Economy thermal imager?

[Archimedes' Lever]

On Thu, 23 Jul 2009 20:47:27 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 23 Jul 2009 18:39:02 -0700, Archimedes' Lever
OneBigLever@InfiniteSeries.Org> wrote:

On Thu, 23 Jul 2009 15:28:42 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 22 Jul 2009 13:04:01 -0700, Fester Bestertester <fbt@fbt.net
wrote:

There are instructions on the 'net I've seen for removing the IR filter from
a digicam's digitizer chip so as to allow recording of the IR spectrum.

Commercial thermal imagers are thousands of dollars. They don't seem to be
anything more than a digicam with a broader spectrum (IR) sensor.

Can such a modified camera be used as a cheap thermal imager for industrial
purposes, such as looking for hot spots in equipment? Some application where
the temperature difference is large.

Thanks.

There's a reason a FLIR costs $10K.

John

Yeah... it is called greed, and the knowledge that most of your buyers
are rich government funded factions.

The detector is pretty exotic, and the retail price of the germanium
lens is a couple of $K.

FLIR recently bought Extech, and has a new, lower-price thermal
imager.

John

It would not surprise me if they did not buy Mikron next. I am sure
they are feeling the sting. Mikron's owner invented the resistor
bolometer transducer in 1960.. k. Irani.

OOOPS!

In 2007, Mikron Infrared was acquired by LumaSense Technologies, Inc.
With offices around the world, LumaSense Technologies develops ...

Just found that. Anyway, Mr. Irani was a cool guy.

Mikron is a cool company. Well, hot really... on average.

http://www.mikroninfrared.com/downloads/mikronproductportfolio.pdf

They got some pretty tight stuff, and I think FLIR buys gear from them
for calibration.

Best looking outdoor camera housings I have ever seen.
Really nice instruments now, compared to the Aluminum tube days when I
was there. We did not have powder coat back then.

Wow. Nice info sheet!

http://www.mikroninfrared.com/AboutBlackbodySources.aspx

and this one has some nice references at the bottom.

http://www.mikroninfrared.com/contentnonav.aspx?id=232

 

[John Larkin]

On Thu, 23 Jul 2009 23:48:33 -0700, Archimedes' Lever
<OneBigLever@InfiniteSeries.Org> wrote:

On Thu, 23 Jul 2009 20:47:27 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 23 Jul 2009 18:39:02 -0700, Archimedes' Lever
OneBigLever@InfiniteSeries.Org> wrote:

On Thu, 23 Jul 2009 15:28:42 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 22 Jul 2009 13:04:01 -0700, Fester Bestertester <fbt@fbt.net
wrote:

There are instructions on the 'net I've seen for removing the IR filter from
a digicam's digitizer chip so as to allow recording of the IR spectrum.

Commercial thermal imagers are thousands of dollars. They don't seem to be
anything more than a digicam with a broader spectrum (IR) sensor.

Can such a modified camera be used as a cheap thermal imager for industrial
purposes, such as looking for hot spots in equipment? Some application where
the temperature difference is large.

Thanks.

There's a reason a FLIR costs $10K.

John

Yeah... it is called greed, and the knowledge that most of your buyers
are rich government funded factions.

The detector is pretty exotic, and the retail price of the germanium
lens is a couple of $K.

FLIR recently bought Extech, and has a new, lower-price thermal
imager.

John


It would not surprise me if they did not buy Mikron next. I am sure
they are feeling the sting. Mikron's owner invented the resistor
bolometer transducer in 1960.. k. Irani.

OOOPS!

In 2007, Mikron Infrared was acquired by LumaSense Technologies, Inc.
With offices around the world, LumaSense Technologies develops ...

Just found that. Anyway, Mr. Irani was a cool guy.

Mikron is a cool company. Well, hot really... on average.

http://www.mikroninfrared.com/downloads/mikronproductportfolio.pdf

They got some pretty tight stuff, and I think FLIR buys gear from them
for calibration.

Best looking outdoor camera housings I have ever seen.
Really nice instruments now, compared to the Aluminum tube days when I
was there. We did not have powder coat back then.

Wow. Nice info sheet!

http://www.mikroninfrared.com/AboutBlackbodySources.aspx

and this one has some nice references at the bottom.

http://www.mikroninfrared.com/contentnonav.aspx?id=232

Thermal imagers are getting cheaper, and will continue to do so, I
hope. Sub-$1000 one of these days maybe. Even a 100x100 pixel imager,
maybe with a plastic lens, could be mighty useful.

Our FLIR cost about $10K a couple of years ago and it was worth it.

ftp://jjlarkin.lmi.net/IR_0026.jpg

ftp://jjlarkin.lmi.net/IR_0032.jpg

John

 

[Archimedes' Lever]

On Thu, 23 Jul 2009 22:35:32 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Thu, 23 Jul 2009 20:47:27 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
The detector is pretty exotic, and the retail price of the germanium
lens is a couple of $K.

FLIR recently bought Extech, and has a new, lower-price thermal
imager.

http://www.extech.com/instrument/products/alpha/IRC40.html
$2,700 to $3,000 from many sources. Still kinda pricy but getting
there. Thanks.

Mikron. More pricey, but an order of magnitude better instrument.

 

[Archimedes' Lever]

On Fri, 24 Jul 2009 09:01:54 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Thermal imagers are getting cheaper, and will continue to do so, I
hope. Sub-$1000 one of these days maybe. Even a 100x100 pixel imager,
maybe with a plastic lens, could be mighty useful.

Our FLIR cost about $10K a couple of years ago and it was worth it.

Harbor freight has about five different sense model, but I wouldn't be
surprised if a cheap Chinese model didn't show up eventually.

Back when all there was was what we had, and that was only 4 fps and
ran on a 386, and had no out to NTSC option that we could even record
from... it was $90k+

At the same time, Fischer Price sold a B&W "toy" camera for $150 that
could have a Ge or Pyrex lens or filter put on it, and it would do IR
very nicely.

We all thought that was pretty funny, thinking how mere color added
$89.9k to the price.

Well... that and a nice arrangement with a little cup of Liquid
Nitrogen. It was a real thermal imager.

Now, they have room temp jobs. Your $10k unit is cheap by comparison.
The industry sure has come a long way since '87.
I'll bet you could get a real nice one from Kikron. Those ones with the
LCD "flap" in the back... They looked nice. Likely about $7k though.
Still a very REAL instrument, where some seem more like an "also show" to
the party. Quantifying accurately should be a requisite. Mikron does
that well. Not sure about some I've seen. I am sure that FLIR also does
nice, and true (as it were) instruments.

There are some though that are wide in their accuracy window, and
calibration seems and end to end thing, instead of being corrected along
the entire scale. Mikron sells nice Black Body calibration sources too.
Many can be made NIST traceable.

I still find it funny that I used to dope up a basketball with high
temp silica "plaster of Paris", then wrap nichrome coils around that,
then more "plaster". Then slice the sphere in half to get the ball back
out. Reform the sphere. bake it all. and add a radiation port to it (a
ceramic tube). Surround the outside with fire brick used for kilns.
Suspend all that in a big rack with the tube pointing out the front.

Add about 2500 Watts, a line cord (X Large) and a pid controller,and you
have a precision black body source that can make about 4000 degrees F
inside the globe. Those high temp sources are far more precise. They
vary little. A small black body source that uses a painted surface
(which they also sell) is harder to keep a uniform temp across it.
They have FLIR calibration sources now that are cool (hot) and stable
across a huge surface, which is very hard to do.

Mikron Or their parent, has also seemingly found a way to get better IR
focus than others.They have some pretty crisp detail going. I do not know
if you looked at the pdfs or not.

 

[Dave]

On Jul 22, 4:04 pm, Fester Bestertester <f...@fbt.net> wrote:

There are instructions on the 'net I've seen for removing the IR filter from
a digicam's digitizer chip so as to allow recording of the IR spectrum.

Commercial thermal imagers are thousands of dollars. They don't seem to be
anything more than a digicam with a broader spectrum (IR) sensor.

Can such a modified camera be used as a cheap thermal imager for industrial
purposes, such as looking for hot spots in equipment? Some application where
the temperature difference is large.

Thanks.

thermograms is what you referring to...... cmos and ccd found in
digital cameras are only sensitive to the NON-Thermal or
NIR.<900nm.... Focal-plane array (FPA) respond the longer wavelengths
900-14000nm

Its also difficult to see the Economy heat up through any electro-
mechanical device...Bernake is at a loss as well

 

[JosephKK]

On Wed, 22 Jul 2009 22:45:22 +0100, Martin Brown
<|||newspam|||@nezumi.demon.co.uk> wrote:

Fester Bestertester wrote:
There are instructions on the 'net I've seen for removing the IR filter from
a digicam's digitizer chip so as to allow recording of the IR spectrum.

Commercial thermal imagers are thousands of dollars. They don't seem to be
anything more than a digicam with a broader spectrum (IR) sensor.

A *much* broader spectrum out to long wave IR and also a silicon lens.
Glass does not transmit the wavelengths needed for thermal imaging.
There is a good reason why they are so expensive.

You can do near IR with a modified digicam and a low pass filter -
enough to make diagnosis of foliage diseases in trees perhaps but
nothing like enough to see temperature differences unless the target was
almost at red heat. I doubt if a soldering iron would show up at all on
a normal digicam CCD in the IR.

IIRC many common soldering irons run in the 600 F to 700 F range (not
all that far from visible glow) and may well output enough for a
modified digicam. I swear, that as child i could see the soldering
iron by its own glow (in the middle of the night when i woke for some
reason) some night when i left it plugged in.

Can such a modified camera be used as a cheap thermal imager for industrial
purposes, such as looking for hot spots in equipment? Some application where
the temperature difference is large.

You can buy relatively cheap non contact IR thermometers and use a servo
to scan one to build up a low resolution thermal image.

Regards,
Martin Brown

 

[Paul Keinanen]

On Wed, 22 Jul 2009 13:04:01 -0700, Fester Bestertester <fbt@fbt.net>
wrote:

There are instructions on the 'net I've seen for removing the IR filter from
a digicam's digitizer chip so as to allow recording of the IR spectrum.

Commercial thermal imagers are thousands of dollars. They don't seem to be
anything more than a digicam with a broader spectrum (IR) sensor.

The peak wavelength for a black body radiator is simply inversely
proportional to the absolute temperature. At 6000 K (e.g. the Sun) the
maximum wavelength is about 0.5 um (yellow). The silicon camera
sensors have some kind of response to 2-3 um, corresponding to 1500 ..
1000 K. Humans and room temperature furniture are around 300 K, thus
the radiation peak is around 10 um, thus, ordinary glass is not very
transparent and ordinary silicon cells are useless.

The radiation drops quite quickly, when going from the peak towards
shorter wavelengths, but still this explains why the eye (400-700 nm
response) can detect some weak dark reddish radiation from object with
only 1000 K temperature (black body radiation peak at 3000 nm).

Paul

 

[Lostgallifreyan]

"JosephKK"<quiettechblue@yahoo.com> wrote in
news:jk4o751192hv04vit912ulhssfp7048dsl@4ax.com:

IIRC many common soldering irons run in the 600 F to 700 F range (not
all that far from visible glow) and may well output enough for a
modified digicam. I swear, that as child i could see the soldering
iron by its own glow (in the middle of the night when i woke for some
reason) some night when i left it plugged in.

I believe it. I've seen them at times. I can't tell when sight ends and
imagination begins though, I tried.. > I have a nice temperature controlled
iron now so I'll try this again some time. Helps to try to see it with the
rods, not the cones, so at least thirty degrees to the side.

 

[Lostgallifreyan]

Paul Keinanen <keinanen@sci.fi> wrote in
news:ct7o75lrjq0424dc8fumn7hsl821he9nmb@4ax.com:

weak dark reddish radiation from object with
only 1000 K

? 727°C? I remember from pottery classes in school that you get into a fierce
cherry red by that high. Nice confirmation here:

http://drjudywood.com/articles/aluminum/alumpics/htchar1.gif

 

[Phil Hobbs]

Lostgallifreyan wrote:

Paul Keinanen <keinanen@sci.fi> wrote in
news:ct7o75lrjq0424dc8fumn7hsl821he9nmb@4ax.com:

weak dark reddish radiation from object with
only 1000 K

? 727°C? I remember from pottery classes in school that you get into a fierce
cherry red by that high. Nice confirmation here:

http://drjudywood.com/articles/aluminum/alumpics/htchar1.gif

White light at 1200 C? What a maroon. I think she applied the
conversion table backwards. Tungsten bulbs run 2800-3300 K.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

 

[Lostgallifreyan]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in
news:7vednQzvUYfK1eHXnZ2dnUVZ_jZi4p2d@supernews.com:

weak dark reddish radiation from object with
only 1000 K

? 727°C? I remember from pottery classes in school that you get into a
fierce cherry red by that high. Nice confirmation here:

http://drjudywood.com/articles/aluminum/alumpics/htchar1.gif

White light at 1200 C? What a maroon. I think she applied the
conversion table backwards. Tungsten bulbs run 2800-3300 K.

I didn't think much about it. That temp would look yellow I think. The red is
right though. I remember being taught to gauge the temperature of a pottery
kiln by the colour. There are even little gauges used for furnaces that are
adjusted till a small element fades to a match with the backgound colour, and
a reading is made of the temperature that way. I think some slack can be cut
re that white. It's wrong, but look at all the colours that pass for white.
You can stick a load of video monitors together in a room and see greater
errors than hers.

 

[Phil Hobbs]

Lostgallifreyan wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in
news:7vednQzvUYfK1eHXnZ2dnUVZ_jZi4p2d@supernews.com:

weak dark reddish radiation from object with
only 1000 K
? 727°C? I remember from pottery classes in school that you get into a
fierce cherry red by that high. Nice confirmation here:

http://drjudywood.com/articles/aluminum/alumpics/htchar1.gif
White light at 1200 C? What a maroon. I think she applied the
conversion table backwards. Tungsten bulbs run 2800-3300 K.


I didn't think much about it. That temp would look yellow I think. The red is
right though. I remember being taught to gauge the temperature of a pottery
kiln by the colour. There are even little gauges used for furnaces that are
adjusted till a small element fades to a match with the backgound colour, and
a reading is made of the temperature that way. I think some slack can be cut
re that white. It's wrong, but look at all the colours that pass for white.
You can stick a load of video monitors together in a room and see greater
errors than hers.

Nowhere close. You don't even get out of the deep reds until 1500 K or so.

http://hancocktechnologies.com/Support/Black%20Body.jpg

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

 

[Lostgallifreyan]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in
news:Y4GdnXaF4Y0CGeHXnZ2dnUVZ_sKdnZ2d@supernews.com:

Lostgallifreyan wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in
news:7vednQzvUYfK1eHXnZ2dnUVZ_jZi4p2d@supernews.com:

weak dark reddish radiation from object with
only 1000 K
? 727°C? I remember from pottery classes in school that you get into
a fierce cherry red by that high. Nice confirmation here:

http://drjudywood.com/articles/aluminum/alumpics/htchar1.gif
White light at 1200 C? What a maroon. I think she applied the
conversion table backwards. Tungsten bulbs run 2800-3300 K.


I didn't think much about it. That temp would look yellow I think. The
red is right though. I remember being taught to gauge the temperature
of a pottery kiln by the colour. There are even little gauges used for
furnaces that are adjusted till a small element fades to a match with
the backgound colour, and a reading is made of the temperature that
way. I think some slack can be cut re that white. It's wrong, but look
at all the colours that pass for white. You can stick a load of video
monitors together in a room and see greater errors than hers.

Nowhere close. You don't even get out of the deep reds until 1500 K or
so.

http://hancocktechnologies.com/Support/Black%20Body.jpg

Or you could try this one:
http://wpcontent.answers.com/wikipedia/commons/thumb/b/ba/PlanckianLocus.png/
303px-PlanckianLocus.png

That shows you can follow the curve right down to around 800K and still
expect to see visible red. The only difference is that they resolve a tad
deeper into the red end.

This one shows that it's a matter of how much visible red, not whether it's
there or not:
http://www.techmind.org/colour/colourtemperatureannotated.png
In a furnace there's a lot of energy, even a 'cool' one has enough visible
red to see it.

So it's a question of how much, and also how sensitive we are to it.. Red is
the largest perceived part of the visible sprectrum, I mean it covers more
actual nanometres than the other colours, by far, and when looking at furnace
colours we're not looking at a single line either, which is why I posted that
second link. Also, we're likely to compare the red against a dark background.
(kiln view ports are designed to exclude external light).

Putting it another way, take a small butane flame with inducted air (blue
sharp-tipped flame), it can just about reach 1500K and will melt a copper
wire (ignoring flame colour) with a orangey-yellow ball forming at the end,
but if you try to put that copper on a red hot cooker ring it just sits there
oxidising. The ring IS evidently glowing a strong cherry red, but it's not
anything like the 1356.6K needed to melt copper. And yes, I have thought of
poor thermal transfer there, but I've seen overheated copper soldering iron
bits glowing, but not melting.

I'm sure a lot of this is very subjective but it's still true. That
subjectivity is why the hot-wire gauge is used to make simple meaures of
furnace temperature. A 'red' that glows against one hot glowing body will
look black against another brighter one. And within that very broad range
you'll likely find that every one of them appears red to someone.

Basically even a deep red looks cherry red if you have enough of it and
nothing for it to compete with..

 

[Lostgallifreyan]

Lostgallifreyan <no-one@nowhere.net> wrote in
news:Xns9C60E3699100Azoodlewurdle@216.196.109.145:

A 'red' that glows against one hot glowing body will
look black against another brighter one.

Meant hotter, not brighter.. (and darker, rather than 'black')

 

[Lostgallifreyan]

Lostgallifreyan <no-one@nowhere.net> wrote in
news:Xns9C60E6D33A5F5zoodlewurdle@216.196.109.145:

Lostgallifreyan <no-one@nowhere.net> wrote in
news:Xns9C60E3699100Azoodlewurdle@216.196.109.145:

A 'red' that glows against one hot glowing body will
look black against another brighter one.

Meant hotter, not brighter.. (and darker, rather than 'black')

And just to show how it can't really be said to be either right or wrong,
what colour IS 'cherry' red? Some of them look deeper and darker than the
little red spot seen in a bare IR laser diode... The dark ones taste better
too.

 

[John Larkin]

On Fri, 07 Aug 2009 11:29:27 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

Lostgallifreyan wrote:
Paul Keinanen <keinanen@sci.fi> wrote in
news:ct7o75lrjq0424dc8fumn7hsl821he9nmb@4ax.com:

weak dark reddish radiation from object with
only 1000 K

? 727°C? I remember from pottery classes in school that you get into a fierce
cherry red by that high. Nice confirmation here:

http://drjudywood.com/articles/aluminum/alumpics/htchar1.gif

White light at 1200 C? What a maroon. I think she applied the
conversion table backwards. Tungsten bulbs run 2800-3300 K.

Cheers

Phil Hobbs

What color temperature is maroon? I bet her face is red.

John

 

[Paul Keinanen]

On Fri, 07 Aug 2009 08:28:06 -0500, Lostgallifreyan
<no-one@nowhere.net> wrote:

Paul Keinanen <keinanen@sci.fi> wrote in
news:ct7o75lrjq0424dc8fumn7hsl821he9nmb@4ax.com:

weak dark reddish radiation from object with
only 1000 K

? 727°C? I remember from pottery classes in school that you get into a fierce
cherry red by that high. Nice confirmation here:

http://drjudywood.com/articles/aluminum/alumpics/htchar1.gif

The human eye is very bad in detecting absolute colours, due to the
"automatic white balance adjustment" so you really need a reference
chart at the same illumination level to reliably compare colours.

Apparently the Draper point at 798 K seems to be some standard for
visibility of hot objects, but I have not found any references how
this is actually determined.

To get a good general overview how the radiation behaves on short
wavelengths, it is a good idea to plot the black body radiation on a
log/log scale, e.g. figure 10 in
http://ceos.cnes.fr:8100/cdrom-98/ceos1/science/baphygb/chap3/chap3.htm

Look at the 800 K (Draper point) curve, which has a peak at about 3,5
um and look at intensity at 0.7 um (the nominal limit of human
vision), the magnitude is about 5 orders of magnitude (50 dB) below
the peak.

At 1500 K, the peak is at 2 um and the intensity drops only 1.5 orders
of magnitude (15 dB) at 0.7 um but the absolute level at 0,7 um is 5
levels of magnitude (50 dB) above the value for 800 K at that
wavelength.

Compare this with the sensitivity of the eye
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/efficacy.html

For normal illumination levels (photopic vision) the nominal 700 nm
limit is somewhat arbitrary, since the sensitivity is doubled (+ 3 dB)
every 10 nm when going from 770 nm down to 670 nm, a total increase
about 30 dB. In that wavelength band the 800 K black body radiation
intensity drops about 12 dB and the 1500 K radiation drops about 5 dB
in that region, so the stimulus is strongest near the shortest end
(670 nm) of that band.

However, the absolute level of 800 K and colder objects is very low
and a dark adapted eye (scotoptic vision) is required, however the eye
is insensitive to deepest red at these levels. Practical values
starting at 700 nm and the response doubling every 10 nm down to about
580 nm.

The scotopic vision is of course black and white and if in a dark room
an object is heated, a glow will be observed, when there is sufficient
power below about 700 nm, but you can just tell that something is
glowing, but you can not determinate the colour due to the scotopic
vision.

When the temperature is further increased, the absolute power levels
below 770 nm are increased significantly and sooner or later the
photopic vision will be smoothly activated and it becomes possible to
determine colours.

The level at which the transition occurs, depends of the absolute
level reaching the eye and hence also of the angle of view.

Paul

 

[Lostgallifreyan]

Paul Keinanen <keinanen@sci.fi> wrote in
newsp2q75tunrt3tr1fncp0t6ee6p4ou9isod@4ax.com:

The human eye is very bad in detecting absolute colours, due to the
"automatic white balance adjustment" so you really need a reference
chart at the same illumination level to reliably compare colours.

That's what I was getting at (rather verbosely). That and the fact that
cherries actually have so many colour variants you can plot most of the
visible black body spectrum with them.

 

[Lostgallifreyan]

Paul Keinanen <keinanen@sci.fi> wrote in
newsp2q75tunrt3tr1fncp0t6ee6p4ou9isod@4ax.com:

However, the absolute level of 800 K and colder objects is very low
and a dark adapted eye (scotoptic vision) is required, however the eye
is insensitive to deepest red at these levels. Practical values
starting at 700 nm and the response doubling every 10 nm down to about
580 nm.

That's interesting because when I looked at that page I saw I was apparently
wrong when I said that to spot a dimly glowing overheated soldering iron bit
it helps to use the scotopic vision. It IS less sensitive to light at
longwave red. Even so it did seem to help, and your comment that scotopic
vison is needed seems to back this up. Maybe it has more to do with the
effort of trying to see it, balancing the two types of vision by arranging
the object at certain angles to our centre of vision. Maybe our brains can
make more of it by this comparison than by either type of vision alone.

Anyway, looking at the rest of what you wrote, it seems that a claim to see a
glow from something as 'cool' as 500°C is viable (though the standard you
mentioned states 10° higher), if seen in darkness. I thought I'd managed it
at 450°C once but I'm not prepared to back that up. Next time I have a
controlled way to set that temperature I'll try it. I did once try this with
a kiln observation hole but I can't remember what the temperature was when it
became visible. It was thirty years ago, and I think I got distracted and
revisited the kiln too late to see the first light anyway.

 

[Hal Murray]

Apparently the Draper point at 798 K seems to be some standard for
visibility of hot objects, but I have not found any references how
this is actually determined.

wikipedia for John William Draper
http://en.wikipedia.org/wiki/John_William_Draper
says:

In 1847 he published the observation that all solids glow red at
about the same temperature, about 977 F (798 K), which has come
to be known as the Draper point.

It has a couple of references if you want more info.

--
These are my opinions, not necessarily my employer's. I hate spam.