Narrow-Bandwidth Television Association

My first laser cut Nipkow disc http://www.taswegian.com/NBTV/forum/viewtopic.php?f=35&t=2452 worked really well but the apertures have a thickness of 1.5mm so there is considerable parallax when using a magnify glass as the focal point is close to the disc. When no magnify glass is present there is very small parallax occlusion. I even get overlap at this point.

Here is a comparison of the same disc. Left side no magnifying glass, right side magnifying glass



This brings me to the idea that I can laminate some 0.38mm polypropylene to some clear acrylic to get a rigid/thin aperture accurate disc I will give this a go when I get some time.

I cut the smallest hole I could in 0.38mm polypropylene, it turned out right on 0.2mm if you discount the intended round hole overlap. Here is a pciture. I will have 0.38mm black polypropylene in stock next week. I tested the 0.6mm black and it has no detectable light leakage



Mircons haha... Microns woops

Cheers
Keith

Keith, it looks as if you had square holes, because I see horizontal structures at the ends of, and in the lines.

In the past Dominique made computer simulations of different shapes of holes, to see the artifacts and the difference in visibility. I will attach a picture of four different shapes. Try to download the picture and watch it enlarged. You will see that some are sharper than others, while others reproduce slanted line better than some.

There is quite some theory behind it. I include the article on that as .pdf as well. As far as I remember, it was never published in the Newsletter.

Yes Klaas thats exactly what I had, rectangular holes to be precise as documented here. http://www.taswegian.com/NBTV/forum/viewtopic.php?f=35&t=2452 Thank you for some interesting information, I had not seen that before.

What I have found is the thickness of the disc which causes parallax occlusions is far more critical than a few percent error in the aperture size and shape. Well it is in my case anyway. When you use a magnifying glass it moves the observation point forwards toward the Nipkow disc which increase the parallax error (makes the apertures appear smaller due to parallax)

This is why I am giving the 0.38mm Polypropylene a go. It is effectively 1/4 of the thickness of my current apertures so the parallax will be massively reduced. I am quite excited to try it out as it will be quite quick and easy to make discs if this method works. Obviously the best disc possible would have zero thickness

How thick are the typical metal discs you talk of?

Cheers
Keith

I measured mine, once made by the late Mart Schouten, to be 0,5 mm. It is a 28 cm disc. And I have a large disc, 50 cm diameter, made by the late Dennis Assemann, which is 0,4 mm thick. Both discs are made of aluminium. Both discs are punched with square holes.

Keith, if you can produce square holes by laser cutting, you should also ba able to make the "Cat's Eye" apertures. I think that they are about optimal. Small displacements horizontally are not observable, which is not the case with squares. And they give a better rendering of slanted lines than squares. Until now nobody could make them, but with laser cutting.....

Good luck

Hi Klaas

I am glad you have mentioned this because I will be testing the 0.38mm polypropylene soon. Can you give me some dimensions of the cats eyes? e.g. lets say my apertures needed to be 1mm wide for a 32mm wide display using 32 lines. What would the cats eye dimensions be? Can you give a drawing?

I like the idea of "optimal apertures" thanks.

Cheers
Keith

Look at the bottom bar of the pictures above. Then you see that the square is 1 x 1, which just gives scanning lines without black or white overlap lines. The Cat's eye is 1 x 1.6, that is a height of 1 square and a peak to peak width of 1.6 square. This gave the best supression of the overlap of lines. I remember that the surface of all four structures is almost the same, so you get the same light output.

For a better view of the results download the .pdf and look at it at a zoom of 200% or more. Then you see the effects of the different shapes best.

Good Luck,
Klaas

I had a few goes at cutting a cats eye. The pdf was quite misleading as the aspect ratio of their "drawn" cats eye was way off. I made a 0.7mm aperture which is 0.7x1.55= 1.085mm with a height of 0.7mm

It came out in a squashed diamond shape which is probably close to optimum. Actual size is 0.975mm x 0.65mm so I could tweak it slightly bigger. Here is a picture of the result. The cool thing is these discs take no time to cut. The bad thing is they take a while to draw, unless I write a macro. Maybe I should do that next.

This what the cad looked like


This is what cut looks like




Cheers
Keith

This shape is rather oplimal. As you might have seen is the diamond shape also quite well. I am curious about the final results.

Klass, I am making slow progress at the moment, too many things on but I did have an idea that I am sure will interest you.

How about a Nipkow disc with no holes? I was thinking if cut a clear acrylic disc, then engraved the apertures, just 100 microns deep. I could then shine the light into the side of the disc e,g, light pipe it. And where the engravings are, the light will leak out nicely.

Cheers
Keith

I fear that the efficiency will be far too low.