Krashen wrote:
“Input is transparent if the acquirer understands every word. This is, of course, a crude definition. We could definite extreme transparency as a conscious understanding of not only every word but every grammatical marker and morpheme.
“Transparent is not the same as “comprehensible.” If input is transparent, it is comprehensible, but input can be comprehensible and not fully transparent, that is, it could contain some as yet unacquired language that does not interfere with comprehension (“noise”).”