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PICList Thread
'Pyroelectric sensors'
1997\11\15@132100 by Ram Krishnan

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face
Does anyone know how non-contact thermometers work? They are now coming
down in price - there's even one for domestic use that stick in your ear.

1997\11\15@234629 by Steve Baldwin

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> Does anyone know how non-contact thermometers work? They are now coming
> down in price - there's even one for domestic use that stick in your ear.

They use the same sensor as the garage door opener.

Everything above absolute zero emits radiation that is dependant on its
temperature. In the case of stars (which are really really hot), the light
is visible. Blue stars being hotter than yellow stars, etc. At temperatures
that are a bit more comfortable, that light is in the infrared range. The
temperature of the object can be determined from the amount and wavelength
of the emitted radiation.

The sensor is a crystal (Lithium Tantalate is the most common) that will
generate a voltage as it heats up from the IR radiation that falls on it.
The voltage leaks away very quickly, so the output is as if it was
capacitively coupled. The voltage is very small so the part usually has an
integrated JFET amplifier.

Because of the AC coupling aspect, the output is measurable if the IR
radiation changes rapidly. In the case of the garage door opener, a
multi-faceted lens is put in front of the sensor. As the subject moves
across the field, the IR image will move into the field of view on one
facet, then out of field, then into field of the next, and so on. The
effect is an IR image that appears to the sensor as on-off-on-off, etc (ie
a square wave). The derivative of that is a series of spikes that trigger
the door opener.

To measure temperature with the same system, the standard approach for
years was to put a mechanical shutter in front of the sensing element.
Something that would alternatively expose and hide the sensor quite fast
compared to the droop rate of the voltage. By measuring the voltages in
both the covered and uncovered state, you can determine the amount of heat
reaching the sensor from external sources.

It took me ages to find out how this was done without any moving parts, but
it's pretty obvious now.
You can put the sensing element into an oscillator, much like a quartz
crystal except that the frequency will vary with the amount of IR
radiation. Then you have something to measure. Simple really.

Steve.

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1997\11\16@193417 by Sean Breheny

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At 05:30 PM 11/16/97 +1300, you wrote:
{Quote hidden}

How do they measure temperature, though? It seems to me that the
temperature would govern the wavelength directly, not the intensity of the
IR radiation. The intensity is related to the quantity of radiant heat,
which would be a function of the surface that it was being radiated from,
etc. So how does one get temperature from measuring the magnitude of the
voltage generated on the sensor? It seems to me that this is like saying
that you can sense color by measuring the amount of voltage generated by a
solar cell.

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
Web Page Under Construction!
.....shb7KILLspamspam@spam@cornell.edu

1997\11\17@045413 by Steve Baldwin

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face
> How do they measure temperature, though? It seems to me that the
> temperature would govern the wavelength directly, not the intensity of
the
> IR radiation. The intensity is related to the quantity of radiant heat,
> which would be a function of the surface that it was being radiated from,
> etc. So how does one get temperature from measuring the magnitude of the
> voltage generated on the sensor? It seems to me that this is like saying
> that you can sense color by measuring the amount of voltage generated by
a
> solar cell.

The pyroelectric sensor doesn't actually measure the wavelength. The
voltage it produces is proportional to the amount of (IR) light that falls
on it. It just happens that its response is fairly flat between 1 and 20
microns. (A silicon photodiode response is < 1 micron)
The radiation isn't a single wavelength for a particular temperature, but a
broad emission with a peak value that moves acording to the temperature.
The radiant intensity also increases and is more pronounced than the shift
in peak wavelength. If you heat a piece of metal, it is brighter when it is
white hot, than it was when it was red hot.

 |____________________
 |    __________________\ 400
 |   /     ________________\ 300
 | /      /   _______________\ 200
 |/___/__/________________\___
A     B  C     D    E              F

In the graph above, the y axis is radiant energy, the x axis is wavelength.
Each of the lines represents the radiation at a particular temperature
(200K, 300K, 400K). These should really look more like skewed, inverted
parabolas, so please think of them as curves. At 200K, the light emitted is
between wavelengths C and F. So the midpoint (peak) is at about E.
At 400K, the intensity is higher and the emission is between A and F, with
a midpoint at D. So as the temperature rises, the peak intensity gets
higher, the bandwidth increases and the peak moves to the left.
This is Planck's Radiation Law. I'm afraid (I think) I know enough to
understand it, but probably not enough to explain it very clearly
If the sensor has a flat response, the bandwidth and peak movement have no
effect and the temperature can be derived from the intensity.

You do have to compensate for the surface that is doing the emitting. The
curves of temperature vs. peak wavelength and radiated energy are for a
theoretical "black body". To compensate for the material, a fiddle factor
called the "emissivity" must be applied.

Steve.

1997\11\17@223314 by Ram Krishnan

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If the pyro sensor is sensitive to intensity & not wavelength, two sources
at different temperatures , placed at different distances - if the hotter
one is more distant, could produce the same total flux at the sensor,
causing the same output. I think two sensors are needed with bandpass
filters in front of each one, and the ratio of the two outputs would bear a
relationship to the black body temperature. Any thoughts ?

1997\11\18@051906 by Steve Baldwin

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face
> If the pyro sensor is sensitive to intensity & not wavelength, two
sources
> at different temperatures , placed at different distances - if the hotter
> one is more distant, could produce the same total flux at the sensor,
> causing the same output. I think two sensors are needed with bandpass
> filters in front of each one, and the ratio of the two outputs would bear
a
> relationship to the black body temperature. Any thoughts ?

Good point. I'm not really sure of the answer. The sensors do come with
various bandwidth limiting filters but they are still quite broad. The more
expensive and thermal imaging types do use a differential method. But they
also have (often cooled) narrow band sensors and a restricted temperature
measurement range
The field of view is a specified parameter and the measurement is the
average over the field of view.
Hmmm. If you take a step back, the intensity drops with the square of the
distance but the emitting surface in the field of view has gone up by the
square of the distance. So they cancel. Is that right ?
This is a bit of a leap for my poor old brain. From 2k of ROM to the laws
of the universe. Must be time for bed. :-)

Steve.

1997\11\18@080232 by Sean Breheny

face picon face
At 11:22 PM 11/18/97 +1300, you wrote:
{Quote hidden}

Come to think of it, I think I have heard of remote temp. sensors using
laser beams. Maybe they use a red laser beam to measure distance and
reflectivity (surface nature) and then use these factors to correct the
reading from the pyro sensor.

About the inverse square law canceling with the increased field of view,
this is only true if the object that you are looking at takes up more than
your entire field of view! Otherwise, clearly, if by stepping back farther
from the object, other things which are emitting less IR start entering
your field of view, the intensity is going to decrease. In fact, as you get
farther away, the source becomes more like a point, and therefore, obeys
the inverse square law morr closely as you get farther away.

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
Web Page Under Construction!
shb7spamKILLspamcornell.edu

1997\11\18@154932 by Steve Baldwin

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> Come to think of it, I think I have heard of remote temp. sensors using
> laser beams. Maybe they use a red laser beam to measure distance and
> reflectivity (surface nature) and then use these factors to correct the
> reading from the pyro sensor.

The laser beam is simply a pointing tool.
Since the device is averaging the field of view, a design goal is to keep
the field as narrow as possible. So that makes it important to point it in
the right direction. If you miss, you get the wrong answer.
Also, the reflectivity for visible light isn't the same as the emissivity
in IR. Even the emissivity for a particular surface may not be constant
over a wide temperature range. One of those things where the real world
spoils a good  law of physics.

Steve.

1997\11\21@185523 by Pierce Nichols

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On Tue, 18 Nov 1997, Sean Breheny wrote:
>
> Come to think of it, I think I have heard of remote temp. sensors using
> laser beams. Maybe they use a red laser beam to measure distance and
> reflectivity (surface nature) and then use these factors to correct the
> reading from the pyro sensor.

       If you have a laser system that good, why not use Thompson
scattering to measure the temp?

       -pn

"I have a work order for the immediate demolition of your reality tunnel."

       -Bob, RAW Construction Corp.

--Begin Geek Code Block--|------Begin Goth Code Block------
v3.12                    |v3.1
GAT[GCS,GS] d-@ s+: !a   |GoCS2)$PS3$ TJtAn PGL B10Bk!
c++@>$ u+>+++>$ P+>+     |CDbrp8S V6s M4 ZGoPuExgExtClMe
L+>++ E+>+ W+ N++ !o K+  |C7ome a20 n5F b56 h180 g6T??95F/A
W-- O- M- V-- PS+ PE+ Y+ |m1Ea@Z3? w6TA V3 r6EP p54565Rd
PGP t+@ 5+ X++ R+[GURPS] |D37 h6 sM9P SsYy k6Z N??91OE
[DARKSUN] !tv b++>+ D I+ |RfsS Lus9
!D G++ e>+++++ h r% y++  |-------End Goth Code Block-------
---End Geek Code Block---|
------------Begin Magick Code Block, v0.92------------
M[WITCH] S* W+ N+ P[CE][GR][OT]++ Dr a- C++ G !Q 666 Y
---------------End Magick Code Block------------------

1997\11\21@202331 by Sean Breheny

face picon face
At 06:53 PM 11/21/97 -0500, you wrote:
>On Tue, 18 Nov 1997, Sean Breheny wrote:
>>
>> Come to think of it, I think I have heard of remote temp. sensors using
>> laser beams. Maybe they use a red laser beam to measure distance and
>> reflectivity (surface nature) and then use these factors to correct the
>> reading from the pyro sensor.
>
>        If you have a laser system that good, why not use Thompson
>scattering to measure the temp?
>

Please excuse my ignorance, but what is Thompson Scattering?

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
Web Page Under Construction!
.....shb7KILLspamspam.....cornell.edu

1997\11\22@112519 by Pierce Nichols

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face
On Fri, 21 Nov 1997, Sean Breheny wrote:
>
> Please excuse my ignorance, but what is Thompson Scattering?

       It's usually used to measure the temperature of plasmas; you run a
laser beam through the plasma to be measured, and measure the doppler
shifts caused by scattering off of moving particles. I'm not actually sure
whether it would work for a solid surface.

       Pierce

"I have a work order for the immediate demolition of your reality tunnel."

       -Bob, RAW Construction Corp.

--Begin Geek Code Block--|------Begin Goth Code Block------
v3.12                    |v3.1
GAT[GCS,GS] d-@ s+: !a   |GoCS2)$PS3$ TJtAn PGL B10Bk!
c++@>$ u+>+++>$ P+>+     |CDbrp8S V6s M4 ZGoPuExgExtClMe
L+>++ E+>+ W+ N++ !o K+  |C7ome a20 n5F b56 h180 g6T??95F/A
W-- O- M- V-- PS+ PE+ Y+ |m1Ea@Z3? w6TA V3 r6EP p54565Rd
PGP t+@ 5+ X++ R+[GURPS] |D37 h6 sM9P SsYy k6Z N??91OE
[DARKSUN] !tv b++>+ D I+ |RfsS Lus9
!D G++ e>+++++ h r% y++  |-------End Goth Code Block-------
---End Geek Code Block---|
------------Begin Magick Code Block, v0.92------------
M[WITCH] S* W+ N+ P[CE][GR][OT]++ Dr a- C++ G !Q 666 Y
---------------End Magick Code Block------------------

1997\11\22@153103 by Steve Smith

picon face
In a message dated 22/11/97  03:33:42, sombody write:

<<  If you have a laser system that good, why not use Thompson
>scattering to measure the temp?
>

Please excuse my ignorance, but what is Thompson Scattering?
 >>
If you put it in your ear it must be a 'Bable Fish' !

Of coures you havent got one cause else you would know all about Thompson
Scattering.

There is a fuller explanition in a very good reference book..
Hitchikers Guide to the Galaxy

Cheers Steve.....

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