Any lens to take various angle of incidence light striking it and then align them all?

[shane21]

Assume a flat one square foot vertical plate attached 6 feet up, outside on a wall.

Dozens of yards away are various solar reflectors angled to reflect sunlight onto that plate.

Problem is, the plate requires to be only struck straight on at a 0 degree angle of incidence.

And, all the reflected sources are striking it at some other angle up to 45 degrees off center.

Is there any lens or dome or reflective funnel arrangement, or something else altogether, that
could be put in front of the plate to take all those different angled incoming sunlight sources
and align them all, to next be exiting it to strike the plate behind it straight on at a 0 degree
angle of incidence?

Thank you for any thoughts or suggestions.

- Shane

 

[IRstuff]

Why does it need to be at 0 deg? If the plate were Lambertian, almost any angle would work

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[shane21]

Flat plate target would be 'hot mirror', requiring 0 degree angle of incidence.

 

[IRstuff]

Why is that? Almost all optical coatings work over a fairly large angle of incidence with little loss in performance

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[DanStro]

If it's a hot mirror and the angle of incidence is 0 degrees then it will reflect the heat straight back towards the sky. Are you trying to use the heat or removing it?

Han primo incensus

 

[shane21]

Hot mirrors, according to suppliers, claim they require 0 degree angle of incidence.

However, you bring up a good point, a convex lens or hollow dome coated hot mirror,
would always be presenting O degree AOI for some small % of sunlight, regardless angle.

 

[DanStro]

What are you trying to do with that light? Reflect it to someplace that you want to use it or remove from the rest of the light?

Han primo incensus

 

[shane21]

DanStro,
Remove as much as possible the IR component above 700nm, letting pass the remaining cooler visible light.

These hot and cold mirrors, represented by the flat plate in OP, that do so are pricey per cm, so looking
at reflecting & concentrating more solar outside to be directed at and through it's smaller aperture, to
where it'll reflect the IR, letting pass the visible light.

Some outfits have done so with Cassegrain parabolic collectors, directing solar through a hot mirror, so
they don't melt the fiber optic cable that next transfers light deeper into building. I don't want to fool
with all that complexity and expense, just direct through wall mounted hot mirror and diffused inside.

Challenge is, these hot mirrors require 0° AOI (cold mirrors 45° AOI), and they don't perform well off it,
so I'm inquiring here if there is any way to get AOI better aligned in front of it, before contact with the
mirror. Closer to 0°, with it coming in from different reflectors at varying angles throughout the day,

 

[IRstuff]

Can you provide a link? A mirror is a mirror. You can see reflections of things in a normal mirror from well past 45º incidence. Unless you're using louvers, there's going to be a fairly large angle of incidence range for operability. A hot mirror is nothing more than a band rejection filter, and anything in the cost range you've put yourself in is going to have a wide angle of acceptance.

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[IRstuff]

We once had someone develop a specialty coating, and it was extremely difficult to get a narrow angle imposed on the coating. That coating cost about $20k per element; I doubt that these guys could routinely make something that could only accept zero degrees; and with what tolerance? That's just not the nature of optical coatings. Moreover, why would they design it for zero degrees? The only ones that operate that way are for projectors, and even at that, the acceptance angle is actually more like 45 degrees, when you consider the size of the source.

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[IRstuff]

I think you've been misreading or someone else has been steering you wrongly:

http://www.edmundoptics.com/optics/optical-mirrors/hot-cold-mirrors/hot-mirrors/1492/

shows both 0 and 45 degree AOI, but that's just the nominal and optimum angle of the coating, but the typical design for projection equipment is around f/3 or f/2, so the acceptance angle is quite large.

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[shane21]

IRstuff,
I hope you are right and I'd been reading this all wrong and AOI not as narrowly critical.

I'd been talking with Edmund Optics, originally about UV/IR filters and they'd written back; "Furthermore, the angle of incidence is critical to the performance of the filter. If the angle is off, the efficiency will not be the same as the published specifications." They also said heat would be an issue with the filters, which had me then looking at the hot/cold mirrors. I wrote same tech guy back about AOI range clarification for the mirrors, awaiting word back. Also, have similar inquiry into Thorlabs about AOI for their hot/cold mirrors.

 

[shane21]

These specialty hot mirrors have layers of materials to optimize reflecting of specific wavelengths (like IR) while allowing pass through of others, like 400-700nm visible light. Digging deeper, the more the optic path is at an angle off center, the more these substrate layers and specified 1/4 or 1/2 wavelengths for reflecting IR are then 'out of tune' to original designed intent.

I'm still waiting to hear from manufacturers exactly what the efficiency drop off is as you go away from straight on 0° AOI.
Anybody here know or have link to more about that particular aspect?

 

[DanStro]

Working for one of the suppliers of these I can tell you that it is unlikely they'll be able to give you an accurate set of data, it will be more like "you lose X% at angles greater than Y degrees".

Han primo incensus

 

[IRstuff]

These coatings are not simple 1/4 or 1/2 wave plates, since those are only effective at a single specific wavelength. The bandpass characteristic means that there's a multilayer coating that's doing the job, since the flatness of the top and steepness of the transition automatically imply multilayer interference. Therefore, it's most likely that shifting the angle moves the bandpass left or right, but does not drastically affect the top.

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