**Units of Wavenumbers**

Yet a third spectral unit,
commonly used in spectroscopy, is wavenumber, the number of waves per cm: σ = ν* */100*c* cm^{-1}. Converting (1) to these
units gives

.

(13)

Again, the peak is where the derivative with respect to wavenumber vanishes:

so

. (14)

The peak value is

. (15)

The spectral photon radiance is found by dividing *L*_{σ}
by the energy of a photon, 100*hc*σ* *:

. (16)

We next find the wavenumber at the peak of the spectral photon radiance:

and . (17)

The peak spectral photon radiance is

. (18)

Fig. 3 shows plots of *L*_{σ} and *L _{σ}*

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**Fig. 3** - Spectral
radiance, *L** _{σ }*, (

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