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Luminous flux problem

  1. Jun 25, 2006 #1
    Please help me with this problem.
    # An isotropic point source of 100 candela is fastened to the ceiling of a room. What is the total luminous flux falling on all the walls and floor?
    I solved it in the following way:
    Here Luminous intensity(I) = 100 cd, Total solid angle(w) = 2 (pi) steradian
    Total luminous flux falling on all the walls and floor = I x w
    = 100 x 2(pi)
    = 100 x 2 x 3.14
    = 628 lumens
    Is it right?
  2. jcsd
  3. Jun 25, 2006 #2

    Andrew Mason

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    The walls and floor subtend a solid angle of [itex]\pi[/itex], not [itex]2\pi[/itex].

  4. Jun 26, 2006 #3
    I thought the walls and the floor subtended a hemispherical solid angle at the ceiling. Could you please explain how the solid angle is (pi)? :confused:
  5. Jun 26, 2006 #4


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    How many radians in half a circle?
  6. Jun 26, 2006 #5
    There are (pi) radians in a half circle. But I think there are 4(pi) steradians in a sphere. So there are 2(pi) steradians in a hemisphere.
    Solid angle(w) = Area of hemisphere/R^2
    = [2(pi)R^2]/R^2
    = 2(pi) steradians

    I think radian is a 2 dimensional unit whereas steradian is a 3 dimensional unit of angle. I may be wrong.
    Last edited: Jun 26, 2006
  7. Jun 26, 2006 #6

    Andrew Mason

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    No. You are right. A sphere subtends [itex]4\pi[/itex] steridians, so the hemisphere is [itex]2\pi[/itex]. Sorry about confusing you.

  8. Jun 27, 2006 #7
    That's ok. Thanks.
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