underexpose has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is overexpose.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
expose to too little light
“The film is underexposed, so the image is very dark”
expose insufficiently
“The child was underexposed to language”
Synonyms, antonyms and the broader and narrower relations for underexpose come from Princeton University’s WordNet. Full sources and licences.