As promised, I paid a visit to banstead today, and here's the result
OK, it's been over 40 years since I did any TD, but the answers are all here, and it's massive
All diameters are calculated to pi as best I can, so not perfect by definition, but they are very, very close. One thing I hadn't noticed before, is that the flange above and below the fresnels were not sunk in (The lens moved very easily). Holding a straight edge against the side, the "Bulge" and outer lip of the fresnels were on the same plain, but to a whisker. One thing's for sure, our ebay selling friend was MILES out, if he thought his lens was "Real"



KB

Wow, that's fantastically detailed, KB - you must have a big vernier caliper for that!

Thanks for taking the time to head out there, and take the measurements for us

Wow - that is absolutely massive! Must weigh a ton.

Always thought the real thing was on the order of 7 inches or so at the bulge, so this is quite a revelation. Well done chriskingbees!

I think you said you'd be using a cloth tape to measure around and then calculate the diameter from the circumference. Did you happen to compare your cloth tape to another ruler or steel tape to check its accuracy?

Well, yeah... this thing is huge, but it is, after all, for an actual Met Box. We know the one on the TARDIS isn't quite as big and can thank DoctorWho8 for his measurements confirming that. (And of course, other members who have posted overhead grabs and publicity photos of the New Series TARDIS, especially the ones with the Gardman Lantern on top showing how the Gardman's roughly 7.25-inch diameter base overhangs the sides of the little box under it by just a smidgen. And with the Smith TARDIS being built to the same plans - with some adjustments for a taller TARDIS base and varying thicknesses of the corner posts - the little box under the lamp seems to be the same size as before.)

So people building TARDISes, don't fret if you think you've got a lens that's too small. This one here is a Met Box lens and it's fracking huge!

Thanks for the dims, KB! I always wondered what the actual size was. (Even the Trench Plans didn't show those.) Also an interesting tidbit about the lens "flange" not being sunk into a recess to hold it in place. (Maybe casting a grove for the lens flange in the concrete when they made the boxes wasn't working out that well, so they said the heck with it, thinking the pressure of the cap, once the bolts holding it to the base were tightened, would be enough to keep the lens solidly enough in place?)

Dino.

Quote from: hb88banzai on Apr 14, 2014, 11:32 pm


I think you said you'd be using a cloth tape to measure around and then calculate the diameter from the circumference. Did you happen to compare your cloth tape to another ruler or steel tape to check its accuracy?

Yep, two tape measures, two strings, and a "Hold up the tape measure for a visual" just for luck, all came out to more or less the same
I measured to supporting struts, too, and they are are 3/8" thick, and actually touch the glass, making the lens secure, more or less.



KB

The Crich lamp for a quick comparison --



Here's a pic of what is believed to be the standard type of cast iron mounting plate/flange/base (here the spare at Crich) that was used to attach the lamp assembly to the roof (photos courtesy of Mark) --



The shallow groove at the centre that the Fresnel would sit in and on measured 19.5 cm at the widest (per Mark), which would be a bit over 7-5/8", so it would indeed be a touch loose for the measured 7-1/2" ends of the lens. You can see the outline of the holes for the struts (filled in by gunk) just outside the area of the groove. The central bar/plate would be the mount for the light fixture.

Note: Zoom in and you can see in the above photos (both of Banstead and Crich) that the cap does indeed have a cutout so it can fit down over the end of the lens, but the clearance visible is far more than the 1/16" or so on each side in the base. This also means that the actual overall height of the Fresnel would probably be a bit more than the measured 6-1/4" as that was only the part that was exposed (probably the length of the struts based on the drawing).

Another view --



And you can see it on the floor under the phone/first aid cupboard here --



In the outside photo of the Crich lamp you can see a rectangular drainage hole in the side at the bottom of the mount (there was one on each side per the Barnet photos). The idea is that the whole base and lamp assembly is separate from the concrete roof, and is what they've attached to the top of the brickwork at Banstead.

It does indeed appear to be a stock Met Box lamp assembly, base and all.

This is very interesting. I had assumed that, as I can see the the flange at the top and bottom, and the fact that it was "Loose" as such, the whole of the lens was in view at Banstead. This adds greatly to the (My) theory, that as these are miniature buildings, built on site, probably by different contractors, there would be all sorts of anomalies, like roof differences etc. Maybe, just maybe, even the lenses are slightly different, coming from different sources, or indeed, if from the same source, different batches.

KB

Looks like the lens goes up inside quite a bit past the flat bottom of the cap --



Interestingly, per the Trench plans (which for various reason I would date to circa 1935, during the redesign for the Mark 3 (Barnet type) boxes) the top light assembly is specified as not being part of the structure to be provided by the builder --



"Tender" here being a reference to the bid by a builder for the contract for a particular set of Box installations (usually by Division).

This means this component was provided by either the GPO or the Met themselves. As it was apparently (per Bunker) designed "in house," I suspect it was the latter. So they were probably a bit more consistent than the Boxes themselves, but that said we have seen some minor variations in them in the photos even after the "reactor" globes were retired.

KB, do you happen to have any photos of the top of the cap? I'm curious what the top of the strut bolts look like (at least I presume they go all the way through).

After Several Years of attempts I finally managed to get out to Banstead for a bit of Research.
I've put the Site Visit photographs on my Flickr Site: https://www.flickr.com/photos/etas/sets/72157646185221721/.

I went with my favorite silicone molding putty, my trusty Vernier Calipers 2" and 6" and a new 12" Vernier Caliper, as well as a contour device. While my primary goal was to secure the Fresnel Glass Profile, I also took the time to photograph the site, as well as take contour measurements, and caliper measurements of all that I had time for. There about 82 Photos of the Site, more than half of them are of the caliper measurements to puzzle out the exact measurement later.

Lantern Profile - Link


3D Model and Dimensions - Link


Field Notes:
Profile Page 1
Profile Page 2
Measurements

It would have been nice to make a huge mold of the lantern to be able to cast it properly, but as always we were running late, and I only had enough time to make a copy of about 1" strip of the lantern. Even then it took forever for it to all harden so I could pull it off. I think that the top prism suffered for this impatience however and has a kink in it. Previous research on the Northwood Police Post Fresnel Glass has allowed me to inference that the two profiles Top/Bottom are mirrored so only one side needed to be perfect anyway. Although please note that the heights of the flats at the top and bottom are different. this seems true for all sorts of Fresnel Lantern Glasses.

Cheers.

Starcross, great details.

Is the 1/4" glass thickness at the top something you were able to confirm, or just an assumption?

Did you get definite dimensions for the diameters that Chris labelled B & D?  (Centre bulge of lens, and vertical flange at base).  Or do they need extrapolating from your curved profile lines...?

I recently got a mould off the Crich lens, which has some interesting differences from this one, so will post diagrams shortly, to compare them...

Quote from: starcross on Aug 23, 2014, 06:54 pm
....  Although please note that the heights of the flats at the top and bottom are different. this seems true for all sorts of Fresnel Lantern Glasses.

Is this different height thing definite...?  From my Crich mould, I can definitely place the height of the top flange, but the bottom one is set into the metal base, and has been painted over, so it's hard to tell where the glass ends.  I had assumed it would be the same height as the top one, but maybe not...

Quote from: matt sanders on Aug 24, 2014, 12:12 pm
Is the 1/4" glass thickness at the top something you were able to confirm, or just an assumption?

Its an assumption I'm afraid, 1/4" and 5/16" seem to be fairly normal for Lantern glass. My PA1 Lantern Glass is 5/16" for example. Mostly the 1/4" was used for closing the profile so I could use Revolve to make the 3D model.

Quote from: matt sanders on Aug 24, 2014, 12:12 pm
Did you get definite dimensions for the diameters that Chris labelled B & D?
(Centre bulge of lens, and vertical flange at base). 
Or do they need extrapolating from your curved profile lines...?

I used my contour device to take profiles of a few dimensions, they should be on the field Notes pages as to which. B I ignored because where could the true centre line be? As for D it was too moss covered on the opposite side.

All is not lost however as I can cast a full positive of my 1" strip and pull the contour from that casting. I just need to use something more solid than the Plaster, such as my sculpy clay.

Once the contours are in the computer I just match a 2 Point circle to the edge to get a full diameter.
For the purposes of the 3D model I used KB's 7 1/4" on measurement D at the base of the glass to make the profile revolve.

Quote from: matt sanders on Aug 24, 2014, 12:16 pm
Is this different height thing definite...?  From my Crich mould, I can definitely place the height of the top flange, but the bottom one is set into the metal base, and has been painted over, so it's hard to tell where the glass ends.  I had assumed it would be the same height as the top one, but maybe not...

Yes this is definitive. As KB said the lantern moved freely so when I squished the putty in I lifted the lantern so it would sit on the putty. It was crazy thin and I almost lost it when I pulled the mold off but its there and accurate. Sadly as you can see the top kinked but the flat stayed true. Its possible the kink is a flaw in the glass, although I don't have the photo angles to really confirm it.

In that regard the lantern may be a 1/16" or so shorter than what I show on the computer profile.
Mind you I'll double check the cast profile to see what the difference is and let you know.

Quote from: starcross on Aug 24, 2014, 02:50 pm
For the purposes of the 3D model I used KB's 7 1/4" on measurement D at the base of the glass to make the profile revolve.

Actually, KB gave diameter D as 7_1/2", not 7_1/4".  And I can confirm the Crich one is EXACTLY 7_1/2" there too.  This would make sense, if the cast metal bases are the same at both locations.

I suspect that this is the ONLY dimension that both lenses share...

Just a comment about Fresnel lenses.  The objective of them is to reduce the thickness/mass of a large lens by eliminating unnecessary material.


In the two lenses I own, I can see the curvature of all the segments and that they are not all the same height.  When I look a the pictures of the Banstead or Crich lamps I'm not seeing much curvature or height differences between the segments, but I'm wondering if these are present?  If there isn't, it could be that whomever manufactured these lenses just didn't bother trying to maximize their efficiency.

Colin

I've posted an accurate profile of the Crich lens in the Crich Lamp thread in this section, and yes, the ribs have straight profiles, rather than curved.

But of course, the light source being refracted by the lens is quite a large bulb, not a pin-point of light, so it probably wasn't worth the extra effort to put those curves in, as it would not make any discernible difference to the end result.