It can be tall buildings, chimneys, masts, cranes, high bridges or similar. There are two major international standards for obstruction lights, ICAO (International Civil Aviation Organization, a UN organization) and the FAA (Federal Aviation Administration, USA). The standards differ slightly, they are normally used as a base for local regulations. Regulations in Sweden are based on ICAO and Swedish Transport Agency’s regulations TSFS 2020:88.
Depending on the height, the object should be marked with low intensity, medium intensity or high intensity light and/or be marked with colour. Both ICAO and FAA give general recommendations, but most countries have adapted local regulations. General recommendations in Sweden are
Modern low intensity obstruction lights use LED as a light source. LED has many advantages, lower energy consumption, longer service life and reduced maintenance requirements. One disadvantage is that LED are not visible to pilots using Night Vision Device (NVD). Therefore, Swedish Transport Agency require that in addition to visible light, obstruction lights with LED should also be equipped with IR light (infrared light) that is visible with NVD equipment.