4 ms·

IIRC, the papers they're working from mention that lime works very nearly as well as the baryta they're using. Guess what people have been painting their house

by kragen 11mo ago

IIRC, the papers they're working from mention that lime works very nearly as well as the baryta they're using. Guess what people have been painting their houses white with for several thousand years?

marcosdumay 11mo ago
Not with the optimal mixture for maximum-packing of limestone nanospheres. That effect is almost not perceptible in normal milled limestone.
- kragen 10mo ago
  It would be very surprising if that were true, given the LWIR emittances in the NASA paper cited in https://news.ycombinator.com/item?id=46212591 https://news.ycombinator.com/item?id=46212591. I totally believe that you can get significant percentage improvements with the right mix of particle sizes, but the LWIR emittance of just about all nonmetals was upwards of 75% the emittance of an ideal blackbody. That puts a ceiling of about ⅓ (4:3) on the possible improvement on that end. And plain old whitewash has a reflectivity upwards of 90% in the visible, while the best known materials, such as polished silver, are something like 96%, so you might get 3:2 on that end—but probably won't even get 2:1. https://nvlpubs.nist.gov/nistpubs/bulletin/07/nbsbulletinv7n2p197_A2b.pdf https://nvlpubs.nist.gov/nistpubs/bulletin/07/nbsbulletinv7n... is one older reference on this, but https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=903 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=90... gives plots of silver mirrors' reflectance by wavelength, polarization, and angle of incidence. As a side note, what people have been painting their houses with for thousands of years isn't normal milled limestone; it's slaked lime, which forms limestone by absorbing CO₂ from the air over about a month. Modern whitewash has milled limestone mixed into it, but the morphology of the final surface isn't very similar to milled limestone in a transparent binder.

by kragen 11mo ago

IIRC, the papers they're working from mention that lime works very nearly as well as the baryta they're using. Guess what people have been painting their houses white with for several thousand years?

marcosdumay 11mo ago

Not with the optimal mixture for maximum-packing of limestone nanospheres. That effect is almost not perceptible in normal milled limestone.

kragen 10mo ago
It would be very surprising if that were true, given the LWIR emittances in the NASA paper cited in https://news.ycombinator.com/item?id=46212591 https://news.ycombinator.com/item?id=46212591. I totally believe that you can get significant percentage improvements with the right mix of particle sizes, but the LWIR emittance of just about all nonmetals was upwards of 75% the emittance of an ideal blackbody. That puts a ceiling of about ⅓ (4:3) on the possible improvement on that end. And plain old whitewash has a reflectivity upwards of 90% in the visible, while the best known materials, such as polished silver, are something like 96%, so you might get 3:2 on that end—but probably won't even get 2:1. https://nvlpubs.nist.gov/nistpubs/bulletin/07/nbsbulletinv7n2p197_A2b.pdf https://nvlpubs.nist.gov/nistpubs/bulletin/07/nbsbulletinv7n... is one older reference on this, but https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=903 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=90... gives plots of silver mirrors' reflectance by wavelength, polarization, and angle of incidence. As a side note, what people have been painting their houses with for thousands of years isn't normal milled limestone; it's slaked lime, which forms limestone by absorbing CO₂ from the air over about a month. Modern whitewash has milled limestone mixed into it, but the morphology of the final surface isn't very similar to milled limestone in a transparent binder.