OPT101 as simple pyrometer

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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

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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

Most pyrometers, AFAIK, operate at long-wave IR -- 8 to 12 microns. Photo diodes and photo transistors operate at near-wave IR -- around 1 micron. They're not going to help you measure temperature much until you start looking at things that are almost hot enough to visibly glow.

I doubt that your dichroic splitter would slice the near IR off from the visible; it's not going to do you much good. You _might_ get this to work for you since you're operating at such high temperatures, but it's not going to work like a 'normal' pyrometer.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

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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?

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 range of 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 to set this up and what to look out for. I should also mention I have an old pyrometer 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.

Try these maybe?

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

pyrometer? This

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practical purposes ( such as flue gas ) with IR emissions over a small sub-band?

Sure thing, but you have to have enough signal, and you have to worry about stability. For instance, the filter in the old pyrometer will have some temperature coefficient of transmission vs lambda. It's sitting on the steep slope of the black body curve, so small variations get magnified.

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

01 photodiode w/ amplifier in a "simple" optical setup as a low cost pyrome= ter? =A0This 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 range of temperatures. =A0By calibration I mean comparison with a thermoc= ouple embedded in the material (temp range of ~600-1000 C). =A0I don't have= much experience in analog electronics so I would like to know how hard it = would be to set this up and what to look out for. =A0I should also mention = I have an old pyrometer housing with intact optics and a dichroic beam spli= tter which allows visible wavelengths to pass for sighting but IR reflected= into a sensor (missing). =A0Let me know if I am getting in over my head. .

omega is expensive stuff, doesn't seem to fit with hacking together something from a photodiode

something like a MLX90616 maybe, don't know where you can get them but digikey has the MLX90614 but it only goes to 380'C

-Lasse

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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

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If the cavity is very nearly closed, it matters much less what the surface emissivity is. You get an error of the order of (1-epsilon)*(escape probability per bounce), which if you have a lot of bounces, tends to go away pretty rapidly.

A germanium PD would probably help. For a one-off, you could can-open a surplus Ge transistor and use it as a phototransistor. (Somebody on SED suggested that recently.)

Thanks! Read it in good health!

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