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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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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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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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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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