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I want to know how much lux the sun emits on a bright day - I don't mean when one stares directly at the sun, but rather when one walks casually outside when the sun is shinning brightly.

Now the reason for my confusion is that Wikipedia states here: http://en.wikipedia.org/wiki/Lux

that "direct sunlight" is between 32,000 and 100,000 lux. I understood this to mean "staring at the sun directly" - is this correct?

The issue is that I've read other articles that state there are places that receive 40,000 lux of light.

Basically, is 32,000 - 100,000 when staring at the sun or not?

Qmechanic
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Jim Tim
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  • I don't understand the question. All the numbers seem consistent to me. Depending on where you are, conditions, season of the year, etc, the illuminance at the earth's surface when exposed directly to the sun will be 32,000 - 100,000 lux. Is the issue the word "direct"? Non-direct would mean outside on a sunny day, exposed to the sky, but not exposed to direct sunlight. – garyp Feb 27 '15 at 17:00
  • OK.. thanks! This is exactly where my confusion was.. In case your interested in knowing why I care so much it's because I'm trying to determine how much "lux of light" would be safe for the human eyes (for my own experiment) and my presumption is that if we can get 100K lux from the sun it would not be harmful to create this amount of light artificially.. – Jim Tim Feb 27 '15 at 17:06

4 Answers4

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32,000 - 100,000 lux is the typical range of illumination that the Sun provides. You don't have to look at the sun, you look at the world it illuminates. Lux is a "per unit area" quantity - not a "per solid angle" quantity. The variation in values mostly depends on the position of the sun in the sky - when it is low, there is significant scatter of sunlight (most noticeable around dawn/dusk when the sun turns red) which reduces the intensity of the illumination (see for example this earlier answer )

There are three closely related units of "brightness".

First, there is the candela - "the light of one candle". If you look at the light of a 1 cd source on a sphere that is 1 m radius (area $4\pi m^2$), it gives you $4\pi$ lumens. At the surface of that sphere, the intensity of light (per unit area) is 1 lux. If you make the sphere bigger, you will have the same number of lumens (lumina?), but the illuminance (lux, lumen/area) will be smaller.

For reference, a 100 W light bulb has an output of about 1600 lumen; if you wanted a "light as bright as the sun" you would need about 2 kW - and you would have the same illumination at 1 m distance.

Since the total power of sunlight per unit area is about 1 kW (round numbers), that is actually remarkably consistent (especially given the fact that a light bulb is cooler than sunlight and therefore emits less of its radiation in the visible part of the spectrum)

cdane
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Floris
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  • Thanks for the in depth explanation. So if I understood your answer correctly when wikpedia writes 32,000 - 100,000 they are NOT referring to staring at the sun, rather it is the amount of lux that we receive when "we look at the world" or walk casually.. correct? – Jim Tim Feb 27 '15 at 17:04
  • Yes that is correct. – Floris Feb 27 '15 at 17:04
  • I presume its 32,000-100,00? Can you tell us why the range and what it depends on? Again, I presume it is how high the Sun is in the sky. – ProfRob Feb 27 '15 at 18:20
  • @RobJeffries - I made some updates. And thanks for catching the typo. – Floris Feb 27 '15 at 18:51
  • Hi Floris, this doesn't seem to make sense. When you look 'at the world' (i.e. to the horizon) on a clear day, unlesss the sun is at a very low angle, the surface of your eye is not illuminated directly by the sun, only reflected sunlight from the atmosphere, ground and objects. The 32,000-100,000 lux figure is referring to a horizontal surface illuminated directly by the sun. Surely then, the only way the surface of your eye could be illuminated by 32,000+ lux would be if you looked straight up at the sky so that the surface of your face/eye was horizontal? – Felixs Jul 19 '15 at 09:18
  • Otherwise, what would the lux value be on your eye/face if you did do that? Presumably much higher. The wikipedia article (and others) give the value for 'full daylight' (clear day but no direct sunlight) as 10,000-25,000 lux. Is that not the actual lux value roughly on the surface of the eye, when we look out 'at the world'? If not, then what conditions and surface are they referring to when they say 'full daylight'? Thanks for your time. – Felixs Jul 19 '15 at 09:19
  • @Felixs yes this is for normal illumination- that is how you usually define the strength of a light source. If you have a non-normal surface you need to account for that - but even if you are taking a photograph, that effect is expected and necessary to see the contours of an object. There is a $\cos\theta$ term that you can add to account for the deviation from normal incidence. – Floris Jul 19 '15 at 10:44
  • So our eye only recieves a maximum of 25,000 lux outside on a clear day when we look normally? You seemed to imply to Jim Tim that we could recieve 100,000 lux when looking normally. – Felixs Jul 19 '15 at 13:16
  • "The eye receives" - not sure what you mean. Lux describes the illumination of a scene - when the sun is beating straight down on a cloudless day the illumination can be 100,000 lux. Most of the time it will be less - sun at an angle, etc. – Floris Jul 19 '15 at 13:18
  • I should have said 'the lux value on the surface of our eye' will not exceed 25,000 lux on a clear day (even when the sun is right overhead), unless we look at the right into the sun. Is that correct? – Felixs Jul 19 '15 at 13:22
  • @Felixs I don't know where you get your factor 4 from. It will be less than 100,000 but why do you drop it all the way to 25,000? There is no justification for that. The exact value is a function of many variables. It is hard to just pick a number. – Floris Jul 19 '15 at 13:38
  • Because the surface of your eye is shaded from direct sunlight by your forehead. Look at the wikipedia article for 'lux' or 'daylight' - "Shade illuminated by entire clear blue sky, midday: 20,000 lux". Or here: http://stjarnhimlen.se/comp/radfaq.html#10 "Full daylight (not direct sun): 10000-25000 lux" It seems for a surface to be illuminated by more than 25,000 lux, direct sunlight (no shade) is needed. Thanks for your reply. – Felixs Jul 19 '15 at 15:35
  • I think I finally understand the confusion we were having. Yes if your eyes are shaded and receive only indirect light they will get much less than 100,000 - the value of 20,000 for shade seems reasonable, since in photography you usually shoot 2-3 stops faster in the shade (4-8x less). – Floris Jul 19 '15 at 15:53
  • Thanks for clearing this up. I should have been more clear. – Felixs Jul 19 '15 at 17:39
  • Re: your light bulb example, assuming a 2 kW light bulb gives about 32 klm, this would typically be much less than 32 klx at 1 m distance. (Assuming uniform emission in all directions, the factor is 1/4π.) – Ulrich Stern Dec 14 '15 at 20:44
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The total light emitted by a body, be it the sun or a fluorescent tube or an LED is measured in lumens. If you place a 1m² surface near the emitter, some of the light will fall on the surface; if 1000 lumens hit the surface, we say it receives 1000lux. If you move the surface twice as far away (assuming it is a point source emitter), it will receive 1/4 of the lumens *so will be illuminated to 250 lux.

The apparent brightness of the emitter is a function of the amount of light being emitted in relation to the apparent area of the emitter which is why even a small LED torch is painfully bright when you look directly at it but may provide very poor illumination of something you aim it at (very small area of LED with relatively few lumens being emitted)

*surface area of a sphere is PIx r²;double the radius >> same number of lumens passing through 4 x the area.

Brian
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If you want your eyes to receive 32000~100000lux, you need to stare at the sun in a sunny day.If you stare at the ground in a sun-exposure area, your eyes receives much less than 32000 lux. If you stare at a shedded ground in a sunny day, your eyes receive slight less than 10000lux. human eyes could not tolerate 32000lux more than 5 secounds

user100027
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  • Your answer seems to contradict Floris's answer and contains some unverified statements (e.g. human eyes could not tolerate 32000lux more than 5 seconds). – Kyle Kanos Nov 30 '15 at 11:42
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In term of nits (candelas per meter squared; $\rm cd/m^{2}$) direct sunlight can exceed the range of my photometer which is 999,999 $\rm cd/m^2$. This is if you point it directly at the sun at mid-day on 1st November, which was at the time of shooting, a clear day.

Kyle Kanos
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Dave Kane
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