourier-transform spectrometers

The impact of optical aberrations in the input collimator on the instrument line shape of a Fourier-transform spectrometer is modeled using an analytic line shape model. Each third-order aberration is considered separately. Design guidelines are obtained stating which aberrations are more important and allowing to choose the sign of some aberrations to minimize their impact on the line shape.

Diffraction and line shape of Fourier-transform spectrometers

The effect of diffraction the instrument line shape of a Fourier-transform spectrometer is studied with an analytical line-shape model. The expression for the instrument line shape of a diffracted point source is obtained. A simple condition on the throughput of the instrument is derived under which diffraction is negligible when compared with the field-of-view-induced line shape. The effect of diffraction is illustrated and compared for various instruments.

NIR All-Fiber Supercontinuum Frequency Comb Spectrometer

We discuss the realisation of an all-fiber spectrometer working on the principle of heterodyne measurement using two high brightness frequency combs exhibiting supercontinua in the NIR (1.2 µm - 1.9 µm).

State-of-the-art imaging Fourier-transform spectrometer with CCD camera

Imaging Fourier-transform spectrometers can quickly produce massive amounts of raw data, especially when paired with large focal plane arrays. As the spatial resolution is increased, overwhelming amounts of data must be managed properly. A suitable design of the data processing chain is thus required to minimize the dataload and deliver processed information in real-time. This paper reviews the work being done to tailor data processing pipelines for Fourier-transform spectrometers (FTS) coupled with externally triggered CCD cameras. Various sampling techniques as well as spectral calibration and line shape correction approaches will be reviewed. Since traditional sampling techniques are not well suited for an FTS operating with a CCD camera, a hybrid time-position sampling approach is presented to reduce the number of samples per pixel. Furthermore, the approach enables a sampling jitter correction algorithm that can account for velocity fluctuations and channel delays, such as the CCD integration time. A fast spectral calibration approach is also demonstrated, based on a rapid line shape integration scheme. The calibration algorithm brings all pixel spectra on the same spectral grid and allows the user to directly compare spectral features between pixels. Moreover, the correction method offers software field-widening capabilities by binning pixels after spectral calibration. A large single-pixel detector can thus be emulated from the CCD array, allowing the user to broaden the field of view and to increase the SNR.

New Sampling Approach Suitable for Imaging Fourier Transform Spectrometers with Integrating Detector

A new approach to interferogram sampling is presented. The method reduces data load and processing overhead while allowing post-correction of sampling errors. It is particularly well suited for continuous-scan spectrometers equipped with a CCD camera.

New Spectral Calibration Approach Suitable for Imaging Fourier Transform Spectrometers

A new approach to line shape correction is presented. The method uses line shape integration to calibrate the spectral grids of pixels and is thus convenient for spectrometers equipped a CCD camera.

Theory of partial coherence as a framework for the optical modeling of Fourier transform spectrometers

The theory of partial coherence [1] is an ideal framework to properly model the optical design of Fourier transform spectrometers.