Encoded photometric infrared spectroscopy


General Principle

The EP-IR system contains the encoder-spectrograph, as well as a real-time embedded micro-processor for signal analysis, data treatment, and Chemometrics (quantitative) analysis in a single small-footprint enclosure.

Operation Principle

An infrared polychromatic beam is emitted from an external source (e.g. glow bar) onto the sample. Then it is either transmitted or reflected by means of appropriate transfer optics into the spectrograph and is focused through the entrance window (CaF2) of the EP-IR spectrograph.

The incoming focused beam is transferred directly onto the fixed holographic grating (which together with the detector determines the encoded wavelength range) The dispersed radiation (2.5 to 5.0 μm, 2000-4000cm-1) is then projected through a slot aperture onto a spinning encoder disk as shown in Figure 4.

The encoder disk features 256 or 128 concentric modulator tracks2 located in an annular region to evenly cover the entire wavelength region dispersed by the fixed grating.

The composite reflected beam is imaged on a single element detector that generates a signal that forms a Discrete Interferogram, hence preserving multiplexing advantage. The intensity contribution for each region of wavelengths corresponding to the bandwidth of each photometric channel is obtained by applying a Fourier Transform to this interferogram.

The MC5000’s encoder disk consists of 128 or 256 extremely accurate “halftone representations” of smooth orthogonal sinusoidal functions referred to as tracks.

These tracks are made of deposited metal formed via micro lithographic procedure and serve as the same purpose as optical bandpass filters to define the center wavelength and bandwidth of the encoded radiation of the photometric channel.

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