Distance/Focusing of IR Beam

[A.J.710]

TL;DR Summary

Feasibility of focusing heat from IR source over several meters.

I have been researching all night and can't find much useful relevant information on this subject. I have gone back to the physics roots and played with ray diagrams and specific heat calculations but all of these theoretical possibilities have tons of real world variables and that's where I'd like some insight from here.

Without getting into too many details. Basically I have a small 1 sq. ft. portion of a gutter drain that I want to keep warm to prevent from freezing and filling with snow. I thought of IR heating because I want to avoid running heating tape or any other wires to the area and at the same time, this would be a very fun project if it works. The problem is distance; ideally around 50 feet. What is the feasibility of having an IR source and focusing the beam to heat an object 50 feet away without tremendous power losses?

I wanted to do this with LEDs to keep the source beam angle narrow and simplify the drive electronics aspect but it seems that to get the same amount of power as halogen type bulbs would be astronomically expensive. Would reflecting the full 360 degree beam of a halogen bulb and focusing it with lenses actually be able to radiate heat over a distance of ~50 feet? If not what would something more reasonable be?

With enough reflectors it looks messy but doable in ray simulations but this barely counts for real world divergence and energy loss over the distance. 250 watts of power should be more than enough to keep the object at an above freezing temperature. For the source, I'm willing to use a 220V/50A line which is way more than enough. This thing will be running only a few hours per month so electricity cost is not a concern. I can coat the gutter drain with some sort of IR absorbing material to boost efficiency as well.

Please provide any insight you may have, especially if this is even possible. I know it seems like a pretty crazy idea but it seems theoretically possible so I just want to see what other information I may be missing or if this is even worth a shot at trying.

Thanks

 

[Baluncore]

Make a reflector that has the shape of a partial ellipsoid, with one focus at the source of IR and the other at the distant receiving gutter. There are no lenses required.

Find out how to draw an ellipse with two pins, a loop of string and a pencil.
https://en.wikipedia.org/wiki/Ellipse#Pins-and-string_method
Then tie a string to the gutter and to the IR source position, draw and cut a template for the part nearest and illuminated by the IR source.

Rotate the template about a vertical axis to generate the partial ellipsoid in a pit of sand.
Cast the reflector in concrete or fibreglass. Paste IR reflective Al foil onto the inner surface.

 

[berkeman]

Summary:: Feasibility of focusing heat from IR source over several meters.

Basically I have a small 1 sq. ft. portion of a gutter drain that I want to keep warm to prevent from freezing and filling with snow.

But if it's snowing, won't that interrupt your IR beam?

 

[A.J.710]

But if it's snowing, won't that interrupt your IR beam?

I suppose slightly depending on how hard it’s snowing but I’d like to make the source exceedingly powerful to overcome obstacles like that. I 3D printed a parabolic reflector today and wrapped it with aluminum foil. I bought a 1kw 14” quartz tube that if placed properly in the custom reflector, should theoretically collimate the beam for the most part. With this quick prototype, I can see if it’s even worth continuing or not because I will get a good idea of how much heat is actually transferred over distance with a reasonable setup.

 

[Baluncore]

I 3D printed a parabolic reflector today and wrapped it with aluminum foil.

I think you are not understanding the difference between a parabolic and an elliptical reflector.

A parabola converts light radiating from a point source into parallel rays. At the receive end, another parabola takes those parallel rays and focuses them to the target point. But that second parabolic reflector must be behind the target.

On the other hand, an ellipse focuses rays from one focal point onto it's other focal point. You only need an ellipse that surrounds most of the IR source. The other focal point will be the target, which does not require any reflector behind it. Unlike parabolic reflectors, the rays traveling from one ellipse focus to the other are only ever reflected once.

 

[berkeman]

I suppose slightly depending on how hard it’s snowing but I’d like to make the source exceedingly powerful to overcome obstacles like that.

Oh, you mean like this system...

 

[A.J.710]

I think you are not understanding the difference between a parabolic and an elliptical reflector.

A parabola converts light radiating from a point source into parallel rays. At the receive end, another parabola takes those parallel rays and focuses them to the target point. But that second parabolic reflector must be behind the target.

On the other hand, an ellipse focuses rays from one focal point onto it's other focal point. You only need an ellipse that surrounds most of the IR source. The other focal point will be the target, which does not require any reflector behind it. Unlike parabolic reflectors, the rays traveling from one ellipse focus to the other are only ever reflected once.

Thank you for the advice. I am still trying to grasp how this would work though. At a distance of 50 feet, won't the elliptical structure have to reach past the midpoint to reflect onto the second focal point? At that point, then the reflector would be over 25 feet long. Ideally it would be nice to focus all of the light to one focal point but at such a large distance, if that big of a reflector would be needed, it would be impractical.

A parabolic reflector can collimate the light and depending on how tight that beam is, I figured it would be close enough to transferring the majority of energy across the distance. It would make for a larger spot size than a direct focal point but the reflector in theory could be fairly small and only slightly bigger than the IR source itself. If I am overlooking something and a small elliptical reflector can in-fact focus on an object a large-distance, please let me know.Here's what I have done so far to try to prove this out:
(This is purely to test the concept, I know the energy requirements are very low for the actual project intention)

14" long quartz tube, 0.5" diameter. Running at 120V, 7A. (Capable of double)
This is like a long cylinder light so the reflector would be a parabola or ellipse and just extended in the 3rd dimension rather than a cone structure.

3D printed and coated with aluminum foil a parabolic reflector with a 1.5" focal point for a 6" beam collimation.

I set it up in a dark room near a wall with half reflected and half open. It was clear when moving my hand across that the heat was significantly more when in front of the reflector. Furthermore, a colored rectangle spot was apparent on the wall for the reflected portion.

Next I'm going to try to make a parabolic reflector with a much more concentrated beam of maybe 2" instead to see if I can have the energy be directed even further from this source without dissipating. The issue I will run into now is that when doing math, the parabola works when assuming a point source. In reality, this is a cylinder ring about 0.5" in diameter so when going to a smaller size reflector, the offset of the actual illumination from the center will probably throw off the reflection by a huge amount. Are there any resources/simulators that can account for this is adjust the reflector shape accordingly?

Also are there any fairly cheap tools that can be used to measure the Watts/meter^2 so I can quantify how much is actually being radiated onto a surface when testing?

Thank you

 

[Baluncore]

If I am overlooking something and a small elliptical reflector can in-fact focus on an object a large-distance, please let me know.

Consider your parabolic reflector, then change the figure of the surface very slightly to that of an ellipse. The reflector will then concentrate the energy at the remote focal point. You can generate the figure of the elliptical reflector with a fixed length string tied to the IR source and the target.

 

[Baluncore]

The issue I will run into now is that when doing math, the parabola works when assuming a point source. In reality, this is a cylinder ring about 0.5" in diameter so when going to a smaller size reflector, the offset of the actual illumination from the center will probably throw off the reflection by a huge amount.

The wavelength of IR is very short, so increasing the diameter of the reflector will not reduce the beam width. However, as the physical size of the source increases, you will need to increase the size of the local reflector in order to concentrate more energy onto the target.

 

[A.J.710]

Consider your parabolic reflector, then change the figure of the surface very slightly to that of an ellipse. The reflector will then concentrate the energy at the remote focal point. You can generate the figure of the elliptical reflector with a fixed length string tied to the IR source and the target.

I understand now. I can see how bending the surface slightly can get the light to hit a a focal point. I just played with bending my aluminum foil reflector a bit and could see the beam on the wall as it converged or split in two based on the angle or distance.

This image also shows a good representation of the rays reflecting on the different surfaces.

The ellipse I assume would be very eccentric in order to keep the reflector from extending so far out and being impractical. In terms of the slope being 0 at the midpoint for the ideal reflection of all light, what slope value would typically suffice in a reflector of this type so I can see where to cut off the remainder of the ellipse? The midpoint would mean the reflector for this project would have to extend out 25+ feet. I would like to keep that as small as possible. Probably closer to 1 or 2 feet.

Thanks

 

[Baluncore]

The ellipse I assume would be very eccentric in order to keep the reflector from extending so far out and being impractical.

You are happy to use a small parabolic reflector that certainly does not continue half way to the receiver. I can't see why you believe the elliptical reflector will be any different. You can make the reflectors exactly the same depth or diameter. The focus of either could be inside the reflector.
The problem you have is the efficient illumination of the reflector by a non-point-source. If the source was a cylinder, mounted on the reflector axis at the prime focus, the second focal point would be concentrated on the target.
To understand the geometry you need to draw an ellipse with the help of two pins and a loop of string.

 

[Tom.G]

Since your IR lamp is an extended source rather than a point, how about making the elliptical reflector a trough, i.e. a 2-dimensional elliptical cross section that extends the length of the lamp. Since your target is also a linear object, you can align the axes for even heating.

Another minor(?) variation is to warp the trough in the lengthwise direction in a parabolic curve. After all, with 3D printing 'anything is possible', assuming you can work out the math!

Cheers,
Tom

 

[sophiecentaur]

At a distance of 50 feet, won't the elliptical structure have to reach past the midpoint

What you are worrying about applies to more or less every time a reflector is used to 'focus' a beam. For microwave antennae, it's usual to use a directional 'feed' which has a broad beam and which reflects most of the transmitted energy into the dish. Your led array could be placed at the focus with a reflector to direct most of the 'wasted' power into the dish.

There are some other issues that I think you are needlessly worried about. If your source array is smaller than the target then an imperfect image is that you in fact want so the optics is not very critical. Making the reflector elliptical or paraboloid will not make much difference. The recommended way of forming an ellipse with foci separated by 50 feet is not convenient. Why not just make a paraboloid with a focal length of, say 54 feet and then adjust the position of the source so that it's spread over the target? You can use a visible light source to set it all up with.

I remember using a 0.9m comms reflector, coating the inside with kitchen foil and using it very successfully to make a solar heater to boil a can of water. Defocussing the Sun's image gave the whole of the can a share of the heat. It worked very well for the classes I was teaching.

 

[Frodo]

Use two car? lorry? headlight reflectors? Army surplus searchlight reflectors would be good.

The bigger your energy source the bigger the reflector should be - a parabola only produces parallel rays for a point source.

Is there a health hazard of being struck by a 250W beam? In the eyes?

 

[chemisttree]

Why not try something like https://www.napaonline.com/en/p/MHTMH32TTC?cid=paidsearch_shopping_dcoe_google_monthly-special_202101&campaign=GSC-Promos&campaign_id=6478906937&adgroup_id=78844950918&adtype=pla&?

 

[sophiecentaur]

Why not try something like https://www.napaonline.com/en/p/MHTMH32TTC?cid=paidsearch_shopping_dcoe_google_monthly-special_202101&campaign=GSC-Promos&campaign_id=6478906937&adgroup_id=78844950918&adtype=pla&?
etc.

The optics looks all wrong to me. The forward facing part of the red hot area is radiating over about a hemisphere. Not what you want but great for heating a room. You need a way of firing most of the radiated heat into the main reflector and then focusing it onto a small area. Also, turning the radiating element the other way round could melt overheat the holder and the gas pipery.

I can't easily find examples of IR reflectors but there are many websites about microwave antennas which work on exactly the same principle. In this link there is an image of the sort of technique that would be applicable. The arrangement in the first figure produces an 'image' at infinity but moving the (directive) feed outwards will bring the image closer. Easy to do experimentally and not particularly critical if the source is not too extended.
Actually the dimensions of the source are very relevant. The wider the source, the larger the dish diameter required, if you want to avoid spillage. The secondary reflector (and the radiant element too) will block the main beam from the dish but that's a small price to pay for the added 'gain' (directivity) of the system.

Would reflecting the full 360 degree beam of a halogen bulb and focusing it with lenses actually be able to radiate heat over a distance of ~50 feet? If not what would something more reasonable be?

I just read this again. A 200W halogen PAR bulb would be the ideal feed source to fire into a large dish. There is a built in reflector so you could fill the area of the dish very efficiently. On the subject of LEDs, I'm not sure that an LED source would be what you want, at all. LEDs produce a lot of light without wasted Heat but what you want is a lot of heat. There could be a snag with light pollution but perhaps one of those IR lamp, used by chicken breeders would do the job without piercing the night sky with LP.

 

[sophiecentaur]

Possible nonsensical alternative. Why not use a fan heater at ground level and pipe hot air up to where you need it. No problems with snow blocking the beam. (Alternatively, a flow / return system could take warm water (with anti-freeze) up there).
This would all depend on how accessible the ground under the gutter may be.

PS This project could be of interest to @OmCheeto ?

 

[chemisttree]

The optics looks all wrong to me. The forward facing part of the red hot area is radiating over about a hemisphere. Not what you want but great for heating a room. You need a way of firing most of the radiated heat into the main reflector and then focusing it onto a small area. Also, turning the radiating element the other way round could melt overheat the holder and the gas pipery.

I can't easily find examples of IR reflectors...

I assure you the optics are all right! Here’s another example sold at Lowe’s. This produces a fairly concentrated beam. Not sure how strong it is at 50’ though. It’s only rated for indoor use.

 

[sophiecentaur]

Not sure how strong it is at 50’ though.

Exactly. There would be no point in designing something that would direct all the heat at one patch on an indoor wall. So I have doubts about its suitability.
But that picture is not of the gas heater in the original picture. It's more like the design that I was suggesting but still not suited for a beam of only a few degrees (50' projection range).
The required application is probably not one that can be satisfied with something off the shelf.
A 200+W source could need a large reflector and a long focus to avoid cooking parts of a reflector that's not metallic. Quite a lot of fabrication work involved, even without the requirement for surface quality. Perhaps a fibreglass shell with strips of shiny aluminium fitted inside. Definitely do-able if you fancy a bit of work.

On a whim, I looked at Solar Cookers on eBay and there are a number of reflectors, around 1m diameter and some some about 100GBP. With a modified focus position, you could find that would work perfectly. Not a 'free' / DIY solution.

@A.J.710 From what you say, you do not have easy access to the gutter and you probably don't want to get up there at all, in any case. But you would need only very infrequent access for a totally satisfactory electrical system. Perhaps you could think again??