Optical trickery?

Do you recognise this shiny graphic?  It’s one of those things our engineers love to put on datasheets. But do you know why it is so special?

It shows the inside of the optics used in our ambient air CO2 sensors. This is the gold component that is a very visible part of many of these sensors.  To understand what it’s for, you need to take a step back to how a Non-Dispersive Infrared (NDIR) CO2 sensor works.  

We detect CO2 by monitoring how much the gas molecules absorb infrared light, which it does between 4.2 and 4.4 microns. To do that, we need to get the light to pass through the gas and, because low levels of CO2 do not absorb very much, we need to shine the light a long way before we can see the effect. The challenge is getting the right balance. Our physicists require a longer optical path to measure lower levels of CO2 more accurately - which potentially means a larger sensor component. On the other hand, in ambient air environments - such as schools, offices and homes - the demand is for ever smaller components to fit neatly inside compact sensor units.    

So what’s the solution? Well our engineers came up with a clever way of reflecting the light by curving the optics into a horseshoe shape (which we often refer to as a 'waveguide'). The longer the path, the more chance that the CO2 molecules can absorb the light as it passes through it if they are present.  So a long optical path means greater sensitivity. Essentially, the more light that reaches the detector, the less CO2 there is present. And the less light that reaches the detector because of absorption, the greater the concentration of CO2 there is.

The blue line in the diagram shows one of the many paths that the light can take – bouncing off the walls as it goes from the light source to the detector. It’s a lot of bounces, but that way we can build some of the smallest NDIR sensors in the world by having a very long optical path in a very small space to provide a high level of sensitivity.  This design was so innovative that we patented it. 

And why the gold coloured optic? Well sadly, it isn’t solid gold… aside from cost, the weight would be prohibitive.  It’s actually a coating of pure gold applied to a precision optical-grade plastic moulding.  We use a gold coating because it’s extremely reflective of infrared light at the wavelengths used in CO2 detectors – that’s between 4.2 and 4.4 microns. Other metals, such as platinum or aluminium would have worked well enough, but gold remains highly reflective as it does not oxidise or corrode, resulting in a very long lifetime. 

You may have spotted that our wide-range sensors use a different style of optic – our patented dome shape.

Our MinIR™ and CozIR™ wide-range sensors are designed for applications with higher levels of CO2 gas. These would typically be used in industrial or safety environments - such as breweries, food packaging or diving applications. That’s because when you’re dealing with higher concentrations of CO2, there are plenty of CO2 molecules to absorb the light on its passage through the optical path. Therefore, a shorter path length can be used with the result that the light source is closer to the detector, giving you a quicker and more accurate reading.

So there you have it – no trickery, just very clever physics! As you can see the optics form a key component of our CO2 sensors.  It’s this patented technology that helps us deliver world-beating miniaturisation and accuracy performance. And, thanks to advanced manufacturing techniques, we’ve managed to engineer the cost down to a level that was unimaginable a few years ago. 

Sound interesting? You can find out more about our range of CO2 sensors HERE. There are dedicated sensors for different applications and CO2 concentrations. If you're not sure which sensor is right for your application, use our product selector - gassensing.co.uk/products/ 

And if you’re after something a bit special, talk to us about your requirements. Our team of engineers and physicists are experienced in bespoke design development, so please get in touch - sales@gassensing.co.uk