Case Study: 4.55 µm Long Pass Filter

Case Study:

Customer Challenge: The customer was using suffering from the interference of the CO₂ gas band which was affecting the function of their instrument. CO₂ has a very strong mid infrared absorption band around 4260 nm, approx. 200 nm wide. The measuring region for this particular device was above 4260 nm so it was identified that a long pass filter would be the most suitable and cost-effective way to remove this interference.

Vortex Solution: CO₂ can have a big impact in optical systems due to its presence in the atmosphere. Reliable measurements due to air disturbance drafts and even exhaled breath could cause large spikes in the measured data and lead to inconsistent results. The filter was designed to start to transmit at 4350 nm which is just past the CO₂  band. It was also important to maintain good transmission in the passband of the filter.

Result: The filters worked and successfully blocked the CO₂ ,the instrument was able to work accurately in the Transmission band. Fig 1 above shows the Transmission graph of the filter and the uncoated Sapphire window that was originally installed in the equipment and the CO2 band. It demonstrates how the CO2 band will interfere with any measurements made.