Well, I guess it depends on what kind of effect you are going for. Are you trying to recreate the effect of a real lighthouse pre-19th century? Or do you want to mimic the power of a modern lighthouse with magic?
"Until the beginning of the 19th century tallow candles, coal fires, and oil lamps were used as illuminating agents; coal gas followed, to be succeeded by acetylene."
To mimic this effect you simply need a light spell. Though the spell's radius is only 20' roughly the same as a torch, remember that the 20' radius is only for illuminated vision range. The light created would be able to be seen to the limit of the visual ability of those looking for it (and weather conditions) - most likley to a few thousand yards at least. If this were combined with Mirrors and Lenses, the range of visibility would be increased.
"In modern lighthouses there are three kinds of lighting systems: the catoptric system, in which rays of light are reflected from silvered mirrors to form a parallel beam visible at a distance; the dioptric, or refractive, system, in which the rays pass through optical glass and are refracted as they enter and emerge from it; and the catadioptric system, in which rays are both refracted and reflected."
If you want to mimic this style of lighthouse there would be two options.
1) MagiTech: Use of mirrors and lenses with the daylight spell.
2) Pure Magic: Use of the Sunbeam spell.
Hope that helps a little.