I'm just wondering how good is the throw compared to a traditional lighthouse bulb.
Yes, indeed. That's what I was wondering also. How great, the
Sealite datasheet mentions the key spec, the
luminance: "Nominal Average Luminance (cd/cm2): 1165". That's 11.65 cd/mm2. Curiously, this is the only relevant number for
throw in the numerical sense, that is, for
max lux in the spot. Different sources may have different diameters, but the lighthouse
lense is still same, and so max spot lux only depends on luminance.
On the other hand, in general, a wider spot almost always makes a
huge difference for an observer looking out in the field. In this case it's simple: it's important that the spot hits a ship at all: The spot has to be wide enough to reach the horizon (upper edge of spot) as well as a nearby ship in dense fog, which would be quite a bit "under" the horizon from the lighthouse keeper's view (lower edge of spot). The spot diameter simply relates 1:1 to the source diameter. So I guess that's a good bet with the relatively huge Sealite led array.
Important fact: Sealite sources are "designed to replace traditional lamps or lighthouse lights in classical lighthouse optics"
[link] . So it's not about competing with todays high performance sources (1-2 kW xenon arc).
Now, according to Thomas Tag's great collection of "
lighthouse lamps through time", the light sources originally were candles/oil lamps, followed by acetylene burners, (oil vapour) mantle lamps, and later electric sources (hotwires and ultimately arc lamps). And after reading that page I fully understand what is to be achieved with a sealite source. It's not about challenging performance but about replacing an original, way more complicated, effort-/cost-intensive source.
Yet, what about the
throw? What are typical luminance values of classical light sources? As far as I know (and luminance is one of my major
interests), typical luminance values of other sources are:
- again, for comparison: the sealite source seems to have a good 11 cd/mm² according to their specs
- candles / oil lamps: about 0.05...0.075 cd/mm²
- acetylene: about 7 times a candle (my own measurement), i.e., up to 0.5 cd/mm²
- hotwire: roughly 1-15 cd/mm²
- halogen hotwire: roughly 10-30 cd/mm²
- carbon arc: roughly 600-1000 cd/mm² (classic arc)
- xenon arc: roughly 500-2000 cd/mm² (modern arc)
luminance and throw relates 1:1:
Same luminance -> same max spot lux.
Twice the luminance -> twice the max spot lux.
So you have an idea: If I understand it correctly, it's like a good hotwire or a medium halogen hotwire. But probably with equal/wider spot and way less effort. Now we only need a quote :- )
I believe the huge cooling fins just serve a good job even if it's a burning hot summer with no ventilation in the lighthouse. After all it's just 200 W input to the source and roughly 3/4 of that as heat, i.e. 150W. The fins look spectacular but might just serve for chilled operations, without fans (as yet another source of error), and perhaps just also for a good look&feel (marketing ;- ), well I like them.
To compare
spot size of the original and the sealite source, we would have to know the very light source which were to be replaced. But for
throw (max spot lux), we'd actually only need the luminance, that is, the general type of the original source.
The gaps between the LEDs would be projected into the field with a perfect lense, which could be a problem. But in practice I believe the slight deficiencies of the cylinderical fresnel lens sytem compensate by blurring slightly.
