Banstead Police Station, Lamp Dimensions

[galacticprobe]

As one who's seen many types of Fresnel lenses over the years, both types exist for different purposes. The "straight" kind (those "facets" are called segments) is mostly used on things like ship's running lights and anchor lights. It refracts the light well enough so other ships can see it and be aware that you're moving in one direction or another, or not moving at all. For most ships (pardon the pun) passing in the night, being noticed from about five nautical miles (NM) away is a fairly clear berth (providing you don't have a moron driving your ship - we'd lost several Coast Guard Cutters and a few Coasties because of such morons; of course you could have a moron on the other ship as well). And when you consider that from a ship's point of view Line of Sight (or LOS) is only about 10 NM, even the faint glimmer on the horizon from the straight lens is enough to let you know that you are not alone on that wide ocean. And unless you do have a moron at the Conn, then you can easily steer a wider berth if you want more than a five-NM separation.

The "curved" segment kind of lens is mostly used in lighthouses to refract the light into a tighter beam so it can be seen farther away. This sort of a tighter beam is visible for up to 30 NM away (depending on height of the lighthouse and the order - or size - of the lamp and lens). In this case you would certainly want to see that light from as far away as possible because in some of the places I've been to, there are shoals as far out as 15 NM from shore.

Now moving into a metropolis such as London, with its buildings and street corners (since light doesn't bend around corners unless it's reflected - or you're walking with a black hole), a straight-segmented lens would more than suffice as a Police Box would not be more than a couple of blocks or so away, and the flashing light would be refracted more than sufficiently for an officer to see the signal even in the daytime. A curved segment lens would certainly have been overkill for that use, and as rightly pointed out, more expensive to make.

Dino.

[hb88banzai]

Wonderful summary, galacticprobe!

From everything I've seen, I believe the Met just used stock, readily available Anchor Lamp Fresnels of the appropriate size. The differences between the Banstead and Crich lenses seems to be one of supplier and/or vintage. All the early photo's I've checked that are of high enough resolution (including Barnet) seem to show the Banstead type of lens, while the Crich type was on those Mark 5's that were restored and sold a couple years ago (though they called them Mark 3's, of course).

[dr hue]

Comparison of Mk5 Beacon and my Marine Lens



Mk5 seems to have straight Facet segments and Marine curved.

[WonderWheeler]

The curved sections are better, especially for things expected to be seen miles away. In the US we have a small super cheap version of a glass porch light we call "jelly jar" style that is even worse looking.

The Fresnel lens, (pronounced fruh-NELL) was originally developed from earlier lighthouse lens designs and has widespread uses. Its based on a semicircle design, cut down  into segments and put together as one fairly thin cylinder. As shown earlier in this thread. Adopted early on in France, the designer might have been Belgian. https://www.youtube.com/watch?v=fQufI-QJLZg

I have seen these lenses used in railroad applications also. In more of a spotlight style as an indicator red versus green, pointing down the tracks, seen for a long ways even with a dim bulb.