You need to add signals first.

来源: 于 2007-02-22 10:38:53 [博客] [旧帖] [给我悄悄话] 本文已被阅读：次

When you superimpose photos, the noise do not add up, but cancel out partially, so that
1/sqrt(9) + 1/sqrt(9) ＋ 1/sqrt(9) = 1 is invalid.

More procisely,

(total noise)^2 = sum of (individual noise)^2