EEVBlog #869 - Counting LED Photons!

[westfw]

BTW, there was some fascinating data on the sensitivity of the human eye that I vaguely recall from a course I took in "vision" some years ago.
Um...  https://en.wikipedia.org/wiki/Absolute_threshold

[Fungus]

Yes, the Rigol is gaining free features, and free bugs too! (But what do you expect, it's only 400 dollars...... innit?)   

I wonder what would happen if somebody like Rigol made a scope without firmware (or just very basic firmware)

They could just publish the hardware specs and a dev. kit and let the programmers of the world go for it.

[Fungus]

Ah... no, not quite. The "Pluses" count is limited by the resolution of the screen data. 597 "Pluses" seems to be the maximum I can get it to display.

Looks like it's working from the screen data.

The DS1000Z seems to do that sort of thing a lot (eg. serial decoders).

I think there must be a limitation in the hardware where the main CPU doesn't have direct access to the captured data. The captured data is probably scaled/zoomed into a buffer using hardware and the main CPU has to work work with that. There's no direct path from CPU to captured data.

[Fungus]

Not every recombination of an electron and hole produces a photon, as there are competing non radiative mechanisms. Though some LEDs can be very good at that point. The more difficult part is getting the light out, before it is reabsorbed.

Good point.

How do they get the light out?

Is it just a case of making the PN junction as thin as possible?  Is that what determines LED efficiency?

[mux]

To a large extent - yes. Modern LEDs have extremely thin metallization on top of the bandgap area (or none at all, and just the bulk material - see illustration below), with the total active region only being a micron or so across. At that point the substrate is very transparent towards its surface. There are other cool things that are being done, like patterning the sapphire substrate to more efficiently reflect back stray photons, adding microlenses or micro-waveguides...



All this has led to ridiculous external quantum efficiencies nowadays. I've made a calculator a while ago that predicted a 75% LER (lighting efficiency, i.e. amount of light power out vs power in, meaning that a 10W LED at 75% LER only outpuyts 2.5W of heat). Including the phosphor losses, that would mean the underlying blue LED would need to essentially have nearly 100% quantum efficiency (about 91-93% calculated). Sure enough, when measuring over an integrating sphere this is actually true. If we'd only need lots of monochromatic blue light in our world, we would pretty much have the perfect light source already.

[ion]

If you want to gather data in the microamps, try using a blue LED.  The detector seems to be much less sensitive to those wavelenghts so it shouldn't saturate as quickly.

[Kleinstein]

Getting the light out there are several tricks, like having several layers with graded refractive index, patterns and multi layer structure in such a way that the lowest band-gap is in the junction region only. So outside the junction region the material is largely transparent and lower refractive index. As a downside a higher voltage is needed - one can actually see that difference: modern high efficiency leds usually need a higher voltage (at least for the same current).

With the white LEDs the phosphor layer is directly on top of the LED crystal. So the light does not need to get out of the solid to be absorbed from the phosphor. It's only the yellow light that needs to get out, but this is at a wavelength that is not absorbed very much anymore.

The observed pulse count should not saturate the detector itself - it's more like Dave's scope is saturated with to many pulses. So the pulse more might be able to detect short pulses, but maybe not more than 20000 per second.

[Someone]

The observed pulse count should not saturate the detector itself - it's more like Dave's scope is saturated with to many pulses. So the pulse more might be able to detect short pulses, but maybe not more than 20000 per second.

Well caught, how far does the memory depth reduce when using the peak detect mode on that scope? I didn't see the sample rate shown during the video. At a guess with 1Ms/s for the 1 second span I get the attached simulated pileup for that situation, which still wouldn't explain the magnitude of drop off seen in Daves data (where we might expect a few %).

*Edited with simulations including the overhead from the scopes measurement function needing to see two edges to make a count.

[Vgkid]

Another video that I enjoyed, thanks Dave.

[wolf32d]

The observed pulse count should not saturate the detector itself - it's more like Dave's scope is saturated with to many pulses. So the pulse more might be able to detect short pulses, but maybe not more than 20000 per second.

Well caught, how far does the memory depth reduce when using the peak detect mode on that scope? I didn't see the sample rate shown during the video. At a guess with 1Ms/s for the 1 second span I get the attached simulated pileup for that situation, which still wouldn't explain the magnitude of drop off seen in Daves data (where we might expect a few %).

*Edited with simulations including the overhead from the scopes measurement function needing to see two edges to make a count.

I couple of years ago I had to characterize a single photon avalanche diode for my optoelectronics thesis ... We directly read the voltage across a series 50 Ohm shunt (with the aid of a fast opamp). The current spikes had roughly a FWHM = 5 ns; we used a beefy LeCroy WaveMaster 8Zi-A scope to sample the signal! We could sample 250 ms @ 1GSa/s  , that bit of gear proved really useful

[vlad777]

Don't you have a photon counter?

[vlad777]

I have been thinking about this experiment in 2013.

http://www.electro-tech-online.com/threads/photon-by-photon.132888/

[max_torque]

For better LEDs, I remember that a high efficiency green one was visible at 100 nA -  nearly useful at 10 µA.

If you sit in a dark room for 30-40 minutes and fully dark adapt your eyes, you'll see it at much lower drive currents (having gained several orders of magnitude gain in your eyes).

If i sit in a dark room for 30-40 mins i won't see anything, because i'll be asleep........   

[German_EE]

Quantum physics and quantum mechanics are way beyond my abilities but I am still enjoying the video and the comments here. I now have new respect for that little white LED on the front of my monitor.

[mtdoc]

Cool video!

For better LEDs, I remember that a high efficiency green one was visible at 100 nA -  nearly useful at 10 µA.

I suspect that is due to the fact that the wavelength of green light is closest to the peak sensitivity of the rods in the human retina, which are more sensitive (fewer photons needed to excite) than cones.

As a point of reference, it only takes a single photon can activate a rod!  Though more than one rod needs to be activated in order for the sensation of a visual stimulus to be perceived - but still on the order of only 10 rods or so IIRC.

The power of evolution to produce a molecule (the rod photopigment rhodopsin) that can respond to one photon is impressive.

[Brumby]

The power of evolution to produce a molecule (the rod photopigment rhodopsin) that can respond to one photon is impressive.

I put the dynamic range up as a strong second.



The brain's image processing is something else.  That is simply awesome.

[Dubbie]

Thanks to all the thread contributors, especially OP. Really fascinating stuff. It's almost worth a follow up video with some whiteboard action!


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[f4eru]

Two remarks :
- The non linearity in the high range is probably from the sensor, or the scope for that matter, who both have a blank time directly after a pulse.
- The LED not emitting for 10 seconds at first is probably due to the capacity of the LED slowly charging when switching on...