9 ms·

> As far back as the 1970s, scientists and manufacturers had been developing whiter and whiter paints to help reflect sunlight and keep buildings and other surf

> As far back as the 1970s, scientists and manufacturers had been developing whiter and whiter paints to help reflect sunlight and keep buildings and other surfaces cool The thing I am pondering: if the objective is to reflect as much as possible sunlight shouldn't the solution be more like a... mirror than a white paint? According to wikipedia a very complex dielectric mirror can reflect up to 99.999% of the light incident upon it while simple mirrors may reflect 99.9% of the light. Cost and practicality will significantly lower these figures but there is still a gap versus the current 98.1% white paint record. I recall seeing various gold tinted mirrors covering buildings but they seem out of fashion.

carlmr 4y ago
Specular reflections will blind pilots, drivers and even pedestrians, melt things if the mirrors aren't completely planar, and cause all kinds of issues.
- college_physics 4y ago
  you could reduce that by making the surface locally rough or having random geometric segments - whatever is easier to manufacture. Since this is not a mirror that must preserve image integrity anything goes.
  - awestroke 4y ago
    Sounds like you're pitching white paint
    - college_physics 4y ago
      ha ha, size (of your reflectors) matters
  - ominous_prime 4y ago
    You literally described diffuse reflection, which is "white paint"
  - slackdog 4y ago
    > you could reduce that by making the surface locally rough or having random geometric segments That actually makes the problem much worse. Normally with a few flat reflective surfaces around, you'll catch occasional reflections of sunlight and flinch away from them. But with a very reflective rough surface, you catch reflections of sunlight every direction you look and it will burn your eyes. This is how "snow blindness" works. Fresh white snow is very reflective and very rough, on a bright day with fresh snow, you catch sunlight reflections every direction you look and if you don't limit your exposure, you can get painful UV burns on your eyeballs. Sunglasses or traditional snow goggles become essential if you're outside for long in these sort of conditions: https://en.wikipedia.org/wiki/Snow_goggles https://en.wikipedia.org/wiki/Snow_goggles It can also happen when you're on the water on a bright day, depending on the state of the waves. At somewhere around sea state 1 or 2, you start catching sunlight reflections off waves every direction you look and if you don't have sunglasses you'll be in for a shitty time. When it happened to me, it felt like having glass powder trapped under my eyelids.
    - college_physics 4y ago
      > That actually makes the problem much worse the more directed the light source the more damaging for retinas (all other things being equal). that is why you have prohibitions on powerful laser pointers. what you are describing is the effect where diffuse reflections can integrate over time (since the impact is not immediately felt) and cause damage this way
      - slackdog 4y ago
        > the more directed the light source the more damaging for retinas (all other things being equal). Yes certainly, but in the real world the human blink reflex means that all other things aren't equal. If you walk past a shiny glass building, you might catch a reflection of sunlight but only for a brief instant. Turn another direction and you'll see another shiny building, but you probably won't get a sunlight reflection off that shiny building because the angles are different and the sunlight is being reflected somewhere else. You can walk around a mirrored city all day and your eyes will be fine. But when you're surrounded by diffuse reflectors, now can't turn away from that reflected light. A city of pure white reflective buildings would be likely be quite painful on a bright day.
- brookst 4y ago
  So there must be some downsides?
- roter 4y ago
  Exactly. Water surfaces or solar panels near airports or buildings near expensive cars [0]. [0] https://www.bbc.com/news/uk-england-london-23930675 https://www.bbc.com/news/uk-england-london-23930675
lm28469 4y ago
> According to wikipedia a very complex dielectric mirror can reflect up to 99.999% of the light You skipped the "but only for a narrow range of wavelengths, ranging from a bandwidth of only 10 nm to as wide as 100 nm" part > while simple mirrors may reflect 99.9% of the light. Simple as in "enhanced silver mirror" ? Not that simple https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=8159 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=81... The regular silver mirror part is: > Silver is expensive, soft, and quickly tarnishes, but has the highest reflectivity in the visual to near-infrared of any metal. Silver can reflect up to 98 or 99%
- samstave 4y ago
  >>"but only for a narrow range of wavelengths..." Curious ; what wavelengths 'carry' the most heat? or capable of carrying the most heat? And what reflective material/color is best at reflecting most heat? I ask because I used to have a friend who was a physicist at Intel and he couldnt tell me what heat actually was... Do we have an answer to this?
  - agileAlligator 4y ago
    >what wavelengths 'carry' the most heat? or capable of carrying the most heat? Most of heating is from infrared iirc
  - dekhn 4y ago
    It's a bit complicated, but heat is expressed in several ways: in the form of kinetic energy of a system (which is measured by a thermometer), and in the form of infrared radiation emitted by hot objects (which is measured through radiation).
    - samstave 4y ago
      Thank you, but what, specifically is "heat" What property of matter is 'heat'
      - 4y ago
Vt71fcAqt7 4y ago
Most mirrors are covered in glass which greatly reduces the effectiveness[0] (also now you live in a glass house..) If you don't use glass you will have aluminum oxide which is much less reflective. So paint will always be better. SSS doesn't really matter (and is prefered, imo) as long as the light is being reflected out of the material in the end. [0]visual proof: https://i.imgur.com/Ar80FYZ.jpg https://i.imgur.com/Ar80FYZ.jpg
s_dev 4y ago
I would imagine it comes down to the fact mirrors need to be cleaned and can break easily from rapid temperature changes -- a hot day and suddenly cool rain comes down can break a mirror.
- hutzlibu 4y ago
  Or that it would be a quite surreal (and dangerous) experience walking through a town made of mirrors. It would be blinding and burning and visually be confusing.
the8472 4y ago
To shed heat you want it to be reflective in the parts of the spectrum that represents sunlight and emissive in the IR windows so it can radiate into space.
benj111 4y ago
Surely solar would be a better tradeoff than mirrors? More expensive but you get energy out. White paint is at least cheap.
- college_physics 4y ago
  yes, definitely part of the mix (of what to cover building surfaces with) but a decision that probably happens higher in the flow chart. Mirrors vs paint in some sense closer substitutes. Besides optimizing reflective performance versus monetary cost one would have to also take into account the full product lifecycle (dependence on fossil fuel, recyclability etc). I know what "a fresh lick of paint" means, no idea what "a fresh mirror coating" would look like :-) But overall a fascinating topic which like the converse (insulation of buildings in cold climates) has a major footprint

carlmr 4y ago

Specular reflections will blind pilots, drivers and even pedestrians, melt things if the mirrors aren't completely planar, and cause all kinds of issues.

college_physics 4y ago
you could reduce that by making the surface locally rough or having random geometric segments - whatever is easier to manufacture. Since this is not a mirror that must preserve image integrity anything goes.
- awestroke 4y ago
  Sounds like you're pitching white paint
  - college_physics 4y ago
    ha ha, size (of your reflectors) matters
- ominous_prime 4y ago
  You literally described diffuse reflection, which is "white paint"
- slackdog 4y ago
  > you could reduce that by making the surface locally rough or having random geometric segments That actually makes the problem much worse. Normally with a few flat reflective surfaces around, you'll catch occasional reflections of sunlight and flinch away from them. But with a very reflective rough surface, you catch reflections of sunlight every direction you look and it will burn your eyes. This is how "snow blindness" works. Fresh white snow is very reflective and very rough, on a bright day with fresh snow, you catch sunlight reflections every direction you look and if you don't limit your exposure, you can get painful UV burns on your eyeballs. Sunglasses or traditional snow goggles become essential if you're outside for long in these sort of conditions: https://en.wikipedia.org/wiki/Snow_goggles https://en.wikipedia.org/wiki/Snow_goggles It can also happen when you're on the water on a bright day, depending on the state of the waves. At somewhere around sea state 1 or 2, you start catching sunlight reflections off waves every direction you look and if you don't have sunglasses you'll be in for a shitty time. When it happened to me, it felt like having glass powder trapped under my eyelids.
  - college_physics 4y ago
    > That actually makes the problem much worse the more directed the light source the more damaging for retinas (all other things being equal). that is why you have prohibitions on powerful laser pointers. what you are describing is the effect where diffuse reflections can integrate over time (since the impact is not immediately felt) and cause damage this way
    - slackdog 4y ago
      > the more directed the light source the more damaging for retinas (all other things being equal). Yes certainly, but in the real world the human blink reflex means that all other things aren't equal. If you walk past a shiny glass building, you might catch a reflection of sunlight but only for a brief instant. Turn another direction and you'll see another shiny building, but you probably won't get a sunlight reflection off that shiny building because the angles are different and the sunlight is being reflected somewhere else. You can walk around a mirrored city all day and your eyes will be fine. But when you're surrounded by diffuse reflectors, now can't turn away from that reflected light. A city of pure white reflective buildings would be likely be quite painful on a bright day.
brookst 4y ago
So there must be some downsides?
roter 4y ago
Exactly. Water surfaces or solar panels near airports or buildings near expensive cars [0]. [0] https://www.bbc.com/news/uk-england-london-23930675 https://www.bbc.com/news/uk-england-london-23930675

lm28469 4y ago

> According to wikipedia a very complex dielectric mirror can reflect up to 99.999% of the light You skipped the "but only for a narrow range of wavelengths, ranging from a bandwidth of only 10 nm to as wide as 100 nm" part > while simple mirrors may reflect 99.9% of the light. Simple as in "enhanced silver mirror" ? Not that simple https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=8159 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=81... The regular silver mirror part is: > Silver is expensive, soft, and quickly tarnishes, but has the highest reflectivity in the visual to near-infrared of any metal. Silver can reflect up to 98 or 99%

samstave 4y ago
>>"but only for a narrow range of wavelengths..." Curious ; what wavelengths 'carry' the most heat? or capable of carrying the most heat? And what reflective material/color is best at reflecting most heat? I ask because I used to have a friend who was a physicist at Intel and he couldnt tell me what heat actually was... Do we have an answer to this?
- agileAlligator 4y ago
  >what wavelengths 'carry' the most heat? or capable of carrying the most heat? Most of heating is from infrared iirc
- dekhn 4y ago
  It's a bit complicated, but heat is expressed in several ways: in the form of kinetic energy of a system (which is measured by a thermometer), and in the form of infrared radiation emitted by hot objects (which is measured through radiation).
  - samstave 4y ago
    Thank you, but what, specifically is "heat" What property of matter is 'heat'
    - 4y ago

Vt71fcAqt7 4y ago

Most mirrors are covered in glass which greatly reduces the effectiveness[0] (also now you live in a glass house..) If you don't use glass you will have aluminum oxide which is much less reflective. So paint will always be better. SSS doesn't really matter (and is prefered, imo) as long as the light is being reflected out of the material in the end. [0]visual proof: https://i.imgur.com/Ar80FYZ.jpg https://i.imgur.com/Ar80FYZ.jpg

s_dev 4y ago

I would imagine it comes down to the fact mirrors need to be cleaned and can break easily from rapid temperature changes -- a hot day and suddenly cool rain comes down can break a mirror.

hutzlibu 4y ago
Or that it would be a quite surreal (and dangerous) experience walking through a town made of mirrors. It would be blinding and burning and visually be confusing.

the8472 4y ago

To shed heat you want it to be reflective in the parts of the spectrum that represents sunlight and emissive in the IR windows so it can radiate into space.

benj111 4y ago

Surely solar would be a better tradeoff than mirrors? More expensive but you get energy out. White paint is at least cheap.

college_physics 4y ago
yes, definitely part of the mix (of what to cover building surfaces with) but a decision that probably happens higher in the flow chart. Mirrors vs paint in some sense closer substitutes. Besides optimizing reflective performance versus monetary cost one would have to also take into account the full product lifecycle (dependence on fossil fuel, recyclability etc). I know what "a fresh lick of paint" means, no idea what "a fresh mirror coating" would look like :-) But overall a fascinating topic which like the converse (insulation of buildings in cold climates) has a major footprint