Wednesday, January 25, 2017
fresnel lens 3
Video from rimstar.org about Fresnel lenses. He discusses the question of which side of the lens should face the sun.
From about 2:20 to 4:50, he measures the focal length of his lens in both orientations. His is the same 32 inches both ways. He then does a heat gain test with both orientations (to 6:10) and finds that with the grooved side facing the sun, he gets significantly more heat. His test is to heat 150 ml of water by 20F degrees at the focal point and measure the time in minutes. Shorter time indicates better heating.
Grooves facing sun: 1:55 (one minute, 55 seconds)
Flat side facing sun: 4:40
He provides an explanation of why this effect occurs - spherical aberration. He goes on to explain the difference between a linear and a spot lens from 8:00.
A very interesting site is rimstar.org. Have a look around at some of the other impressive work shown there.
NTKJ Co., Ltd. (Nihon Tokushu Kogaku Jushi) Fresnel lens supplier: an illustration showing the orientation of their Fresnel lens for solar concentrator with the statement: "Our standard fresnel lenses can also be used for the same purposes but with the opposite facing design. Namely its plano side faces focus and the fresnel surface faces parallel light source."
Perhaps this applies only to NTKJ lenses?
Their website is a useful catalog of different types of commercially available Fresnel lenses. I don't know if this company manufactured either of my Fresnel lenses.
The Green Power Science store is a source of Fresnel lenses of different sizes and types.
A video description of my temporary optical test bench used for measuring focal length of Fresnel lenses.
I was able to show myself that both of the lenses I have (your mileage may vary) focus much more precisely if the parallel light enters the lens on the Fresnel side but this not always have to be the case hence you might consider testing your lenses in a similar manner?
It is easy to understand why this is the most common approach. You need a rigid frame around the lens to keep it straight and what better way to add a pivot than at the center point of the frame?
I would imagine that the movement of the sun over time and the wind would make the this type of stand problematic.
The other degrees of freedom are #2 which allows the work table to rotate so different sides of the work could face the sun and #3 allows the work table to be raised and lowered, moving the work vertically in the focus.
Sorry if that is not more clear but it is only a concept that I hope to refine as I build something.
For now, I need the benches free so I must finish with these focal length measurements.
Thank you for your interest.
Lion's Head, Ontario, Canada