The rooftop experiment

In November 2014, a team of Stanford engineers placed a small device on a rooftop in Stanford, California. It was a disk of glass and silicon, topped with seven alternating layers of hafnium dioxide and silicon dioxide, each measured in nanometers. In direct sunlight exceeding 850 watts per square meter, the surface cooled itself to 4.9 degrees Celsius below the temperature of the surrounding air. It consumed no electricity. Its heat sink was outer space.

That result, published in Nature by Aaswath Raman, Marc Abou Anoma, Linxiao Zhu, Eden Rephaeli, and Shanhui Fan, demonstrated something that had been theorized but never achieved: passive daytime radiative cooling below ambient temperature. The idea is simple in principle and deranged in practice. Earth’s atmosphere is opaque to most infrared radiation, but it has a narrow transparency window between 8 and 13 micrometers where thermal radiation passes straight through to the cosmic background at 3 Kelvin. If you can engineer a surface that reflects nearly all sunlight while emitting heat at exactly those wavelengths, the heat leaves the surface, punches through the atmosphere, and dissipates into the void. You are, in effect, air-conditioning with the universe.

The year before, the same group had published the theoretical design in Nano Letters — a paper that has since accumulated over 1,100 citations. That paper drew on photonic crystal engineering techniques developed for fiber-optic telecommunications: multilayer dielectric stacks tuned to manipulate specific wavelengths. Fan, whose career had been built in computational photonics, had essentially taken the same tricks that telecom engineers use to manage light in optical fibers and pointed them at the sky.

From rooftop to patent office

The USPTO has since granted 127 patents for materials specifically engineered for daytime radiative cooling — surfaces designed to combine high solar reflectance with high thermal emissivity in the atmospheric transparency window. That is more than the 39 years before 2015 produced. In 2024 alone, 21 such patents were granted, the most in any single year. Through September 2025, another 14 have landed. Eighty-three distinct organizations hold these patents.

Stanford leads the assignee table with 10 patents since 2015. Behind it: Xerox’s Palo Alto Research Center with 6, Columbia University with 6, MIT with 5, and Osaka Gas with 5. The commercial layer is thinner but growing: SkyCool Systems (Stanford spinout, 3 patents), Ningbo Radi-Cool in China (3), and a scattering of startups and corporate labs including Toyota, Boeing, and John Deere.

What they are actually building

The Stanford prototypes were lab-fabricated multilayer films. What the patent trail shows is the technology splintering into at least four distinct material platforms, each chasing a different market.

Paint. In 2021, Xiulin Ruan’s lab at Purdue demonstrated an ultrawhite barium sulfate paint that reflects 98.1% of incident sunlight — enough to cool a surface below ambient temperature in direct sun. The key: barium sulfate particles of deliberately varied sizes scatter a wider range of solar wavelengths than uniform particles can. The paper, published in ACS Applied Materials & Interfaces, has drawn 563 citations. Applied to a 1,000-square-foot roof, the paint delivers roughly 10 kilowatts of cooling power. The Guinness Book of World Records certified it as the whitest paint ever made.

Textiles. At least five patents since 2024 describe radiative cooling fabrics — materials you can wear. Ningbo Radi-Cool, a Chinese manufacturer, holds patents for fabrics with 80%+ emissivity in the 7–14 μm window and 80%+ solar reflectivity, already sold commercially as hat and outerwear material. City University of Hong Kong has patents on electrospun ceramic-polymer textiles (US 12,221,721 and US 12,152,843). Stanford’s own US 11,925,226 describes a spectrally selective textile with 40%+ infrared transmittance at 9.5 μm — a fabric that lets your body’s own thermal radiation escape to the sky.

Metamaterial panels. PARC’s six patents describe a manufacturing approach using anodized aluminum oxide self-assembly to create tapered nanopores that emit in the 8–13 μm window with near-unity emissivity. Their US 10,060,686 explicitly targets “dry cooling systems suitable for large power plants” — radiative cooling as industrial infrastructure, not a consumer product.

Smart coatings. The strangest patent in the set belongs to Toyota and the University of Illinois. US 11,999,152 describes a thermoresponsive hydrogel that autonomously switches behavior based on ambient temperature. Below its critical threshold, the coating turns transparent and absorbs solar heat. Above it, the coating becomes reflective and radiates heat to space through the atmospheric window. The patent names electric vehicles — including airborne, seaborne, and ground vehicles — as target applications. A car that heats itself in winter and cools itself in summer, with no energy input, using a single coating.

Why the telecom connection matters

This is not a story about white paint getting better. It is a story about photonic engineering — a discipline built to manage light inside fiber-optic cables — escaping its original habitat and colonizing materials science. The techniques that let telecom engineers tune a Bragg reflector to pass 1550-nanometer light while blocking everything else are the same techniques that let Shanhui Fan’s lab build a surface that reflects sunlight while radiating heat at 10 micrometers.

Fan, for the record, was elected to both the National Academy of Engineering in 2024 and the National Academy of Sciences in 2025. He co-founded SkyCool Systems, which has raised $5 million and deployed panels with grocery stores and refrigerated warehouses, claiming 10–40% efficiency gains on commercial HVAC systems. His co-author Aaswath Raman, who ran the original rooftop experiment as a PhD student, is now an associate professor at UCLA and co-founder of the company.

The trajectory from a lab curiosity to 83 patent-holding organizations in a decade is fast for a materials technology. And the timing is not incidental. The International Energy Agency projects that global cooling demand will triple by 2050, driven by rising temperatures and urbanization. Space cooling already accounts for nearly 20% of electricity used in buildings worldwide, according to the IEA. A zero-electricity cooling supplement — even one that shaves 10–20% off a building’s cooling load — addresses one of the largest energy line items on the planet.

The materials that cool themselves using outer space are not, yet, a product category that anyone reports quarterly earnings on. But they are a category with 21 patents in a single year, 83 assignees, a Stanford-to-startup pipeline, a Purdue paint in the Guinness Book, a Toyota coating for electric vehicles, and a PARC design for power plants. The pieces are assembling faster than the press has noticed.

Method note

• Patent corpus: 9.3M US utility grants sourced from USPTO bulk grant XML, searched by full-text keyword combinations requiring both “radiative cooling” and at least one spectral-engineering term (emissivity, emittance, daytime, subambient, atmospheric window/transparency, or solar reflectance/reflectivity). This filter is deliberately strict to exclude generic thermal management patents.
• Literature: 357M scientific papers from OpenAlex, searched by keyword and ranked by citation count.
• Time window: grants issued through late September 2025. Literature citations as of the most recent OpenAlex snapshot.
• Assignee counts: each organization’s total combines variant spellings and subsidiary filings. “83 distinct organizations” is after this normalization.
• Caveats: patent grant dates lag filing dates by 2–4 years, so the 2024 spike reflects applications filed roughly 2020–2022. The actual pace of new filings is likely higher than what the grant data shows. Not all patents in this set describe commercially viable technologies — patent filing is a signal of R&D investment, not product readiness.