Just for fun, with all this talk about flashlights, minimum lumens, etc., I decided to flip this question on its head. (See how I spend a vacation day?:))

My question is what's the minimum number of lumens the human eye can actually see? That's the flashlight I want!

We know the human eye can reliably respond to an input of 90 photons arriving irregularly or a single photon arriving regularly, which we'll say reasonably corresponds to our definition of a flashlight. (D.A. Baylor, T.D. Lamb, K.W. Yau, "Response of retinal rods to single photons." Journal of Physiology, Lond. 288, 613-634. 1979.)

So, how to translate that to lumens?

Small numbers of photons will be detected scotopically by the rods in your eyes (as opposed to the cones), and each rod has a diameter of approximately 0.002mm, giving each an area of 0.0063mm^2.

Now, let's consider a 1 photon flashlight, sure to be coming to a store near you real soon now. (Can we get a Doug Ritter model???)

We'll suppose the photon has a frequency of 540nm, which would make it "greenish" on a larger scale.

Then, the rod would have an incidence of 2.39x10^13 photons per second per lumen. Working backwards, we have that a single rod can detect 4.18x10^-14 lumens, assuming the flashlight were outputing one photon/second.

And there ya go. So keep yer fancy 200 lumen lights! I'll be more than satisfied with my 0.00000000000004177 lumen model! (It probably runs for 100,000 years on a single battery. And if you're willing to use a radioactive battery, it can run for millions of years on a single "charge." Try that with your Surefire!)

Photons, aren’t those what they shoot each other with on Star Trek??

I forget what they were called but there are crystals that use a small radioactive source encased in phosphor doped glass. The radiation excites the phosphors and they emit a dim but visible light for a very long time.

Used to be the light source of choice for elite special forces types. The advantages where that the solid chunk of glass was pretty much indestructible and the half-life of the radioactive source was measured in decades. Useful output was said to be beyond the lifespan of an average human.

The problem was that these units were expensive and their output was minimal. With very good and young eyes you could read a large-print book without too much trouble. The light output was all-round and couldn't be turned off. To eliminate the light you stuffed it back into a pouch. With practice you could make your way down a path holding the light source in the palm and facing the palm away from you. There was just enough light to keep from stumbling over a log, getting clotheslined by a vine or stepping off a cliff. You could make out shapes at six or eight feet if your eyes were good.

Back in the 60s these units were a big deal. Having one was considered in some circles to be the mark of an elite operator. Those were when batteries were less efficient and bulbs used a lot of power. These units were supposed to be issued to people who had to go very long times between resupply. People who lived out of a pack for weeks, sometimes months.

They were never much more than marginal for doing anything. But with one of these you always had light and for some people a very dim light was better for not attracting attention.

Technology and times have changed. Batteries have improved a lot. LEDs produce light much more efficiently than the old incandescent bulbs. A couple of high capacity lithium batteries connected to a low-draw LED will run for months and produce far more light. Also people are far more leery about toting around radiation sources. Even ones that are encased in thick glass that acts as a shield.

I'm guessing you mean tritium tubes. They're inexpensive and easy to buy today, and companies even ship them through the mail.

I've read about a couple of people who've put them on key chains, but I prefer my Glo-Toob!

You can still get hold of Betalights here at;

http://www.penrithsurvival.com/penrith_survival/55/mia/d/betalight/pid/242045

They are expensive but last many years and require no batteries. The light output is minimal though i.e. just enough to read a map etc. I don't know if you can get them in the US as a 'normal' purchase. I think Lofty Wiseman recommends one for a PSK in his SAS Survival Handbook.

Yes, the rods and cones can detect a very, very small number of photons exciting it. However, the difference between a cell responding to a single photon, and the brain recognizing and processing that into a picture, is a world of difference. If nothing else, that 1 photon means that you're essentially blind in one eye.

The other problem, just practicality, of course, is that a flashlight that emitted such a small number of photons would be fairly useless because A) you can't gaurantee that the photons emitted would be reflected directly back to your eye, and B) even if they were, you'd only see the one thing it bounced off of. Meaning, you'd miss the tree in your way, only to walk off the cliff.

On the other hand... that's some pretty fancy math work!

Friend, this was not intended to be a particularly serious thread.

I just wanted to know the answer to the question and thought I'd share it. (Although the numbers are accurate, you'd probably want 10 photons/sec rather than at 1Hz for the stream to be consciously detectable. It's physiologically detectable at lower frequencies.)

Also, you'd have to shine this flashlight directly into your retina and hold it very, very still. Used as a regular flashlight, the chances of any of these photons actually bouncing off something and returning to your eye are roughly zero. (The odds are better that you'd be hit by every lightening strike ever.)

Much more could be written about why flashlights are inherently incredibly inefficient, but I don't want to run the risk of taking this thread too seriously myself...

Oh, I know it's meant in jest. As I said, I'm rather impressed with your math skills! that's about the only serious part.

I once woke up next to a woman and, in retrospect, I wish that (a) the light source had been only one Photon and (b) the minute amount of reflected light would not have come back to my visual receptors. Now that I'm much older and dependent on bifocals, I'm safe from that type of sunrise surprise. O^O

http://www.youtube.com/watch?v=XkCFwVe34IY