TT #159 · Parshas Tazria · Aliyah 3 · Passuk 21
English: If the priest examines it and there is no white hair in it, and it is not lower than the skin but is faded, he shall isolate the person for seven days.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:21
Torah Temimah cites Sifra (Toras Kohanim) for the requirement to see the entire lesion at one time.
Sifra — Vayikra 13, Midrash Sefer
English: The priest must see the whole lesion at once.
Textual derivation
The priest must view the entire lesion together, not assess separated parts at different times.