Themes > Science > Physics > Molecular Physics > Molecular Spectra > Molecular Orbitals > Infrared and optical spectra of molecules > Infrared Point Spectrometer (IPS)

Photo of IPSThe Infrared Point Spectrometer (IPS) reveals the mineralogy of rocks and soils from the near field to the horizon. Its wavelength range is sensitive to aqueously-deposited minerals such as clays and carbonates in addition to igneous minerals.



FIDO's Infrared Point Spectrometer

When matter is exposed to the electrical and magnetic components of light, the atoms and molecules within it are affected. The minerals in rocks absorb and reflect infrared light of different wavelengths depending on the composition of each mineral. Because every mineral absorbs and reflects IR differently, each has its own spectral signature in these wavelengths. If our eyes could see infrared light, every mineral would have a unique color.

Principle of IPS Operation

 Near-infrared spectrometers, such as the IPS used on FIDO's Integrated Instrument Suite, can spectrally record the mineralogy or reveal the mineral contents of (planetary) rocks. This is possible because there are combination and overtone molecular vibrations in the crystal structures of rock in the infrared range at which the IPS operates. Optical spectra originate from electromagnetic radiation generated by these optically active molecules, which are best modeled as damped harmonic oscillators. Reflected light from sunlit planetary surfaces, when measured in the 1300-2500 nm region of the electromagnetic spectrum, contains sufficient energy, intensity, and width (of the spectral features) to allow the identify of the minerals to be revealed in the remotely-sensed reflectance spectra.

Acousto-Optic Tunable Filter

 An AOTF, most simply, is like a diffraction grating device used as the dispersive element in an optical spectrometer. It is an electronically tuned optical filter that operates on the principle of acousto-optic diffraction in an anisotropic medium. It is "tunable" because the "grating" constant (given by the acoustic wavelength) can be electronically controlled, allowing it to rapidly scan across the entire operational spectral range.

IPS Reflectance Spectrum
When an acoustic wave propagates in a transparent material, it produces a periodic modulation of the index of refraction via the elasto-optical effect. As a series of frequencies are applied to the IPS's AOTF crysal, the wavelength of infrared light that is diffracted in the direction of the infrared detector is modified, sweeping the infrared spectrum from 1.3 to 2.5 microns, and generating a spectrum of the reflected infrared light from the target.






 IPS Reflectance Spectrum

How the IPS Works

 Infrared light reflected from a target rock is collected in the IPS fore-optics mounted on FIDO's mast head. The light is passed through an optical fiber down the mast into FIDO's electronics enclosure. The infrared light arrives at the AOTF crystal where the applied radio-frequency sweeps the infrared spectrum of the rock across the detector. Signals from the detector pass through analog-to-digital conversion within the IPS electronics, where the infrared spectrum is built up and then passed into FIDO's data stream.

IPS Specifications
Spectrometer Technology
Acousto-Optic Tunable Filter (AOTF)
Technology Vendor
Brimrose
Wavelength Coverage
1.3 to 2.5 Microns
Spectral Resolution
Approximately 8 Nanometers
Spectral Optics Window
1 Millimeter Thick Sapphire
Detector
InGaAs, 2-Stage TEC Cooled
Digitization
16-bit
Projected Field of View
9.3 Milliradians (~25 x 32 Pancam Pixels)


Information provided by: http://fidoinstruments.jpl.nasa.gov