How "bright" is a lumen?

[KDM]

Re: Stupid question, but what is a lumen?

Hogo is right, this place is an enabler for lumen junkies. It's going to be hard for anyone to describe to you what only you can perceive. One beauty of having multiple output levels is that you can turn it up or down for your needs. They are also like Pringles, bet you can't stop at one. Good luck.

 

[LedTed]

I'd like to find a table that shows what lumens you would expect for various tasks.Bill

EXTECH has published the following list ...

Typical Light Levels

Type
Lux
Foot Candles

Emergency Stairs
20-75 (Lx)
2-7 (FC)

Egress Passages
75-150 (Lx)
7-15 (FC)

Packing Work
150-300 (Lx)
15-30 (FC)

Production Work
300-750 (Lx)
30-75 (FC)

Inspection Work
750-1,500 (Lx)
75-150 (FC)

Detailed Assembly
1,500-3,000 (Lx)
150-300 (FC)

 

[AnAppleSnail]

Lumens make it easy to sell flashlights. "100! 320! 680!" But they aren't helpful for seeing things. It's a long boring discussion, though:

"We see reflected photons, so we are really interested in the intensity of light on the object in question, compared to the intensity of light in the surrounding area, giving an acceptable contrast and signal to your retina to process an image."

So we sell flashlights by lumens. A lumen is:


Plenty to see in a dark room if you're young.
Enough to read a book in bed.
Not enough to see under your car in a lit parking lot
Enough to find dropped keys in a dark parking lot
Nothing at all compared to a streetlight
Plenty to walk on a very familiar sidewalk in total darkness
Not enough to walk through most forests

 

[magicstone12]

light one little candle in the dark ,this is one lumen,maybe

 

[StarHalo]

light one little candle in the dark ,this is one lumen,maybe

A candle flame averages 13 lumens, actually. Focused into a single direction, that's about the output of an incan MiniMag on worn batteries, hence the mirrored/lensed candle holders of old.

 

[RemcoM]

A lumen is not brightness, a lumen is output. Let's go with output per area (Photons per area, so to speak, accounting for human sensitivity). Given that 1 lux is 1 lumen per square meter, use this table:

Source from Wiki

120,000 lux	Brightest sunlight
110,000 lux	Bright sunlight
20,000 lux	Shade illuminated by entire clear blue sky, midday
10,000 - 25,000 lux	Typical overcast day, midday
<200 lux	Extreme of darkest storm clouds, midday
400 lux	Sunrise or sunset on a clear day (ambient illumination).
40 lux	Fully overcast, sunset/sunrise
<1 lux	Extreme of darkest storm clouds, sunset/rise

For comparison, nighttime illuminance levels are:

<1 lux	Moonlight[3]​
0.25 lux	Full Moon on a clear night[4]​[5]​
0.01 lux	Quarter Moon
0.002 lux	Starlight clear moonless night sky including airglow[4]​
0.0002 lux	Starlight clear moonless night sky excluding airglow[4]​
0.00014 lux	Venus at brightest[4]​
0.0001 lux	Starlight overcast moonless night sky[4]​

So if you have a 1 lumen light shining evenly on 1 square meter, then it's about like bright full-moon light. That same lumen spread over a 1cm square will look as bright as an overcast day.

Hi Anapplesnail, and others,

Can a flashlight with 350 kcd...klux, be more intence at 1 meter distance, than bright direct sunlight?

 

[leon2245]

How "bright" is a lumen?

Exactly 1/100 as bright as a Luman.

100 lumens = 1 Luman.

 

[AnAppleSnail]

Hi Anapplesnail, and others, Can a flashlight with 350 kcd...klux, be more intence at 1 meter distance, than bright direct sunlight?

Not many flashlights will. I've charred cardboard with DIY LED arrays touching that cardboard but it is difficult to do with a flashlight instead of a bare LED.


To double sunlight's intensity on 0.0001 m^2 (1cmx1cm) would require "only" 24 lumens (120000 lux / (0.01m x 0.01m) of photons. But to do this on a hand-sized thing would take 1200 lumens. The bezel of this 204 kCd light is about 48mm. That means we could expect about (204KCd / (0.024^2*Pi))/120kCd x sunlight, or 0.939x Sunlight on average.

On the other hand, a Cree XM-L at 10W puts out about 1000 lumen. If its 5mm diameter die is 3mm from cardboard, we have a contact area of roughly 10mm diameter. 1000 lumens / (0.004m^2*pi) = 20 million lux. That's 100 suns.

There are some optical laws of physics that mean the peak intensity of a source is always right at the source - Never further away. We use lenses and reflectors to get high intensity further from the flashlight.

High-end custom searchlight: 0.94 suns at the bezel, less everywhere further

Bare Cree XM-L: 100 suns at the LED dome.

Edit: line breaks, wording.

This is why bare LEDs can cause fingertip burns.