Lucid

1. clear; easily understood
2. mentally rational; sane
3. bright, luminous, translucent or transparent

(easily understood): clear, perspicuous, straightforward; comprehensible

(mentally rational): coherent, sane

(bright): brilliant, light

(luminous): glowing, radiant; See also Thesaurus:shining

(transparent): clear, pellucid, see-through, transparent; See also Thesaurus:transparent or Thesaurus:translucent