Economy thermal imager?

Mon Aug 17, 2009 11:21 am   

On Sun, 16 Aug 2009 11:11:46 -0700, John Larkin
<jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:



"Can't work"? You're an idiot. The first IR imagers were CCD. They
came out before CMOS image planes were even around.

As for the lenses, they are specifically for narrowing a spectral
response.



OK. So you found a site that gave you a good picture of what passes
through Germanium. So what?


So what? The FP device did as well. All that was needed was a bit of
gain increase after placing the filter.

Mon Aug 17, 2009 12:18 pm   

Archimedes' Lever wrote:

I'm with John on this one. The first CCDs were largely in the visible
with a strong peak sensitivity in the *near* IR and with a very big push
to get them out to much longer wavelengths on somewhat exotic materials.
I knew an astronomy imaging group that were given new military chips to
test because they could turn one into a fully working prototype camera
way faster and cheaper than the approved contractors.

Rough graphs of the typical sensitivity for silcon CCDs with front and
back thinned window construction are online at:

http://www.andor.com/learn/digital_cameras/?docid=315

Their response does not extend much beyond 1000nm which is still an
order of magnitude short of the roughly ~10um IR wavelengths needed for
thermal imaging at ambient temperatures.

Eventually they did get longer wavelength CCDs working, and they stopped
at a particular point. As the man said "what we have is good enough to
see what *we* need to see". Astronomers were a bit disappointed that
after that they had to pay for their own chip R&D. It didn't stop
terahertz sensors eventually being made though. Strangely the terahertz
image of my favourite object Cass A appears to have been removed from
the web.

That only gets you near infra red. The thermal band for things in the
temperature range 0-100C is much more tricky and generally involves
exotic doped materials, germanium lenses and cunning optical design
since the emissions from the casing start to be almost as bright as the
target. Some form of multistage thermoelectric cooling is usually
employed or LN2.

Regards,
Martin Brown

Mon Aug 17, 2009 12:39 pm   

On Mon, 17 Aug 2009 13:18:36 +0100, Martin Brown
<|||newspam|||@nezumi.demon.co.uk> wrote:



Our first imagers came out in '86, and they were LN2 cooled 16 color, 4
frame per second devices that sold for $95k each.

If the imaging plane on them was something other than CCD, I was
unaware. I suppose it could have been a micro-array of bolometers.

Mon Aug 17, 2009 3:26 pm   

On Mon, 17 Aug 2009 04:21:51 -0700, Archimedes' Lever
<OneBigLever_at_InfiniteSeries.Org> wrote:


So now you find a site that shows the spectral response of silicon,
and draw a conclusion about the combination.

John

Mon Aug 17, 2009 3:28 pm   

On Mon, 17 Aug 2009 13:18:36 +0100, Martin Brown
<|||newspam|||@nezumi.demon.co.uk> wrote:


CCD detector + germanium lens = zero response.

John

Mon Aug 17, 2009 4:15 pm   

Martin Brown wrote:

I have a bit of experience with the radiometry of thermal imaging, both
in the uncooled (pyroelectric) and cryogenic (InSb) ends.

The biggest, irreducible problems with thermal detectors are their
thermal mass, which slows them down, and the fluctuations in thermal
conductivity, which makes them noisy.

A thermal conductor has fluctuations exactly like Johnson noise in a
resistor, and for the same reason--classical equipartition of energy
applied to the heat capacity of the pixel (Even the formula is almost
the same--noise power flux per hertz is sqrt(4kT**2/R_th).) That means
that the power in the fluctuations goes as T, so a room temperature
pixel is 4 times noisier than one at 77K for the same thermal
conductance. However, the thermal resistance of materials tends to
become very large at low temperature, and so does that of vacuum (the
effective thermal conductance of vacuum goes like T**3--it's d/dT of the
total thermal radiation, which goes as T**$).

Thus thermal detectors become really dramatically better at lower
temperatures. Interestingly, it's a big win to increase the insulation,
even though that slows down the response--the slowdown is due to 'bass
boost' rather than 'treble cut', so you get more signal everywhere, as
well as lower fluctuation noise. You just have to filter afterwards to
crispen up the temporal response a bit.

Quantum detectors, e.g. Si, Ge, InSb, and HgCdTe (pronounced
'mercadtell' and sometimes abbreviated as MCT), are limited by their
band gaps--there has to be a filled electron level around h*nu below the
conduction band, or else nothing happens when you shine light on it. To
see IR radiation of room-temperature objects, you have to go out to at
least the 3-5 um band (InSb), and it's easier in the 8-14 um band
(HgCdTe). The radiometry in the 3-5 um band is getting really
difficult at 300K, because there's an exponential falloff of photon flux
on the blue side of the radiation peak. The good news about 3-5 um is
that there's a lot more contrast there--small temperature differences
give rise to bigger intensity shifts, another consequence of that
exponential falloff. You just have to have low enough noise.

There are cryogenic silicon-based detectors that are sensitive in the
very long wave infrared (out to 100 microns or further), but they're
extrinsic photoconductors--the relevant band gap is then between the
impurity levels and the conduction band, so you have to run them at very
very low temperatures so that the carriers freeze out. No silicon
junction device (e.g. a photodiode, CCD, or CMOS imager) can work beyond
about 1.1 microns, because at room temperature the impurity states are
all ionized, so there aren't any valence electrons available that are
closer to the conduction band than that.

On the other hand, people have made very low sensitivity IR detectors,
good enough for measuring the power of CO2 lasers for instance, by
forward biasing a photodiode and looking at the change in the forward
voltage caused by the laser heating.

I'm expecting to start work soon on free-space IR detection in both
bands, using modified versions of my antenna-coupled tunnel junction
gizmos. They're sort of halfway between the thermal and quantum
devices--they work by internal photoemission of electrons from one metal
across the junction to the other metal. If the metals are sufficiently
different, I think you can make them into zero bias detectors.

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

Mon Aug 17, 2009 4:33 pm   

On Sun, 16 Aug 2009 11:11:46 -0700, John Larkin
<jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:


If you look at the black body spectrum, the power density drops quite
quickly at wavelengths below the black body peak wavelength. The cheap
IR sensors measure the amplitude around 1-2 um and if you have a idea
of the spectral distribution in that area, you should be able to
(gu)estimate where the spectral peak is and from the Wien law what the
temperature is.

Paul

Mon Aug 17, 2009 5:01 pm   

Archimedes' Lever <OneBigLever_at_InfiniteSeries.Org> wrote in
news:enji85519tgtddakm80f6u5fu8cdn74mps_at_4ax.com:


No idea what was in the thing I first saw, but it was a lot earlier than
1986, more like 1978 or so. I was a young kid, taken to the Science Museum in
London. The imaging wasn't great, just contoured moving blobsfo false colour,
but shapes were well defined, the refresh rate better than most simple
webcams. It could see an electric iron, and it could see me vaguely, and my
hand fairly well when I waved it open-palmed in front of the camera. I don't
remember the camera being especially bulky. It was definitely picking up
emissions, this wasn't reflection of near IR light. Maybe the mechanism was
described, but at that age I wouldn't know.

Mon Aug 17, 2009 5:07 pm   

Lostgallifreyan <no-one_at_nowhere.net> wrote in
news:Xns9C6AB76B6DCE2zoodlewurdle_at_216.196.109.145:


Actually I think I remember it being based on thermocouples, some kind of
fine array of them.

Mon Aug 17, 2009 8:33 pm   

John Larkin wrote:

You need to specify *silicon* CCD + germanium lens = zero response.

QWIP arrays work from 1um down to around 12um from the open literature.
Even has a Wiki entry.

http://en.wikipedia.org/wiki/QWIP

Earlier systems were more like bolometers using doped germanium or PbS.
Infrared "the new astronomy" was hot in 1975 and got hotter as the
sensors became ever more sensitive and better resolution.

Regards,
Martin Brown

Mon Aug 17, 2009 8:53 pm   

On Mon, 17 Aug 2009 21:33:19 +0100, Martin Brown
<|||newspam|||@nezumi.demon.co.uk> wrote:



OK, but the Fisher-Price toy was a silicon CCD.

Incidentally, Andor is an OEM customer of ours! Nice folks.

John

Tue Aug 18, 2009 12:44 am   

On Mon, 17 Aug 2009 08:28:35 -0700, John Larkin
<jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:


You + logic = water + oil.

Tue Aug 18, 2009 12:51 am   

On Mon, 17 Aug 2009 12:01:58 -0500, Lostgallifreyan <no-one_at_nowhere.net>
wrote:



This one could see the remnant of heat your hand would leave on an
ambient formica desktop after ten seconds. It could still see the
differentials nearly a half hour later. It had a 0.1°C sensitivity.

Tue Aug 18, 2009 1:28 am   

On Mon, 17 Aug 2009 17:44:26 -0700, Archimedes' Lever
<OneBigLever_at_InfiniteSeries.Org> wrote:


So do you think that a germanium lens would make the FP toy into a
thermal imager?

Yes or no?

John

Fri Aug 21, 2009 1:56 am   

On Mon, 17 Aug 2009 18:28:47 -0700, John Larkin
<jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:


A window does not "make" anything into anything. Windows are used for
filtration, just like say a "bandpass filter".

The camera has optics already. All it would need is a windows of the
appropriate spectral range for the desired intended use.