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Old 31st Dec 2006, 12:03 pm   #1
Panrock
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Default Colour mixing in mechanical television

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Sure thing Steve. The main page is here: http://www.earlytelevision.org/color...w.html#12-21aI am very anxious to see what kind of results Steve M. gets from this project. The 30 line mirror screw sets I've seen operating are very impressive.Darryl
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I'd have probably not tried to persuade light from LEDs into optical fibres like Steve McVoy did, but would have tried a different approach, namely having a stack of narrow-angle ultra-brights some distance behind a vertical slit (probably bearing a perspex/frosted 'mixing rod') at the end of a light box, with the LEDs adjusted to beam toward the slit and mixing there. I would have thought this would be more efficient and give a brighter picture. It's a variation on the arrangement I currently use. Steve
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Steve - I'd like some more details of the arrangement you use for your mechanical light source, and in particular of the perspex/frosted 'mixing rod'. It may well be a better solution than the fiber optic one.
Steve, the method you have adopted to mix the colours for your 60-line mirror-screw monitor looks superbly engineered as always, and I offer my own suggestions with due respect. My idea would not be perfectly efficient either, but I think it might be more so.

The problem is how to efficiently concentrate the modulated light from your super-bright coloured LEDs onto the long narrow slit (necessary for illuminating mirror-screw displays) and also how to effectively mix the light from the three coloured primaries.

As I see it, the key to the solution is to exploit the narrow angle beams of your LEDs (typically only 15). This means that almost all their light will be concentrated in a patch 3 times smaller across than the distance from the LEDs (1/tanθ). So my colour light mixing box consisted of a mosaic of red green and blue LEDs at one end of quite a long box, with a ground glass screen at the other end. This is an efficient arrangement. Using 15 LEDs the box should be about 3 times longer than it was wide; then virtually all the light from the coloured LEDs would then arrive on the screen and be thoroughly mixed.

The problem then becomes how to adapt this arrangement to concentrate the light to mix on a narrow slit, rather than a broad square. I therefore think the LEDs should be arrayed in RGB sequence along a line rather than as a mosaic (in my case), set some way back from the exit slit. Their natural beam overlap should provide effective colour mixing at the exit slit.

In passing, I see that 8 super-brights are available in the Maplin catalogue (UK). So then the light would spread out 7 times less than the distance it is from the LED. Basically, the tighter the LED beams - the better...

But still, a lot of the light will get lost where it misses the narrow exit slit. If two reflective plates were added either side of the diverging beam, positioned half way between the slit and the LEDs, then more of the light should emerge from the slit (and then diverge again). Using parallel reflective plates, the natural divergence angle of your LEDs then becomes the viewing angle.

On further reflection, I think a perspex rod placed just behind the exit slit might collect more light but it would also act as a strong diverging lens. This might only become worthwhile if the viewing angle were otherwise too narrow.

Steve, I emphasise I am thinking off the cuff here so my ideas may well have drawbacks I haven't yet thought of!! I tend to just go ahead and try things out to see what happens in practice, rather than plan properly beforehand!

Anyway, I think your existing plans are already well thought-out and will give very impressive results.

Steve
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Old 31st Dec 2006, 1:12 pm   #2
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Default Re: Colour mixing in mechanical television

As is often the case when designing a solution for a new problem, the first idea isn't always the best. Your solution seems simpler and probably better than the one I've chosen.

I'll complete my fiber optic source, then play around with Steve's idea and compare the results. Since the light source needs to be 3-6 feet from the mirror screw, and the screw is about 5 inchs wide, the angle is relatively small, so I think that I won't need a perspex rod.

I wonder if this approach would also work for our lens-disk scanner:

http://www.earlytelevision.org/weste...storation.html

Here the problem was getting a point light source (20 mils) with a dispersion angle of about 20 degrees. I'm using an original crater lamp now, but hate to demonstrate it, since there are very few of these around. Couldn't a bright LED be used with 4 parallel mirrors and a hole 20 mils in diameter?

Darryl has completed the color version of his Aurora, and built a light source for his 60 line scanning disk set. The results are amazing:

http://www.earlytelevision.org/color...ew.html#12-31a

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Old 31st Dec 2006, 1:51 pm   #3
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Default Re: Colour mixing in mechanical television

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I wonder if this approach would also work for our lens-disk scanner:

http://www.earlytelevision.org/weste...storation.html

Here the problem was getting a point light source (20 mils) with a dispersion angle of about 20 degrees. I'm using an original crater lamp now, but hate to demonstrate it, since there are very few of these around. Couldn't a bright LED be used with 4 parallel mirrors and a hole 20 mils in diameter?
Instead of 4 parallel mirrors I should think an internally polished cylinder (or large converging lens) would be better.

However why not simply saw off the top of a bright LED and then polish the resulting flat emitting surface? Wouldn't the result then be an instant 'crater lamp' ?

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Darryl has completed the color version of his Aurora, and built a light source for his 60 line scanning disk set. The results are amazing:
http://www.earlytelevision.org/color...ew.html#12-31a
Terrific stuff! Congratulations to all concerned - can't wait now to see the pictures with your mirror screw!

Steve
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Old 31st Dec 2006, 1:56 pm   #4
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Default Re: Colour mixing in mechanical television

I tried sawing off a bright LED, but the surface is about 40 mils for the LEDs I used.
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Old 31st Dec 2006, 2:24 pm   #5
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Default Re: Colour mixing in mechanical television

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I tried sawing off a bright LED, but the surface is about 40 mils for the LEDs I used.

Have you tried sawing a 3mm LED ?

Steve
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Old 31st Dec 2006, 2:59 pm   #6
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Default Re: Colour mixing in mechanical television

It's been a while, but I researched a number of LEDs, and all seemed to have 40 mil surfaces. The super bright ones got their brightness by putting a number of 40 mil elements together. For 45 lines resolution, I need something no larger than 20 mils.

If anyone comes up with a LED that might work, please let me know.
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Old 31st Dec 2006, 6:07 pm   #7
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Default Re: Colour mixing in mechanical television

I've just had a look at a random couple of LEDs, one a 3mm standard and one a 5mm super-bright, ground them down and lit them up...

It wasn't possible to make precision measurements so these are approximate. Each LED looked like it had a 0.3mm (12 mil) central emitting surface, surrounded by a 1.2mm (47 mil) reflector for the smaller LED and a 1.5mm (59 mil) reflector for the larger.

If you could get in really close to the emitting surface (for the sake of the emitting angle) then screen out the concentric reflector with black paint, might this not work - giving a 12 mil light source ?

Steve
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Old 1st Jan 2007, 1:08 pm   #8
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Default Re: Colour mixing in mechanical television

A very quick search found these http://www.elexp.com/opt_rgbc.htm. You might be able to find better, as I am sure there must be others designed for LED based screens.
Interestingly, the output is not the same for each colour but I guess these must be balanced. You might want to synchronously switch them to allow more output, by reducing the duty cycle, which would also have the added benefit of reducing unwanted light.

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Old 1st Jan 2007, 10:40 pm   #9
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Default Re: Colour mixing in mechanical television

It looks like a great project but being an old Physicist I have a couple of observations although I know absolutely nothing about scanning mirror receivers.

The mirror screw must have been designed to work with a specific modulated light source set at an exact distance from the mirror.

From what I understand it needs to be a slit source and the slit width will determine the horizontal resolution of the receiver.

The light source must be able to illuminate with equal intensity from all possible viewing angles the entire area covered by the rotating mirror and sufficiently outside the area so as to allow for viewing angle.

I don't think tight beams are going to work well from 6 inches away you need something that evenly disperses the light with a mask over it.
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Old 2nd Jan 2007, 1:46 am   #10
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Default Re: Colour mixing in mechanical television

PJL - You are correct. The light source needs to be 3 to 6 feet away, and the horizontal resolution is determined by the slit width. The .75 mm fibers I have chosen will result in about 170 lines of resolution, which is more than enough. The fiber optic approach will provide equal intensity from all viewing angles, and after thinking about it I have concluded that the recessed LED approach will require a ground glass (or some other object) at the slit to work. The question is which approach will provide the most illumination.
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Old 2nd Jan 2007, 10:21 am   #11
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Default Re: Colour mixing in mechanical television

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and after thinking about it I have concluded that the recessed LED approach will require a ground glass (or some other object) at the slit to work. The question is which approach will provide the most illumination.
Yes, I must think more about this. Perhaps the recessed LED method with parallel plates would produce a light field which was bright in the centre then bright again at the edges where the reflections from the parallel plates kicked in, but somewhat lacking in between? If this were so, it should be possible to overcome this by bending the plates (as they approach the LED) into a parabolic curve. No lossy ground glass needed then...

Steve
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Old 2nd Jan 2007, 7:18 pm   #12
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Default Re: Colour mixing in mechanical television

The mirror will not be perfect, leading to a backgound light. This is not a contrast ratio as we know it ,as it is the modulated source so might be distracting in intended black areas of the 'screen'.
Assuming the mirror is set in a blackened box, it is possible the optimal light level may not be that difficult to achieve...
all the best Peter
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Old 3rd Jan 2007, 5:05 pm   #13
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Default Re: Colour mixing in mechanical television

I am not convinced the calculation for the line resolution is correct as I would expect it to be dependent on the distance of the light source from the mirror...needs some thought to work through the maths.
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Old 5th Jan 2007, 2:28 am   #14
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Default Re: Colour mixing in mechanical television

I've done further experimentation with my fibre optic approach and the mirror screw. The individual fibres are visible in the mirror screw, and I'm convinced that some sort of diffusion material is needed to blend the colours. This will significanly decrease the light output, since the diffuser will have to be placed some distance from the ends of the fibres to avoid hot spots. I think that Steve's suggestion of using recessed LEDs will probably result in a higher light output, but a diffuser will be needed at the slot.

I've ordered some high power 5mm LEDs (9000-16000 mcd) which I'll try.

http://www.earlytelevision.org/color...crew.html#1-4a

http://www.earlytelevision.org/color...crew.html#1-4b

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Old 5th Jan 2007, 8:18 am   #15
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Default Re: Colour mixing in mechanical television

Since when viewing the picture the eye will be focussed on what makes it up: the pixel source at the slit, and the colour beams will be overlapping at the slit anyway, I can't see why any diffusing material will be necessary.

In other words, the eye will automatically do your 'diffusing' for you by virtue of being focussed on the slit.

Steve


PS. I've now been to see your updated article and I think I understand why you propose a diffuser; this is because you are using the colour LED rows side by side, and not in a line. Without a diffuser this might give a red, a green and a blue picture side-by-side. Instead, I think they would be better all in a tight line, arrayed in RGB sequence. Then you shouldn't need the diffuser.
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Last edited by Panrock; 5th Jan 2007 at 8:44 am. Reason: More information arrived
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Old 5th Jan 2007, 1:27 pm   #16
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Default Re: Colour mixing in mechanical television

Your eye sees directly to the LEDs, off the mirror of the screw, through the slot, to the rear of the box. Arranging the LEDs in a row won't help, as my experiment with the fibers demonstrated. You can see each fiber (or LED), creating a confusing bunch of red, blue and green lines in the picture.

If it were possible to get extremely small LEDs (.1mm or so) then arranging them vertically would work. That was what I was trying to accomplish with the fiber optic approach, but the .75mm diameter of the fibers was too large.
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Old 5th Jan 2007, 1:42 pm   #17
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Default Re: Colour mixing in mechanical television

Could try this instead. Box needs to be reflective inside and the deflection plate would need to be painted in something purpose made? Alternatively you could try the colours side by side and a delay in the RGB signals?
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Old 5th Jan 2007, 6:52 pm   #18
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Default Re: Colour mixing in mechanical television

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Your eye sees directly to the LEDs, off the mirror of the screw, through the slot, to the rear of the box. Arranging the LEDs in a row won't help, as my experiment with the fibers demonstrated. You can see each fiber (or LED), creating a confusing bunch of red, blue and green lines in the picture.
Right, I stand corrected. I guess I imagined that the slit box's length would occupy a substantial part of the total light travel distance so the focal planes would be different and easily distinguishable. In fact this is not how things are arranged.

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Could try this instead. Box needs to be reflective inside and the deflection plate would need to be painted in something purpose made? Alternatively you could try the colors side by side and a delay in the RGB signals?
Nice one Peter! To capture all the light of the LEDs the slit would have to be near the focus of the reflector. All the coloured beams could be arranged to converge at the slit, but then they'd diverge and separate again. So you'd still need a diffuser/light mixer.

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Old 6th Jan 2007, 9:56 am   #19
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Default Re: Colour mixing in mechanical television

I was thinking of some kind of white reflective paint that would disperse the light - they have glass beads in them.
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Old 2nd Feb 2007, 12:11 pm   #20
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Default Re: Colour mixing in mechanical television

You might be interested in the progress:

http://www.earlytelevision.org/color...rew.html#1-24c
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