| new guy, few questions - Click HERE for Original Thread |
| Johnboy233 |
| hi, after reading through a bunch of the FAQ's, i still have a few unanswered thoughts. for example, what does a fresnel lense do? the difference between a fresnel lense and a regular magnifying lense? I ask this becuase i have a large magnifying glass that i plan on using as a lense. Some preliminary testing has displayed a beautiful (upside down) candle on the wall when adjusted to the right focus. I think i have enough materials to make a unit, jsut not the know how. I plan on disassembling my Game gear for the LCD, keep the electronics intact as it has a video input on the TV tuner. (Anyone tried this?) I am aware it is a passive matrix screen and will be blurry with low update frequency, but as a first project.... Any ideas??? thanx. Also, what is the purpose of a mirror in the home made units i see? does the mirror enhance anything? or jsut allow the unit to be more compact??? thanks |
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| bobharry |
| A fresnel lens simply changes the direction of the light. The first half changes diverging light into parallel rays, then the second half changes the parallel rays into converging rays. The difference between a fresnel and an ordinary magnifying glass is that a fresnel is flat, and this might be incorrect (could someone correct me if it is) but I think it is different as well because it makes the equivalent of two convex lenses with the bumps facing each other, while in a magnifying glass they are facing in opposite directions. If you want a very basic bit of fun you could use a gamegear screen and the magnifying glass as the objective (projection) lens, but you probably won't be very impressed with the results, but it will give you an idea of the concept of projection, then you will have a bit of know-how for your next project, assuming there is one. A mirror doesn't enhance the image, it will actually make it worse by causing ghosting if you don't use a front-surface one, they are just used to make the pj more compact, so if you are going to use a mirror, and you want good results, make sure you get a front-surface one. |
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| Johnboy233 |
| ok, that helps a bit, so why use a fresnel then, can you get more magnification than the same size magnifying glass? i don't see the advantage yet. is it better quality? Say i disassemble my gamegear, project a bright light behind it, through my magnifying glass, now the image will be inverted, so my next step is to revese the LCD so its upside down, and backwards and the magnifying glass with then flip it around properly on the wall, am i on course here? now do i need some other kind of lense? for example, a concave lense now for size adjustment, or can all this be done with adjsuting the distance of the one lense. (planning to put everything in series for first attempt, no mirrors) make it nice and big. |
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| Aidan Isaac Lev |
| in laymans terms, fresnels are not used for projection. what thay do is take a source of light (a light bulb) and spread it evenly over your LCD and then compress it again so that your progection lense (NOT a fresnel) can colect the hole (evenly lit) image from you LCD and project it onto your screen. |
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| Mr.Kh |
*Cough* Search *cough*
But now that I'm reading this ANYWAY I'll try to shed some light.
Why fresnel? - Well now you know what a fresnel does, why would you want one? LCD's pass primarily perpendicular(to the LCD surface) light rays, and block light rays that are off angle. When you use a bulb and no fresnel, you will see some image because even without the fresnel some rays are going in the correct direction, and LCD's pass some light from off angles.
You are correct about flipping your display upsidedown and putting the front of the screen facing the bulb. Without mirrors this is the only way to set it up unless you watch while in handstand position.
Adjusting the size of the projected image(without moving the projector furrther from the wall) requires the use of more than one lens, what type of lens is dependant on what size and distance from the screen. I'm sure you'll be able to search mroe on this if you really are interested. |
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| Johnboy233 |
| ok, so I'm coming around here with the fresnel lense theory, does this mean that the light from the bulb must be put through the fresnel so that the light beams are travelling parallel instead of diverting. like this "0<|=" 0 is the light source, < is the light beam coming from the light source, |is the fresnel and = is the now parallel light beams, am i on the right track? now do you need another fresnel lense to bring to converge the light back? or now that the light is "=" it can be sent right to the focusing lense?.... also an update on the gamegear project, i accidentlally fried it last night with an overvoltage, which i blame on a previous project. (don't ask). I think I"ll start fresh with a LCD display panel for an over head projector for a few hundred and make that work.... ideas? thanks |
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| Johnboy233 |
| sorry, I mean "o<|= then LCD then focus lense" how far off track am I? |
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| bobharry |
| If you are using a split fresnel (you can find a lot of opinions about whether or not to use a split one) you are right about what the light does, and you will have to use the other half of the fresnel after the lcd (with a gap between them, to stop rings appearing on the image) to make the light converge to the objective lens. |
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