||A fresnel lens consists of a transparent plastic plate,
with circular grooves. Each grooves is a segment of a lens, and together
the grooves form a large magnifying glass.
When placing a fresnel lens in front of a monitor, two things happen: the screen is enlarged, but also the screen looks farther away. This creates a sense of depth. Only outside view (scenery) should have added depth, therefore fresnel lens can best be added to a split screen system: The scenery view monitor with lens, and cockpit monitor without lens.
Below drawing shows the optical magnification when adding a lens in front of an object:
Fresnel lenses are used in overhead projectors (small size and rather coarse grooves) but also large screen projection TV's (both triple CRT and micro display type). Especially the projection TV kind is very suitable for flightsim screen enhancements. One day I came across an "old" projection TV on the junkyard and have done several experiments with this 93x44cm (40" diagonal) lens. Its focal length is 56cm.
Finding shops that sell large Fresnel lenses is not easy. Sometimes they
are used as screen magnifiers for TV sets. Someone recommended
3Dlens.com also sells Fresnel lenses,
F550 type seems most used with LCD's. Update: 3DLens.com no
longer sells the F550 type lenses. I have been told that TV magnifier lenses
also can be used.
Adding a fresnel lens will create some screen distortion and color aberration, depending on the amount of magnification. Therefore, the distance lens-monitor and lens-viewpoint needs to be tweaked for best compromise between magnification and distortion.
Experiments with my 93x44cm lens with 56cm focal distance have shown that for optimal results, this lens had to be about 33cm way from the LCD screens. Viewing distance from the lens should be equal or less than the focal length. I'm sitting about 55cm from the lens. Larger viewing distance will result in magnifying distortion at the edges.
A fresnel lens can also be added to spanned monitors. See Parhelia solution