What are High-intensity Obstruction Lights?
The brightest obstacle lights: flashing white lights for very tall towers, stacks and long spans, above about 150 m.
High-intensity lights are the brightest class of obstacle light. They are used on very tall fixed objects, typically above about 150 m, where lower-intensity lights would not be seen from far enough away.
Both types are flashing white and work by day, at twilight and at night. Type A is about 200,000 cd by day and Type B about 100,000 cd. Both step down to about 20,000 cd at twilight and about 2,000 cd at night.
Lights go at the top and at intermediate tiers, aimed so the beam peak is near horizontal and the object is seen from every direction. The night step-down keeps the light from dazzling people on the ground, and glare near an airfield needs particular care.
| Light | Mode | Day / twilight / night (typical) | Where it is used |
|---|---|---|---|
| Type A | Flashing white | ~200,000 / ~20,000 / ~2,000 cd | Very tall objects, above about 150 m |
| Type B | Flashing white | ~100,000 / ~20,000 / ~2,000 cd | Towers supporting overhead wires or cables, where the study finds lights essential or markers are impracticable |
| FAA L-856 | Flashing white | Same order as ICAO Type A; stepped down for twilight and night | FAA high-intensity light on very tall structures |
Confirm the need
Height above about 150 m, and an aeronautical study that finds day recognition essential, put the object in the high-intensity class. Otherwise medium-intensity lights may serve.
Choose the type
Type A for objects above about 150 m where day recognition is essential; Type B for towers carrying overhead wires or cables.
Set tiers and aim
Place lights at the top and intermediate tiers, aimed with the beam peak near horizontal.
Control and check glare
Use a light sensor to step intensity, keep flashing in step, and check glare near the airfield.
High-intensity and medium-intensity Type A look alike, because both are flashing white and both show about 2,000 cd at night. Tell them apart by day brightness and height, because at twilight and night their ICAO intensities are the same:
- High-intensity (A / B)→ flashing white, ~100,000 to ~200,000 cd by day, above about 150 m
- Medium-intensity Type A→ flashing white, ~20,000 cd by day, about 45 m to 150 m
- Low-intensity (A / B)→ fixed red, night only, under about 45 m
Also remember that full day intensity is far brighter than a night light needs to be. The step-down is what makes these lights acceptable around people and aircraft.
ICAO High-intensity Obstacle Lights
ICAO Annex 14 Volume I, Chapter 6, provides high-intensity obstacle lights Type A and Type B. Both are flashing white and are used by day, at twilight and at night, with the intensity stepped down as the light level falls. Type A has a higher day intensity than Type B. Type A is the choice for very tall objects; Type B is intended for towers supporting overhead wires or cables, where the study finds lights essential or markers cannot be fitted to the wires. They flash at 40 to 60 flashes per minute. They are used on very tall objects, typically above about 150 m, placed at the top and at intermediate levels, about 105 m apart at most, so the object is visible from every direction in azimuth. Type A is called for where the object exceeds about 150 m and an aeronautical study finds the lights essential for recognising it by day. Because of their brightness, the standard asks for care where the lights could dazzle pilots in the vicinity of an aerodrome, within roughly 10 km, where a dual lighting system may be used. Take the type, intensities, aiming, flash rate and layout from the regulator's determination and the current edition of the standard.
FAA High-intensity Obstruction Lights
The FAA covers this in AC 70/7460-1, Obstruction Marking and Lighting. Its high-intensity light is the L-856, a flashing white light used day and night with intensity stepped down for twilight and night. Other FAA lighting styles, such as dual systems and lighting for overhead wires, use their own fixtures and rules. Confirm the current edition of the advisory circular and the style the FAA determination requires for your structure.
Frequently Asked Questions
Q1What are high-intensity obstruction lights?
High-intensity obstruction lights are the brightest class of obstacle light. They are flashing white lights used on very tall fixed objects, such as towers, stacks and long spans, typically above about 150 m, where a lower-intensity light would not be enough.
Q2What is the difference between Type A and Type B?
Both are flashing white and both work by day, at twilight and at night. Type A is brighter, about 200,000 cd by day. Type B is about 100,000 cd by day. Both step down to about 20,000 cd at twilight and about 2,000 cd at night. Type A suits objects above about 150 m; Type B is intended for towers supporting overhead wires or cables.
Q3Why do they change intensity?
A light bright enough to be seen in daylight would dazzle people on the ground and pilots at night. A light sensor therefore steps the intensity down as the daylight fades, in the stages of day, twilight and night.
Q4Can high-intensity lights be a problem near an airport?
They can. At full intensity they may dazzle pilots on approach, so in the vicinity of an aerodrome (the ICAO text suggests within roughly 10 km) the regulator may require another arrangement, such as a dual system using red lights at night, or may limit where they are used. The determination for the site governs.
Q5How are they different from medium-intensity Type A?
Both are flashing white and show the same ICAO levels at twilight (about 20,000 cd) and at night (about 2,000 cd). Medium-intensity Type A is about 20,000 cd by day as well, so the two differ by day. High-intensity lights are about five to ten times brighter by day (Type B and Type A) and are used on taller objects, above about 150 m, where the light must be seen from much further away.
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