[HN Gopher] How white can white paint get?
___________________________________________________________________
How white can white paint get?
Author : mhb
Score : 62 points
Date : 2023-03-02 13:08 UTC (9 hours ago)
(HTM) web link (gizmodo.com)
(TXT) w3m dump (gizmodo.com)
| denton-scratch wrote:
| > cannot cool a surface below the ambient temperature.
|
| Only active cooling can cool anything below the ambient
| temperature. 100% reflectivity can prevent it absorbing more
| heat, if you can achieve it, but it doesn't cool anything down.
|
| > "Barium sulfate has reflective properties that allow us to hold
| below ambient temperature," said Felicelli
|
| That's a bit more like it: 3/4 of the way down the page, someone
| finally points out that paint can't cool anything, it can only
| stop it warming up.
| planede wrote:
| > Only active cooling can cool anything below the ambient
| temperature.
|
| It's more complex than that. Through thermal radiation objects
| on the surface of Earth are also thermodynamically connected to
| the surface of the Sun (~6000K) and Space (~4K).
|
| The way to cool below ambient temperature with passive cooling
| is to isolate from the Sun, radiation from nearby objects and
| and conducting heat from surrounding air as much as possible,
| while making the thermal connection to Space as strong as
| possible.
|
| 1. How to isolate from the Sun: surface reflective in the
| spectrum where the thermal radiation from the Sun is the
| strongest, which is not too surprisingly the visible light
| spectrum (hence this article)
|
| 2. How to isolate from surrounding objects: insulation against
| conduction + surface reflective in the spectrum where the
| surrounding objects' thermal radiation is strongest (somewhere
| in infrared).
|
| 3. Thermal connection with space: you want to emit in the
| infrared spectrum as strong as possible.
|
| 2 and 3 are seemingly in contradiction, as a surface can't be a
| good reflector and good emitter at the same wavelength at the
| same time. But just have a geometry where the surface
| satisfying point 2 is pointed towards surrounding objects,
| while the surface satisfying point 3 radiates towards space.
| dahart wrote:
| What's the difference in practice, in summer weather? If the
| ambient temp is 30C in the morning and heats up to 40C in the
| afternoon, for example, then a paint that keeps things at 30
| could replace active cooling on a red house with a brown roof
| whose surfaces exposed to the sun might even heat up to 50. It
| seems fair to call paint that effectively reflects the sun's
| heat a "cooling" method, because it's cooling relative to what
| you'd have otherwise, right?
| jcims wrote:
| >Only active cooling can cool anything below the ambient
| temperature.
|
| Not exactly:
|
| https://en.wikipedia.org/wiki/Radiative_cooling
|
| Couple of videos explaining the effect
|
| https://www.youtube.com/watch?v=N3bJnKmeNJY
|
| https://www.youtube.com/watch?v=5zW9_ztTiw8
| orev wrote:
| An object in a room at ambient temperature would slowly warm up
| to equal the same temperature from the conduction of the warmer
| air around it. If the temperature of an object can "hold below
| ambient", it means it must be able to allow the object to shed
| heat energy, at least at the same rate or faster than it
| absorbs the ambient heat. I.E. cooling.
| OscarCunningham wrote:
| Would this actually look like bright white paint? Maybe because
| they're also trying to reflect light outside the visible spectrum
| it might not be as bright in the colours we can actually see.
| franciscop wrote:
| > The concept of using white paint to deflect heat isn't new. As
| far back as the 1970s, scientists and manufacturers had been
| developing whiter and whiter paints
|
| In the Mediterranean (at least, probably many other areas) this
| has been done for thousands of years by painting the houses white
| to keep the heat out as much as possible, as well as a variety of
| other techniques like smart air management. So I think "as far as
| the 70s" is fairly misleading, it should be more like "for
| thousands of years...".
| fractallyte wrote:
| ...and the 'paint' is made using basic ingredients:
| https://en.wikipedia.org/wiki/Whitewash
|
| Easy to apply, long-lasting, biodegradable, and (from the
| linked article) it "produces a unique surface glow due to the
| double refraction of calcite crystals."
| franciscop wrote:
| NEW RELEASE: bio natural organic paint made from seashells to
| cool down your house 2023 release innovation startup
|
| Mediterraneans: get out of here
|
| Edit: how to make lime/limestone in a low-tech way from
| Primitive Technology:
|
| https://www.youtube.com/watch?v=Ek3aeUhHaFY
| aidenn0 wrote:
| And it's so fun to apply that people will pay to do it for
| you!
| dmitriid wrote:
| > So I think "as far as the 70s" is fairly misleading
|
| As far as the 70s refers to efforts to create whiter and more
| reflective paint.
| [deleted]
| nomel wrote:
| This assumes that the white paint, used on those houses, had
| an unrelated use, and was never improved in the context of
| the purpose of why the paint was applied. This doesn't seem
| like a safe assumption, with the intent required to keep it
| going for so long.
| Vt71fcAqt7 wrote:
| It only assumes that a new paint was never created in the
| context of the purpose of why paint was applied, more
| accurately, which is possible. The article doesn't know,
| and I don't think you know either out of hand. That's why
| it reads "as far back as the 1970s."
| uoaei wrote:
| Yours is a bald-faced assumption that humans aren't
| constantly innovating on existing technologies. You
| demand evidence in place of such assumption.
|
| Yours appears to me to be a view that is very strange not
| to extend back beyond the 1970s.
|
| We know how people work, in societies, at this broad-
| stroke level. This level of pedantry gets us nowhere.
| Vt71fcAqt7 wrote:
| >Yours is a bald-faced assumption that humans aren't
| constantly innovating on existing technologies.
|
| No it isn't. Mine is an asumption that "white" was just a
| color before we understood what light is, which doesn't
| happen until centuries later. Searching for ways to
| produce "the whitest paint" is unrealisric given that we
| are talking about objects that reflect invisible light
| here. Not "white" the color. And there is also a
| difference between using something and creating something
| for that purpose. Saying that scientists are _now_
| searching for the whitest paint does not imply that
| nobody was _using_ white paint before.
|
| >This level of pedantry gets us nowhere.
|
| It gets us to the question of whether ancient societies
| looked for ways to produce paint that reflect more light
| in order to prevent heat absoption which is what has been
| hapening since at least 1970.
| D13Fd wrote:
| In context it's pretty clear they are talking about modern
| efforts rather than historical ones.
| Vt71fcAqt7 wrote:
| Is there any evidence they where developing whiter paints for
| this purpose?
| harywilke wrote:
| In Greece, the iconic whitewashed island house has a public
| health angle to it. "In 1938, a national order mandated the
| painting of island homes in blue and white. At the time, Greece
| was suffering an outbreak of cholera during the dictatorship of
| Ioannis Metaxas. In an effort to curb the disease, he ordered
| citizens to whitewash their homes. This might sound strange
| today, but the whitewash used to paint the houses contained
| limestone. Limestone is a powerful disinfectant, and not many
| others were in common use at the time. Greek citizens thus
| whitewashed their homes to help sanitize them and reduce the
| spread of cholera." and older
| erybodyknows wrote:
| This reminds me of the blackest acrylic paint from Stuart Semple.
|
| https://culturehustle.com/products/black-3-0-the-worlds-blac...
| S_Bear wrote:
| Stuart Semple also has a 'white 2.0' that looks like neon white
| compared to regular white.
|
| https://www.culturehustleusa.com/products/white-20
| Pet_Ant wrote:
| If you want an idea of how this might feel on a room scale,
| here are Action Lab's whitest & blackest rooms:
|
| Whitest: https://www.youtube.com/watch?v=DjdFn1RYHDE
|
| Blackest: https://www.youtube.com/watch?v=p6q54q2iam8
| wppick wrote:
| I once bought a paint so white it thought sliced bread was too
| spicy
| cobbaut wrote:
| And yet men will not be able to tell the difference. I have a
| flat with white painted walls, or so I thought... until a woman
| told me they were cream, milk and eggshell :-/
| wjnc wrote:
| So we recently went from RAL 9010 to RAL 9016 for the walls
| (with colours and walls to create a nice atmosphere) and to my
| horror the difference between the RAL 9001 (cream) doors and
| frames is suddenly unbearable. This coming from a person who
| has a hard time naming the less than basic colours.
|
| Not to get me started on the millions of colours you paint your
| walls in. I did have a laugh when I found out that proprietary
| paint colours are a big thing, a few weeks after reading on HN
| about Adobe "cancelling" a colour. Proprietary colours are a
| thing I can suddenly talk about with repair guys. Car paints
| are proprietary. Kitchen panel paints are proprietary. It's a
| whole new world. It's not just RGB, there's gloss and sparkle
| to consider.
| themitigating wrote:
| Maybe you can tell the difference but to express your
| masculinity pretend not to. Why? Because caring and beauty are
| feminine.
| msm_ wrote:
| > Because caring and beauty are feminine.
|
| Is this something you (or people you're familiar with)
| believe? Never encountered this norm before.
| yowzadave wrote:
| I believe the comment was meant as an ironic dismissal of
| GP's assertion that men can't tell the difference between
| colors, but women can.
|
| As an architect, I can confirm that there are many men who
| know the difference between cream/milk/eggshell!
| brookst wrote:
| This is true, and we should be suspicious of gender
| stereotypes. But it is also true that on average men are
| worse at seeing color gradations, just because the
| incidence of color deficiency is 10x higher in men.
|
| So any particular man may see color as well as any
| particular woman, but if you had to pick one person at
| random to save the world in the galactic color
| identification game show, you'd best pick a woman.
| kayodelycaon wrote:
| So... everyone that works with graphic design is feminine?
| sircastor wrote:
| Familiarity with these sorts of decorative details are often
| considered to a non-masculine trait. Though I suspect that
| most of the time it's just that some people don't care. And
| it's possible that tracks with gender norms, but I think it's
| probably that many men don't feel like they're allowed to
| care.
|
| A reminder that you're allowed to care about whatever you
| want.
| jwestbury wrote:
| This is a complicated subject. Color perception is influenced
| by culture[0], so how someone is raised can impact their
| ability to discern color differences. It's quite possible
| that men in (Western|American) cultures are raised in such a
| way that their color perception is more limited than that of
| women despite no biological differences.
|
| Of course, there's plenty of research[1] showing women are
| better at color discrimination than men.
|
| 0. https://ec.europa.eu/research-and-innovation/en/horizon-
| maga...
|
| 1. https://pubmed.ncbi.nlm.nih.gov/21675035/#:~:text=Color%20
| wi....
| S_Bear wrote:
| I started painting miniatures again a few years ago and my
| appreciation (and vocabulary) for colors has dramatically
| expanded, especially when I started mixing primaries from
| scratch. I literally see the world in a different, richer
| way. I wish I would have realized this sooner.
|
| Maybe gutting art programs in American public schools was a
| bad idea.
| dekhn wrote:
| I can second this: spend a lot of time working with
| subtle color variations (not just frequency, but the
| various matte/shiny bits). I learned I LOVE highly
| saturated colors. Recent car paint was also a revelation
| to me (https://www.blackbirdspyplane.com/p/why-do-new-
| cars-look-lik...).
| phicoh wrote:
| I wonder what the effect is of language. If on average men
| know only a few names of colors, then how can they remember
| or talk about the variations in between?
| rayiner wrote:
| There is a biological difference in color perception, as well
| as hearing and smell: https://www.nbcnews.com/health/body-
| odd/men-women-really-do-...
|
| > Abramov explained in a statement these elements of vision
| are linked to specific sets of thalamic neurons in the
| brain's primary visual cortex. The development of these
| neurons is controlled by male sex hormones called androgens
| when the embryo is developing into a fetus.
| dekhn wrote:
| here's the actual science paper, which is always less
| exciting than the public press. https://bsd.biomedcentral.c
| om/articles/10.1186/2042-6410-3-2...
| jjulius wrote:
| ... just men?
| doikor wrote:
| A lot of people have a terrible conception of what proper white
| is.
|
| Often what many think of as white is white mixed with blue.
| This is super evident after properly calibrating the white
| point of a (good) TV. Most casual viewers will say that the
| whites are now yellow/orange but in reality they are just used
| to always looking at white with a lot of blue mixed in.
|
| I've had similar experience with a interior designer friend of
| mine who told me my new white walls were not white but cream. I
| have a spectrophotometer that pretty clearly said that my walls
| were closer to white then his walls that he insisted were
| proper white (they were white mixed with blue)
| jwestbury wrote:
| > Often what many think of as white is white mixed with blue.
| This is super evident after properly calibrating the white
| point of a (good) TV. Most casual viewers will say that the
| whites are now yellow/orange but in reality they are just
| used to always looking at white with a lot of blue mixed in.
|
| "White" is tricky, because it's dependent on color
| temperature of the light which is illuminating the given
| surface. If someone is used to viewing white objects under
| daylight, but you're calibrating their TV to match the color
| temperature of their indoor environment (which proper
| calibration should do), then the "white" displayed on their
| TV will be a fundamentally different color from the "white"
| they see reflected off a white surface outside.
|
| This effect is going to be found to be doubly true if you're
| calibrating to indoor lighting in the evening, but they watch
| their TV at noon in the daylight with the windows open --
| because the color temperature of the environment will now be
| much higher than the color temperature of the screen.
| euroderf wrote:
| It seems to be a kind of common knowledge that a bit of
| blue makes a white look whiter. I wonder if there's any
| studies done on this tho.
| brookst wrote:
| That's color temperature. A bit of blue makes white look
| more like white does in direct sunlight, which we think
| of as whiter.
|
| A but of orange will make white look more like white does
| in the evening, or in firelight.
| joncrocks wrote:
| Might be relevant:
|
| https://en.wikipedia.org/wiki/Optical_brightener
| ghusbands wrote:
| > A lot of people have a terrible conception of what proper
| white is.
|
| Color is a perceptual phenomenon that is not independent of
| surroundings and is especially tricky when you're talking
| about emission. Just as a piece of white paper in a candle-
| lit room is very different to a piece of white paper outdoors
| in sunlight, what is perceived as white on a TV can differ
| greatly depending on the room it is in or even what else is
| displayed on-screen.
|
| Rather than insulting people's conception of what "proper
| white" is, perhaps consider that it's not quite as simple as
| you think.
| sp332 wrote:
| Are you calibrating the TV to 5000k when they expect 6000k?
| doikor wrote:
| 6500K is what one should be calibrating for. A TV straight
| from the shop in some cases is more like 7500 or 8000K or
| even more (well into the blue range)
|
| Most content is mastered for this 6500K as that is what the
| D65 standard is and the tv/film standards like Rec709 and
| Rec2020 use D65.
| morcheeba wrote:
| There are also cultural differences, too. Asian companies
| preferred colder whites, while our US customer wanted a
| warmer white. We were told this made the different skin
| tones more attractive.
| dekhn wrote:
| This seems like an unnecessary stereotype to state in a comment
| on Hacker News.
| fwlr wrote:
| So there's these three sets of cells in the eye, each tuned
| to a different wavelength of light, and the difference in
| relative activations of each cell type in response to a given
| light is what provides color information. Which wavelength
| each set of cells is tuned to varies quite a bit person to
| person. If your wavelength "tunings" for two types of cells
| are too close together, then there's less relative difference
| in their activation for any given wavelength of light, and
| thus - in the most rigorous information-theoretic sense -
| there is literally less color information available.
|
| We pick an arbitrary threshold of diminished color
| information and call that CVD, color vision deficiency.
| Depending on which threshold you pick for CVD, males are
| anywhere from 10 to 20 to 50 _times_ more likely to have it
| than females. Of course, this "has / does not have CVD"
| binary is an artifact of us picking a threshold; the real
| underlying mechanism is that the distribution of wavelength
| tunings in the male population is much more heavily weighted
| towards "overlapping".
|
| Put another way, you have two overlapping bell curves of "how
| much color information can your eye cells theoretically
| generate" and if you drop a single point on that graph you
| can capture 1% of females and 10% of males to the left of it
| (for that 10x figure), or 0.02% of females and 1% of males to
| the left of the chosen point (for that 50x figure), or
| anywhere in between. And the fact that you can do this tells
| us the two bell curves very likely have different means and
| standard deviations.
|
| The search term is "spectral sensitivity" if you're
| interested in finding out more.
|
| (I do know that "but it's true" isn't considered to be a good
| rebuttal to "it's a stereotype", but it kinda seems like it
| should be, and I don't really know why it isn't. Also, as
| someone with extreme CVD, color vision is just a
| fantastically interesting topic regardless of sex
| differences. The way those EnChroma glasses that "treat"
| color vision deficiency work is they block out a small
| section of the light wavelengths where CVD tends to have high
| overlap. This artificially increases the relative difference
| in activation, meaning the brain can extract more color data,
| with the practical result that people with CVD put on these
| glasses and see colors with an intensity they've never seen
| before. But it gets even cooler! For instance, did you know
| if we develop the technology to directly stimulate those
| different wavelength receptors independently of each other,
| we can make those receptors report "differences in relative
| activation" that are literally impossible to achieve with any
| combination of light wavelengths? That is, the cells will
| happily report perfectly well-formed color data to the brain,
| except that it's data in a range that no brain has ever
| received before. The visual cortex seems like it does fairly
| general information processing, at least at the lower levels
| before we start tagging that information with e.g. emotional
| responses or linguistic names, so on some level our brain
| would probably be able to make immediate sense of this
| previously "out of band" information. It must be able to do
| so, or else CVD sufferers wouldn't get such vivid reactions
| to EnChroma glasses. So it's very plausible that this
| procedure would instantly unlock actual "new color
| experiences" for everyone. How cool is that?)
| chowells wrote:
| "But it's true" isn't a good rebuttal to a stereotype
| that's based on social factors.
|
| For instance, "Jewish people are good with money" is a
| social construct, caused by the Catholics banning their
| adherants from working in banks for centuries. There's no
| inherent difference between groups as far as being able to
| do the job. It's the result of centuries of social
| distortions applied to who can do what job. If the social
| distortions (which still happen today, often propagated by
| the stereotype itself) ended, the difference would cease to
| exist.
|
| It's important to distinguish the difference between
| intrinsic differences and socially-constructed differences.
| And we keep discovering more differences are socially
| constructed than we had previously thought. Even the idea
| that men are worse at color perception has a significant
| social component. That's what the OP was referring to - the
| social construct caused by telling children that caring
| about color distinctions isn't a masculine trait. Sure,
| there's also an intrinsic difference, with color perception
| having a sex-linked genetic component. That is real. But
| it's not the entire story, and I'd be wary of making
| blanket statements that assume the intrinsic property is
| the only one.
| dekhn wrote:
| I'm a biologist with a PhD in biophysics and work in
| optics, you don't need to explain the biology.
|
| My complaint was with the way it was said, it looks
| immature and promoting stereotypes about color perception
| in daily life.
|
| With the exception of color blindness, the difference in
| perception between women and men is much smaller than the
| variation across all humans. There are many things where we
| can measure a difference that is "significant" but in
| reality, insignificant (IE, passes a t-test with bonferroni
| or other multiple tests correction, but doesn't matter in
| any way at all during life).
|
| Much more interesting than "seeing"/"perceiving" colors
| that most humans don't (example: tetrachromancy) is seeing
| polarization. I can see polarization but it was self-taught
| (see the page on Haidinger's brush and then go outside and
| look at the sky with polarized glasses on/off.... after a
| bit, you will realize you actualyl can "see" the
| polarization.
| nunodonato wrote:
| Got me wondering, how viable would it be to paint your house
| white in the summer and with a darker color in the winter? From a
| cost perspective (paint cost+work vs heat/cooling savings). And
| how much does it matter if you have good insulation in the walls?
| is the paint still relevant?
| bombcar wrote:
| The "normal" way this is done is with trees. During the summer,
| they have lots of leaves and shade the house (especially if you
| have them in the right place for the summer sun) and in the
| winter the leaves fall off and the winter sun can shine
| through.
|
| But insulation matters much more in the winter; the color/shade
| only reduces the energy load, it doesn't dissipate it.
| nunodonato wrote:
| yes, its ridiculous how in my country people can't plant
| trees to close to houses due to the summer fires
| bombcar wrote:
| Those firebreak rules make sense, but they should be more
| along the line of "within X feet of the building there must
| be a firebreak of Y feet width" or similar - so you can
| have shade trees and still have a break.
| awestroke wrote:
| Painting is labor intensive
| albert_e wrote:
| e-ink roofs :)
|
| press a button to turn the walls from black to white :)
| dylan604 wrote:
| "Down the road, Ruan said, he's also interested in creating
| reflective paint in different colors, as well as paints
| that could switch from reflective to absorptive, depending
| on the season--like a building-size version of drinkware
| that changes color when you pour hot or cold water into
| it."
|
| From the TFA
| [deleted]
| A_D_E_P_T wrote:
| What's especially interesting is that the coloring agent they hit
| upon -- hexagonal boron nitride -- is structurally analogous to
| graphite.
|
| Graphite and its compounds are important black dyes.
|
| To go a step further: Vantablack, like most other carbon
| nanostructures, is functionally graphitic (sp2 bonding). And it's
| the blackest pigment known.
|
| I wonder if the boundaries of hBN can be pushed in the same
| manner. Perhaps hBN nanostructures can make for even whiter
| pigments? It'll be interesting to find out.
|
| As an aside, I had once heard that beryllium oxide is the whitest
| known pigment, but it's so rare and toxic that it can never leave
| the lab under any reasonable circumstances.
| inkcapmushroom wrote:
| >I had once heard that beryllium oxide is the whitest known
| pigment, but it's so rare and toxic
|
| Don't let industry know about it, they'll be putting it in our
| toothpaste!
| yellow_lead wrote:
| > Eventually, Ruan's team came up with a paint that used high
| concentrations of barium sulfate particles of varying sizes,
| which was able to reflect 95% of sunlight.
|
| This substance is also used as a radiographic contrast agent and
| turns your poop white.
| DeathArrow wrote:
| [flagged]
| college_physics wrote:
| > 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.
| Vt71fcAqt7 wrote:
| 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
| s_dev wrote:
| 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 wrote:
| 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 wrote:
| 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.
| lm28469 wrote:
| > 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=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 wrote:
| >> _" 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 wrote:
| >what wavelengths 'carry' the most heat? or capable of
| carrying the most heat?
|
| Most of heating is from infrared iirc
| AlotOfReading wrote:
| Simplifying _heavily_ , Heat is essentially the kinetic
| energy of the molecules and atoms of a material. It's
| transferred radiatively at all wavelengths, but shorter
| wavelengths carry more energy per photon. When those
| photons are absorbed by a material, the kinetic energy (and
| thus its temperature) rises. If you want to minimize
| radiative transfer, you have to maximize the amount of
| light reflected in the spectrum that's transferring energy
| and minimize absorption. When something is white, it's
| diffusely reflecting the entire visible spectrum. That
| works pretty well for a house in a sunny place, but you
| don't want to paint the mirrors in your optical cavity
| white though because there's only a narrow spectrum of
| light that needs extremely high reflectivity to be usable.
| For those we use specialized mirrors that can achieve much
| higher reflectivity over narrow ranges tuned to the
| application.
| zwieback wrote:
| Depending on the temperature of the object there will be a
| characteristic distribution of wavelengths. It's tricky
| because a hot object will only radiate specific
| wavelengths, based on how the heat is generated.
|
| That's why the concept of a black body, an idealized thing
| that radiates a continuous spectrum is used to reason about
| heat transfer from radiation.
|
| Bottom line, there's no single wavelength that carries most
| heat, it depends on how the heat is generated, what the
| emissivity of the generating body is and what the
| absorption of the receiving surface is.
| samstave wrote:
| So you can tune a body to the emitting source, if it is
| understood - thus revealing stealth capabilities if you
| know the radar emission freq?
|
| How much is geometry vs material science?
|
| We know that anachoic chambers have pyramid-ic structures
| of foam.
|
| So I propose the most stealthy paint would be tiny
| pyramids of aero-graphene in a spray with a carbon based
| paint, such that the pyrs absorb bot heat and sound and
| radio waves.
| dekhn wrote:
| 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 wrote:
| Thank you, but what, specifically is "heat"
|
| What property of matter is 'heat'
| sbaiddn wrote:
| The photon energy is proportional to its wavelength, but
| its not relevant.
|
| Look up Plank's Law. Its an equation for black body
| radiation (like, effectively, the Sun). That'll tell you
| how much heat the Sun radiates at any given wavelength.
|
| That's not the same spectrum as falls on your house, the
| atmosphere has absorbed or scattered some of it. But the
| distribution that falls on your house is well known.
|
| A dielectric mirror is fairly narrow band, compared to this
| distribution. A dielectric mirror might work well for a
| factor of two bandwidth, but solar radiation is significant
| for about a factor of ten.
|
| Also dielectric mirrors are: 1. Hard to design 2. Expensive
| relative to paint 3. Rather fragile compared to paint.
|
| Point 3 is important. You can't just pressure wash your
| siding to clean it without stripping the dielectric mirror.
| But if you don't clean your siding the mirrors can't
| reflect the sunlight!
|
| As to heat, heat is _technically_ thermal energy
| transferred from system A to system B by a temperature
| difference. It has units Joules. In practice it also refers
| to the thermal energy a system has.
|
| Thermal energy is the kinetic energy a material's atoms and
| molecules have because they're vibrating about.
|
| Finally, these are hard questions because they start to hit
| upon the fundamental definitions in physics and everyday
| misuse of definitions by physicists themselves. Furthermore
| laymen have used the word "heart" far before physicists
| stole the term to refer to something similar but technicals
| in their field. They did the same thing with the words mass
| and weight when we needed to distinguish between the force
| and the mass in the 17th century (its _perfectly_ Ok to say
| "weight" to refer to mass in most situations including most
| engineering, chemistry and physics applications)
| samstave wrote:
| Thank you, but what, specifically is "heat"
|
| What property of matter is 'heat'
| shrimp_emoji wrote:
| Basically, heat is the thing you say to describe your
| detection of a transfer of internal energy from an object
| to its surroundings.
|
| E.g., an object's atoms are vibrating a lot, and you
| touch it, and that vibration spreads to your atoms, so
| you say you've noticed "heat" in that object. Or the
| vibrations are emitting radiation (like infrared light)
| to you, and the radiation begins vibrating your atoms, so
| you might say "heat is coming off of that object".
|
| > _Heat is energy in transfer to or from a thermodynamic
| system, by a mechanism that involves the microscopic
| atomic modes of motion or the corresponding macroscopic
| properties._
|
| > _In the kinetic theory, heat is explained in terms of
| the microscopic motions and interactions of constituent
| particles, such as electrons, atoms, and molecules.[57]
| The immediate meaning of the kinetic energy of the
| constituent particles is not as heat. It is as a
| component of internal energy. In microscopic terms, heat
| is a transfer quantity, and is described by a transport
| theory, not as steadily localized kinetic energy of
| particles. Heat transfer arises from temperature
| gradients or differences, through the diffuse exchange of
| microscopic kinetic and potential particle energy, by
| particle collisions and other interactions._ [0]
|
| 0: https://en.wikipedia.org/wiki/Heat
| benj111 wrote:
| Surely solar would be a better tradeoff than mirrors? More
| expensive but you get energy out.
|
| White paint is at least cheap.
| college_physics wrote:
| 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 wrote:
| Specular reflections will blind pilots, drivers and even
| pedestrians, melt things if the mirrors aren't completely
| planar, and cause all kinds of issues.
| brookst wrote:
| So there must be some downsides?
| college_physics wrote:
| 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 wrote:
| Sounds like you're pitching white paint
| college_physics wrote:
| ha ha, size (of your reflectors) matters
| slackdog wrote:
| > _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
|
| 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.
| aksss wrote:
| _Black Sabbath has entered the conversation_
| college_physics wrote:
| > 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 wrote:
| > _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.
| ominous_prime wrote:
| You literally described diffuse reflection, which is "white
| paint"
| roter wrote:
| Exactly. Water surfaces or solar panels near airports or
| buildings near expensive cars [0].
|
| [0] https://www.bbc.com/news/uk-england-london-23930675
| lkbm wrote:
| > The paint is also being perfected and tested to ensure it can
| resist dirt and can last as long under UV light as commercial
| white paints.
|
| Having stored some old closet doors on the porch near a small
| urban road for a few months, this was my first thought. The grime
| that accumulated from exhaust was incredible. (There were leaf
| blowers running across the way a couple times a week, so I'm
| guessing those were the main culprit.)
|
| Things turn dingy in the city remarkably fast. Cleaner cars and
| elimination of two-stroke engines will help, but for now I think
| that for most exterior uses, "resists smog" will be more
| important than "starts out very white" unless you clean it
| frequently and thoroughly.
| bombcar wrote:
| A surprising amount of that "grime" is rubber dust from tires.
| The exhaust helps in getting the items a bit "sticky" and then
| dust and dirt clings much easier to it.
| mhb wrote:
| There is also two part IR reflective paint. With an IR reflecting
| bottom layer and a colored top layer. Doesn't say what percentage
| IR is reflected though:
|
| https://www.newscientist.com/article/2241717-infrared-reflec...
| olivierduval wrote:
| 98,1% of the heat is reflected... to where ?
|
| It's nice to have a room "not too warm"... unless it's a lot
| warmer in the street and you cant go outside!
|
| Let's imagine a "totally white" town (white on walls, streets,
| roofs): the air will be quite hot, or am I misunderstanding
| something ? So it look to me that we will trade one problem (warm
| outside and inside) for another one (cooler inside and warmer
| outside)
| Rexxar wrote:
| Light is reflected, not heat, and a part of it goes directly in
| space.
| dahart wrote:
| What's the problem with cooler inside and warmer outside? How
| much warmer outside do you suppose this white paint makes the
| outside?
| planede wrote:
| In any case, it's better to keep the heat outside than letting
| it in then pump it out with an AC with some additional heat on
| top.
| CorrectHorseBat wrote:
| Heat mostly travels through air, air gets hot from being in
| contact with hot things. The heat will be reflected back to
| space.
| roter wrote:
| What you seek to do is increase the bulk albedo of the
| ground/surface/neighbourhood/city resulting in the net
| reflected solar radiation approaching the incoming leading to
| minimal absorbed shortwave and thus less compensating emitted
| longwave (sigma * T_surf*4). If you increase albedo, you're
| definitely going to lower the air temperature during the day.
| Rays are going to bounce around and eventually leave without
| warming anything. Other than your body if you haven't painted
| yourself white.
|
| However, with all these reflective surfaces you do get
| concerned with direct solar i.e. skiing in shorts & glare e.g.
| the "walkie-talkie" building [0].
|
| [0] https://www.bbc.com/news/uk-england-london-23930675
| hammock wrote:
| "The air" is big, and warming affects winds which move the air
| elsewhere.
|
| I would also mention the ground (very big, planet-sized) as a
| heat sink, but in cities where it's all covered with concrete
| and pavement it does reflect a lot of heat back which is one
| reason why we observe cities hotter than the country otherwise
| would be
| Syzygies wrote:
| Two decades ago I painted my apartment with artist grade titanium
| white acrylic. It still looks new.
|
| A temperament that I share with everyone on HN is a suspicion of
| convention. So many common practices are dumb that it's hard to
| see each one coming in advance. One is always on edge waiting for
| the next shoe to drop. I'm supposed to spend days clearing my
| apartment, so the bottom rung of the construction trades can come
| in using paint that only lasts four years, dabbing hanging peels
| back in place with their brushes? Why did I actually have to go
| through this once to see this is the poster child for "stupid"?
|
| I've also never understood the fashion choice of "off white".
| Titanium white is brighter, a better lit room day or night. It
| can become a color mirror, where a corner looks red because a red
| object is nearby.
|
| For anyone tempted to try this, the student grades have less
| pigment, and the most expensive brands have the most pigment. It
| would also appear in an art supply store that there's only one
| kind of primer, and mediums vary only by gloss. Going to the
| manufacturer's sites, there are variations intended to survive
| the outside wall of a building. One could probably wind up
| someone on a phone call, and get excellent advice on which
| products to use. On the other hand, I used what I found at Pearl
| Paint in NYC, and it held up fine.
| JohnFen wrote:
| > I've also never understood the fashion choice of "off white".
|
| Personally, if it's a room I'm going to be relaxing in
| (bedroom, living room, etc.) as opposed to a work room, I
| strongly prefer an off-white over a bright white. It's more
| muted and relaxing.
| brookst wrote:
| I think the argument is that you can produce a relaxing shade
| by using pure white and controlling the lighting / decor,
| rather than relying on paint color to subtract certain colors
| out.
|
| For instance, bright white paint + 2800k lamps may vibe
| similar to eggshell + 3200k lamps.
| DannyBee wrote:
| If your main concern is lighting effect, you should just be
| using LRV.
|
| Sheen has almost no effect on LRV and thus how much light
| reflected from a fixture is bounced in the room.
|
| While it is true that matte finishes have particles that
| are diffusing light so that the surface does not look as
| shiny, no sheen is actually specular enough to reflect a
| meaningful amount of light.
|
| Especially compared to the color
|
| (This is crazy for people to accept at first but it is in
| fact correct and you can prove it to yourself if you want)
| JohnFen wrote:
| I see. That seems like an overly complex solution to a very
| simple problem, though. Not to mention the unnecessary use
| of electricity.
| MichaelZuo wrote:
| Your using electricity in either case, and they're both
| of the same complexity.
| JohnFen wrote:
| Well, I'm only using electricity at night. During the
| day, the sun lights everything. But now that I say that,
| I realize that's not going to be the same for everybody.
|
| But adjusting the wall color through lighting is more
| complex than just having the walls be painted the shade
| you want.
| MichaelZuo wrote:
| All lighting adjusts the 'wall color'. It's impossible
| for it not to.
|
| There's no man made source that exactly replicates the
| sun.
| JohnFen wrote:
| Well, sure, I'm speaking in approximations. My only point
| is that my personal preferences mean that a bright white
| room would drive me crazy. I could fix that with various
| methods, sure, but it seems to me the easiest and best
| solution is just to paint it a color I would prefer.
|
| I'm certainly not saying that anyone else is wrong for
| having tastes that differ from mine.
| silisili wrote:
| If you're prepping a home to sell, sure.
|
| Stark white in a brightly lit room can be overwhelming.
| Sure you could dim the whole room, but now you've dimmed
| everything. For something like a gathering room, where a
| person may want to be reading, such a solution doesn't
| work.
|
| And I'm not using 'soft' white bulbs that make everyone
| look jaundiced to hide the wall color.
|
| Much easier all around to go offwhite(or anything but
| white) on the walls, using neutral 3kish bulbs to your
| preferred brightness.
| brookst wrote:
| Dimming and color tone are not the same thing.
|
| Yes, using paint color to control room light is easier.
| It's just less flexible. If you just want to throw a
| bunch of furniture and lights in, paint color is simpler.
|
| If you want the flexibility for the room to present as
| daylight white sometimes and cozy amber evenings at other
| times, it takes more work in lighting and for furniture
| choices, and neutral white walls increase flexibility.
|
| Neither answer is "better" in some objective sense. It's
| just a simplicity vs power choice.
| daniel_reetz wrote:
| The primary ingredient in housepaint is rutile titanium dioxide
| particles, which have a very high refractive index. The
| particles are suspended in a carrier which, when drying,
| creates air voids. The air/TiO2 refractive index change, as
| light bounces around inside, is what gives white paint it's
| hiding/wall covering power.
|
| In other words, artist paint and house paint use the same base
| pigment. Almost all commercially available "white" housepaints
| are based on titanium dioxide pigment. The special thing about
| artist paint is the extraordinarily high concentration of
| pigment.
| jjeaff wrote:
| I would think that high end construction paint also has more
| pigment. Not sure how it would compare, but I know painting a
| large area with artist grade paint could get exorbitantly
| expensive.
| dieselgate wrote:
| Why the negativity towards and when did painters become the
| bottom rung of construction trade? (Thought those were earth
| workers but I'm just kidding - question still remains)
| _a_a_a_ wrote:
| Because it's the chromatic equivalent of happy hardcore techno.
| Sometimes you just not in the mood for that.
| Syzygies wrote:
| Once I recognized that art supplies can substitute for
| construction materials, this opened up a world for me. For
| example, though miscommunication I had many holes to fill in
| some period kitchen cabinet doors in my house. The normal
| sequence would be filler, primer, cabinet paint. Instead I
| played with stiff acrylic modeling compound. I got the paint
| store to give me a sample of what they squirt into the paint,
| and I played around with the pigment and different mediums
| until the color and gloss was an exact match. Now I could fill
| each hole in one pass, no painting, and no one could find the
| hole afterwards.
| johnohara wrote:
| The sad but interesting story of Pearl Paint.
| https://www.artistsnetwork.com/magazine/pearl-paint/
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