hakonjdjohnsen

A fun fact about Cherenkov radiation is that research on making efficient Cherenkov detectors in the 1960s led to the development of optical principles still used to design illumination systems and solar concentrators today.

The late professor Roland Winston worked on this problem and discovered a geometry that could concentrate the light from a diffuse source like Cherenkov radiation to a detector with near-ideal performance ( https://doi.org/10.1063/1.1720428 ). It turns out that efficiently transferring light from diffuse sources has applications far beyond detecting Cherenkov radiation, so Winston founded the field of Nonimaging Optics and spent much of the rest of his carreer on developing the foundations of the field and on bringing together a community of scientists who would work on carrying the field forwards.

I do research in this field myself, and I find the optics and principles behind it endlessly fascinating

Agreed, that was a really good talk! It is kind of scary how simple some of these exploits that find their way into smartwatched for children are

This video by AlphaPhoenix is absolutely incredible! I do research in (nonimaging) optics and I am used to thinking about the propagation of light. Still, there is something amazing about seeing a real recording of the propagation of a real beam of light. I also love the fact that you see artefacts due to how long light from different parts of the scene takes to reach the camera

Thermophotovoltaics is really cool. It is an old idea but recently several groups (including the group behind fourth power) have shown much better experimental performance than before, towards the level where this is starting to look like a real solid-state heat engine.

The idea is to use a photovoltaic cell (“solar cell”) to convert thermal radiation to electricity. A regular solar cell has limited efficiency because the sun has a wide spectrum and a single material is not efficient across the whole spectrum. With thermophotovoltaics, the hot surface is so close to the cell that you just reflect the “bad” photons back to the hot surface to recycle them instead of losing their energy.

In theory, a more efficient alternative to a traditional solar cell is to use the sunlight to heat a surface to ultra-high temperatures and then run a thermophotovoltaic cell on that hot surface, but this is easier said than done.

As an outsider I do think it looks like the competitor Antora Energy has a simpler approach: instead of pumping the heat using high-temperature liquid (with lots of moving parts), they just use thermal radiation to transfer the heat inside their battery.

I think there is at least some plausible interpretation of this that points to more than marketing fluff.

You want to count particles per volume of air, so conventional sensors use a fan to have a constant volumetric flow and then count particles per second to infer particles per volume.

The way I interpret the above marketing language is that they use the optical sensor not only to count particles but also to measure the particle movement and infer airflow. So as long as there is some natural movement in the air, they can measure both particle count and volumetric flow, and thus infer particles per volume.

Yeah good point!

When I came across this amazing project and wanted to share it to HN, I was debating whether to post the youtube link or the project page. I decided to post the project page and mention the youtube link in the description for those who prefer video, but somehow that description got posted as a comment instead (not sure how that happened?). Anyway as you said the video is embedded in the project page so it wasn't really necessary

See also the youtube video about the project: https://youtu.be/8s9TjRz01fo

I found this presentation from the Disobey 2025 conference to be a really good and entertaining watch!

Last year the presenter also wrote a blog post about the attack, which received some discussion at the time:

https://news.ycombinator.com/item?id=41608949

Cool concept! I do research in solar concentrator optics, so I enjoy seeing such completely different applications of concentrated sunlight. Still, I am not fully convinced in this specific case. I wonder if it is not a lot easier to provide the missing spectrum ourselves instead of running fiber optic bundles from the roof?

If I understand correctly, your two main benefits are broader spectrum and lack of PWM flicker. Did you measure the spectrum of the light from the prototype monitor? The light goes through several filters - first I assume the daylighting system has an IR filter to prevent overheating. Then it goes through the LCD itself, and the color filter array in front. Are you still left with a lot of IR (or the frequencies are considered beneficial) after all this?

The reason we get a lot of light from the sun is not that the sun is particularily "bright" (high radiance) compared to other stars, it is because the the sun has an absolutely huge apparent size in the sky compared to all the other bright objects we can see.

Let's say you go to one of the illuminated areas that paid for reflectorbital-light and look up. What would you see? You would see a tiny bright spot flying past, with an angular size of about 10^-10 steradians [1].

This tiny spot has the same "brightness" (radiance) as the sun, because a mirror preserves radiance. However the mirror looks about 10 000 times smaller than the sun from your perspective (the sun has an angular size of about 10^-5 steradians). This means that the satellite would only give you 0.01% of the light compared to the real sun.

If you could somehow take 10k satellites and use them to illuminate the same spot, you could technically get it to resemble real sunlight. But imagine what this would look like: These satellite would need to be many enough / huge enough to cover a very significant portion of our sky, on the order of the apparent size our actual sun. They would be spread out in a sun-synchronous orbit, so they would be visible at dusk with this size, from all points on the earth. Would we really want that?

The founder has been thinking about using mirrors to collimate the sunlight to get around this problem, but it won't work. The collimator design he showed in a 2022 article [2] would decrease the focal spot from a 5km diameter to some smaller diameter as intended, but it would do so by throwing away light, not by increasing the brightness in this smaller spot. This is given by conservation of ètendue, one of the fundamental laws of nonimaging optics (where I do research).

[1] They are planning a 100sqm mirror at 600km altitude, which gives a solid angle of (100 m^2)/(600e3 m)^2

[2] https://www.vice.com/en/article/this-man-is-trying-to-put-mi...

> You make a big ring (roughly of the same area of the spot you are trying to make on the ground)

Unfortunately, this is not how the size of the spot on the ground is decided. Sunlight, even if reflected by a perfectly shaped mirror, spreads by approx 1 meter every hundred meters. At the "edge of space" at 100km, your spot already has a 1km diameter, in reality with a higher orbit and imperfect mirror & tracking it will be much larger. The size of your (ideal) mirror decides the brightness of the spot, not the size of the spot.

Liquid mirrors in space seem like a cool concept though!

So the birdkill is actually from a separate focal spot near the absorber where standby heliostats are focused? Interesting, I did not know that! Makes me wonder why standby heliostats would need to be focused at all? Couldn't their aimpoints be randomized over a much larger volume near the receiver, while still being standby and able to quickly move back onto the receiver when needed?

By the way, I’m happy to find someone with CSP knowledge on HN. Are you working in the field?

Looks really cool! I look forward to reading your paper. Do you know if a recording of the talk is/will be posted somewhere?

Interesting! I added an email address to my profile now

Yes, exactly. I have not looked at Mitsuba 2, but Mitsuba 3 is absolutely along these lines. It is just starting to be picked up by some of the nonimaging/illumination community, e.g. there was a paper last year from Aurele Adam's group at TU Delft where they used it for optimizing a "magic window" [1]. Some tradeoffs and constraints are a bit different when doing optical design versus doing (inverse) rendering, but it definitely shows what is possible.

[1] https://doi.org/10.1364/OE.515422

This, very much this!

I do research in a subfield of optics called nonimaging optics (optics for energy transfer, e.g. solar concentrators or lighting systems). We typically use these optical design applications, and your observations are absolutely correct. Make some optical design software that uses GPUs for raytracing, reverse-mode autodiff for optimization, sprinkle in some other modern techniques you may blow these older tools out of the water.

I am hoping to be able to get some projects going in this direction (feel free to reach out if anyone are interested).

PS: I help organize an academic conference my subfield of optics. We run a design competition this year [1,2]. Would be super cool if someone submits a design that they made by drawing inspiration from modern computer graphics tools (maybe using Mitsuba 3, by one of the authors of this book?), instead of using our classical applications in the field.

[1] https://news.ycombinator.com/item?id=42609892

[2] https://nonimaging-conference.org/competition-2025/upload/

There are also two systems in Sydney, the big tall one on Rhodes Central [1] and the slightly smaller on One Central Park [2].

I think the idea is that by removing some of the shadowing from the high-rise buildings, the local authorities allow the buildings to be taller, which makes this type of system pay off.

[1] https://www.abc.net.au/news/2021-09-13/heliostats-bringing-l... [2] https://good-design.org/projects/one-central-park-heliostat-...

This is known as a charge pump, and is the third concept described in the linked article. The article only mention one flying capacitor, but you can use more than one and connect them in series to get a higher multiple of the input voltage.

Ben Eater also did a nice introduction to charge pumps by building a simple one on a breadboard: https://www.youtube.com/watch?v=4alV5LzHLE4&t=704s

This is a really cool concept! The term used for connecting mirrors to rotate together is a ganged heliostat. Most papers about ganged heliostats don't do anything fancy like what you describe, but there are some patents that show a nice way of connecting the rods to give the correct angles using a slightly different concept than what you are describing [1]. I previously made a visualization of how that concept works (click and drag the sun) [2].

I've not seen exactly what you describe published anywhere, but it sounds very smular to something I thought about as well. See this 2d illustration [2]. Is this the same as what you are describing?

I had a masters student try to make it mechanically. It turns out that though it's an elegant concept, you still end up with quite a few moving parts so it's a bit tricky.

[1] https://patents.google.com/patent/US20060060188

[2] https://folk.ntnu.no/haakonjj/ganged_heliostat_illustration....

[3] https://folk.ntnu.no/haakonjj/ganged_heliostat_gear/

Great question, and this shows why we could never get a 100% efficient solar panel. Otherwise your scheme would brak thermodynamics.

The most efficient possible way to convert sunlight to electricity is ~86% and is related to the second law of thermodynamics. So we use the heat flow from a hot reservoir (sun) to a cold reservoir (earth) and are able to convert some of that heat into work (electricity) which can then be used to heat something else to a higher temperature without breaking the second law.

> Each cable is actually wrapped around the motor axis, then passed through the pulley and tied to a fixed ring in the corner of the panel.

Thanks, when seeing the video again now it makes sense! I didn't catch the counterweight the first time I saw it. Nice! In the Heliogen concept I mentioned previously they got around having to use a counterweight by attaching the other side of the cable to another part of the panel, such that the cable length stays approximately constant. Then they used a spring to compensate for the small changes in cable length that are inherent to the geometry.

> I use one bolt that pulls the mirror holder in the center and 3 bolts that push it in the corners. By screwing or unscrewing the corner bolts you can precisely orient each mirror independently.

Nice! Even in large heliostat fields it is often done in a similar way. It becomes quite labor intensive when you have thousands of heliostats in a field, with 10+ segments each, so there are ongoing efforts to find ways to do it automatically or to get around the need for doing it in the first place.

> ps. It's like making a VPN tunnel to the surface of the sun :-)

Wow, I love that! Great analogy!

> Does the black body radiation send the energy back out?

Exactly, this is the issue. If an object is able to absorb sunlight, it is also able to emit blackbody radition back towards the sun. When the temperature limit is reached, these two exactly cancel each other. The object will emit blackbody radiation with the same brightness as the surface of the sun.

Another way to look at it is to imagine yourself standing at the center of the concentrated sunlight and looking out towards the concentrator. The concentrator makes the sun look "bigger" from your perspective, and this is what makes the sunlight concentrated. The limit to this effect is if the sun fills all directions in the whole hemisphere above you. Now it will be as if you are standing on the surface of the sun, and all you can see in any direction is sunlight. Normally, the solar disc fills 1/45000th of the hemisphere above you here on earth, thus the limit of 45000 suns concentration.

Yes, and it is super-interesting!

The fundamental limit is given by the 2nd law of thermodynamics - you can never reach higher temperatures than the surface of the sun, or around 5800 K. We have the atmosphere that absorbs and scatters some of the light, so on the surface of the earth it is a bit lower, but not by a huge amount.

This means that there is a fundamental limit to how small and intense you can make the focal spot in a solar concentrator. The limit is around ~45 MW/m² or 45000 "suns" (which is plenty high, but far from infinite).

Concentrators used for eletricity generation use much lower concentration than this, on the order of 25 suns to 1000 suns depending on the type. There are also solar furnaces designed for reaching much higher concentration by using a different type of optics. The most impressive one is the huge Odeillo solar furnace [1]. I would guess that they could melt tungsten, but I have not actually run the numbers.

I did a talk last week about a concept we are developing for reaching furnace-level concentration ratios with conventional heliostats [2].

[1] https://en.wikipedia.org/wiki/Odeillo_solar_furnace

[2] https://folk.ntnu.no/haakonjj/talks/2024-08-19-nonimaging-fr...

Yeah exactly!

I like your cable-drive concept by the way. Did you describe it in more details anywhere? Heliogen were also developing a cable-drive system for their commercial heliostats, but I don't know if they are still working on it.

How are you getting the right mirror orientation for each mirror (aka canting)? Custom spacers for each mirror?

One trick for closed-loop control with many heliostats/panels is to have a few cameras surrounding the receiver. When they look back at the mirrors, they will see the circumsolar radiation (how the sky gets brighter as you get closer to the sun). By comparing the brightness of the sky at different cameras, you can estimate which cameras is "closest" to seeing the real sun, and get an estimate for the real position of the sun.

Cool work! I do research in nonimaging optics, the optics of achieving high concentration ratios (or wide tolerances to errors) in solar concentrators.

I like that you are implementing closed-loop control. This is all the rage also in large-scale heliostat fields. Most traditional heliostats are controlled using open loop, which places very strict requirements on both the mechanical structure, the actuators, and on the kinematic model, leading to expensive and very stiff heliostats. People are therefore moving towards cheaper heliostats where the tracking precision is achieved through closed-loop control. Implementing closed-loop control is a little bit more tricky when you have overlapping focal spots from thousands of mirrors, but there are approaches that are being developed, e.g. having cameras around the target looking back out over the heliostat field (developed by Heliogen among others).

You mention the challenge of light only being focused for a few hours per day. This is also a problem with large helisotat fields, and is also a field of active research. There's a group at University of Arizona with Professor Roger Angel developing heliostats that actively deform through the day to keep the perfect shape, and there's also an Australian company (Heliosystems) building heliostats that passively deform from gravity to keep as correct shape as possible.

When you are only using a single heliostat, as in your project, you could also consider building it as a Scheffler reflector - placing it on a single-axis polar-aligned tracking axis that passes through your target. Then it only requires single-axis tracking through the day, with some (possibly manual) seasonal adjusting.

I am very happy to see that you are highlighting the inherent risks in concentrated sunlight. There are lots of stories about people accidentally settings stuff on fire if the tracking doen't track correctly.

You mention renewables and especially solar as a contributing factor to these negative prices when combined with capacity constraints. Can you explain how this works?

A PV installation can easily start and stop production as long as the sun is shining, right? Then it seems like PV producers should bid to produce as much as possible whenever the price is positive, and not produce anything whenever the price is negative. Is this how they operate, and if so, how does this contribute to negative prices and not just to bringing the price towards 0?