At left is one of the many different types of detectors used to see visible and Invisible flames which can happen in Petroleum and Chemical production and transportation facility's.
This product from Det-tronics a UTC Fire & Security Company is one of them
When you have a critical need to protect lives and detect a fire conventional detectors may take to long to react or may not even be able to be used in the area where you need to be alerted to a fire or may not even be able to see the condition. This is where flame detectors are used.
They come in many different versions including Ultraviolet , Near In-fared etc. depending on the product needing watched and environment its located.
You can find them at Gas and Oil rig drilling and production sites and fueling terminals and Chemical production and storage facilitates.
Some products like alcohol do not produce visible flames . If you ever watch Formula 1# Open wheel racing on TV you may have seen incidents of these types of fires in the pits.
Thats why all these specialized detectors are needed . There not cheap but they are worth every penny when it comes to saving lives and protecting property's from explosions.