how far lighthouses are visible and what's the difference between range of light and visibility

The range of light is the distance to the lighthouse, where the light would be just observable. Therefore the range of light is only dependent on the power of light (luminous intensity) and the air transparency. The range of light for defined air transparency is noted in nautical charts and lists of lights.

The visibility is the distance where the light is directly visible from a defined height above sea level (at height of eyes). Therefore the visibility is decisive dependent on the height of fire and the height of eyes (because of spherical surface of earth). The visibility can't be greater than the range of light, only equal. The central part of flashing light has to be visible.

the fire is visible:

the fire isn't visible:

For a constant height of eyes a directly proportionality exists between visibility and height of fire (example: table for h=5m).

height of fire (m)

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

visibility (sm)

7.6

8.8

9.7

10.5

11.2

11.8

12.4

12.9

13.4

13.9

14.3

14.8

15.2

15.6

16.0

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