Do all rear projection TVs have a fresnel lens? Update: Evil laugh!!!!

[IronWing]

I want to play with a big fresnel lens and picking up a used rear projection TV might be the cheapest method of obtaining one. But... I want to be sure that before I buy a projection TV that the lens is really in there. Also, if they do, are they generally linear or spot pattern?

Moved to OT on request of OP
-ViRGE

 

[NutBucket]

FYI those lenses are probably plastic. Even in the 80's Sony was using plastic in their high-end front projection TVs.

 

[Knavish]

Did you get your fresnel lens yet? Building a solar oven?

Here's a 13" diameter, 10" focal length fresnel for $8.50.
http://www.anchoroptics.com/catalog/product.cfm?id=24

Or for quite a bit more money, higher quality fresnel lenses:
http://www.anchoroptics.com/catalog/product.cfm?id=402

 

[IronWing]

I picked up a TV off craigslist today. It took about an hour to liberate the fresnel lens and remount it back into the original bezel. The lens is 32x42 inches for an area of 9.3 sq feet (0.87 m^2). The factory bezel, free of the stand, is too flexible to serve so I'll have to build a new frame. Even with the flexing, the lens focuses down to about 2cm and ignites twigs in 3-5 seconds in full sunlight. This is substantially better performance than the parabolic collector we built before as well as being much easier to manage. There are also three pretty nifty projector lenses in there. There is another set of lenses (maybe just color filters) still in the TV but they are oil immersion lenses so I have to read up on the fluid used before opening them up.

 

[IronWing]

I'm working on my frame now. I'm going to sandwich the lens between two pieces of 1/2 plywood. I have the plywood cut into a 3" wide frame to fit around the lens with about 3/4inch overlap onto the lens on all sides to hold it in place. My plan is to lay the lens in place and glue the plywood pieces together using Titebond II wood glue. The lens is ~1mm thick so the plywood won't be quite flat together. Should this work okay for gluing or should I sand down the edge where the lens will fit? I'm also planning to glue the lens to the plywood, also using the Titebond II. I think it should bond to acrylic okay. A schematic:

And with sanded down section:

 

[DrPizza]

You have no idea how upset it was when I found out that my very large rear projection sony television that I took to the landfill had a fresnel lens in it - found out one week after I got rid of it.

Great sadness.

 

[IronWing]

:twisted: Muhahaha. :twisted: <wrings hands> Muhahaha. Ignites wood in less than a second under early dawn light. Muhahaha. Fresnel lenses are amazing.

The glue job is okay, I didn't have sufficient clamps to get things perfectly even but the lens is flat and the glue is holding so it works. I had to notch the wood frame as illustrated above to fit the lens. This afternoon we'll try melting stuff like copper and glass. I'll add handles and design the mount next. Going to get me a bag of marshmallows. :biggrin:


Edit: Running the calcs for Tucson in June, the lens should be dumping ~480W into an area of ~0.2 square inches (1.3 square cm).

 

[lxskllr]

Do you have a practical use for this, or is it just for playing?

 

[Krazy4Real]

You're crazy!

 

[IronWing]

Do you have a practical use for this, or is it just for playing?

If I could figure out the collimating lens then I would have the death ray I've always wanted.

The practical use is annealing pieces of copper and steel plate that are too large to fit in the forge and for melting silver (my wife is a metalsmith). While a torch can be used for this purpose, a lens setup has the benefit of being less oxidizing and the flux won't blow off small pieces as it tends to do when using a torch or the forge.

The nerd in me wants to design a CNC lens mount to automate wood burning. I could sell "Welcome to the the Ironwings" plaques at the swap meet.

 

[jaedaliu]

nothing to see here.

 

[El Guaraguao]

:twisted: Muhahaha. :twisted: <wrings hands> Muhahaha. Ignites wood in less than a second under early dawn light. Muhahaha. Fresnel lenses are amazing.

The glue job is okay, I didn't have sufficient clamps to get things perfectly even but the lens is flat and the glue is holding so it works. I had to notch the wood frame as illustrated above to fit the lens. This afternoon we'll try melting stuff like copper and glass. I'll add handles and design the mount next. Going to get me a bag of marshmallows. :biggrin:


Edit: Running the calcs for Tucson in June, the lens should be dumping ~480W into an area of ~0.2 square inches (1.3 square cm).

Melting copper and glass? I swear, If you dont record this and show us, I will....tell my mom on you

 

[lxskllr]

If I could figure out the collimating lens then I would have the death ray I've always wanted.

The practical use is annealing pieces of copper and steel plate that are too large to fit in the forge and for melting silver (my wife is a metalsmith). While a torch can be used for this purpose, a lens setup has the benefit of being less oxidizing and the flux won't blow off small pieces as it tends to do when using a torch.

The nerd in me wants to design a CNC lens mount to automate wood burning. I could sell "Welcome to the the Ironwings" plaques at the swap meet.

The sun's also cheaper to operate. I like the wood burning idea. Extra credit for using 100% solar energy to operate it. That would add value for selling the art too. The novelty factor will raise the price. You could use steam powered machinery, with the sun being the heater.

 

[IronWing]

Copper Steel sheet

Cinder block

Videos are loading.

Edit: My wife (the blacksmith in the family) corrected me that the disk was steel not copper.

 

[BladeVenom]

Copper
http://www.mineralarts.com/images/fresnelcopper.jpg

If that was a penny, it's mostly zinc.

 

[Baked]

Now some bugs under the bullseye.

 

[IronWing]

If that was a penny, it's mostly zinc.

Steel sheet

 

[phucheneh]

Copper sheet

Look's like the tin man's anus.

Videos. Now.

 

[IronWing]

Look's like the tin man's anus.

Videos. Now.

When did youtube loading get so slow? By the time these videos load you could have walked over to my house and played with it in person.




^^ sig worthy, btw.



Edit: Here's a nickel in the mean time.

 

[BrunoPuntzJones]

How much energy could you harvest using a row of these? Seems like here in TX, with 3 months of 100F+ days, you could get a pretty good amount of power in either steam or molten salts?

 

[phucheneh]

If he uses it to make a generator of the steam turbine variety, I'll have his babies.

I would've never thought the lens in a projection TV was that powerful. If there was no screen in the TV, and you faced the front torwards the sun, would it catch on fire? :hmm:

BTW, Youtube uploads never seemed terribly slow, but if you use it to trim video length or anything, the edits seem to take an eternity.

 

[IronWing]

Videos are up!

First, the safety video

Sheet steel melting 1

Sheet steel melting 2

 

[Shawn]

Videos are up!

First, the safety video

Sheet steel melting 1

Sheet steel melting 2

Looks like it's damaging the camera sensor.

BTW, that is cool. Looks like a good way to take off some fingers if you aren't careful!

 

[IronWing]

More videos:

Melting a nickel 1 (75% copper 25% nickel)

Melting a nickel 2

The camera is fine. One feature of solar concentrators that makes them easy to use but also dangerous is that outside of the focus spot there is very little heat. Stuff in the focus point can be burning or melting while the camera can be inches from the focus without harm.

 

[Shawn]

Must find more stuff to melt!