Coatings for integrating spheres?
JB
sphere of approx 2M diameter. This will only be used in the visible
(400-700nm) and so far I've only found Avian-D (Avian Technologies) and
Spectraflect (Labsphere).
We can get our sphere coated here in the UK, but are there any other options
available as to the coating itself? Most paint manufacturers in the UK don't
seem to have heard of barium sulphate paint.
This will primarily be used for measurement of total luminous flux of
fluoresctent and HID lamps from 14W-400W so I wouldn't have thought that a
reflectance over the visible of >90% would be needed.
Any advice, links, thoughts on this?
Thanks,
JB
Patrick
while ago.
http://www.boss.be/eng/home/home.htm (contact details)
Patrick
"JB" <new...@nilspam.com> schreef in bericht
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Johannes Swartling
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Titanium oxide paint should have about the same reflectivity as barium
sulfate, but is not quite as good a Lambertian scatterer. I don't know how
critical that is to your application. But there must be old papers in the
literature on how to make your own barium sulfate paint.
Johannes
JB
"Johannes Swartling" <johannes....@home.se> wrote in message
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properties of TiO based pigments.
JB
Danny Rich
Do not waste your time on Titanium Dioxide. The situation is just the
opposite of what Mr Swartling has stated. Titanium Dioxide is a nearly
round structure with a refractive index of about 2.7 versus 2.0 for Barium
Sulphate. Thus TiO2 is a better scatterer of optical radiation. However,
it has a strong absorption band in the violet region of the spectrum and so
will not have adequate reflectance in the blue to violet end of the visible
spectrum. Other common pigments include Zinc Oxide (ZnO) and Zinc Sulfide
(ZnS). The Munsell products division of GretagMacbeth sells the former
Eastman White Reflectance Coating (BaSO4 in Polyvinyl Alcohol). They also
sell miniture sprayers. It is not difficult to apply this paint. Most
important is to prime the surface with a white primer such as KILZ (a North
American product) which is a primer and stain covering paint. If you really
want a good white then try getting an organic chemist to synthesize and kiln
some zinc ortho titanate. That pigment is used in space craft coatings and
has excellent UV and VIS reflectance.
Danny
"JB" <new...@nilspam.com> wrote in message
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JB
"Danny Rich" <Dann...@softhome.net> wrote in message
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regards,
JB
Johannes Swartling
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>
> Do not waste your time on Titanium Dioxide. The situation is just the
> opposite of what Mr Swartling has stated. Titanium Dioxide is a nearly
> round structure with a refractive index of about 2.7 versus 2.0 for Barium
> Sulphate. Thus TiO2 is a better scatterer of optical radiation.
However,
> it has a strong absorption band in the violet region of the spectrum and
so
> will not have adequate reflectance in the blue to violet end of the
visible
> spectrum. Other common pigments include Zinc Oxide (ZnO) and Zinc Sulfide
> (ZnS). The Munsell products division of GretagMacbeth sells the former
> Eastman White Reflectance Coating (BaSO4 in Polyvinyl Alcohol). They also
> sell miniture sprayers. It is not difficult to apply this paint. Most
> important is to prime the surface with a white primer such as KILZ (a
North
> American product) which is a primer and stain covering paint. If you
really
> want a good white then try getting an organic chemist to synthesize and
kiln
> some zinc ortho titanate. That pigment is used in space craft coatings
and
> has excellent UV and VIS reflectance.
Danny,
one partly right and one wrong. TiO absorbs in the UV, but in the visible it
has good reflectance. Since I usually work in the NIR with my spheres I tend
to forget about the violet part of the spectrum... TiO reflectance is
probably okay for characterization of lamps in the visible though.
Titanium oxide paints, even the matte types, have more specular reflection
than barium sulfate coating. The scattering cross section of the particles
is irrelevant here. The scattering coefficient of the bulk paint is not
directly related to the specular reflection of the surface (and it has
nothing to do with the diffuse reflectance, as long as the layer of paint is
thick enough). The specular reflection is a function of the surface
microstructure and the difference in bulk refractive index vs. air.
I'm not saying TiO paint is good, but you can buy it cheap in any paint
store so it might be a good first step.
Johannes
Danny Rich
course the absoption is even deeper in UV. This is why all standards on
assessment of the tint strength of paints recommend not using any
wavelengths below 420nm as the slope is so great that assess of colorant
absorption is unreliable. The problem for this application is that most
solidstate photodetectors do not have very poor response between 400nm and
430nm and most commercial lamps do not have high radiances except at the
mercury lines. Thus the sphere loses all efficiency below 430nm and that
amounts to about one-sixth of the visible spectrum.
Danny
"Johannes Swartling" <johannes....@home.se> wrote in message
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Arfty1166
JB,
There has been a lot of discussion, and as the developer of a number of these
coatings (Spectraflect, Avian-D, etc.), I thought I'd put my 2P in...
We do make a pre-mix of the barium sulfate coating (called Avian-B), that you
can apply yourself. We give full instructions on surface prep, how to mix, and
how to spray the coating. It isn't all that difficult but...
Danny Rich mentioned the Eastman 6080 in the spray bottles. This is now made by
GretagMacBeth. Decent coating (it's what Spectraflect was based on, ditto
Avian-B) but the sprayers are HORRIBLE! (Sorry, Danny- I really don't like
them...) To do a really decent job on a sphere, use professional spray
equipment (for touch up or small areas, an small air brush w/ bottle will
suffice). The quality of the coating really does depend on application
technique. You should applyb to thickness of anout 20-24 mils (0.5-0.6 mm),
BTW: Too much coating is almost as bad as too little; the reflectance actually
starts to decrease above 0.6 mm. This should take about 15-20 coats to do this.
You can apply the first few coats heavily, but the last few coats should be
very fine. This is what gives the very good scattering characteristics.
Avian-D is very good in the range your looking at in the the reflectance is
high and it is very tough. The problem is, with you being 'across the pond',
it's expensive to ship due to its flammability. It is NOT barium sulfate based
(it has a proprietary pigment) but has excellent properties from about 330 nm
to about 1250 nm.).
if you have any application questions, please feel free to contact me.
Art Springsteen
Avian Technologies LLC
arts@***avian***technologies.com
(please remove the ***'s to reach me)
Johannes Swartling
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> BTW: Too much coating is almost as bad as too little; the reflectance
actually
> starts to decrease above 0.6 mm.
Interesting. Do you have an explanation for this? Theoretically an
infinitely thick "half-space" of a non-absorbing scattering medium would
give 100% reflectance. Does it have something to with the fact that you have
a highly reflecting metal surface under the coating that prevents too long
photon propagation lengths inside the coating?
Johannes
Arfty1166
>Interesting. Do you have an explanation for this? Theoretically an
>infinitely thick "half-space" of a non-absorbing scattering medium would
>give 100% reflectance. Does it have something to with the fact that you
>have
>a highly reflecting metal surface under the coating that prevents too long
>photon propagation lengths inside the coating?
Hi Johannes,
You might be right but the opaque thickness seemed to be about 0.5 mm (20
mils). After that, the coating plateaued, then fell off- maybe you do get
lateral diffusion losses after 0.6 mm- I don't know. What I do know is that we
tested about 10 samples a day for a couple years and this observation was
consistent. The thickness versus reflectance was very predictable up to about
24 mils, then it started to drop- didn't matter who the sprayer was (there was
a big difference depending on how it was applied- whether it was put on heavy
at first or in even coats, etc.).
BTW: The original 6080 spec'd the reflectance of their coating as a 1 mm thick
layer that was poured and dried. Hardly a realistic procedure!
Cheers!