Mike Verta's CG TY-J Project

[mverta]

The lens "feels" right in any case.  They're all "Fresnel" lenses - there's a lot of slop in the profiles compared to what a true Fresnel lens would be if made with absolute precision. These lenses bleed off a bit of light at the rounded "ridges" but this is doubtless due to allowed tolerances in mass production.

[galacticprobe]

So far, I'm seeing the s18 lens has 3 "steps" like yours does, Mark, and not 4 like the reference I have here. Timerotor's is also a 3-step lens.

Most Fresnel lenses of this size (and some a little larger) have only 3 prisms above and below the "bull's eye" bulge in the center. (More terminology to help avoid confusion, especially when people start talking about "steps" above the TARDIS doors or side walls. Fresnel lenses have "prisms". Moving on...)

Even the original Brachacki's Season 1 Fresnel lens had just 3 prisms on top and bottom of the bull's eye, as does (do?) the lens(es) on the Series 5 on props. From what I've seen of the Newbery's lens in the references, it has 3 above and below, and the housing it sits in is very like the TY-J's (at least its white housing was: http://tardisbuilders.com/index.php?topic=250.msg363#msg363, and http://tardisbuilders.com/index.php?topic=966.msg61197#msg61197 - 4th image down, excellent close-up). I don't have a super clear image of the Hudolin lamp from the TVM TARDIS, but from what I do have, it looks like that one as 3 prisms as well. (Haven't really gotten a close enough look at the Series 11 lamp to count the prisms on that lens, but I'm going to say it also has 3 above and 3 below the bull's eye.)

So my guess is that all of the Fresnel lenses used on the TARDIS props were like the one Mark posted. (I know all of the ones on the ships I was stationed on were like that, and the lens I managed to "acquire" from one of them is like Mark's... almost the exact dimensions, too.)

I hope some of this is helpful.

Dino.

[mverta]

Mark -

I splurged for a full-size shot from the Davison shoot since it had a damn-near-orthographic shot of the lens on it.  I overlaid your own lens on it and it lined up basically exactly.  Here it is beside the original.  If it's not exact, it almost might as well be.

[Mark]

That is almost a perfect fit judging on your comparison picture.

I bought the lens for a potential Police Box build before I found out it was too small.

At least I know it's about right for a TARDIS build should I ever make one!

[mverta]

This is bothering me.  If you realign the prisms to their "original" orientation, it's not a Fresnel lens.  At least, it's not defining a spherical edge, which Fresnel lenses start from.  I have to consult with some optics people.  I want to understand what is going on here.




EDIT:  Got it - It is a Multi-Order lens, which is made of concentric annular Fresnel zones.  This helps reduce aberration and pinpoint a focus point.  Mark's lens prisms are indeed different radii based on the bull's eye radius.

[galacticprobe]

Correct. Those upper (and lower) prisms will be shaped differently because their focal plane is farther from the light source, so they need to refract the light at a different angle in order to focus the light beam on the same plane as the bull's eye. (It's one reason why the light from a Fresnel lens looks so bright when you're looking at it head-on, but from a higher or lower viewpoint it doesn't look as bright.)

Lighthouses (which I've also worked on in my career) have lenses that work the same way, and their lenses' prisms get narrower the higher (or lower) you get from the bull's eye... and why you can safely see that "laser-like" beam from a lighthouse when looking at it from the ground, but would blind you if you looked into it. (I put this in as an added reference since Mike mentioned "multi-order". Lighthouse lenses are designated in "Orders" according to size, and there are 7 Orders. A 1st Order lens is the largest - roughly 6-foot inside diameter - and used on seacoast lights. The lenses get smaller until you reach the 6th Order - inside diameter about a foot - which are used on small river lights. The odd "3 1/2 Order" size, which gives us that 7th Order, is a "bi-valve" lens... it's shaped roughly like a clamshell. I could go into the differences between the facets of a rotating lens - which gives you a real laser beam light and an apparent flashing characteristic - and those of a lens from a "fixed" light - which doesn't have a flashing characteristic, but rather an all round halo - but we'll save than until some Time Lord turns up with a TARDIS disguised as a lighthouse... or someone wants to build a full-size replica of Fang Rock Light. )

Dino.

[Angelus Lupus]

Fascinating! I suddenly want to learn all about Fresnel lenses!

[mverta]

Here is Mark's/Davison Photoshoot Lens in the assembly.  The assembly matches the height and proportions of the Davison photoshoot.  My posts are slightly thinner and will be addressed next.




This lens is dirty and has manufacturing seams but no other text at the moment.

[Angelus Lupus]

Blimey!! 
...what more can I say?

[Mark]

Wow!

I've just looked at my lens from a similar angle and it's spot-on even down to that weird shaped "smiling mouth" shaped selection/refraction.

Out of interest, how do you even go about modelling rust and scratches on something like this?

[mverta]

As long as you get the Index of Refraction correct and the wall thicknesses and angles, physics is physics; a path tracer traces virtual photons through the refraction the same way nature does, so the results look just like they do in life.

For weathering, I build my materials in layers, just like the real objects would have.  So in this case, there's a base metal layer, which has its own character and scratches and whatever, and then I put on the paint layer - clean, just like it's brand new.  Paint has very different optical qualities.  So does dirt, dust, etc.  Each gets its own layer so that each material is itself photoreal.  Then I begin making maps to reveal which layers go where, and how far down things like nicks and scratches cut.  In the case where I cut all the way through to the bare metal and it was exposed, I made a rust material.  So it's many layers of materials, just like in life.  The advantage is that I can change WHERE things are very easily, and I can equally preserve where things are but change the underlying materials as well.  So it would be no problem to change the blue paint job without affecting all the dirt and scratches and years of weathering on top of it.  All of this is more work, but if you want it to look natural, you have to do what nature does!

In the end, all of this is so that it disappears; so that the viewer doesn't question it, or see it as CG.  You know you've done the work when nobody appreciates it.

[mverta]

Did my first turnaround:

Needs work! But it's on it's way...

[bjones]

When you finish it, please output some psd files with a transparent BG and email me them ))

You know you want to.

Barry

[darren79]

My favourite box!

That looks amazing!

[mverta]

I've never painted a Police Box before, and despite being virtual, this is the same process - lots of layers, careful choice in colors, repeat.  I'm going for the battered look seen in Full Circle - it's my favorite weathering.