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Why is intensity $I$ on a graph of the solar spectrum always showed in units of $[\mathrm{W/m^2/nm}]$ instead of simply $[\mathrm{W/m^2}]$? (The y-axis on the graph.)

It is apparently shown as intensity per wavelength, but why add this extra specification? For me it just complicates matters (it is not clear to see at which wavelength the intensity is greatest e.g.), so what is the point?

enter image description here

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    $\begingroup$ This is a lousy diagram. That 5250° C reference curve in that wikimedia diagram is unsourced and is wrong. A better value is 5778 K, or 5505° C. That's wikipedia for you. Sometimes it's great, other times it's lousy. You never know which is which unless you already know the subject matter. $\endgroup$ – David Hammen Dec 12 '14 at 19:55
  • $\begingroup$ Luckily that is not the point of this question. $\endgroup$ – Steeven Dec 12 '14 at 20:29
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    $\begingroup$ Something to ponder: what would the area under the curve represent in the two cases? (Hint: only one case makes any sense.) $\endgroup$ – user10851 Dec 13 '14 at 2:46
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Intensity has units of watts per area: $$ \left[I\right]=\rm\frac{W}{ m^2} $$ where the area is the surface area of the emitting source (in this case, the sun). This tells you the total amount of radiation present (over all wavelengths).

The extra factor of 1/nm in your plot gives the spectral irradiance: $$ \left[\mathcal E\right]=\rm \frac{W}{m^2\,nm} $$ This tells you the intensity at each particular wavelength.

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  • $\begingroup$ If the y-axis simply was intensity $[W/m^2]$ then this graph would show intensity of each wavelenght, wouldn't it? $\endgroup$ – Steeven Dec 12 '14 at 19:50
  • $\begingroup$ $I$ is basically the sum of $\mathcal{E}$ over all wavelengths, which means $I$ is not a function of $\lambda$. Thus, plotting something that's not a function of wavelength doesn't make a whole lot of sense to me. $\endgroup$ – Kyle Kanos Dec 12 '14 at 20:48
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$W/m^2$ would be the total energy emitted, regardless of wavelength. When you use $W/m^2/nm$ you are explicitly saying that it corresponds to a specific part of the spectrum (nm is a unit of wavelength). Which is what the graph you posted is showing. The first one is called "irradiance", the one plotted here is called "spectral irradiance". For more details you can see here.

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