Let us is used in all varieties of speech and writing to introduce a suggestion or a request: Let us consider all the facts before deciding. The contracted form let's occurs mostly in informal speech and writing: Let's go. Let's not think about that right now. Perhaps because let's has come to be felt as a word in its own right rather than as the contraction of let us, it is often followed in informal speech and writing by redundant or appositional pronouns: Let's us plan a picnic. Let's you and I (or me ) get together tomorrow. Both Let's you and me and Let's you and I occur in the relaxed speech of educated speakers. The former conforms to the traditional rules of grammar; the latter, nonetheless, occurs more frequently. See also leave¹.

Did you mean lethality?