Fresnel lenses are fascinating things. They are used in a wide range of devices, from lighthouses to televisions. You can buy A4 sized fresnel lenses to help you read and can even get credit card sized lenses for the same purpose.
We’re interested in the ones we can scavenge from old, rear projection TVs. They are the front screen on these big beast and can easily be removed.
Fresnel lenses are used as one of the front components of the screens to increase the brightness of the viewing area. We use them to get the opposite effect.
When reversed, they gather light from a large area and focus it to a point or line.
These fresnel lenses are flat sheets of plastic with hundreds of concentric rings moulded into them. These rings capture the light and direct it towards a centre point, acting as a normal optical lens but having the advantage of being flat.
There are two kinds of fresnel lens – point focused and line focused. ‘Point focused’ act as we normally expect a lens to act, focusing light to a point. ‘Line focused’ focus the light to a line across the width of the lens.
Both kinds can be used for our purposes but it’s good to think of point focused lenses as being better at burning or rapid heating of a small area while line focused are better at slow heating.
Fresnel lenses don’t (as some folks mistakenly claim) increase the amount of light available. Like all lenses, they can only work with the light falling on their surface. Lenses merely redirect that light.
Another mistaken claim is that stacking fresnel lenses increases the amount of light and therefore heat that they output. Once again, they can only work with the light falling on them.
We’re interested in turning that light into heat by focusing it on an object and using the light’s energy to heat that object.
Being rather large and thin, the fresnel lenses we use are pretty floppy and are difficult to focus. Sometimes it’s tricky to find the best focal point and a big lens can get heavy when you’re holding it at arm’s length. . To overcome these issues, we need to use a frame to keep them flat and some sort of mount to keep them stable.
Wood is the best thing to make frames out of. The ones shown in the pic have aluminium brackets to hold the lens flat. You don’t need to be that extravagant and can use wood for both purposes.
Keep your frame light enough to move around. Make sure you measure the distance from lens to focal point to make focusing easier layer.
One tip is to allow a few millimetres gap around the edges of the lens. Being plastic, they can expand when warm and warp if that expansion is restricted.
I write the focal length on the side of the frame of each lens to aid my poor memory.
I’ve never got temperatures high enough to melt metal as some have on YouTube. It’s quite believable that you can do it though. I reckon it’s just that the lenses I’ve scrounge aren’t quite right for that purpose. One day I’ll buy some new ones and give them a try. Maybe I’ll even invest in some glass lenses.