What are Medium-intensity Obstruction Lights?
Brighter flashing or fixed lights for masts, chimneys, wind turbines and tall buildings, from about 45 m up to about 150 m.
Medium-intensity lights are the middle class of obstacle light. They are used where a low-intensity light would not be conspicuous enough, on objects typically from about 45 m to about 150 m above ground.
There are three types. Type A is flashing white, at about 20,000 cd by day and at twilight and about 2,000 cd at night. Type B is flashing red and Type C is fixed red, both about 2,000 cd and both for night.
Lights are placed at the top and at intermediate tiers so the object is shown from top to base and from every direction. Type A is stepped down by a light sensor as the daylight fades, so it does not dazzle people on the ground at night.
| Light | Mode | Peak intensity (typical) | When it works |
|---|---|---|---|
| Type A | Flashing white | ~20,000 cd day and twilight; ~2,000 cd night | Day, twilight and night |
| Type B | Flashing red | ~2,000 cd | Night; object relies on marking by day. On taller objects it alternates with low-intensity Type B lights |
| Type C | Fixed red | ~2,000 cd | Night; object relies on marking by day |
| FAA L-865 | Flashing white | ~20,000 cd day and twilight; ~2,000 cd night | Day, twilight and night |
| FAA L-864 | Flashing red | ~2,000 cd | Night |
Confirm the class
Height and extent put the object in the medium band, between low- and high-intensity.
Choose the type
Type A for day and night conspicuity; Type B or C where red night lights and day marking are used.
Set the tiers
Place lights at the top and at intermediate levels so every approach direction sees the object.
Control and monitor
Use a light sensor to step intensity, keep flashing in step on groups, and monitor for lamp or sensor failure.
Medium-intensity does not mean one light. The three types behave differently by day and night, so say which type and when it works:
- Type A (Flashing White)→ works day, twilight and night; intensity steps down at night
- Type B / C (Red)→ night lights; by day the object relies on its marking
- High-intensity→ flashing white, far brighter, for objects above about 150 m
Also keep it apart from low-intensity: medium-intensity lights flash (or are brighter fixed red) and carry far more light, so they are never a drop-in swap for a Type A or B low-intensity light.
ICAO Medium-intensity Obstacle Lights
ICAO Annex 14 Volume I, Chapter 6, provides medium-intensity obstacle lights Type A, Type B and Type C. Type A is flashing white, with a higher peak intensity by day and at twilight than at night. Type B is flashing red and Type C is fixed red, both for night use. They are used on objects typically from about 45 m to about 150 m, placed at the top and at intermediate levels so the object is visible from every direction in azimuth. Where a light of this class would not be adequate, high-intensity lights are used. Take the type, intensities, flash rate and layout from the regulator's determination and the current edition of the standard.
FAA Medium-intensity Obstruction Lights
The FAA covers this in AC 70/7460-1, Obstruction Marking and Lighting. Its medium-intensity lights are the L-865 (flashing white, used day and night) and the L-864 (flashing red, used at night). Which lighting style applies, and whether it is used with day marking, depends on the structure and the FAA determination. Confirm the current edition of the advisory circular.
Frequently Asked Questions
Q1What are medium-intensity obstruction lights?
Medium-intensity obstruction lights are the middle class of obstacle light. They are used on taller fixed objects such as masts, chimneys, wind turbines and tall buildings. They come as flashing white (Type A), flashing red (Type B) and fixed red (Type C).
Q2What is the difference between Type A, B and C?
Type A is flashing white, with a peak intensity of about 20,000 cd by day and at twilight and about 2,000 cd at night. Type B is flashing red and Type C is fixed red, both about 2,000 cd and both night lights. By day, an object lit with Type B or C relies on its marking.
Q3Why is Type A white by day and dimmer at night?
White flashing light is very conspicuous by day, so the light is run at full intensity then. At night the same intensity would dazzle and disturb people on the ground, so a light sensor steps it down. Many installations also pair a white day light with a red night light.
Q4How tall an object do medium-intensity lights normally cover?
Typically objects from about 45 m to about 150 m above ground level. Below that, low-intensity lights are normally enough. Above it, high-intensity lights are normally needed. The height band is a guide, and the regulator's determination for the object governs.
Q5Do medium-intensity lights need to flash together?
On groups of structures, such as a wind farm, flashing in step is normally required or strongly preferred, so the group reads as one hazard and does not disturb people on the ground. How this is done is covered in a later entry on flash synchronisation.
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