Hi there, I am currently building another lightbox for my 15" DIY projector and I have a few questions that I cannot find the answers to via the forum search facility.
I have a dichroic reflector from a Elmo A305 and the condensor lens from a A305 also, regaurding these optics I have a couple of questions.
Im using a 15" viewsonic (1024x768).. DIYlabs Fresnells unsplit about an inch away from the LCD (perhaps the cause uneven image?)
1. When placing the Bulbs arc at the reflectors focal point does one put the edge of the Arc at the reflectors focal point or put the centre of the arc at the reflectors focal point?
I have determined the focal point based on Ace3000's wisdom, placing a object in front of it and measuring how far away it is when the object reflected appears the very same size, this was 2cm( from the rim, prob 35mm or so from the bottem), I currently have the outter glass of the Arc of the bulb at 10mm away from the reflector, placing the centre of the arc at the reflectors focal point.
2. When placing the condenser. does one attempt to mount it as close as possible to the bulb? My old lightbox had the condensor glass TOUCHING the bulb glass(eg, less than 1mm) and I run that for atleast 3 hrs, and it did not crack, the focal point of the condenser appears to be about 150mm
3. No matter what I tried, the corners were always dimmer than the centre, not noticably in movies, but at the desktop its quite noticable, eg, the start button is not lit very well, could this be caused by using the fresnells unsplit at about 1inch away from the LCD? without cutting them I cannot try unsplit, and im not keen on getting another one from america if i ruin it.
4. My projection lens(Elmo a305 lens) is very small, the back of the lens element is about 55mm and the front element is about 40mm, could this be causeing brightness issues around the sides? i see HEAPS of light around the lens not making it into the lens.
Where do people get copy lens from, do they come out of photocopiers? cause if they do I'll go into town tomorrow and go and ask a few copier shops.
I have a dichroic reflector from a Elmo A305 and the condensor lens from a A305 also, regaurding these optics I have a couple of questions.
Im using a 15" viewsonic (1024x768).. DIYlabs Fresnells unsplit about an inch away from the LCD (perhaps the cause uneven image?)
1. When placing the Bulbs arc at the reflectors focal point does one put the edge of the Arc at the reflectors focal point or put the centre of the arc at the reflectors focal point?
I have determined the focal point based on Ace3000's wisdom, placing a object in front of it and measuring how far away it is when the object reflected appears the very same size, this was 2cm( from the rim, prob 35mm or so from the bottem), I currently have the outter glass of the Arc of the bulb at 10mm away from the reflector, placing the centre of the arc at the reflectors focal point.
2. When placing the condenser. does one attempt to mount it as close as possible to the bulb? My old lightbox had the condensor glass TOUCHING the bulb glass(eg, less than 1mm) and I run that for atleast 3 hrs, and it did not crack, the focal point of the condenser appears to be about 150mm
3. No matter what I tried, the corners were always dimmer than the centre, not noticably in movies, but at the desktop its quite noticable, eg, the start button is not lit very well, could this be caused by using the fresnells unsplit at about 1inch away from the LCD? without cutting them I cannot try unsplit, and im not keen on getting another one from america if i ruin it.
4. My projection lens(Elmo a305 lens) is very small, the back of the lens element is about 55mm and the front element is about 40mm, could this be causeing brightness issues around the sides? i see HEAPS of light around the lens not making it into the lens.
Where do people get copy lens from, do they come out of photocopiers? cause if they do I'll go into town tomorrow and go and ask a few copier shops.
Heya buddy, i get what your saying about the image being uneven and quite often it sux trying to get it right. Put it this way you will never get it perfect on a big lcd without considerable light loss.
From taking at look at your system more then likley the biggest cause is the lcd being that 1 inch away from the frensel. When having frensels unsplit, the farther away the lcd is from the frensel the bigger the frensel has to be. This is because the light coming from the top frensel behind the lcd is tapering back to a point.
For your lcd you should be able to run it 10mm from the frensel without seeing any rings, try this, it will brighten those corners up.
As for your light engine i cant realys say much as i dont know any real specs. Whats the diameter of your reflector and condenser?
As for your projection lens, are you sure its in the right way round? Normally its the bigger lens being on the outside and the smaller on the in.
Trev🙂
From taking at look at your system more then likley the biggest cause is the lcd being that 1 inch away from the frensel. When having frensels unsplit, the farther away the lcd is from the frensel the bigger the frensel has to be. This is because the light coming from the top frensel behind the lcd is tapering back to a point.
For your lcd you should be able to run it 10mm from the frensel without seeing any rings, try this, it will brighten those corners up.
As for your light engine i cant realys say much as i dont know any real specs. Whats the diameter of your reflector and condenser?
As for your projection lens, are you sure its in the right way round? Normally its the bigger lens being on the outside and the smaller on the in.
Trev🙂
50mm reflector and a 65mm condenser, will try copier shop..
so you think with larger panels light loss around the corners is always going to be something of an issue?
Lens is mounted around the same way as Elmo mounted it on their OHP/.
so you think with larger panels light loss around the corners is always going to be something of an issue?
Lens is mounted around the same way as Elmo mounted it on their OHP/.
tuning the condensor system
If you are seeing tons of light missing the lens, then you should try to tune your condensor system to get it adjusted correctly:
Take your projection lens out and put a piece of white paper where the lens goes. Move it back and forth until you see a minimum-sized focussed image of your lamp arc. That is the point where your condensor system is sending the light. Then move the paper to the normal projection lens position. See how much bigger the arc image gets. You can adjust the condensor system by adding or removing space between the LCD and the field fresnel. If you have to, you can also adjust it by changing the distance between the lamp amd the condensor fresnel, but don't move it too far from it's focal length.
If the minimum arc image is much bigger than your projection lens, then that means you need to get a wider lens!
The corner problem is made worse by using a short focal length condensor fresnel. The angle of the light approaches the critical angle, so more gets reflected. It also gets spread over more area near the corners by the inverse square law. You could change to a design using a longer fl condensor fresnel. On the projection side, changing to a longer throw lens has the same effect.
If you are seeing tons of light missing the lens, then you should try to tune your condensor system to get it adjusted correctly:
Take your projection lens out and put a piece of white paper where the lens goes. Move it back and forth until you see a minimum-sized focussed image of your lamp arc. That is the point where your condensor system is sending the light. Then move the paper to the normal projection lens position. See how much bigger the arc image gets. You can adjust the condensor system by adding or removing space between the LCD and the field fresnel. If you have to, you can also adjust it by changing the distance between the lamp amd the condensor fresnel, but don't move it too far from it's focal length.
If the minimum arc image is much bigger than your projection lens, then that means you need to get a wider lens!
The corner problem is made worse by using a short focal length condensor fresnel. The angle of the light approaches the critical angle, so more gets reflected. It also gets spread over more area near the corners by the inverse square law. You could change to a design using a longer fl condensor fresnel. On the projection side, changing to a longer throw lens has the same effect.
Ok, by simply splitting the Fresnells I solved the dim corner issue, its now pretty much uniform from corner to corner.. however overall brightness is lacking... Here is what I have at the moment, I have checked that everything is centered etc, very carefully and its all with a few mm... obvisoully I've had a play moving the fresnells back and forward etc, but what I have at the moment seems to be the best brightness I can get?
I also managed to get the light to fill the projection lens fully, eg, theres very little now showing either side of the lens... this was fixed by moving the feild fresnell forward a little bit. (after checking all the distances)
Everyone else seems to get even better results with NO reflector or condenser.
What am I doing wrong?
I also managed to get the light to fill the projection lens fully, eg, theres very little now showing either side of the lens... this was fixed by moving the feild fresnell forward a little bit. (after checking all the distances)
Everyone else seems to get even better results with NO reflector or condenser.
What am I doing wrong?
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From LCD to condenser fresnell is around 1cm, or maybe a bit less, and from the Feild fresnell to the LCD is 2cm.
neither of these distances should cause a noticable difference should they? 3cm totel between the two of them.
neither of these distances should cause a noticable difference should they? 3cm totel between the two of them.
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