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I keep getting email asking for more information so I made
this little write up about how to make a projector, this should
be able to answer most of your questions.
LCDs:
Well first off most all new projectors use lcds some have
one and some have three and their easy to work with because
you can find them everywhere with different options like s-video,
composite, or vga input so you can connect it to a pc, vcr,
satellite, DVD player, or game console. One thing you need
to look for in these panels is the response time usually measured
in milliseconds or "ms" a decent lcd has around
30-40 ms response if it gets much higher than that when you
watch a movie that has a lot of action it can be jerky. Another
thing is the resolution bigger is always better I suggest
1024x768 but I would not go below 640X480 there are some panels
out there that are 930x234 and those would not look good at
all imagine you have a image 8 foot wide by 6 foot tall the
pixels that make the image would be 1/3 of an inch tall which
makes for an effect called screen door its like looking out
a screen door you can see a lot of little black lines everywhere.
There is also the contrast ratio this determines how dark
black is when projected on the screen in store bought projectors
this can be up to 3000:1 which is very high it makes it so
that if the lcd was totally black no light will hit the screen
most normal lcds are around 150:1 to 500:1 here again higher
is better I suggest to go no lower than 300:1 or your blacks
start to look like dark blue. The final thing with lcds is
a trial and error step, for a projector to work it needs a
very bright light to shine through the lcd in order to do
this you have to take the lcd apart and take the old backlight
and cover off the back of the lcd so you can get a new light
to shine right through it. Not all lcds will work some of
them you can't take it apart at all or it's not easy and if
you're not careful you could tear one of the ribbon cables
and those are almost impossible to replace or repair. The
most simple way and most popular is to use what is called
a projection panel I'm sure you have seen these being used
in school, you just set it on an overhead projector and presto
you got a big TV. These can be found all over eBay and some
inventory surplus sites but do research on it before you buy
it and make sure the specs are within an acceptable range.
If you got a big wallet you can actually buy real .7"
projection lcds with custom driver boards for around $3000
these work GREAT but as you see quality comes with a big price
tag.
Light Source:
It takes a lot of light to make a bright projected image,
I'm guessing about 60-75% of the light is actually blocked
by the polarizer's in the lcd about 50,000 lumens going through
the screen will make a very bright picture. The tricky part
is if you get a 400W metal halide bulb that puts out 50,000
lumens and just set the bulb behind the screen the actual
light going through the screen is about 5-10% of the rated
lumens because the light is going in every direction. You
need a way to make all the light go through the lcd, this
is done with a reflector this again is a trial and error step
not all reflectors are going to work you just have to try
and see what happens. The best type of light to use is a Metal
Halide these come in all kinds of shapes and sizes you can
go to the "GE" or "Sylvania" website and
see what kind would work for you.
Lenses:
Sitting behind the lcd is a lens called a fresnel lens this
is used to converge the light onto the projection lens its
important to get one that has the same focal length as your
projection lens so it focuses most of the light onto the projection
lens. If its not the right focal length it could only let
half of the light through the projection lens and will make
your image half as dark as it should be.
The final and a very important part is a projection lens the
best ones to use are triplet lenses it is 3 lenses made in
one casing and makes it easy to focus the image, what I mean
by this is if you use a regular convex lens when you try to
focus the center of the image it will be in focus but the
outside edges will be blurry because they are a little bit
further from the lens. A triplet lens fixes this and makes
the outside edges come into focus when the center is in focus.
This next step is another trial and error step there are some
calculators you can use for this but its hard to get an accurate
measurement, the magnification of the lens or also called
focal length determines how big your projected image is say
your projector is 10 feet away from the screen and you have
a 200mm lens it makes the image 8 feet wide and if you use
a 400mm lens at the same distance it makes the image 6 feet
wide (these are not accurate measurements) you just have to
find the right size for your needs. Another factor for this
is the lcd size if your lcd is 10" and you have a 300
mm lens and its 10 feet away from the screen you image is
8 feet wide but if your lcd is 1" and you got the same
300mm lens and same distance your projected image could be
about 3 feet wide. There are so many variables in this that
its very difficult to tell what size you need an easy way
is to go buy a set of like 10 convex lenses that range from
200mm - 600mm and try them out and see about what size you
need you can get these very cheep at places like www.surplusshed.com.
I hope this has got the ball rolling for you I am no expert
in this field so I really can't give a good answer to most
of the tough questions I have received but this should give
you the basic idea behind this. It does take a lot of trial
and error I have been trying all kinds of things I can think
of to make my projector better and I have been searching for
a cheap small lcd source and a small light, when I find reliable
sources you can get them here and Im hoping to make a kit
that has about every thing you need to build your own projector.
This is a basic setup like in my first projector you can
use mirrors to direct the light to make a more compact design.
This way you can save room and if you can find small parts
the unit can be made very small.
Here are some links that will show you how
the optics work.
http://www.glenbrook.k12.il.us/gbssci/phys/Class/refln/u13l1a.html
The basics of optics.
http://www.education.eth.net/acads/physics/light-I.htm
More on optics.
http://ourworld.compuserve.com/homepages/awi_industries/Optic_reflectors.htm
How reflectors work.
http://www.tecdis.com/product.htm
How LCDs work.
http://micro.magnet.fsu.edu/primer/photomicrography/colortemperature.html
Color tempature of light.
http://www.nasatech.com/Briefs/June01/NPO20824.html
Recycling light.
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