fresnel(?) lenses

[ducatisti]

I was standing on the Whitechapel-bound platform of Paddington (H&C) on Sunday and was idly examining the distant?(repeater?) halfway down the platform.
The yellow lense was stepped, wheras the green was a conventional shape. Does anyone know what the function of this is? I presume it's to make the aspect more visible, but I'm slightly confused as to why you'd make one and not the other.

[mrfs42]

Jul 14, 2009 13:02:31 GMT ducatisti said:

I was standing on the Whitechapel-bound platform of Paddington (H&C) on Sunday and was idly examining the distant?(repeater?) halfway down the platform.

Seeing someone off to the West then?

Jul 14, 2009 13:02:31 GMT ducatisti said:

The yellow lense was stepped, whereas the green was a conventional shape.

Hmm; depends from which side of the coin you're speaking from to define 'conventional'.

Obviously, main signal aspects use fresnel doublets to produce a beam of (very nearly) parallel/collimated light, directed at the Motorman. AFAICR, the inner lens is stepped in the air gap between the doublet and the outside surface of the outer lens is smooth with the annular stepping also in the air gap between the doublet.

There are several possibilities:

1. Both aspects were originally fresnel doublets and the clear outer has fallen off for the yellow.

2. Sighting reasons - Motormen would need to see the yellow at a distance - hence the fresnel; the green would not need to be viewed at such a distance, so a fog repeater green (wide diffuse angle) would be used.

3. The green is an LED cluster (unlikely, but possible as the LEDs are slowly creeping across the system).

4. The green is a 'new' style short range colour light signal and is only a single lens, rather than a fresnel doublet.

I suspect the answer lies somewhere in these possiblilities

[Colin]

If it's halfway down a platform it'll be a platform repeater not a distant!!

As I don't know the platform in question to make a categoric statement, the clue will be whether or not an ID plate is attached - no ID plate will mean it's a lowly platform repeater and nothing more.

[mrfs42]

Ah. I'd assumed we were going on about platform repeaters in my reply. Oops.

Was this the one in question: clicky - from 21146.

[Tom]

Jul 14, 2009 14:34:22 GMT mrfs42 said:

4. The green is a 'new' style short range colour light signal and is only a single lens, rather than a fresnel doublet.

That would be my guess. Platform Repeaters tend to be SRCLs

[ducatisti]

I may have been seeing someone off...

That photo is looking in the general direction of the signal, but it's behind the not used staircase towards the far end of the platform.

I suspect the single/double may be the answer, the green lense looked like a conventional signal lense with what I always assumed were grooves in it.

Hmmmm... I'm sure I'll be be back that way soon enough (although next weekend I'm off down there on the infernal device that gives my sig, so not that soon). I'll have another look then.

Thanks all

[mrfs42]

.jpeg? Please? [1] The signal, rather than your fine Italian machinery. I'm intrigued.

[1] if you get chance, rather than a 'much-stamping-of-little-feetses'-type request. You know what those are like.......

[ducatisti]

Will do - won't be for another fortnight or so.