Best reflective surface for skylight shaft?
Don Wiss
At the bottom is a stained glass window. I could paint the shaft sides
gloss white. But what about silver? Is there some reflective sheet metal
that I can buy that is even better? A glass mirror would probably be a bit
difficult to get up there.
Thanks, Don <donwiss at panix.com>.
Edwin Pawlowski
"Don Wiss" <donwiss@no_spam.com> wrote in message
news:s03mg0hab9ph0b53q...@4ax.com...
Bright and shiny. I saw one of the DIY shows where they installed a
skylight that used a round flexible duct to send the light from roof to the
room ceiling. It was bright silvery coated like a mirror. Maybe you could
get some highly polished stainless steel sheet metal or a bright foil
lining. I have no idea where you would buy it, but the silvered Mylar film
would work well. The stuff they make balloons from or line potato chip
bags.
Ed
e...@snet.net
http://pages.cthome.net/edhome
Mark
donwiss@no_spam.com says...
--
Mark
The truth as I perceive it to be.
Your perception may be different.
Triple Z is spam control.
Michael Daly
> I could paint the shaft sides
> gloss white. But what about silver? Is there some reflective sheet metal
> that I can buy that is even better? A glass mirror would probably be a bit
> difficult to get up there.
It would take a very good quality mirror to reflect more light than a good
white paint. Cheap mirrors may only reflect about 70% or less of the light.
Buy a high quality white paint and look for one with a high LRV (light
reflectance value). IIRC, matt white should actually have a higher
reflectance than gloss.
Mike
B
photons out of the sky than plain white paint. White paint will do the job.
-B
"Don Wiss" <donwiss@no_spam.com> wrote in message
news:s03mg0hab9ph0b53q...@4ax.com...
nicks...@ece.villanova.edu
>I think glossy white paint or silver or even mirrors will not pull more
>photons out of the sky than plain white paint. White paint will do the job.
Sure, but I'm looking at a table that says white paint has 80% reflectance,
vs 86% for aluminum foil and 92% for aluminized Mylar. The mirrorlike Mylar
can minimize the number of bounces in the shaft, compared to diffuse white
paint, further increasing the intensity at the bottom. A south-facing
reflective sunscoop over the skylight can also help.
Nick
Edwin Pawlowski
B <nospa...@nc.rr.com> wrote:
>
>I think glossy white paint or silver or even mirrors will not pull more
>photons out of the sky than plain white paint. White paint will do the job.
It is not a question of pulling photons out of the sky. It is a question of
moving the ones that you do have in the most efficient manner. There are
much better materials than white paint to do the job. Depending on the
particular pain and gloss, it is maybe 75 to 80% efficient. I don't have
the specs on a mirror, but I'd guess it to be over 90%.
dadiOH
> I have a skylight shaft that I've like to maximize the light coming
> down. At the bottom is a stained glass window. I could paint the
> shaft sides gloss white. But what about silver? Is there some
> reflective sheet metal that I can buy that is even better? A glass
> mirror would probably be a bit difficult to get up there.
Remember the foil reflecting material used over car dashes? Use that or
similar for max reflectance. However, max reflectance may not give you the
max output at the bottom because they are so directional. Think of a
mirror...you have to angle it around to reflect the source light where you
want it to go. For your purpose, a non specular (not glossy) white or
aluminum paint would probably be best due to the usual shape of a skylight
shaft.
--
dadiOH
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Michael Daly
On 31-Jul-2004, nicks...@ece.villanova.edu wrote:
> Sure, but I'm looking at a table that says white paint has 80% reflectance,
> vs 86% for aluminum foil and 92% for aluminized Mylar.
Considering you can see through aluminized Mylar, I have my doubts about this
claim.
Consider what they cover telescope observatories with. The domes are almost
always painted white with a high-titanium content paint. They are trying to
reflect as much heat and light as possible in order to keep the observatory
from heating up during the day. When have you ever seen them cover it with
a mirror finish?
> The mirrorlike Mylar
> can minimize the number of bounces in the shaft, compared to diffuse white
> paint, further increasing the intensity at the bottom.
Only if you can get it nice and smooth.
Mike
Michael Daly
> Depending on the
> particular pain and gloss, it is maybe 75 to 80% efficient. I don't have
> the specs on a mirror, but I'd guess it to be over 90%.
Paints can be up to 90% reflective. These are high-quality titanium oxide
based paints, of course. There are other oxides as well, but the Ti stuff
is probably easier to find.
Mirrors are highly variable, depending on the quality of the coating. Getting
over 90% is quite difficult and expensive. I remember paying quite a premium
to get a telescope mirror that was over 90% - the standard quality (still
higher than, say, a bathroom mirror) was in the mid 80s.
Dollar for dollar, paint is a much better deal and easier to install.
Mike
nicks...@ece.villanova.edu
>Don Wiss wrote:
>> I have a skylight shaft that I've like to maximize the light coming down...
>
>Remember the foil reflecting material used over car dashes? Use that or
>similar for max reflectance. However, max reflectance may not give you the
>max output at the bottom because they are so directional.
Not according to the usual lightwell calculations.
>Think of a mirror...you have to angle it around to reflect the source light
>where you want it to go. For your purpose, a non specular (not glossy)
>white or aluminum paint would probably be best due to the usual shape of
>a skylight shaft.
I doubt that. Can you find a specific example that makes your claim true?
Nick
10 RHO=.9'lightwell wall reflectance
20 W=4'lightwell width (feet)
30 L=8'lightwell length (feet)
40 C=4'concentration factor
50 EC=.8'concentrator efficiency
60 TFLUX=8000*W*L*C*EC'total flux entering well (FC)
70 FOR F = 1 TO 8'floor number
80 H=8*(8-F)'lightwell depth (feet)
90 R=5*H*(W+L)/(W*L)'well cavity ratio
100 E=EXP(-.0443*R/RHO)'transmission
105 PRINT F,E
110 ATTN=ATTN+1/E'cumulative attenuation
120 NEXT
130 ATA=ATTN/8'average attenuation
140 PRINT ATA'average attenuation
150 AREA=32^2*8'floor area (ft^2)
160 ER=.9'secondary reflector efficiency
170 PRINT ER*TFLUX/ATTN/AREA
180 ER=.6'eg 2 wall bounces, and a bottom reflection...
190 PRINT ER*TFLUX/AREA
Floor Lightwell
number Transmittance
1 .0056940
2 .0119144
3 .0249304
4 .0521655
5 .1091535
6 .2283978
7 .4779098
8 1
Average diffuse well attenuation: 41.9
Illumination from diffuse well: .268 footcandles
Illumination from mirror well: 60.0 footcandles(?)
Nick
nicks...@ece.villanova.edu
>> ...I'm looking at a table that says white paint has 80% reflectance,
>> vs 86% for aluminum foil and 92% for aluminized Mylar.
>
>Considering you can see through aluminized Mylar, I have my doubts about this
>claim.
Full sun is about 10,000 footcandles. We can see in 0.1 FC,
ie we can "see through" something that's 99.999% reflective.
>Consider what they cover telescope observatories with. The domes are almost
>always painted white with a high-titanium content paint. They are trying to
>reflect as much heat and light as possible in order to keep the observatory
>from heating up during the day. When have you ever seen them cover it with
>a mirror finish?
That's not easy to do.
>> The mirrorlike Mylar can minimize the number of bounces in the shaft,
>> compared to diffuse white paint, further increasing the intensity at
>> the bottom.
>Only if you can get it nice and smooth.
I disagree. Wrinkles make little difference.
Nick
Don Wiss
>On 31-Jul-2004, "Edwin Pawlowski" <e...@snet.net> wrote:
>
>> Depending on the
>> particular pain and gloss, it is maybe 75 to 80% efficient. I don't have
>> the specs on a mirror, but I'd guess it to be over 90%.
>
>Paints can be up to 90% reflective. These are high-quality titanium oxide
>based paints, of course. There are other oxides as well, but the Ti stuff
>is probably easier to find.
What is the paint they use when they paint flat roofs? Mine was just
painted silver, and checking out the job just now I find it very
reflective. Certainly paint would be the easiest. Are these titanium oxide
paints readily available in my local hardware store?
dadiOH
> dadiOH <xi...@verizon.net> wrote:
>
>> Don Wiss wrote:
>
>>> I have a skylight shaft that I've like to maximize the light
>>> coming down...
>>
>> Remember the foil reflecting material used over car dashes? Use
>> that or similar for max reflectance. However, max reflectance may
>> not give you the max output at the bottom because they are so
>> directional.
>
> Not according to the usual lightwell calculations.
>
>> Think of a mirror...you have to angle it around to reflect the
>> source light where you want it to go. For your purpose, a non
>> specular (not glossy) white or aluminum paint would probably be
>> best due to the usual shape of a skylight shaft.
>
> I doubt that. Can you find a specific example that makes your claim
> true?
Can you line a skylight chase with mirrors (or even specular reflectors) so
they reflect the skylight opening to the ceiling opening? Didn't think so.
nicks...@ece.villanova.edu
>>> ...max reflectance may not give you the max output at the bottom because
>>> they are so directional.
>>
>> Not according to the usual lightwell calculations.
>>
>>> ...a non specular (not glossy) white or aluminum paint would probably be
>>> best due to the usual shape of a skylight shaft.
>>
>> I doubt that. Can you find a specific example that makes your claim true?
>
>Can you line a skylight chase with mirrors (or even specular reflectors) so
>they reflect the skylight opening to the ceiling opening?
Think "non-imaging." Direct sun aside, the question is: "Can a lightwell
with diffuse reflective walls move more lumens into a room than one with
specular walls, given a skylight that collects a certain solid angle of
diffuse isotropic sun?"
Intuitively, light rays enter the skylight from all directions. If the
lightwell is vertical, vertical rays from overhead sky pass all the way
down the well with no reflections in either case, like shining a laser
down a pipe. Slightly off-vertical rays are reflected from the walls far
down in the lightwell. More tilted rays hit the walls nearer the top.
Specular reflectors pass these rays downwards with little intensity loss.
Diffuse reflectors scatter the light all over the place--down, up, and
horizontally to adjacent walls...
We could make this more mathematical, if you like.
Nick
Michael Daly
> >Considering you can see through aluminized Mylar, I have my doubts about this
> >claim.
>
> Full sun is about 10,000 footcandles. We can see in 0.1 FC,
> ie we can "see through" something that's 99.999% reflective.
I can see thru some aluminized mylar in a dimly lit room. My
guess is that these are no more than 50% reflective. High-
reflective aluminized mylar is harder to get and more expensive.
If you're going to recommend Al mylar, you have to be specific
about the quality and reflectivity.
My solar filter is nowhere near 99.999% reflective and I see
the sun thru it dim and safe. Don't forget that there is absorption.
My solar filter - nickel-steel coated glass - gets quite hot when
in the sun. The titanium-white painted mask around it doesn't.
It's a myth that mirror finishes (metallic coatings, polished
metal etc) are great reflectors of heat and light. Good quality
white paint can outdo mediocre mirror finishes easily and are
often a lot cheaper.
Mike
Michael Daly
> What is the paint they use when they paint flat roofs?
Roof paint. :-)
> Mine was just
> painted silver, and checking out the job just now I find it very
> reflective.
The full moon looks almost white, but is something like 10-15%
reflective and is quite black (check it tonight - it's a blue
moon, BTW).
Better check the actual LRV of the paint.
> Certainly paint would be the easiest. Are these titanium oxide
> paints readily available in my local hardware store?
Many good quality white paints use TiO2. Cheaper ones use clays
and stuff. Contact the paint manufacturers for this info. They
can tell you which ones are the highest reflectance. This data
is available for architects and interior designers.
Mike
dadiOH
> Specular reflectors pass these rays downwards with little intensity
> loss. Diffuse reflectors scatter the light all over the place--
> down, up, and horizontally to adjacent walls...
Agreed. But the difficulty would be to get the specular reflectors to
collect the light from source and direct it to where desired. Keeping in
mind that it is a skylight in (presumably) a home. Actually, if I were the
OP and wanted to light a stained glass window in the ceiling I probably
would have scrapped the skylight in favor of artificial, controllable light.
> We could make this more mathematical, if you like.
Not me, I = R is about my limit :)
JerryMouse
Think LOTS of optical fibers.
JerryMouse
Or a sunlamp in the shaft.
Hmmm. Can one get a REVERSE photocell?
nicks...@ece.villanova.edu
>> Full sun is about 10,000 footcandles. We can see in 0.1 FC,
>> ie we can "see through" something that's 99.999% reflective.
>
>I can see thru some aluminized mylar in a dimly lit room. My
>guess is that these are no more than 50% reflective.
You might see 0.1 FC in a 5 FC room through a 2% transmissive film.
>High-reflective aluminized mylar is harder to get and more expensive.
>If you're going to recommend Al mylar, you have to be specific
>about the quality and reflectivity.
I like Duraflex from Graphic Arts Systems in Cleveland.
About 10 cents/ft^2 in 0.002"x54"x100' rolls.
>It's a myth that mirror finishes (metallic coatings, polished
>metal etc) are great reflectors of heat and light. Good quality
>white paint can outdo mediocre mirror finishes easily and are
>often a lot cheaper.
Duane Johnson gets great results with Mylar heliostat mirrors.
Nick
nicks...@ece.villanova.edu
>nicks...@ece.villanova.edu wrote:
>
>> Specular reflectors pass these rays downwards with little intensity
>> loss. Diffuse reflectors scatter the light all over the place--
>> down, up, and horizontally to adjacent walls...
>
>Agreed.
So a lightwell lined with mirrors acts like a crab trap for entering rays.
Rays enter, but they never come out. They are all passed downwards, with
more reflections for rays from lower sky elevations and fewer for rays from
higher elevations. But diffuse reflectors scatter light back up and out of
the well. That seems like enough to convince most people that specular
lightwells deliver more light, without any math...
>But the difficulty would be to get the specular reflectors to collect the
>light from source and direct it to where desired.
I'm assuming that part is the same for the diffuse and specular lightwells,
that indirect sun rays come equally from all over the sky hemisphere, and
each point on a diffuse reflector reflects each ray out from its wall equally
over its hemisphere, but a specular reflector beams each ray downwards with
an angle of reflection that equals the angle of indicence. Always downwards,
never upwards, because of the way rays enter a lightwell with parallel sides.
>...if I were the OP and wanted to light a stained glass window in the
>ceiling I probably would have scrapped the skylight in favor of artificial,
>controllable light.
That might be nice at night. A specular lightwell with no diffuser at the
bottom might have glare problems in direct sun. SunOptics makes skylights
with lots of "microprisms" that spread direct sun about 30% more efficiently
than white plastics with bubbles inside. They produce enough skylights every
week (Wal-Mart is one of their customers) to offset 1 megawatt of peak
electrical lighting power, without any government subsidies.
Nick
Don Wiss
> Keeping in
>mind that it is a skylight in (presumably) a home.
Yes, a 1892 row house in Brooklyn.
> Actually, if I were the
>OP and wanted to light a stained glass window in the ceiling I probably
>would have scrapped the skylight in favor of artificial, controllable light.
But the shaft is only a couple feet deep (the thickness of the roof). The
amount of light that comes in would cost a fortune to replace with
artificial. And the skylight is orginal, just I replaced the outside glass
with an InsulaDome.
dadiOH
> But the shaft is only a couple feet deep (the thickness of the
> roof). The amount of light that comes in would cost a fortune to
> replace with artificial. And the skylight is orginal, just I
> replaced the outside glass with an InsulaDome.
Well, there's my problem...I was thinking of my own that come down to a
dropped ceiling in hall, etc.. Shafts are 6-8'.