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When selecting LEDs for use as photodiodes, are there general guidelines that one can use to increase the odds that a more suitable LED is selected?

For example:

  1. Does diffuse or clear impact efficiency as a photodiode?
  2. Does a brighter or dimmer LED tend to correlate to better performance as a photodiode?
  3. Can the output spectrum of the LED be directly taken as the sensitivity spectrum of the photodiode?
  4. Does the LED output field of view correlate with the field of view when used as a photodiode?
  5. Are there any characteristics of an LED that would tend to correlate towards larger die-size for increased sensitivity when used as a photodiode?
  6. Would a monochrome LED with a transparent, untinted, clear resin package be sensitive to only its output spectrum when used as a photodiode? Or would it be different?
  7. Would a monochrome LED with a tinted resin package only be sensitive to its output spectrum when used as a photodiode? Or would it also be sensitive to near-IR that can easily pass through the resin?
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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Voltage Spike
    Mar 19 at 21:24
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If you want a color selective photodiode, using LEDs perhaps isn't the best approach.

Spectrum, as discussed in the comments, is different, added to that a wavelength transmission of the chip encapsulant, as well as the primary optic. On top of that, the primary optic doesn't have AR coating, which can start to mess things up.

Then you have temperature dependencies, silicon impurities, and all sorts of effects that will introduce unknown characteristics besides the basic physics of the junction. You may spend time to find an LED that works for your need, but since they are not designed to work in this mode, the manufacturer can at any time modify the characteristics, while still operating as the datasheet, it may change the behavior as a photodiode.

On top of that, the LED chip-making process is more like a kitchen recipe than a set process. They will be differences between wafers, batches, on the silicon level, but also encapsulant, optics, and so forth.

Using a device in a way it wasn't meant to be used is always a bit risky. Then also it depends if you intend to make 1 device or 5'000.

The correct approach rather would be to use a photodiode and to place optical passband filters on top. Those are quite cheap, can be tuned to specific wavelengths, have known cut-off and thermal behavior and you will have a predictable, and stable result.

enter image description here enter image description here


EDIT

More detail on filters:

Optical Filters can be made by many different means:

  • Using material properties like different types of glass or polymers that all have specific spectral transmission properties.
  • Chemically colored glass or polymers.
  • Interferential by deposition of thin-film layers.

It is possible to create a filter by just stacking a few different polymer or glass materials to achieve the desired spectral sensitivity on the photodiode.

Example of colored glass form this Japanese company enter image description here

More detail on low-cost filters:

When one searches for optical filters, filters from Thorlabs and such tends to be the only results google provides. Thorlabs and the likes make high-grade optics that are used for research, military, and special scientific devices, and the cost is high.

However, there are many companies providing much lower cost optics like Schott in Switzerland, or many many companies in China. The optics may not be as high grade as Thorlabs, but they are way-way sufficient for this type of application.

enter image description here

For instance, we had a filter custom made which is RG1000 with additional interferential layers and we pay a few $ a piece for a 20x20mm filter.

The cost is directly dependent on the size, for a photodiode a 4x4mm filter is probably enough.

One may not forget that interferential filters are everywhere, at a low cost. Most camera lenses have one. Another example is the reflector cups of the old Halogen Lamps, which are glass coated with an interference filter called Dichroic, letting the IR through to reduce heat.

Also, a simple colored piece of plastic, like a paper holder, can work.

enter image description here

I worked in a company that was stacking pieces of different types of glass to achieve similar things.

Trade Show

An optics trade show is a great place to go to find a lot of suppliers.

Other Methods

Reflectors: Depending on the optical needs, it is also possible to use the reflectance, rather than the transmission of the material by placing the photo-diode facing a reflector or in an integration chamber. Then you can paint the surface or use metals and other materials' reflectance properties. You need to be careful of oxidation though.

Phosphorus: You can also use phosphor up or down-conversion as well, this is particularly useful if you are trying to detect outside the photodiode sensitivity region.

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    \$\begingroup\$ Love the pretty pictures, but you need to edit your post and give credit to the source of the images. \$\endgroup\$
    – K H
    Mar 19 at 10:46
  • \$\begingroup\$ -1 because mainly because, unless I'm looking at different filters from Edmund and Thorlabs, you say they are cheap but those filters are priced at $30-$50USD for 12mm diameter, unless you know somewhere where you can get them for 1/10th the price. Otherwise I would have pushed down this route a lot harder long ago. Even then my understanding is that they are very bulky Not mentioned in the OP, but this this ultimately might go on a little UAV so anything resembling an optic bench setup with lots of doodads won't work. \$\endgroup\$
    – DKNguyen
    Mar 19 at 17:38
  • \$\begingroup\$ I'm starting to think maybe I should just get some transluscent acrylic rod and cap a narrow FOV silicon photodiode with it. The problem is machining the interior it to fit and machining the exterior so it doesn't mess with the FOV (which would be nice if was just a flat surface so I can just keep the rod as the cylinder it is, but my experience with optics is not quite enough that I cannot rule it needing to be spherical. And to keep a good finish when boring or reaming the interior. \$\endgroup\$
    – DKNguyen
    Mar 19 at 18:28
  • \$\begingroup\$ @DKNguyen If you're trying to figure out how to drill the end of the rod to fit the LED, it might be easier to use a bare LED or dedome/cut the dome flat. The rod would make me think you want directionality but need guidance, the sphere would make me think you want as broad a FOV as possible, although it would still be far from omnidirectional. \$\endgroup\$
    – K H
    Mar 20 at 2:09
  • \$\begingroup\$ @KH True, I did not specify low cost, however the answer says they are quite cheap and then shows a picture of glass filters when they are anything but. I need a narrow FOV. I was thinking that maybe a enclosing it in a sphere would preserve the effects of the original LED's resin lens similar to how if you want to view things clearly you stick yourself inside a dome, although if you only care about view out one side then a flat window works too) that's why I am unsure. \$\endgroup\$
    – DKNguyen
    Mar 20 at 2:11
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I can answer some of the questions that have to do with optics.

Does diffuse or clear impact efficiency as a photodiode?

Yes. Diffusion encourages random scattering, absorbing some extra light in the process, so it's less efficient by nature, but also the path of light is non-reversible, meaning light trying to pass the other direction into the diode will also be scattered and some of it lost without being absorbed.

Does a brighter or dimmer LED tend to correlate to better performance as a photodiode?

I don't know, but it would be easy to test.

Can the output spectrum of the LED be directly taken as the sensitivity spectrum of the photodiode?

The output spectrum is caused by the energy level of photons emitted as charge carriers change energy levels across a temperature dependent but still relatively stable bandgap. Working in the other direction, logically any radiation that can be absorbed and has a sufficient energy level could excite charge carriers. When the LED was made, the lens would be chosen to pass the color that the LED is intended to emit, but it would only intentionally be made to filter other colors if they were an unavoidable emission of that LED, so you can't count on the light reaching the LED being the color you want to detect unless you filter it yourself. I can work out this much, but I lack the skills to guess how much the input and output spectrums might correlate.

Does the LED output field of view correlate with the field of view when used as a photodiode?

Yes for simple nondiffusive convex lenses like LED domes, but not necessarily for more complex lens geometries like fresnel, etc. Implication being the right type of additional optic could help your sensitivity at the cost of being less directional or make your sensor more directional at the cost of being less sensitive.

Would a monochrome LED with a transparent, untinted, clear resin package be sensitive to only its output spectrum when used as a photodiode? Or would it be different?

This seems to indicate as I mentioned above that any photon with sufficient energy can raise an electron into the conduction band so long as the semiconductor is sensitive to that frequency and it doesn't get filtered out.

Would a monochrome LED with a tinted resin package only be sensitive to its output spectrum when used as a photodiode? Or would it also be sensitive to near-IR that can easily pass through the resin?

Tinting could indicate intentional filtering like the tinting you see on an IR photodiode, or it could be an irrelevant side effect of a material chosen to pass some other color of light.

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I've done this a few times. The sensor needs a smaller bandgap than the emitter (with a little margin as there's some sensitivity around the band edge), so a red LED will be sensitive to red, yellow, green, blue, and white, but not IR. (Qs 3 (no), 6 (different), 7 (the semiconductor probably won't be sensitive to IR unless it's an IR LED)). If the emitter is at too short a wavelength, its light won't get to the junction of the sensor very well, but in practice I haven't found this to be a major issue (though I haven't tried deep blue into a red or near-IR diode).

I'm currently using a 5mm yellow LED (bright, but an indicator rather than for illumination) to pick up light from a high-power blue one. It has a yellow package, which helps me here by attenuating the blue that would otherwise be too bright; It might hinder you. The transmission of the packaging materials is rather broad; I wouldn't trust it for actual selectivity. Previously I've used a junk old red indicator to pick up green and white LEDs to test jitter on a dual pulse-generating circuit with a single-channel scope.

When you ask about brighter/dimmer (Q 2): more efficient emitters should be more efficient sensors too, but the really bright ones that emit over a large area will only help you if you illuminate that area (Q 5) - otherwise they'll just give you more sensitivity to stray light, more noise, and slower response. In a related way I'm working with mid-IR photodiodes. One is big, slow, and sensitive because I fill it. The other is smaller, faster, and gives next to no signal because it's overfilled for my current project. With an LED you have the added complexity of a lens you can't easily remove.

The angular sensitivity (Q 4) should be a decent match to the emission angle, and one advantage of an LED over a photodiode is that you can power the LED to indicate where it will pick up from. In terms of diffusing (Q 1), it depends what you want to do. For a fixed source with a predictable output pattern, diffusing should be worse unless your optical arrangement is awful. For a moving source, sensing ambient light levels, etc., diffusing will widen your sensitive area, increasing pickup around the edges.

Personally, for most visible applications, I'd start with a high-efficiency water-clear 5mm red, with a narrow emission angle; this would have next to no spectral selectivity within the visible range (though it would have varying efficiency).

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    \$\begingroup\$ Just trivia, but removing the lenses from many LED types isn't prohibitively difficult. It's called dedoming in hobby circles and it's done to increase the range of "thrower" flashlights. \$\endgroup\$
    – K H
    Mar 19 at 22:43
  • \$\begingroup\$ @KH, I've taken a few off - heat will often allow you to lift the lens off high power models, and I've got access to optical polishing equipment so can flatten the lenses on conventional packages, though in that case you have to stop before reaching the wire bond. \$\endgroup\$
    – Chris H
    Mar 20 at 8:40

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