page_title: Fourier-transform infrared spectroscopy
section_title: Beam splitter
hierarchical_section_title: Fourier-transform infrared spectroscopy / Components / Beam splitter
caption_attribution_description: English: Includes compensator plate
context_page_description: Fourier-transform infrared spectroscopy is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-spectral-resolution data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer, which measures intensity over a narrow range of wavelengths at a time.
  The term Fourier-transform infrared spectroscopy originates from the fact that a Fourier transform is required to convert the raw data into the actual spectrum.
context_section_description: An ideal beam-splitter transmits and reflects 50% of the incident radiation. However, as any material has a limited range of optical transmittance, several beam-splitters may be used interchangeably to cover a wide spectral range. For the mid-IR region the beamsplitter is usually made of KBr with a germanium-based coating that makes it semi-reflective. KBr absorbs strongly at wavelengths beyond 25 μm (400 cm⁻¹) so CsI is sometimes used to extend the range to about 50 μm (200 cm⁻¹).  ZnSe is an alternative where moisture vapor can be a problem but is limited to about 20μm (500 cm⁻¹). CaF₂ is the usual material for the near-IR, being both harder and less sensitive to moisture than KBr but cannot be used beyond about 8 μm (1200 cm⁻¹).  In a simple Michelson interferometer one beam passes twice through the beamsplitter but the other passes through only once. To correct for this an additional compensator plate of equal thickness is incorporated. Far-IR beamsplitters are mostly based on polymer films and cover a limited wavelength range.
caption: Simple interferometer with a beam-splitter and compensator plate
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