Optics: Lens for capturing light from parabolic reflector

[Karnifax]

I was wondering what type of lens would be best for capturing the light incoming from a parabolic reflector into a container? The Fresnel lens looks like a good candidate, but I am still not quite sure. Does anyone have any insight on this subject? Just for info, the parabolic lens redirects incoming light rays to a focal point. What I am trying to do is capture this incoming light into a container, using a lens, but I need to know the best lens in order to do this.

 

[Drakkith]

What do you need a lens for if the mirror is already focusing the light?

 

[Karnifax]

I need an efficient way to capture the light into a closed container, so i can't just leave the aperture open to the environment, it must be closed.

 

[sparkle_pony]

Do you mean a lense that collimates the rays at some arbitrary distance from the focal point?

 

[Drakkith]

I need an efficient way to capture the light into a closed container, so i can't just leave the aperture open to the environment, it must be closed.

How about just a clear window? The mirror is already focusing the light, so a lens would only be useful if you wanted to change the way the light converges into the container.

 

[Karnifax]

The light doesn't have to be focused inside the container, it just need to get inside with the highest efficiency.

 

[Karnifax]

yeah but the photons from the edge of the parabolic mirror will bounce off the clear window

 

[Drakkith]

yeah but the photons from the edge of the parabolic mirror will bounce off the clear window

Only some of them. If you're really worried about it then you can use a window that curves outward on both sides, that way each light ray is entering and leaving the window perpendicular to the surface.

 

[Karnifax]

right, so like a convex lens?

 

[Karnifax]

Only some of them. If you're really worried about it then you can use a window that curves outward on both sides, that way each light ray is entering and leaving the window perpendicular to the surface.

right, so like a convex lens? or a convex lens that is more semi-spherical? Fresnel lenses do a good job with transmitting light as well too tho

 

[Chronos]

A convex corrector lens in front of the primary is called a maksutov telescope. This configuration yields images that rival a refractor, but, at a cost. The lens is heavy, relatively expensive, and not seen in apertures exceeding 7" in the commercial market. A Fresnel lens would suffer a great deal of light loss, rendering it largely impractical. A Schmidt corrector lens is a thin, lightly figured transparent 'window' that is extremely popular in the commercial market. It is somewhat unclear what you are asking about. There are also lenses used within the optical train that are generally classified as correctors [e.g., barlows and field flatteners]. It sounds, however, like you want the optical tube enclosed given your comment about not wishing to leave the aperture exposed to the environment. I assume by aperture you mean the primary light gathering element [objective].

 

[Drakkith]

right, so like a convex lens? or a convex lens that is more semi-spherical? Fresnel lenses do a good job with transmitting light as well too tho

No, I mean a piece of glass that has the same curvature on both sides so that it has no optical power. A meniscus. The curvature of both surfaces can be chosen so that the converging light rays hit the surfaces at a normal angle (perpendicular).

 

[Karnifax]

A convex corrector lens in front of the primary is called a maksutov telescope. This configuration yields images that rival a refractor, but, at a cost. The lens is heavy, relatively expensive, and not seen in apertures exceeding 7" in the commercial market. A Fresnel lens would suffer a great deal of light loss, rendering it largely impractical. A Schmidt corrector lens is a thin, lightly figured transparent 'window' that is extremely popular in the commercial market. It is somewhat unclear what you are asking about. There are also lenses used within the optical train that are generally classified as correctors [e.g., barlows and field flatteners]. It sounds, however, like you want the optical tube enclosed given your comment about not wishing to leave the aperture exposed to the environment. I assume by aperture you mean the primary light gathering element [objective].

Sorry for not explaining myself well. What I am trying to make is a solar still. I am going to have a parabolic mirror and at the focal point I am going to have a container with a hole in it, so that the photons will enter the container (through the hole) and then scatter inside the container and be completely absorbed inside. The inside of the container will be painted black in order to more effectively absorb the light and then transfer the heat to the water. I am leaving out other details in terms of steam leaving, water entering, and solar tracking because they are not important for this conversation. What I need insight on is at the "hole" there will be a lens (so that I can have a closed container in order to keep the steam inside) now which lens would be the most efficient in allowing the light to enter the container and not be reflected is my question.

 

[sophiecentaur]

So you are not talking about optical quality and you are not even worried too much about efficiency (as long as it's in the high 90%s). Overall efficiency will also be affected by heat loss and other factors so you need not worry too much about the optics. The window could be a mylar (or cheap plastic) sheet.
If you want the best out of your parabolic area then an offset reflector could be worth considering. (Here we go, redesigning your pet project; that's PF for you.) How 'good' will the image be from your paraboloid?
Are you starting with a reflector that you already have or would you be starting from scratch? As long as the image of the sun, where the light falls through the window is smaller than the window, you don't need any other optics. Your tracking should take care of any problem with a small window.

 

[Karnifax]

So you are not talking about optical quality and you are not even worried too much about efficiency (as long as it's in the high 90%s). Overall efficiency will also be affected by heat loss and other factors so you need not worry too much about the optics. The window could be a mylar (or cheap plastic) sheet.
If you want the best out of your parabolic area then an offset reflector could be worth considering. (Here we go, redesigning your pet project; that's PF for you.) How 'good' will the image be from your paraboloid?
Are you starting with a reflector that you already have or would you be starting from scratch? As long as the image of the sun, where the light falls through the window is smaller than the window, you don't need any other optics. Your tracking should take care of any problem with a small window.

Yeah but if the window is a flat plastic sheet then I'm going to get a lot of reflection of the photons that are entering from the outer edges of the parabolic mirror, and they will not enter the container. This is because the angle of incidence will be close to 90 degrees.

 

[sophiecentaur]

Yeah but if the window is a flat plastic sheet then I'm going to get a lot of reflection of the photons that are entering from the outer edges of the parabolic mirror, and they will not enter the container. This is because the angle of incidence will be close to 90 degrees.

What is the fraction of light coming in at the edge of the window? Just make it bigger. I think it's time for a diagram with some indication of dimensions here. What efficiency are you aiming for? This is a very relevant question because just making the thing a bit bigger would achieve whatever you want. Is there a size limit?

 

[Karnifax]

What is the fraction of light coming in at the edge of the window? Just make it bigger. I think it's time for a diagram with some indication of dimensions here. What efficiency are you aiming for? This is a very relevant question because just making the thing a bit bigger would achieve whatever you want. Is there a size limit?

I am talking about the angle of incidence at that angle would be so great that the light would simply bounce off the plane of glass. Think about it, the light approaching the focal point from the outer edges of the parabolic reflector will be close to parallel with the flat plane of plastic(which will be located at the focal point) thus not having hitting it directly enough for the photons to enter, they will bounce off. the focal point is about 1 inch in diamter, and i will be using a 24" diameter parabolic reflector

 

[davenn]

agree with Sophiecentaur, Definitely time for a diagram !

karnifax

do you realize that anything put near the focal point of the mirror is going to get extremely hot !
you won't be able to use a plastic, or glass window or lens. Its going to either melt (plastic/Perspex) or crack (glass). I doubt that you would find anything easily available that could withstand the temperatures involved

It seems you are going an odd way about making a solar water heater. You don't need the light focussed into a container. You should have the water IN the container and the container being placed at the focal point of the mirror so that the OUTSIDE of the container is heated

cheers
Dave

 

[Karnifax]

What is the fraction of light coming in at the edge of the window? Just make it bigger. I think it's time for a diagram with some indication of dimensions here. What efficiency are you aiming for? This is a very relevant question because just making the thing a bit bigger would achieve whatever you want. Is there a size limit?

And this is a dumb question but here we go. There is a critical angle when light is going from a medium with an index of refraction that is greater than the medium that it is about to enter. Is there a critical angle for the opposite? Like I am imagining in my head that if I shot a laser, barely grazing the surface of a piece of glass then the laser would bounce off. Am I wrong? (the laser would be going from air to glass)

 

[davenn]

read my post, you probably missed it as we posted at the same time

 

[Karnifax]

agree with Sophiecentaur, Definitely time for a diagram !

karnifax

do you realize that anything put near the focal point of the mirror is going to get extremely hot !
you won't be able to use a plastic, or glass window or lens. Its going to either melt (plastic/Perspex) or crack (glass). I doubt that you would find anything easily available that could withstand the temperatures involved

It seems you are going an odd way about making a solar water heater. You don't need the light focussed into a container. You should have the water IN the container and the container being placed at the focal point of the mirror so that the OUTSIDE of the container is heated

cheers
Dave

the lens won't crack, there will be a continuous supply of fresh water which will keep the lens <100 degrees Celsius. Only if I allowed the container to run out of water then it would get hot enough for the lens to crack, but if there is water, then it won't get too extreme. hitting the container from the outside is not as efficient because you don't capture the photons that don't get absorbed and you can't insulate the container ;)

 

[davenn]

You are getting way way too hung up on this photon business

D

 

[sophiecentaur]

I am talking about the angle of incidence at that angle would be so great that the light would simply bounce off the plane of glass. Think about it, the light approaching the focal point from the outer edges of the parabolic reflector will be close to parallel with the flat plane of plastic(which will be located at the focal point) thus not having hitting it directly enough for the photons to enter, they will bounce off. the focal point is about 1 inch in diamter, and i will be using a 24" diameter parabolic reflector

I did "think about it" and my mental picture, in the absence of a diagram from you, had a longer focal length for the paraboloid. Your problem of reflection would be much less with a longer focus. Btw, have you actually calculated the loss that you are worried about? Is it really significant? This is not an optical instrument where blooming is a requirement.. As with all Engineering, the actual numbers are important and you should tyres to estimate the actual efficiency.

 

[Karnifax]

I did "think about it" and my mental picture, in the absence of a diagram from you, had a longer focal length for the paraboloid. Your problem of reflection would be much less with a longer focus. Btw, have you actually calculated the loss that you are worried about? Is it really significant? This is not an optical instrument where blooming is a requirement.. As with all Engineering, the actual numbers are important and you should tyres to estimate the actual efficiency.

yes increasing the focal length would be a really good solution. I have not been able to calculate the efficiency because I am having a really hard time writing equations that relate the angle of attack and the tangent of lens.

 

[sophiecentaur]

I think you are worrying too much about that aspect of the design, actually. You could easily measure, to an acceptable degree of accuracy, the amount lost by glancing incidence with a digital camera and comparing a scene, reflected in a mirror and reflected on a flat sheet of your plastic. I bet there is well under 5% reflected at reasonable angles of incidence. If it is more and that turns out to be a crucial bit of the design, go for a domed window, which would be easy to arranged by making it inflatable.
You really need to do a 'budget' for the power loss, including all contributions and then work on the big ones first.

PS Have you a draft design with a diagram? We really can't go much further without one (and nor can you).