We've defined what flux and luminosity mean, but there is one point
where it can get a little confusing.  If you want to measure
the amount of energy that goes onto a particular object from a light
source, you multiply the flux from that light source by the area of the
object.  The answer is in Watts, but it's not the luminosity of the
light source; it's the luminosity incident on the object!  Don't mix
up these two different "luminosities."

   e.g. If you're looking at a candle flame and its flux on your eye
        is 0.01 W/m^2, and your dilated pupil has an area of about 
        0.000025 m^2, then the luminosity that gets into your eye is

        L = F * (area) = 0.01 W/m^2 * 0.000025 m^2 = 2.5 * 10^(-7) W.

   e.g. If that candle is one meter away from your eye, its luminosity is

        L = 0.01 W/m^2 * 4 * pi * (1 m)^2 = 0.126 W.


REMEMBER THIS: to help keep the concepts of luminosity and flux clear,
keep in mind that 

   (1) the LUMINOSITY of an object doesn't change no matter how 
       far away you are from it; luminosity is a property of the object.

   (2) the FLUX onto an object doesn't change no matter how big it is;
       flux is a property of distance from the luminous source.