Laser projector

[remp]

Hi My name is Richard. I live in New Zealand. For a while now I have been thinking about building a large screen projector using either LCD panel(s) or lasers. So I looked everywhere for LCD panels prices information etc and found they were quite expensive.
I had on hand a rotating polygon with 6 facets, a 5mw red laser diode and a few first surface mirrors.

I thought build a small laser system and see if it works and if it does scale it up.

First problem. How fast does the polygon have to rotate to display a raster. We have a 625 line system here using interlace. The line rate is 15625 Hz. There is one full frame (625 lines) every 1/25 of a second so the lines per second is 625 times 25 equeals 15625 times 60 to get lines per minute equeals 937500.
This means if I had a polygon with one mirror it would have to rotate at 937500 revs per minute.
My polygon has 6 mirrors so speed drops way down to only 156250 revs per minute.
My polygon ex a laser printer said it could do 7701 revs per minute. Big difference between 156250 and 7701 rpm.

This problem seemed insurmountable however if I had a polygon with 625 mirrors each rotation of the polygon would be drawing a full frame of 625 lines therefor its speed would only need to be 25 frames per second times 60 equeals 1500 revs per minute. Well within my polygon speed range. There are no polygons available with 625 mirrors but what if I used stationary mirrors instead of rotating mirrors.
I did not fancy the idea of cutting and mounting such a large number of mirrors so how about using the 6 mirrors on the polygon to assist the stationary mirrors. That way I only need 600mirrors approximately divided by 6 equeals only 100 mirrors at 1500 revs per minute. Still too many mirrors. 50 mirrors speed goes to 3000 rpm and 25 mirrors speed of the polygon becomes 6000 rpm.

I tested this out with 5 mirrors arranged in a radius around the polygon in a special way which I will describe later and obtained 5 raster lines on top of each other. A slight tilt of each mirror gave me a 5 line raster.

That showed the idea was feasible and would work. The mirrors only have to be small about 1/2 an inch square.

I will be fitting a frame polygon so the mirrors dont have to be tilted but that one operates at a reasonably low speed so I am not worried about that.

Part 2 tommorow enough light. I have not solved this problem yet but will post how far I have got.

Richard

[remp]

Before getting on to the light aspect I should mention an advantage of using a rotating polygon in conjunction with a small number of mirrors.
The first advantage is to lower the speed required from the polygon from very high rpm to the much easier speed range of 6000 rpm.
Second advantage A polygon with 6 sides covers an arc of 120 degrees
360/6 times 2 equeals 120. This wide angular coverage was much too great because, using a laser beam I thought I would project directly on the screen 18-20 feet 6 meters away without needing a projection lens.
To check this I approximated the required angle using two string lines and a protractor and it came to 10-12 degrees.
By a happy coincidence the polygon with surrounding stationary mirrors as descibed previously also put out a deflected beam of 10-12 degrees sweep.

So far I was quite happy with progress. The polygon could be used at low speed and the sweep angle of the beam was the correct angle to put a beam from one side of my projection screen to the other.

LIGHT
The next thing was, could I get a visible raster with a 5 mw laser diode. The short answer is no. With the curtains drawn and all lights out and testing at midnight in pitch darkness I could easily get one raster line but as soon as I tried for several raster lines the beam faded away and became invisible. I had thought that persistence of vision would allow my eyes to retain the image of a raster line sufficient to see some sort of a raster. No visible raster at all. Very disapointing that was. Moving the screen to 4 feet from the projector gave me a raster but the raster was only a foot wide. And the beam from the laser diode was far too thick and not a good round beam. A little bit more light and I would have had a raster. Need more work on this.

I now had 3 options
1 Forget the whole thing
2 Buy a much more powerful laser
3 Use a powerful white light source.

1 is out. 2 is out because powerful lasers are expensive, use a lot of electricity, may need cooling water supply and can be dangerous and I would need 3 one for red blue and green.

I thought about the new high power leds that Vince tried in his DIY LCD projector. While thinking about a suitable light source I hooked up my 2000 watt electric heater to a pair of carbon rods. When the rods were touching the heater worked normally. I left the heater on for a few minutes to heat up and then separated the carbon rods. An arc formed with a tremendous brilliance similar to a welding arc. It is advisable to take all electrical safety precautions when doing this including eye protection with a welding helmet because there is a lot of UV from an arc. Arcs can also form carbon monoxide so be very careful with ventilation.
I positioned a headlamp reflector close to the arc and got a very strong beam of light which just about burned the wallpaper off the wall.

What I need for this laser projector is a very powerful light source. It must be from a point source so I can collect/collimate or focus to a small spot and it must stay as a small spot until it hits the projection screen 20 feet away. Also the light has to be divided into three colours red green and blue. These colours have to be modulated and then recombined back into one beam to go through the mechanical projection equipment.
There are numerous point source arc lamps available but are expensive, need correct control gear, probably need a fan for cooling and do not seem to last very long.
At the moment I am investigating a system which collects light from many small light souces such as car headlamp bulbs and guides the collected light to a glass fibre. This is the reverse of lighting systems which illuminate signs evenly from one powerful light source.

I think I will buy a red laser diode about 100 mw to replace the 5 mw laser I have been using and continue testing. Might even buy a green pointer. I want to see a full raster at 20 feet, I have to modulate the laser diode with video to see a picture and I have to prove the polygon and mirrors are working correctly.


Richard

[Fiat1]

This type of system you are proposing was first invented in the early 1930's. I am interested in mechanical television and so thought about using some old technology such as this. Here are some links regarding some early systems. They give good technical info

Mihaly Traub
http://pyanczer.home.mindspring.com/Tour/mhalytrb.html

Mirror Screw
http://pyanczer.home.mindspring.com/Tour/mirscrws.html

VIBRATORY SCANNERS
http://pyanczer.home.mindspring.com/Tour/vibrscan.html
This type of scanner could be made with a programed chip the produce an of signal that could drive the electromagnets so you don't get a sine wave shading effect.

Scophony System
http://pyanczer.home.mindspring.com/Tour/scophony.html

The Eidophor Television System
http://pyanczer.home.mindspring.com/Tour/eidophor.html

Hope this helps, I think the vibratory scanner is the most promising. I will be starting work on that in the next 2 weeks.

[remp]

Thanks for the info Fiat1
Peter Yanczer's site is most interesting as are the others you listed.
One of the problems with mechanical Projection is the much higher line rate used today. Early mechanical projection television rarely went over 180 lines. The Scophony system could go to 405 lines and even higher.
If you haven't done so already, having a look at US patent office database is useful.
Many thanks for your posting.

Regarding my small scale laser projector,(5 mw red laser diode) I am getting ready to test out the polygon speed, stability etc by modulating the laser diode with a 625 line horizontal sync pulse. If all goes well I am expecting to see one very small gap in a raster line corresponding to a sync pulse. I am hoping there is enough polygon speed stability for the pulse to sit on the line without moving left or right too much. If it does I could then look at a start of scan from the laser into a detector compare its position with the tranmitter sync pulse and try and lock the motor at the correct speed and phase.
Will report later.

[pa]

Hi !
This is a cool project, but i think that your first idea to use Laser was right...
I had post a reply in the project of diy videoprojector wich use lcd, and ask why not a DLP or Laser,
A student in Paris i had met on a list was working on the same thing, he use a Laser tube of 5 or 8watts.
If you want to make as good as a Tritube, you will need more power than 100mw...
But the laser as an advantage, for large screen the spot of the laser will be very small (little dispersion of the spot) and the resolution could be better than all existing systems, there is also laser tubes wich can be modulated for several colors, if the modulation is quick, we could use only one modulated tube. It may be expensive but high quality !
Pa

[Fiat1]

Early television rarly went about 180 lines due to limited bandwidth. The motor will have to be synced some how to either the horizontal or vertical picture frequency. I have long been a hobiest in mechanical television and have buitl many expamples of early televions, it is very exciting to build a cammera and reciver and get television pictures on them! Laser projection ususally works by servo and has two to produce the raster. I still think that a vibratory mirror would be the best, it is like a DLP but using on mirror to do the whole lot, look at my previous post for a link.

[Fiat1]

You should produce colour with a 3 colour wheel to make it cheaper ans possibly easier. You can not modulate light at such high frequencies as modern television uses. That is why the scophony system has a jeffer cell to do this. Modern laser projectors do this:
Light Modulation

Once the proper red, green and blue beams are produced, each must be modulated with its correctly decoded video signal. This is done with one acousto-optic modulator for each beam. An acousto-optic modulator operates by the interaction of a very high frequency sound wave traveling through a crystal material. The laser beam passes through this crystal at approximately 90 degrees to the traveling sound wave. The sound wave is varied by the incoming video signal causing distortions in the crystal and the light diffracts off the variations as if they were, in effect, a solid diffraction grating. The angle at which the light is deflected is very small and different for each color and the light is only deflected when a video signal is present. The electronic circuitry of the projector supplies the individual red, green and blue signals to the modulators in response to the video input.

[remp]

Pa Thanks for your post. The reason I am at present using only A 5 mw laser is to make a small scale projector, see if it works and if it does then I can think about scaling it up.
Fiat1 Thanks for your post. You hit the nail on the head when you said it is exciting to make these things and actually get them to work and not only work but sometimes work better than commercial products. Things have changed a lot from when a group of us used to build our own Hi Fi amplifiers. All valves like output tubes EL84 6v6 EL34 KT66 and KT88 even 807 transmitting tubes. We had one big advantage then. There were far more stores catering for DIY projects and most electronics magazines had heaps of circuits. The parts themselves were not all that expensive and guys would turn up at my place with huge amplifiers 4 KT88 output stage which were so heavy. One chap even built an amplifier with no output transformer using 40 6v6 output tubes. Worked too.

Today I find not so many shops catering for the DIY enthusiast and magazines more interested in advertising commercial products. Changes happen and you got to work with whats available but the internet is a great thing where people share their ideas with others.
As you said, DIY is really exciting.

[pa]

If you use only one ray (all colors output mixed on the same ray), and not 3 separate rays, you could set the size of the screen with a "zoom" or move your videopro, if you use 3 rays (like crt's) it will be harder because you have to modify angles of rays and convergance will be long... (i know this i'have a crt videopro..) this could be an advantage for laser projection compared to crt's.

[pa]

3 light source -> easy settings of colour balance, (could be laser or 1 white lighte source and filters ? (not laser)
I don't know But with 3 source you can imagine both a red and blue ray simultaneously...
ah... other thing, someone said me there is a lots of patents and the subject published in big us databases in us patent office and maybe IBM. Not useable for commercial applications but super for diyers :-).
maybe you should take a look at this.