2026-04-22

Let There be Light

Light can mean different things to different people at different times. For OPSIS, light is primarily a tool for measuring concentrations. But what exactly is light, and how is it possible to measure something using light?

The answer lies in physics. Many of us have probably at least touched the subject at school, but it may have been a long time ago. Join us on a journey through the world of basic physics to refresh old memories or perhaps learn something new!

What is light?

The simple answer is “it depends”. In the world of physics, we create models of what we want to describe. When it comes to light, there are two models to choose from, and the choice depends on what you want to know.

• In some cases, it is practical to describe light as electromagnetic waves. This makes it easy to explain, for example, how a rainbow is formed or how light is refracted in glass or a liquid.
• In other cases, photons (“light particles”, “light quanta”) are the concept that is easiest to work with. In that case, we are talking about quantum optics, which describes, amongst other things, how light interacts with some of our smallest building blocks, namely atoms and molecules.

Wavelengths

Light as a wave motion is usually not too difficult to grasp. Just like waves on the sea, light waves also have a wavelength, that is, the distance between two successive crests. This wavelength is a measure of the colour of the light.

Visible light is what we perceive with our eyes, comprising the primary colours red, green, and blue, with other colours interspersed between them. However, there is much more than just visible light, including:

• Infrared light (IR), which has longer wavelengths than red light.
• Ultraviolet light (UV), which has shorter wavelengths than blue light.

The wavelength range from infrared through visible to ultraviolet is called the optical range.

Light and energy

Electromagnetic waves contain energy. However, we must also introduce the concept of photons into the discussion to understand what is happening.

Light energy is quantised, meaning that each individual photon carries a specific amount of energy. The energy of a photon is linked to the frequency of the light, and thus to its wavelength. Energy is required to create light, that is, photons. When light strikes matter, the surface heats up, and it is then that the photons transfer their energy to the atoms and molecules of the surface.

Atoms, molecules, and energy

To understand how light can be used in measurement technology, we must also look at how atoms and molecules interact with light, or more precisely, the energy in light.

It turns out that atoms and molecules can absorb and emit energy, but they can only exist in certain specific energy states. The energy absorbed or emitted when the energy state changes is therefore also quantised. It only occurs at certain specific energy levels.

Energy levels

Energy levels differ between different types of atoms. This is fundamentally dependent on the number of electrons surrounding the atomic nucleus, but also on the number of protons and neutrons in the nucleus.

In a molecule, moreover, the bonds between the molecule’s atoms give rise to other types of energy states, known as vibrational and rotational energies. All the different types of energy levels that can exist in an atom or molecule form a complex pattern, but for the same type of atom or the same type of molecule, the pattern is the same and, moreover, usually unique.

Taken together, this results in a signature of energy levels that exists only in a specific type of atom or molecule. This also yields a signature of energy that can be absorbed and emitted.

Interaction between matter and light

With this, the stage is set to demonstrate how gas concentrations can be measured using light.

If we send a photon towards a molecule (or atom, but let’s simplify the text and just talk about molecules), the photon’s energy may be close enough to the difference between two of the molecule’s energy levels for the energy to be transferred to the molecule. The photon is absorbed and the molecule is excited – it ‘heats up’.

The more molecules through which photons of matching energy are trying to pass, the more photons will be absorbed.

So much for what happens at the quantum level, but now it is time to return to the light beams and their waves.

A very large number of photons with the same energy moving in the same direction is the same as a light beam of a certain wavelength. When the light beam strikes the gas of matching molecules, some of the photons in the beam will be absorbed. The beam’s intensity is attenuated, and we have light absorption at the macroscopic level.

From one wavelength to all

So how does OPSIS make use of all this? Let’s focus on the measurement systems based on DOAS technology (differential optical absorption spectroscopy). Here, we have a white light beam created by a xenon lamp and a mirror. White light comprises ‘all’ visible wavelengths at the same time, i.e. a massive quantity of photons of ‘all’ energies. The beam also contains IR and UV wavelengths.

As it travels through the gas, the light beam encounters all manner of molecules, each of which can absorb some of the energy in the light. The amount of absorption varies for different wavelengths depending on the type of molecule and how many molecules are present.

...and on to DOAS

What remains of the light beam after passing through the molecules is collected in a receiver and sent to an analyser, in OPSIS’ DOAS case, one of the instruments in the AR series.

If we compare the emitted spectrum of light (i.e. the light energy we had for each wavelength at the xenon lamp) with the spectrum remaining after the light beam has passed through the molecules, we will see the molecules’ absorption signatures (“fingerprints”).

A particular fingerprint becomes deeper the more molecules of that specific type there are along the light beam. Add a bit of maths and we can calculate the concentration of a particular type of molecule in familiar units such as µg/m³ or ppm. We can also do this for many different types of molecules using a single measurement system.

And with that, we have our “DOAS”. We look at how light and matter interact (spectroscopy, the S) and, more precisely, at how much energy is lost along the light beam (absorption, the A). We stick to the optical wavelengths (from IR to UV, the ‘O’), and we compare the original spectrum from the xenon lamp with the spectrum remaining at the receiver (the difference, the ‘D’). S-A-O-D – DOAS!