From <span class="searchmatch">subvisual</span> + -ly. <span class="searchmatch">subvisually</span> (not comparable) Below the threshold of vision....
From sub- + visual. <span class="searchmatch">subvisual</span> (comparative more <span class="searchmatch">subvisual</span>, superlative most <span class="searchmatch">subvisual</span>) That is too dim to be seen unaided....