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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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This note was uploaded on 11/17/2011 for the course PHYS 121 taught by Professor Burgeson during the Fall '11 term at BYU.
- Fall '11