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1.4: Radiant Intensity, I

Not all bodies radiate isotropically, and a word is needed to describe how much energy is radiated in different directions. One can imagine, for example, that a rapidly-rotating star might be nonspherical in shape, and will not radiate isotropically. The intensity of a source towards a particular direction specified by spherical coordinates (\(\theta\) , \(\phi\)) is the radiant flux radiated per unit solid angle in that direction. It is expressed in \(\text{W sr}^{-1}\), and the standard symbol is \(I\). In astronomical custom, the word "intensity" and the symbol \(I\) are commonly used to describe a very different concept, to which we shall return later.

When dealing with visible radiation, we use the phrase luminous intensity rather than radiant intensity, and the unit is a lumen per steradian, or a candela. At one time, the standard of luminous intensity was taken to be that of a candle of defined design, though the present-day candela (which is one of the fundamental units of the SI system of units) has a different and more precise definition, to be described in section 1.12. The candela and the old standard candle are of roughly the same luminous intensity.