How much lumen difference to be meaningful?

Sorry for the awkward phrasing of the question. This topic may have been covered before, but I'm wondering:

1. What lumen multiple and
2. What lux multiple is noticeable to the average eye and
3. What lumen multiple and
4. What lux multiple is meaningful for a flashlight?

In other words, can the average person distinguish between 65 and 100 lumens, for example? Even if he/she can, is it a meaningful difference between two flashlights? I ask the same question about candela/lux.

The answer would sound something like, "People generally can discriminate a 1.5x (50%) increase in lumens, but it would probably take a 2x (100%) increase to be useful/worthwhile."

I was playing (I mean conducting empirical testing /ubbthreads/images/graemlins/blush.gif) with my E2e and C2 the other night. By every simple method I could devise, the E2e was brighter, whiter and more "lumenous" than the C2 to my eye. According to Quickbeam's charts, the C2 should throw further and produce more light. I even switched batteries and got the same result.

In a store today, I compared an L2 to an L4 and could not see any significant difference.

It depends on whether you are comparing 2 different brightnesses of one light, or two lights of different brightnesses side by side.

I've read that some pilots (good eyes) can discern a 2% difference in brightness -- but that's only between two light sources side by side.

You should be able to see a little more with a little more light, but that is a completely different thing than being able to conciously notice the difference. That's why people by Ultra Clear Lenses. No ones's going to notice the extra ~10% of light that gets out through the UCL, but they figure that they'll still be able to see more with the increased output.

I've read a few times that output must double (from one light) to be noticeable, which is probably why the most meaningful run time tests go to 50% brightness.

Lightraven,

In my experience, side by side, you can notice a 20-30 percent difference in total output (lumens), if comparing like to like (i.e. incan to incan, with similar beam profiles), but a 50 percent increase is needed to make the difference clear to the average, non-highly motivated viewer.

Lux (intensity/brightness) is another story, and I don't the answer.

Get out some standard lightbulbs and try 40,60,75,100 watts and see if you can easily tell the difference. The lumens should follow the wattage in proportion. I myself can tell the difference between 60 and 75 watts but not as easily as 40 to 60 or 75 to 100 watts. So by my observations 25% is discernable (60-75) 33% is more discernable (75-100) and 50% is even more noticeable (40-60).

For household bulbs, watts do not follow lumens.

For example, a 75W bulb puts out about 1100 lumens, whereas a 100W bulb puts out 1700 lumens. The difference in wattage (33%) does NOT track the difference in output (54%).

A 60W bulb puts out about 800 lumens, so 60->75 may be a 25% difference in wattage, but it's a 37% difference in output.

A 40W bulb puts out about 300-400 lumens. If we take the high side, then 40->60 may be 50% increase in power, but it's a 100% increase in lumens, so it's quite easy to see the difference in that case.

So you really need to know the actual lumens output to compare.

Slightly off topic - calculating the efficiency from above, you can see that it's insane to use 40W bulbs for general lighting - considering you can get the same output from a 60W bulb as two 40W bulbs, for less energy. Likewise, you can replace 2 60W bulbs with a single 100W bulb, again, saving energy. So in the end, a single 100W bulb can replace 4 40W bulbs for the same light output, but using only 62% of the power.

This is how I tell the difference:
I got into Luxeon LEDs for use as a bicycle helmet mounted light, I use minimags as the body. There have been three different "sandwiches" in my minimag over the years and I calculate output by how fast I can confortable ride at night.

N?? 500mA NX05 optics 11 MPH
Q3L 416mA NX05 optics 13 MPH
R2H 500mA Fraen LP optics 17 MPH

Part of the boost for the R2H is the Fraen LP optic but there is a considerable difference between riding at 11MPH and 17MPH!

How I calculate safe riding speeds is I pick out a painted line on the street at XX MPH and completely stop by the time I hit the line. Consider it takes longer to stop going 17 MPH than 11MPH and that is the difference between the two.

I calculate the following for my three lights:
N bin 18 lumens
Q bin 32 lumens
R bin 55 lumens

My dream helmet light is the size of a minimag with 27mm reflector sized head. Give me 100 lumens outta two 2500mAH NiMH AA batteries and I will be happy! I think by 2006 I should get there. Hope that helps.

Let me add something-
a camera is a lot more sensitive to variations in light intensity than your eyes, meaning put 2 lights side by side and take a beamshot photo. The camera will detect differences in intensity that your eyes can't see as easily. Where this will show up the most is if you have 2 lights with a lot of difference in intensity. Your eyes can see the spots from both lights but your camera might not be able to record the dimmer one.

Thanks for all the replies. I think I am trying to talk myself out of buying another flashlight that I really don't need. For me, there isn't much point to buying a flashlight (especially the more expensive ones) unless it is a reasonably significant upgrade to what I already have.

I'm the sort of person who drinks tap water instead of buying bottled water cause I can't see a difference. /ubbthreads/images/graemlins/smile.gif

off topic - about the water comment - it is what you can't see that we kill you... Especially in communities with karstic geology and in old houses with rusty pipes. Also all those fancy water filters do not kill some of those nasty critters....

With that being said, drinking tap water is still cheaper than buying flashlights /ubbthreads/images/graemlins/wink.gif at least for now.....