Fresnel Lens

does anyone know where i can a fresnel lens that can fit a D-cell maglite? i guess the diameter is a appor 52 - 55mm (2.047 - 2.165in)

i have been searching on the web and most round fresnel lens are much larger, i cant find any small ones. i'm thinking of getting a square fresnel lens and shape it accordingly

if i started this thread in the wrong section will the mods pls move this to the appropriate seection. thanx

Have you tried Edmund Scientific?
Good luck, though. There aren't very many small Fresnel lenses out there.
Fresnel lenses are big by design. They are meant for large applications (lighthouses, for instance) because they use less material (glass, plastic) in order to make large diameter lenses.
Why can't you use a more conventional lens?---Marc

Marc,

thanx for the tip. i have searched Edmund Scientific and apparently they have only large square Fresnel lens.

there are small square fresnel lens which i guess with an apropriate tools i can shape it if there is no detrimental effects on the lens. but i would still prefer something off the shelve.

if i were to use a conventional lens i guess i won't be able to stand the maglite on it's face and i guess these convex lens would be more prone to wearing

Moved to the Homemade and Mod flashlights forum.

OK then.
How about a plano-convex lens? It has one side flat and one side spherical.
You could place the spherical side toward the bulb and have a flat surface on which to stand the light.---Marc

Originally posted by MicroE:
You could place the spherical side toward the bulb and have a flat surface on which to stand
the light.---Marc

Click to expand...

<font size="2" face="Verdana, Arial">Um... but then, it wouldn't focus into a tight beam. To do a tight beam you need the spherical portion on the outside.

Could you cut out a circle from one of those pocket fresnel magnifiers? They can be had for pretty cheap so experimenting on a few wouldn't be too costly. American Science & Surplus used to sell 4 wallet sized card for about $2.

Sam

Marc,

as hotfoot has pointed out placing the convex of the lens inwards wounldn't focus it into a tight beam and besides the maglite head isn't really that spacious

Sam,

yea i have purchased fresnel lens magnifier from my neighbourhood bookshop but it didn't focus the light. did some research, i think magnifiers are negative fresnel lens. what i need is a positive fresnel lens to focus the light. these magnifers also have dense grooves, i.e.more grooves per mm making it inefficient for 'light collecting'

On some of the old military green (olive drab) flashlights with the "bent" head, they had a filter assortment in the bottom of the battery holder, and I thought one of them was a Fresnel Lens for the right diameter.

You might look for these at a Military Surplus Store

Mike
www.inretech.com

"...Um... but then, it wouldn't focus into a tight beam...."

Actually I was thinking that you could use a plano-convex and place the filament at the focal point. This would yield a flashlight beam that was parallel with no point of convergence (a la MagLaser).

"...and besides the maglite head isn't really that spacious..."

Now you've done it. You made me pull out my calculator.
The lens would fit. Too well, I'm afraid. I just eyeballed my Maglite and the filament is about 1.9 cm from the bezel. Thus the required lens would have a focal length of 1.9cm and this means that it's radius will be less than 1 cm. In other words, you could get the lens in place but it would be too SMALL. It would only collect about 20% of the light. Oh well, on to plan B.
Plan B==> Use a conventional lens and put it INSIDE the Mag bezel.
Hey. Why are we doing this, anyway? The mag already has a focus system.

One way to get more space in the mag-head is to sacrifice things. The switch-module is easily removable, and you can glue the movable post the bulb is mounted on at it's lowest level. Then use the head-travel to focus, instead of the 'patented cam-action.' If I can I want to get a pile of misc Mag-size lenses (plano-convex, convex-convex, fresnel, acrylic-ball, etc) for my LED-Mag experiments.

The Mil-type angleheads don't really have a fresnel-lense, it's really more of a beam-diffuser thant a true magnifier. Gets rid of the annoying irregularities, and keeps the beam about as tight as before.