Perhaps I can explain:  That's starting to be a lot of conversion functions, and every time this equivalency is used those functions have to be redefined with the outer function is called.  I'm not sure how aggressively Python caches those.  Right now it's not a big deal and I'm sort of off in premature optimization land, but it might make sense to define all those converters outside the actual `spectral_density` function.  And I was thinking, in order to keep things organized, might as well make it a class.  Something like:

```
class SpectralDensityEquivalencies(object):
    @staticmethod
    def converter_photlam_fla(x):
        return ...
    etc...

    def __call__(self):
        return [ equivalencies list...]

spectral_density = SpectralDensityEquivalencies()  # Basically a singleton
```