Flame Detection - Sunlight

Sunlight

The sun emits an enormous amount of energy, which could be harmful to human beings if not for the vapours and gases in the atmosphere, like water (clouds), ozone, and other vapours and gases in the air, through which the sunlight is filtered. In the figure it can well be seen that "cold" CO₂ filters the solar radiation around 4.3 µm. The Infrared detector which uses this frequency is therefore solar blind. Not all manufacturers of flame detectors use sharp filters for the 4.3 µm radiation and thus still pick up quite an amount of sunlight. These cheap flame detectors are hardly usable for outdoor applications. Between 0.7 µm and approx. 3 µm there is relatively large absorption of sunlight. Hence, this frequency range is used for flame detection by a few flame detector manufacturers (in combination with other sensors like ultraviolet, visible light, or near infrared. The big economical advantage is, that no expensive sapphire detector windows but quartz windows can be applied. These electro-optical sensor combinations also enable the detection of none-hydrocarbons like hydrogen fires without the risk of false alarms, caused by artificial light or electrical welding.