Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2021-02-23.
A spectrometer separates incoming light into individual wavelengths so that a spectrum can be recorded. The key element is a grating: a mirror carrying a very fine, regularly repeated pattern of parallel lines, thousands per millimetre. Light striking it is mostly reflected as from any mirror, but part of it leaves in other directions where the wavefronts interfere constructively — diffraction. The angle of the diffracted light depends on wavelength, with blue leaving closest to the mirror direction and red furthest away. A prism would separate colours too, but a grating spreads them much further, which is why a high-resolution spectrometer can be built small. The rest of the instrument can be simple: light enters a light-tight box, is directed onto the grating, and diffracted light leaves at different positions onto a detector. In a common design the grating rotates about an axis perpendicular to the incoming beam, which selects the wavelength window the detector sees. What remains is practical engineering — light budget, F-number, grating resolution, geometry, detector sensitivity — plus folding mirrors to keep the package compact. OPSIS UV-DOAS analysers use a Czerny-Turner arrangement.
Resolution and usable wavelength window decide which components an analyser can measure and whether neighbouring absorption lines can be separated in a real flue gas. That is a selection question, not a maintenance question, and it is worth settling before a project is quoted. For Chinese emission sources the CEMS design, acceptance and operation follow the HJ 75 and HJ 76 series, with further requirements for ambient stations in the HJ 654 series; all of them assume that the optical core stays stable between calibrations. In hot, dusty and humid installations, keeping the optical path and the dispersive element clean and thermally stable is what protects data availability — a dirty window shows up first as drift and only later as an obvious fault. Purge air quality, purge flow and window access for cleaning belong in the mechanical design, not in the commissioning checklist.
Inside the Gas Analyser: the Spectrometer — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2021-02-23. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
Send the gases, ranges, duct or path dimensions and required certification — we will respond with a technical proposal.