IR Photodiode project

shane ren said:

Thanks very much John, never heard of that before. Sounds very interesting to experiment with. But like yu said earlier, I think what the lecturer wants to see is hard experimental work put into the project, so even if my project isnt able to perform the way I want it to, well then thats okay. As long as I can show meaningful ways in which I tried to overcome all these problems. By having the method which you just informed me about sounds mind blowing, I really think they would like to see would that benefit the sensor in anyway. I know photodiodes might not be the right tool for the job, but I am looking forward to investigating into these in further detail. Thank you very much to John and Dknguyen for all your help, really would not be half as comfortable as I am about the project now. I also have a nice few bits to look back on and to inseert into my project. Keep up the good work and once again THANK YOU.

Click to expand...

It will probably be of some effectiveness if you can get the target ridiculously hot. We're talking visibly, glowing red hot. Higher temperature = higher energy = shorter wavelengths which means at some very high temperature near-IR becomes more feasible. The minimum temperature where you would get anything even remotely useful would be when steel starts to get red hot. That's where the radiated thermal energy is entering the red spectrum near the visible range.

Or you could point it at the sun. According to jpanhalt's website, 3000-5200 Kelvin which is right where a photodiode would measure.

Ya that is an option, but trying to Heat something up to 300C within a classroom could be a slight hazard. Is there any way that photodiodes can detect radiation within the mid region? That’s the region which I need, the low region is pretty much for visible light only with no thermal radiation being given off,

shane ren said:

Ya that is an option, but trying to Heat something up to 300C within a classroom could be a slight hazard. Is there any way that photodiodes can detect radiation within the mid region? That’s the region which I need, the low region is pretty much for visible light only with no thermal radiation being given off,

Click to expand...

No. What makes a photodiode a photodiode is the physics of of the way it works. Photodiodes work by having photons knock electrons out of the orbits in the material they are made of. Long-IR photons do not have enough energy to knock electrons out of the orbit of of any known material. Short of a material completely unknown to science, it's not happening.

That's why we use photodiodes for short wavelengths and thermopiles for longer wavelengths, and an antenna for even longer wavelengths. They're all E-M waves, but we use very different methods to produce and detect them because of the limitations of the materials that exist. It's kind of like asking if you can use a light sensor to pick up a radio transmission.

Have you ever seen a laser lens made of salt? You can't get it wet. Why do we use such a crappy material? Because it's the best material available that actually passes the wavelengths of interest. That happens a lot in optics. We're super lucky that we have so many materials that are transparent to our eyes, not the least of which is whatever our corneas are made of.

shane ren said:

Ya that is an option, but trying to Heat something up to 300C within a classroom could be a slight hazard. Is there any way that photodiodes can detect radiation within the mid region? That’s the region which I need, the low region is pretty much for visible light only with no thermal radiation being given off,

Click to expand...

More like 3000°C. They won't work.

Haha I'd be warming up the whole college with that kind of heat, okay I think its time to start looking into thermopiles, would they be my best option for measuring IR radiation in the mid region?

I also need a thermopile with a low enough FOV seeing as there will only be a space of less than 10cm.

shane ren said:

I also need a thermopile with a low enough FOV seeing as there will only be a space of less than 10cm.

Click to expand...

A space in which direction? Between heat sources or between thermopile and heat source? The closer you are, the wider FOV you can get away with and still measure a small target area. It's pretty hard to find thermopiles with special traits like narrow FOV without special ordering in bulk. Search Mouser and Digikey. Most are ~50 degrees for the 50% half angle.

At Digikey and Mouser, this is the only one currently available with narrow view and it's not going to get much narrower than that:
https://www.digikey.ca/product-deta...nt-specialties/G-TPCO-019/223-1579-ND/5277315

https://ca.mouser.com/ProductDetail/Measurement-Specialties/G-TPCO-019?qs=/ha2pyFaduiAPF7NUNjJI/10AR4qMyLYjm9HEuqM7LicUJO4hVupxw==

A system I was involved with used a sensor similar to these: https://www.raytek-direct.com/stores/raytek-ci-series-compact-ir-sensors

It may have been a sensor with adjustable emissivity and laser sighting. I know I was asked to pick out a CCD image sensor.

In class today and I had to go over why I could not use photodiodes in order to measure the required temperatures which I need which happen to be within the mid IR region, I dont think they are impressed with me going ahead with the thermopile and not the photodiode hah. The peltier is split into 16 squares each with its own temperature. Is there any material which could help me on focusing my thermopile onto each of these squares?

You could use a lens that transmits mid-IR wavelengths to focus. Zinc selenide (ZnSe) is a common substance used for that, and it is insoluble in water. However, lenses can be made from other salts, such as NaCl (table salt) and KBr, but they are soluble in water and easily ruined. Go with ZnSe.

John

Okay so the ZnSe seem to be a bit too expensive for the college to pay, so its pending. At the moment all I can do is pick out some thermopile options, I definitely will not be able to use the TS105-10L5.5MM. The reason being someone else in the group has already chosen this and the lectures will not allow both. So now I have to determine what sensor will give me the best results at what distance away, and what fov. The peltier is 4cm by 4cm, so that leaves me with 16 regions of interest, all 1cm squared. I know it was mentioned earlier that a larger fov doesnt matter once you are close enough to the area of interest, but surely it would be hard to focus on that 1cm^2 region of interest without dipping into another region of interest. I really haven't done any optics since my first year of college which was over 4 years ago now.

I have used a tube (rolled paper or aluminum, i.e, a light tube) to "focus" or restrict the FOV of a near IR photodiode. I am not sure what the best material would be for 10 microns. Many things are effectively opaque at that wavelength, but that approach may allow you to sense only a small area (say 0.5 cm^2) and then step your sensor in 1-cm steps. Rather than Al, I might try something with a high thermal mass, such as ceramic or closed cell foam. Maybe try both (i.e., low and high thermal mass).

John

Is this an undergrad-type project or one where you have a larger budget? I mean, if you really want to push limits I suppose you could go with the pyroelectric sensor. I wouldn't try this unless it was the entire project though (and not just one small part of a larger project). If it's educational you might be able to negotiate a cheaper price with the supplier. If functionality and practicality is not the objective of the assignment but experimentality is, it could be worth it. You'll have lots of that with pyroelectrics.

The fact you need to chop or keep the target temperature changing in order for the pyroelectric sensor to register anything might lend itself to mapping if you can think of a novel way to chop the target heat source. That said, thermopile is still the most straightforward and practical approach by a large margin. The main reason anyone would go with a pyroelectric sensor is it's super response time to a thermopile, but most temperature applications don't heat up in the 100s or even 10s of Hz.

There would be a mechanical design element to this since you need a motor and chopper disc or something else to do the same thing. (no solid-state choppers exist yet as far as I am aware). Pyroelectric amplifiers are also tricky to manage noise due to the low signal level and high resistors used. I've never tried building one but this is what the materials said from when I was looking into it.

If you want to consider this, definitely start by reading app notes to see if you think you can pull it off at all and trying to source a pyroelectric sensor because you may not be able to reasonably get ahold of one.
https://www.infratec.eu/sensor-division/sensor-division-knowledge/application-notes.html

https://www.farnell.com/datasheets/...MIwP-untqH2QIVR5kbCh218QSGEAkYAyABEgKQgvD_BwE

What are your thoughts on this thermopile? Would it be suitable for my project?

shane ren said:

https://www.farnell.com/datasheets/...MIwP-untqH2QIVR5kbCh218QSGEAkYAyABEgKQgvD_BwE

What are your thoughts on this thermopile? Would it be suitable for my project?

Click to expand...

Thermopiles are fairly easy to pick. Most of the ones available are pretty much all the same except for FOV and time constant (dependent on the area of the die). Area is probably not going to matter too much for your application so don't worry about it. The last big differentiator is the lens but you're limited to what you can get without special order and all the ones you can commonly get measure temperature to varying degrees of selectivity so don't worry about that either. You would have to go out of your way and special order to get one with a very selective filter that isn't suited for your application. Just pick one you think works that you can get and and go with it. It won't be make or break things for you. Just make sure the datasheet tells you the output vs temperature of the thing.

Thanks ya maybe I am pondering too much. Would you agree with johns theory on trying to focus more on a particular FOV? If I was able to experimentally show that I was able to focus the fov onto the region of interest, im guessing I would get a lot of marks for it.

shane ren said:

Thanks ya maybe I am pondering too much. Would you agree with johns theory on trying to focus more on a particular FOV? If I was able to experimentally show that I was able to focus the fov onto the region of interest, im guessing I would get a lot of marks for it.

Click to expand...

I'm not sure tbh. I always think diffraction might come into play and do the reverse so I'm always wary about doing stuff like that. I think what John is really saying is get the thermopile up close and use blinders around it to stop adjacent areas from being picked up, rather than using a tube to act like a lens or anything like that so you can use the thermopile from far away like you could with one that actually has a narrow FOV. So you would still have to get up close. It's just sounds like extra insurance for when you do get up close to me.

It's easy enough to test though and remove if it doesn't work. Stick a tube on and hold a hot pin in and around the opening at different angles and distances. Or maybe you can actually find the math for a pinhole lens and plug in 8um or 10um into it to see if you can just make what you need. Or trial and error with a sheet of some IR opaque material with a bunch of different sized needles or micro drill bits and distances in front of the thermopile until you find one that works.

https://photo.stackexchange.com/questions/46489/how-to-calculate-the-optimal-pinhole-size

I get ~0.5mm for a focal length of 1cm when using c=1.414 at 10um. That seems doable.

How close can you get to the peltier?

I might have spoke too soon about needing to go out of your way to find a thermopile that won't work. You might have run by this thermopile on Mouser:
https://www.mouser.com/ds/2/418/NG_DS_TS418-1N426_A-1074562.pdf

Do not use it. It has a filter for 4um and is designed for CO2 detection. A lot closer to the 8-14um for measuring heat than an IR photodiode, but not what you want. Though nothing is stopping you from trying to see how well it works as long as you know what you're getting yourself into.

On your post about how to select the perfect diameter for the pinhole, des it matter about the distance on the pinhole to the inrared source? Or is it just the focal length between sensor and pinhole that matters?

https://www.mouser.com/ds/2/18/AAS-920-548A-Thermometrics-ZTP-101T-032114-web-850486.pdf

I think I am going to go ahead with this thermopile