Hi,
I'm a newbie in the field. This is the first time I'm venturing from audio field to this one.
I've got a Few questions:
* Can any one tell me why fresnel lenses are used?
* Are they a must in a LCD Projector?
* Should they be used always in pairs?
* what are the specs for a Fresnel lens to be used in conjunction
with a 7" LCD panel?
I'm a newbie in the field. This is the first time I'm venturing from audio field to this one.
I've got a Few questions:
* Can any one tell me why fresnel lenses are used?
* Are they a must in a LCD Projector?
* Should they be used always in pairs?
* what are the specs for a Fresnel lens to be used in conjunction
with a 7" LCD panel?
Q: why fresnel lenses are used?
A: They are used to direct light through the LCD and into the projection lens. With no system to direct the light, you get a very dim image or an image only lit in the center.
Q: Are they a must in a LCD Projector?
Q: Should they be used always in pairs?
A: No. The first fresnel (condensor fresnel) is used to refract the diverging cone of rays from a point-source lamp into a parallel beam of rays going through the LCD. If you use a reflector that generates such a parallel beam, then you don't need (or want) a condensor fresnel. The second fresnel (field fresnel) is used to refract the parallel set of rays from the LCD pixels into a converging cone with a focal point at the center of the projection lens. This gets most of the image light onto the screen. If you use a huge CRT-projector lens that is larger than your 7" LCD, then you don't need the field fresnel. So you can use 2, 1, or 0 fresnels, depending on your projector design. Reflectors and huge lenses tend to be expensive, so a two-fresnel design might be the most cost-effective.
Q: what are the specs for a Fresnel lens to be used in conjunction with a 7" LCD panel?
A: The fresnels need to be a bit wider than the LCD in each dimension. The focal length of the fresnel to use depends on your projection lens focal length, the desired throw distance, and the desired image size. The most common combination is a 200-220 mm fl condensor fresnel, a 330 mm fl field fresnel, and a 320 mm fl overhead projector triplet projection lens. This is the same combination used in most overhead projectors, so the optical parts are easy to find. (You can salvage them from an old overhead projector, or buy them online.) Some people building small-format LCD projectors have also used 2 or 3 fresnel page magnifiers as their fresnels with some success. You can get those at office supply stores.
Online sources:
https://secure.lumenlab.com/shop/group.php?id=2
http://www.exclusiv-online.com/shop/index.php?main=product&art=OSET1
A: They are used to direct light through the LCD and into the projection lens. With no system to direct the light, you get a very dim image or an image only lit in the center.
Q: Are they a must in a LCD Projector?
Q: Should they be used always in pairs?
A: No. The first fresnel (condensor fresnel) is used to refract the diverging cone of rays from a point-source lamp into a parallel beam of rays going through the LCD. If you use a reflector that generates such a parallel beam, then you don't need (or want) a condensor fresnel. The second fresnel (field fresnel) is used to refract the parallel set of rays from the LCD pixels into a converging cone with a focal point at the center of the projection lens. This gets most of the image light onto the screen. If you use a huge CRT-projector lens that is larger than your 7" LCD, then you don't need the field fresnel. So you can use 2, 1, or 0 fresnels, depending on your projector design. Reflectors and huge lenses tend to be expensive, so a two-fresnel design might be the most cost-effective.
Q: what are the specs for a Fresnel lens to be used in conjunction with a 7" LCD panel?
A: The fresnels need to be a bit wider than the LCD in each dimension. The focal length of the fresnel to use depends on your projection lens focal length, the desired throw distance, and the desired image size. The most common combination is a 200-220 mm fl condensor fresnel, a 330 mm fl field fresnel, and a 320 mm fl overhead projector triplet projection lens. This is the same combination used in most overhead projectors, so the optical parts are easy to find. (You can salvage them from an old overhead projector, or buy them online.) Some people building small-format LCD projectors have also used 2 or 3 fresnel page magnifiers as their fresnels with some success. You can get those at office supply stores.
Online sources:
https://secure.lumenlab.com/shop/group.php?id=2
http://www.exclusiv-online.com/shop/index.php?main=product&art=OSET1
can the fresnel lens be cut down to fit a 7" monitor without damaging the lens and it's performance
Yes. Trimming the fresnels to size is a typical step in building a projector. A Dremel or similar high speed rotary tool with a thin abrasive disk works well, but makes a mess of tiny sharp bits of acrylic in your workshop. (Do not use a power saw of any kind, they always crack acrylic fresnels.) ALWAYS USE FULL EYE PROTECTION, not just safety glasses when you do this. The little bits get everywhere and it is very easy to accidently rub them into your eyes.
The fresnel must be cut symetrically around the center to work properly. That means you cut a particular amount off both sides. Then you cut another amount of the top and bottom. You end up with a smaller fresnel, but the old center is still in the center of the new shape.
It does not make the projector work any better to have the fresnels trimmed perfectly to just fit the optical path. Too small and you cut off part of the image or light. So try to design your projector to fit fresnels that are a little bit larger than the LCD. Make sure that everthing lines up with the optical center line: Lamp, fresnels, LCD, and projector lens should all be perfectly centered.
The fresnel must be cut symetrically around the center to work properly. That means you cut a particular amount off both sides. Then you cut another amount of the top and bottom. You end up with a smaller fresnel, but the old center is still in the center of the new shape.
It does not make the projector work any better to have the fresnels trimmed perfectly to just fit the optical path. Too small and you cut off part of the image or light. So try to design your projector to fit fresnels that are a little bit larger than the LCD. Make sure that everthing lines up with the optical center line: Lamp, fresnels, LCD, and projector lens should all be perfectly centered.
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