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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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#1 | |||
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,533
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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 3¾ 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 3¾ 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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#2 |
Pentode
Join Date: Feb 2004
Location: Columbus, Ohio, USA
Posts: 150
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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 Last edited by Steve_McVoy; 31st Dec 2006 at 3:39 pm. |
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#3 | ||
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,533
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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' ? Quote:
Steve
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#4 |
Pentode
Join Date: Feb 2004
Location: Columbus, Ohio, USA
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I tried sawing off a bright LED, but the surface is about 40 mils for the LEDs I used.
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#5 | |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,533
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Have you tried sawing a 3mm LED ? Steve
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#6 |
Pentode
Join Date: Feb 2004
Location: Columbus, Ohio, USA
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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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#7 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
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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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#8 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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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. Last edited by PJL; 1st Jan 2007 at 3:27 pm. |
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#9 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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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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#10 |
Pentode
Join Date: Feb 2004
Location: Columbus, Ohio, USA
Posts: 150
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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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#11 | |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,533
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Steve
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#12 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
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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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#13 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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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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#14 |
Pentode
Join Date: Feb 2004
Location: Columbus, Ohio, USA
Posts: 150
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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 Last edited by Mike Phelan; 26th Oct 2007 at 8:13 am. |
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#15 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,533
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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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https://www.radiocraft.co.uk Last edited by Panrock; 5th Jan 2007 at 10:44 am. Reason: More information arrived |
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#16 |
Pentode
Join Date: Feb 2004
Location: Columbus, Ohio, USA
Posts: 150
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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.
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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#17 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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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 colours side by side and a delay in the RGB signals?
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#18 | ||
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,533
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Steve
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#19 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
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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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#20 |
Pentode
Join Date: Feb 2004
Location: Columbus, Ohio, USA
Posts: 150
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