How many photons in sunshine?

This is bound to get mixed up with things in Electronics, check both. Physics-specific stuff here, mostly.

How many photons in sunshine?

Postby Doug Coulter » Tue Apr 19, 2011 7:20 pm

This is a little exercise to help people with perspective and feel. I won't cover the whole world here, y'all chime in on related energies and sensitivities.

A modern phototube, at least a decent one, can count single photons. It's not even hard electronically with the gain they usually have. The problem is getting to dark, so anything they see is signal and tube noise, not signal+noise+leak.

So, for grins and to get in the right sort of scale in thinking, let's look at how many photons/second hit something in sunshine.

In my usual back of envelope style, I'll pick some "brown" numbers -- if you want to change them and refine your estimate, well, that's why I rounded them, to make it easier to do mentally.

Take a number for sunshine. 1Kw / m2 -- 1 kJ/second. This is roughly the number you'd get in low earth orbit. On the planet, it's about half that.

Lets look at a sq cm -- that might equate to some of a phototube face, or the area you're trying to seal light tight to make a detector (my current thinking).
That works out to 100 mW in full sun. So we want to know how many photons that is per second.

Well, I'm going to really be sloppy and assume our photons are about 1 eV. That's on the low side for visible light, but that's also where most of the sun's energy is at the ground. That means the spectrum is actually really sloped for photon count/wavelength, as there's less blue as a power, and for the same power, it takes less blue photons/second.

1 eV is 1.6 e-12 erg, one erg is e-7 joule. Therefore, one eV is 1.6e-19 joule.

We then take .1/1.6e-19 and we get 6.25 e17 photons per second from the sun, looking directly at it. Our light sources are usually dimmer, but this is just to get some perspective on things.

So, to make a detector light tight, all you need is order of e17 attenuation. Depending on how you're working your dBs (I'll use power, 10log()) then that's a "mere" 170 dB you need.

Ahem. That's not an easy number... Things you'd think of as being opaque aren't good in light of this. Carbon black, what makes most things black (spray paint, plastics, tires) is more or less translucent in the near IR...where TiO2 white pigment stops it -- but the binders in either case can leak light around these. You'd think a labyrinth might do it, such as a tight screw type plumbing fitting. My experience is, nope. Black tape -- a joke. Space blankets, nice to reflect 70+% of light back into something, yeah, but light tight? 30% transmission.

My first attempt at photon counting kinds of things used an old RCA 931B phototube -- has near IR sensitivity. That one took a couple weeks to finally get light tight enough to see its own noise for sure. I had a welded steel pipe with a sidearm (it was meant to go on my 10" telescope and look for pulsars), and brazed on pipe cap on one end, soldered on base on the other. I finally got the ends tight enough (no small thing, and divider chain in the dark so few wires had to come through) but that end, gheesh. I made a tight fitting tapered plug out of steel, and drove it in there hard - enough to expand the steel pipe a little. Still not tight...tape and Al foil, finally. It can be that hard.

The tubes we use for radiation stuff tend to be mostly blue sensitive, biAlkali stuff. That's way way easier, and simpler methods will get you there. Since the cathode has a higher work function, there are fewer dark electrons spontaneously coming off it at room temperature as well. Major factor in "can I make a nice detector and have it work well".
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
 
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