Thursday, January 19, 2017

fresnel lens 2

I had written in fresnel lens about wanting to determine the focal length of a large Fresnel lens removed from an old rear projection television. I want to know the focal length to design a mount to aim the lens at a fire-proof adjustable work table in a safe manner.

I am using laser pointers to see where the light beam is bent. I have a measurement for each lens but I am confused about which side of the lens to point toward the sun. I am seeing an unusual effect that I wanted to explore further.

The simple test setup I was using showed me that I was on the right track but I realized that I could improve the setup to make the measurement more accurate and repeatable.

This is a picture (click to enlarge) of my temporary wood shop optical bench setup. There is one Fresnel lens lying flat on the left table. You can see the circular rings. The other is hanging vertically between the two tables. I have made some improvements which I'd like to tell you about.

I would like to use one of my lenses to construct a solar concentrator, primarily to fuse materials. An example by Dan Rojas of Green Power Science.

A very thorough article on using Fresnel lenses for communications purposes.

I had read an instructable which suggested using two laser pointers resting on chairs. I am trying to use the same concept but using a more stable setup. I had some difficulty following the laser beam when I was manually holding the pointer.

As before I am using two benches with the lens suspended between them. Previously, the benches were about the same height. Now one bench is about 1 inch shorter in height. This difference allows space for a sliding cart to hold the laser pointers. The cart rides in a track so that I can smoothly move the laser pointer across the front of the lens.

There are two laser pointers on the cart. The yellow emits a red dot and the black emits a green dot. The pointers are held to the cart with hot glue. The height of the pointers is adjusted before gluing with a pair of tapered shims under each pointer to bring the dot high enough so that it just skims the out table top. If you enlarge the picture, you will notice the red laser dot just at the tabletop height of the out table (the higher one).

The red pointer has a rocker switch which allows it to stay ON. I must hold the button on the green pointer to keep it on.

A view of the side of the cart and it's track. The track is not fastened to the table and may be lined up parallel with the lines I have marked on the tabletop at 5, 10 and 15 inches back from the lens. The lens is on the left side.

You'll notice I haven't got the fronts of the laser pointers at the same distance but I don't think it matters. I have been using only one of the pointers at a time, not both of them. They are fixed down, the button on the green is over the beam so it is less tipsy and they can be slid easily.

I have the sheet to be tested hanging level with its focus (the center of the Fresnel pattern) at table top height (the height of the out table). I have two short strips of wood with hook eyes in the ends that I clamp to each side of the sheet.

In this way, I can easily unhook the sheet and reverse it so that I can send the beam into either the Fresnel side of the sheet or the smooth side. It is necessary to move the out table away to make some space to flip the lens. Normally I push the tables together to clamp hold flat the Fresnel lens at the desired height.

Here are my results so far (shown also in the first picture above):

Fresnel lens focal length
38x29in (96.5x73.7cm) Lens A30.5in (77.5cm)
35x27in (88.9x68.6cm) Lens B28.0in (71.1cm)

Note that my results are with the Fresnel surface of each lens facing the laser pointer (the top of the two illustrations).

I thought the lens performance would be approximately equal in the two orientations. In my observation, the determination of the focal length is less clear when the smooth surface of each lens is facing the pointer. I will do some more work and report shortly.

Thanks for your interest.

George Plhak
Lion's Head, Ontario, Canada

Friday, January 13, 2017

fresnel lens

I have a large flat plastic Fresnel lens hung between two benches that are the same height, more or less. The benches are pushed together so that the sheet is sandwiched between them. The sheet is supported on wires from hooks in the ceiling. I can vary the height by adjusting the wires.

I have two of these sheet lenses, but they are different. I am trying to measure the focal length of each, the distance from the lens to the point at which the light converges. A Frensel lens is like a magnifying glass, except in thin lightweight plastic sheet form.

Mine both came out of discarded rear projection televisions. It does take a bit of effort to free one of these from the set and it isn't for the faint of heart. I generated quite another pile of waste by taking out every part from the frame but perhaps my separation of the waste helps with the ultimate recycling effort?

Both lenses came from the sets as an assembly of two sheets, the Frensel and another sheet with very fine vertical stripes, like a diffraction grating. The two sheets were taped together anong their edges and held in plastic frames. I removed the frames and tape to get the assembly apart to free the Fresnel sheet.

At any rate, I have the sheet hung from the ceiling so that the center is approximately at the table height. I am using a laser pointer to see the direction that the beam is bent as it passes through the sheet.

This is a series of pictures showing the bending at the back of the sheet. The metal ruler is perpendicular to the back of the lens lined up with the main axis (at the center of the sheet).

I have marked lines parallel to the laser beam near both edges of the sheet with a ruler and I find that the lines converge at a point that is 30 inches from the sheet. This is the Focal Length of this sheet.

The sheets are different and seem to give different results depending on which side of the sheet faces the laser pointer.

Fresnsel lenses have application in concentrating photovoltaics with some interesting examples shown by Green Rhino Energy.

Fresnel lenses can also be used as solar concentrators to reach very high temperatures, like this.

Not sure what I am going to do with mine yet.

Thank for your interest

George Plhak
Lion's Head, Ontario, Canada

The donor of one of my lenses, a Sony KP-43T70, made in Westmoreland, PA in February 2000.

Saturday, January 07, 2017

major appliances

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Now that I have one summer and a few months of winter electrical data, I am able to predict the relative usage of my major appliances in these two pie charts. My data is the percent of typical daily kilowatt hours kWh used by these major appliances.

The big light blue section of the chart is everything else (lighting, ovens, washer, dryer, dishwasher, computers, chargers, power tools, plug in heaters, garage door openers etc, etc). None of these are used often enough to deserve their own slice of the pie so I have lumped them together in one slice. I want to focus on my heavy electrical users, the other slices, for now. But the everything else category accounts for over half of my usage!

My spreadsheet (.ods format) is here.

Starting at the top left, water heating is the dark blue segment. My old electric water heater accounts for about 15% of my electrical usage both summer and winter. The water heater actually uses more during the winter because the ambient temperature of the basement, where my water heater is located, is cooler in the winter. My overall home usage goes up in the winter months as does the water heater so it is about 15% the total year round.

The refrigerator (orange) and freezer (yellow) together account for about 23% of summer usage, falling to about 15% together in winter.

The furnace (green) is not active in the summer but makes up a whopping 25% of winter usage. Although I have an oil burning furnace, it uses about 620 watts of electrical energy when running. I last wrote about my furnace here.

Finally, the base load (red), the things that are always ON, accounts for 17% of summer falling to 11% of winter usage. The base load is made up of a whole bunch of things which deserve their own category. I wrote about my base load here.

Thank you for your interest.

George Plhak
Lion's Head, Ontario, Canada

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