Maker Pro
Maker Pro

OPT101 as simple pyrometer

S

scot

Jan 1, 1970
0
This is a newbie question so have at it. Is it possible to use a OPT101 photodiode w/ amplifier in a "simple" optical setup as a low cost pyrometer? This would not be for general use but a specialized application and I would do calibration for a specific material at a specific distance and a rangeof temperatures. By calibration I mean comparison with a thermocouple embedded in the material (temp range of ~600-1000 C). I don't have much experience in analog electronics so I would like to know how hard it would be toset this up and what to look out for. I should also mention I have an oldpyrometer housing with intact optics and a dichroic beam splitter which allows visible wavelengths to pass for sighting but IR reflected into a sensor (missing). Let me know if I am getting in over my head.
 
Trying to use a silicon PD for that temperature range is difficult,

because there's so much more long-wave IR than there is short-wave. The

usual rule of thumb is that a dark surface just begins to have a visible

red glow at 900 F (~500 C).



The most generally useful black body function for this sort of job is

n_lambda(T, lambda), the number of photons emitted per square metre per

second per unit wavelength. It is given by



n_lambda(T, lambda) = 2c/(lambda**4*(exp(h*c/lambda/k/T)-1)).



Integrating that over lambda between 0.3e-6 and 1.1e-6 (the silicon

band, being generous) and over the flat surface of the 0.09 inch square

photodiode (1.25e-7 sq m) gets you the following amount of photocurrent:



T(C) Iphoto(300-1100 nm)

(hemisphere) f/8 aperture

---------------------------------------------

500 8.4e-08 0.33 nA

550 2.5e-07 0.99 nA

600 6.7e-07 2.6 nA

650 1.6e-06 6.2 nA

700 3.5e-06 14 nA

750 7.2e-06 29 nA

800 1.4e-05 54 nA

850 2.5e-05 98 nA

900 4.4e-05 170 nA

950 7.2e-05 0.28 uA

1000 1.2e-04 0.45 uA



Thing is, you can't use anything like a whole hemisphere's worth, or the

apparatus will melt very rapidly, just as though it were attached to the

hot object. So assuming that you can actually manage an aperture of f/8

(0.012 steradian vs pi steradians for the hemisphere), you get the third

column's worth of photocurrent. This of course neglects the visible

light filter and other optical inefficiencies, which will reduce these

numbers some.



Your detector has a built-in 1M feedback resistor, so you're pretty well

down in the mud at the lower end of your range, and will have problems

with ambient light. A chopper wheel will help a lot with the DC drift,

but not with the ambient light, because that'll get chopped as well.



A nice black bolometer (thermistor) will do a better job in this range,

because almost all the photons are outside the detector's range.

Pyrometry isn't that easy to do well.



Cheers



Phil Hobbs



--

Dr Philip C D Hobbs

Principal Consultant

ElectroOptical Innovations LLC

Optics, Electro-optics, Photonics, Analog Electronics



160 North State Road #203

Briarcliff Manor NY 10510

845-480-2058



hobbs at electrooptical dot net

http://electrooptical.net

Isn't there such a thing as correlating a temperature of an object for working practical purposes ( such as flue gas ) with IR emissions over a small sub-band?
 
S

Scot

Jan 1, 1970
0
Trying to use a silicon PD for that temperature range is difficult,
because there's so much more long-wave IR than there is short-wave. The
usual rule of thumb is that a dark surface just begins to have a visible
red glow at 900 F (~500 C).

The most generally useful black body function for this sort of job is
n_lambda(T, lambda), the number of photons emitted per square metre per
second per unit wavelength. It is given by

n_lambda(T, lambda) = 2c/(lambda**4*(exp(h*c/lambda/k/T)-1)).

Integrating that over lambda between 0.3e-6 and 1.1e-6 (the silicon
band, being generous) and over the flat surface of the 0.09 inch square
photodiode (1.25e-7 sq m) gets you the following amount of photocurrent:

T(C) Iphoto(300-1100 nm)
(hemisphere) f/8 aperture
---------------------------------------------
500 8.4e-08 0.33 nA
550 2.5e-07 0.99 nA
600 6.7e-07 2.6 nA
650 1.6e-06 6.2 nA
700 3.5e-06 14 nA
750 7.2e-06 29 nA
800 1.4e-05 54 nA
850 2.5e-05 98 nA
900 4.4e-05 170 nA
950 7.2e-05 0.28 uA
1000 1.2e-04 0.45 uA

Thing is, you can't use anything like a whole hemisphere's worth, or the
apparatus will melt very rapidly, just as though it were attached to the
hot object. So assuming that you can actually manage an aperture of f/8
(0.012 steradian vs pi steradians for the hemisphere), you get the third
column's worth of photocurrent. This of course neglects the visible
light filter and other optical inefficiencies, which will reduce these
numbers some.

Your detector has a built-in 1M feedback resistor, so you're pretty well
down in the mud at the lower end of your range, and will have problems
with ambient light. A chopper wheel will help a lot with the DC drift,
but not with the ambient light, because that'll get chopped as well.

A nice black bolometer (thermistor) will do a better job in this range,
because almost all the photons are outside the detector's range.
Pyrometry isn't that easy to do well.

Cheers

Phil Hobbs

Thanks Phil! I had not considered the problem with ambient light. It
sounds like this OPT101 will not be appropriate for my application. I
was also looking at a narrow band photodiode such as the ODD-95W from
Optodiode Corp. but it sounds like that will have a similar issue of low
signal. My personal experience with a Raytek single color pyrometer
(which used a silicon photodiode) a couple years ago was that it would
only register a signal above ~650 C and that was sighting into a
graphite cavity which has an emissivity close to 1.

I have considered the use of a thermopile or some kind of broadband
radiation detector but wouldn't I have to use a lens transparent deep
into the IR? Perhaps a CaF2 lens? I believe the lens in my pyrometer
housing is BK7 or something similar which only transmits to ~2-3 micron.

BTW, I love your book. I've only read bits and pieces but it is rare to
find anything written from an experimentalist point of view these days.
My background isn't optics but I still appreciate it.

Scot
 
Top