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Homework Help: Radiant Intensity vs. Irradiance

  1. Jan 16, 2013 #1


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    1. The problem statement, all variables and given/known data

    I am having a hard time discerning the difference between radiant intensity and irradiance. I searched PF and found this distinction, but am still confused.

    Radiance is the power from the source per area into a certain solid angle.
    Irradiance is the power onto a surface per area

    2. Relevant equations

    Radiant intensity (Ie) is the output power per steradian, specified in mW/sr.

    Irradiance (E) is the power per unit area at a given distance from a light source, specified in mW/cm2.

    3. The attempt at a solution

    Perhaps my confusion lies in the definition of a steradian.

    I know that a radian is an 2-dimensional angular measurement representing the angle "cut out" of a circle when a length of its subtended arc is equal to the radius.

    A steradian cuts out an area of a sphere equal to r2.

    So, it appears that radiant intensity and irradiance both measure an amount of electromagnetic power per given area. Where is the difference in the two terms?
  2. jcsd
  3. Jan 16, 2013 #2
    Let a source of light give 1mW/sr.
    The Irradiance at 1cm away is 1mW/cm2
    At 2 cm away it is 0.25mW/cm2
    At 10cm away it is 0.01mW/cm2

    This is because area subtended by solid angle = solid angle in steradians x distance squared.
  4. Jan 16, 2013 #3


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    Your example helps make perfect sense of the subject. Thank you!
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