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I How does the luminosity equation work?

  1. Feb 1, 2017 #1
    I've read that L = 4 x Pi x b x d2
    Where L = Luminosity (W)
    d = distance (m)
    b = brightness (W m-2)

    But in a real life applied situation, how does it work, how does one measure the distance and its brightness? And what does the 4Pi do?
     
  2. jcsd
  3. Feb 1, 2017 #2

    berkeman

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    Staff: Mentor

    Hint -- what is the surface area of a sphere in terms of its radius? :smile:
     
  4. Feb 1, 2017 #3

    Drakkith

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    The distance can be measured with a variety of range finding devices. You can probably do a google search and find plenty. The brightness can be measured using a camera, photo-diode, or other device.

    Also, notice that if you move everything on the right over to the left except for brightness, you then get the well known inverse square law.
     
  5. Feb 1, 2017 #4
    4/3 x πr3.

    (I really wish you could easily do fractions)
     
  6. Feb 1, 2017 #5

    berkeman

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    No, the surface area increases as r^2. Try again?

    And you can use LaTeX to do fractions... https://www.physicsforums.com/help/latexhelp/
     
  7. Feb 1, 2017 #6
    Oh, thought you meant volume, I saw you said surface area, then thought of volume for some reason. Don't ask why, the brain is weird. So, its the same as that, but instead of r2, its bd2.
     
  8. Feb 1, 2017 #7

    berkeman

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    So how does the brightness fall off with distance, and why?
     
  9. Feb 1, 2017 #8
    What do you mean by fall off with distance?
     
  10. Feb 1, 2017 #9

    berkeman

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    As you get farther and farther from the light source, it appears dimmer and dimmer, right?

    Maybe do a search on the hint that Drakkith gave you... (actually he even gave you a link to the inverse square law)...
     
  11. Feb 1, 2017 #10
    Oh, I meant a rephrasing, but I get what you mean.
     
  12. Feb 1, 2017 #11

    Drakkith

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    The distance (d) is the same thing as the radius (r) since the object is considered to be at the center of a sphere and the observer (you or an instrument) is some distance from the center. You can write bd2 or br2.
     
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