"Invisible flames" refer to flames that are not easily seen by the naked eye, often produced by certain types of gases or chemical reactions. These flames can occur during combustion processes where the fuel burns at a temperature that emits light in wavelengths outside the visible spectrum. For example, flames from burning hydrogen or methane can be nearly invisible in daylight. These flames can pose significant safety risks, as they are difficult to detect without specialized equipment. Firefighters and safety professionals often use thermal imaging cameras to identify these flames, ensuring that they can respond effectively to potential hazards in environments where invisible flames may be present.