TT #174 · Parshas Tazria · Aliyah 4 · Passuk 25
English: the priest shall examine it. If the hair in the bright spot has turned white and its appearance is deeper than the skin, it is tzaraas that has broken out in the burn; the priest shall pronounce him unclean.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:25
Shevuos 6b.
E-Daf — Shevuos 6b · Shas.org · Sefaria — Shevuos 6b
English: An appearance is perceived as deep by the visual contrast of sunlight against shade.
Textual derivation
The apparent depth of the lesion is judged by the visual contrast described by the Sages.