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# Imagine two radii shooting out from the center

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Unformatted text preview: llow-green). It is measured in lumens. Imagine a light source in the center of a sphere. Luminous flux is the quantity of light that hits the surface of the sphere per unit time. The size of the sphere is irrelevant. If the sphere were larger, the same quantity of light would reach the surface every second, so the flux wouldn’t change. However, this light would be more spread out, so the illuminance of the surface would be less than it was with the same candle in the smaller sphere. Also called illumination, the symbol for illuminance is E, not to be confused with energy, and is defined as luminous flux per unit of surface area: E = Φ / S. The SI unit for illuminance is the lux, which is a lumen per square meter. The illuminance of the sun is about 100,000 lx (lux); for the full moon it’s about 0.2 lx. A common, non-SI unit for illuminance is the foot-candle, which is equivalent to about 10.8 lx. Illuminance vs. Distance A point source at P radiates light in all directions. The pic below shows how light spreads out as it radiates. If the illuminance on the sheet 1 m from P is 1 unit, the...
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