The Search For The Perfect Fresnel - For LCD's Over 15"

Status
Not open for further replies.
Jump to Latest
#61
i was just about to say the same thing, try to get some of the grounding sand and u can grind your own lens by hand, i was going to make one of those scopes once and they are cheap but i didnt have the time to grind the mirror as it takes about a week to do lol, but doing it that way is not hard just patients and its dirt cheap

cheers Trev
 
#63
Large singlet condensing lens.

Guys,

I think that I may have come up with a method to build a relatively inexpensive spherical large lens.

First glue or laminate several 16" - 17" 2x4's or something wide enough to make a lens mold blank with. Then turn the wood or shape it by hand into approximately the right shape for the lens. Get it as accurate as possible but a little oversized.

Sand it down as smooth as possible trying to maintain the proper shape without making any flat spots.

Using as fine a sand as possible make a sand casting using the wood blank to make the cavity. It should be a little larger than the finished lens will be.

Then heat optical grade acrylic pellets in a large enough quantity and pour them into the sand mold and let it cool.

When all is done we should have a lens blank of the approximate size and shape but without adequate surface finish.

Build an inexpensive lens grinding machine like the amatuer telescope makers build and then use the shape of the plastic lens blank to make a pitch lap.

The lap will not be perfectly shaped but as it heats and grinds it should take on a near perfect spherical shape.

Start with a course enough abrasive so that the imperfections will be removed and then quickly move to a polishing grade grit and then somekind of polishing substance appropriate for the type of plastic.

Repeat the process for both sides of the lens if necessary.

After the final polishing test the lens to see if it works and is better or not than a fresnel.

Continue with improvements or abandon the large lens concept.

Hezz
 
#66
Re: The Search For The Perfect Fresnel - For LCD's Over 15"

verbose mustafa said:


Whats the price tag? $34.95 for the small size, and $39.95 for the medium size. This leaves on question though? Is 2X magnification enough?

http://www.optelec.com/ProductDisplay.asp?ProductID=140

An externally hosted image should be here but it was not working when we last tested it.
Click to expand...



For those of you that are interested, I ordered a pair of these types of lenses from another manufacturer, and they have a ~38 inch focal length. They also have a blue tint and are slightly convex. I plan on prototyping a setup using them, but it may take me a couple of weeks. I hope to be able to compare them directly to the OHP fresnels from diylabs.

They also dont work well as TV magnifiers unless you close one eye...I haven't had double vision like that since my 21st birthday.....🙂


BT
 
#68
A couple of questions...

1) were you pleased with the service at diylabs.org? I have heard some bad things about that place...

2) is that fresnel you have in there a double sided one?

3) it seems that you have reflective projector insides. I have heard that many people make the insides as dark as possible so taht there is no diffuse lighting inside giving a sharper image. This make a lot of sence to me... It seems that you said that you went with the white to minimize heat... is that true? why exactly did you opt for the white instead of dull black?

Thanx a lot!
 
#69
Alan is very good at DIYLABS. There was a delay in getting the fresnel that I ordered but he was very forthcoming and delivered it when he said he would. He is very helpful as well.

The fresnel is double sided. My design is very similar to a regular OHP tipped on its side. I don't like to put anything in front of the LCD that will add distortion.

Truthfully, I hadn't considered ambient light as being much of a problem because of the way a fresnel rejects off axis light. There may be something to what you are saying though. I will keep this in mind. Thanks
 
#71
Property said:
Old uncle Alan at DIYLabs is a stand up guy who will do honest business with you. Yeah some of his prices seem a bit high...but other of his prices are resonable, and really he's just providing a service for us.
Click to expand...


Ditto - But I don't think he is 'Old'. Unless of course 'I" am now old...NOOOOOOOOOO!!!:bawling:



BT
 
#72
Thanx

Thanx for the info. Do you guys no if anyone has tried splitting this fresnel? I know some people were trying, but I don't remember who. I might just buy this on eand see how everything works out. If I decide I want to go with two single fresnels instead I can do that later...

Now all I need is a reflector and a projection lens...

Thanx for the help!
 
#75
I read that thread and it seemed like someone said that Alan's fresnel could be taken apart, but is not a very good source because of the 14" focus length of both fresnel halfs. Is this true that they are not good to be usin gin a diyprojector? I keep hearing that this fresnel is bad or this one will not be good because of this, but then nobody says what fresnels are better let alone where to get them... AHHHHHHHHHHHHHHH!!!!!!!!!!!!!!!!
 
#76
Well, fresnels, as all things in life, cannot necessarily be broken down into being either good or bad. If a 14" focal lenght on both sides does not fit your needs (as it doesn't fit mine), then it wouldn't be ideal. But there's nothing wrong with the lenses, and Alan is an excellent guy to buy from.
 
#77
keep it simple

Speaking from experience.....

start with an OHP based project....Yuo get all the parts and you know they'll work together. (Lenses, fresnels, lamp, preset focal length, etc......)

If it all works out you can use the parts in a DIY enclosure.

Starting totally from scratch is much harder.
 
#78
Dwiel said:
I read that thread and it seemed like someone said that Alan's fresnel could be taken apart, but is not a very good source because of the 14" focus length of both fresnel halfs. Is this true that they are not good to be usin gin a diyprojector? I keep hearing that this fresnel is bad or this one will not be good because of this, but then nobody says what fresnels are better let alone where to get them... AHHHHHHHHHHHHHHH!!!!!!!!!!!!!!!!
Click to expand...


I used Alan's fresnel. I tried about 5 other fresnels and his produced the most even lighting with no hot spots and a whiter light that the others. Only the outer sections are attached but why would you want to put anything in front of your LCD? I understand that you are passing collimated light through your LCD and you can get a little more coverage by not having your LCD in the beam as it is starting to "shrink" but aren't you risking adding distortion to the image by putting the fresnel after the LCD?

http://www.mdc.nicsystems.com
 
#79
Smaller Desktop size?

Hi there

I just got a splitted fresnel from Alan. I am a little disapointed since it is no really larger than the one I already had in my OHP. Obviously it is quite hard to find a frensnel lens that is large enough for a 15" display. 14" displays are not common except in Notebooks. What I ask myself is why I could not read on any forum any idea of decreasing the size of the windows or linux desktop so that the pixels not shown simply are not needed anymore? I once had a video adapter where I could seperately set the display resolution and the working area (which had to be same or bigger than the resolution). So obviously in windows a mechanism is provided to set the working area which is normally same than the resolution. Anyone have an idea how to set this manually?

Christian
 
Status
Not open for further replies.
Top Bottom