External Appearance, Outward Appearance, Surface
noun
The outside face to something is the external appearance of that thing. While 外面 can refer to the outer surface of an object, it’s more commonly used to describe someone's outward appearance, behavior, or demeanor on the surface.
がいめん, そとづら
This is a jukugo word, which usually means on'yomi readings from the kanji, so this is がいめん. But note that this can also use the kun’yomi readings, which makes it そとづら. While がいめん is neutral, そとづら is a bit negative, often implying that the person's outward appearance might be deceptive, hiding their true nature.
彼女の外面にだまされない方がいいですよ。
You shouldn’t be fooled by her outward appearance.
彼は外面が良く、人前ではいつも笑顔を見せています。
He has a good outward appearance and always smiles in public.
このメガネは、レンズの外面のカーブをフラットにしているので、周辺部のボヤケやユガミが少ないんです。
These glasses have a flat curvature on the outer surface of the lens, so there's less distortion and glare in the peripheral areas.