It's just that their aboutness can't be translated into non-visual terms.
In the heating of solid bodies to incandescence, this non-visual emission is the necessary basis of the visual.
early 15c., "coming from the eye or sight" (as a beam of light), from Late Latin visualis "of sight," from Latin visus "sight," from visus, past participle of videre "to see" (see vision). Meaning "relating to vision" is first attested c.1600. The noun meaning "photographic film or other visual display" is first recorded 1951.
visual vi·su·al (vĭzh'ōō-əl)
Of or relating to the sense of sight.
Seen or able to be seen by the eye; visible.