Getting the "field length norms"

In ES documentation its mentioned that scripts can access a a variety of information like "field length norms".

The script_score function provides enormous flexibility. Within a script, you have access to the fields of the document, to the current _score, and even to the term frequencies, inverse document frequencies, and field length norms (see Text scoring in scripts).

I could not find any docs mentioning how exactly to get the value for it.

Can this value be accessed like, say, "_index['FIELD'].sumttf()" ?