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Postby AncientBrit » Mon Dec 08, 2008 6:34 pm

Guys,

I am one of those NBTV members that Chris Long refered to recently.
Reasonably clued up on electronics but with very little understanding of optics.

Can anyone give me a (short) explanation of how beads improve the performance of an ordinary aperture disc?

Do they gather more light?
Where does the primary lens have to be focused, still on the surface of the disc?
And if so, how come they do not blur the image by taking optical info from several pixels at once?

Regards,

Graham
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Postby Klaas Robers » Thu Dec 11, 2008 2:34 am

Oh Graham,

also not my job. But I think:

When you look through a glass or plastic sphere to a small light source at some distance from the sphere, you see a small point of light, much smaller than the light source. This point is located very close to the sphere, but it is in between the sphere and your eyes. The sphere can be seen as a lens with a very short focal distance, so there is an image of strongly reduced size of the light source. When you move the lens, the point of light also moves and is still visible, even when the sphere is no more directly in between the light source and your eye.

In this way you see a real image. The light source can be a single LED or a circular neon lamp. The light output is larger than from an aperture disc, as the lens (bead) collects light over a larger area. And there is no diffuser needed.

Disadvantage is that the size of the point is not so easily defineable and there is some stray light because of internal reflections in the spheres.

This is as far as I can imagine. As said, it is not my expertise.
Last edited by Klaas Robers on Thu Dec 11, 2008 7:24 pm, edited 4 times in total.
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Postby AncientBrit » Thu Dec 11, 2008 6:31 pm

Klaas,

Thanks for the explanation.
I knew the process worked but not how.

Kind Regards,

Graham
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Postby Klaas Robers » Thu Dec 11, 2008 7:49 pm

Graham,

I remembered that I was as a teenager whith my class on an excursion at the Dutch meteorologic institute. They measured the number of hours of sunny spells (mostly minutes, if any here in NL) per day with a so called Pyranometer, an instrument invented by Campbell and Stokes. When you search on the web on these words you will find it. But I will include a photograph that I found there.

It is a glass sphere of about 12 cm (5") with a strip of paper mounted close to the sphere. Every day a new strip was inserted. The sun, when directly visible burns a line into this strip. Mearuring the burnt line gives information about the duration and moments of the sun shining. The thing was mounted at the top of their observation tower, where a clear view from horizon to horizon was guaranteed. The explanation on how it worked was so simple and clear that I never forgot this.

So I knew that a sphere acts as a lens with a very short focal distance.

Of course these instruments are no more used and an electronic version is now in use. But they are still called Pyranometer.
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Postby AncientBrit » Thu Dec 11, 2008 11:36 pm

Thanks for the additional information Klaas, most informative.

Regards,

Graham
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Postby chris_vk3aml » Fri Dec 12, 2008 12:30 am

I know this will seem controversial, and many who believe in lens discs or bead-discs will scoff at this analysis. However, for efficiency figures of lens discs viz a viz simple aperture scanning discs, first log in to this forum, and then look up this section of the "Patents and Articles" section of the forum:

http://www.taswegian.com/NBTV/forum/viewtopic.php?t=575

The discussion of lens disc scanners commences on page 208 of L M Myers' book "Television Optics" (viewable on the page above), and continues for about five pages of maths. The author examines real-image lens disc scanners and virtual-image lens disc scanners (like the bead disc).

His conclusion, after deriving an optical efficiency equation for apertured discs (p.170) and the lens disc (p.210) - to quote from page 210-211:

"...this will at once be recognised as identical with that of the aperture disc in the same circumstances. This being the case, the aperture disc would be used in preference on economic grounds alone..."

Hard to believe? Maybe so, but the figures are there. Of course, this is comparing the lens disc system to an ideal apertured disc scanner where the objective lens, condensor lenses and photodetector/light source are all properly matched to the image area of the apertured disc. I have rarely seen this successfully done by NBTVA members.

I bet this starts a raging debate! It seems extremely contra-intuitive, but the maths are provided, with the necessary explanatory diagrams.

Best wishes,

Chris Long VK3AML.
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Postby Klaas Robers » Fri Dec 12, 2008 12:55 am

It might not be impossible Chris. It depends on the premisses that are taken. A bead disc will have a large viewing angle, much better than an aperture disc. You showed us yourself that an aperture disc with a certain thickness gives problems when you are not watching it perpendiculary. The bead disc has these problems hardly, because the out going light angle is very wide, almost 90 degrees (45 + 45).

Yes that includes that a lot of the light is spilled to other places than your eyes. However we have seen rather bright pictures coming from Eddie Greenhoughs bead disc monitor, with only one white LED, at this years Convention. On the other hand, a bead disc monitor has only place for one LED, as more LEDs will give rise to more spots in front of the beads.
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Postby chris_vk3aml » Fri Dec 12, 2008 6:57 am

...and that is precisely how I predicted you'd react!

- Chris VK3AML.
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Postby AncientBrit » Fri Dec 12, 2008 6:21 pm

Thanks for the link Chris,

Regards,

Graham
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Postby gary » Sat Feb 14, 2009 1:59 am

Here is a slideshow of a test run in making a small bead disk on my newly completed homemade CNC machine - I have just used some acrylic sheet I had lying around as a first test - seems to have come out pretty well - anybody know how to paint acrylic? :-)

Process chain:

Create a DXF drawing of disk using in-house written software based on CadLib

Import into free version of CamBam to generate tool-paths and create G-Code file

Download G-Code file to TurboCNC on CNC computer.

Run G-Code and wait half an hour.
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Postby Klaas Robers » Sat Feb 14, 2009 8:49 pm

Marvellous Gary.

I think that you may spray the disc with acrylic laquer. Black matt. Spray both sides to make the black intransparent enough. May be its even better to spray the back side aluminium. In most cases there is real aluminium in the paint and that is intransparent.

Do NOT use black acrylic material for the disc and rely on the blackness. Black plastic is in reality very dark red and is transparent for IR.
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Postby gary » Mon Feb 16, 2009 2:10 pm

Thanks Klaas, The matt black acrylic paint does seem to do the job (with enough coats) - it adheres much better than I would have expected.

But what is this aluminium paint you refer to? Is that faux aluminium coloured paint (like gold, brass, etc) - not one that I have noticed down the hardware store.
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Postby AncientBrit » Tue Feb 17, 2009 12:04 am

Very impressive Gary, those hours in a hot workshop paid off.

What accuracy do you estimate you can achieve with the rig?


Regards,


Graham
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Postby gary » Tue Feb 17, 2009 1:22 am

Thanks Graham, well, the resolution is 1/4000 th of an inch although that could be improved on with a micro step controller, but I expect the runout on the cheap router is more than that I haven't measured it yet, and the backlash according to my dial indicator is about 1 thou - although I think I can adjust that out.

All-in-all, so far, for a first attempt I am pretty pleased with the result, more testing is in order.
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Postby AncientBrit » Tue Feb 17, 2009 6:21 pm

That's impressive Gary, should do for NBTV!

Regards,

Graham
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