Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2026-04-27.
Light is one of nature's most attractive phenomena, and its physical properties connect nature, technology and daily life. For OPSIS, light is first of all a measuring tool, and the reason it can measure gas concentrations lies in physics. Light can be described in two ways depending on the question. Sometimes the electromagnetic wave is the convenient model, which explains how a rainbow forms or how light refracts in glass or water; sometimes the photon is the easier concept, and that is quantum optics, which describes how light interacts with atoms and molecules. As a wave, light has a wavelength — the distance between two successive peaks — and wavelength determines colour. What we call light is usually the visible part, from red through green and blue, but the family extends well beyond it: infrared with longer wavelengths than red, ultraviolet with shorter ones than blue, together forming the optical band, with microwaves and radio waves beyond the infrared and X-rays and gamma rays beyond the ultraviolet. Light carries energy, and that energy is quantised: each photon carries a specific amount that depends on frequency and therefore on wavelength. Atoms and molecules absorb and emit energy only at particular levels, and different elements have different level structures, which is the foundation of every spectroscopic measurement.
This is the article we give a new engineer who asks why a spectrometer can tell one gas from another. The chain is short: energy levels are fixed, absorption therefore happens at fixed wavelengths, and a molecule's set of lines is as characteristic as a fingerprint. Two consequences matter in the field. First, the same physics that makes the measurement possible also makes interference possible — water vapour has absorption lines almost everywhere, which is why spectra are evaluated over a chosen window and why cross-sensitivity appears in every uncertainty budget. Second, because absorption depends on the number of molecules in the light path, the result is a path average that also depends on temperature and pressure unless it is normalised. When a customer asks why an analyser needs a stored reference spectrum and a polynomial correction, the answer starts here.
Let There be Light — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2026-04-27. 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.
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