Admitting light so that objects beyond can be seen
Free from what obscures or dims
Clear to the sight
Clear to the understanding
It was a lucid dream; half awake, half asleep.
Lucid explanations have been most helpful to my understanding.
The poet was reading lucid prose.
He seems to have been an admirable teacher, with a great power of lucid exposition.
The main points remained lucid, straightforward, and well worth listening to.