>>3654283It's not about thermal or not, thermal cameras don't have high enough resolution plus their sensitivity is really far into the infrared.
Dyes used in clothes (as well as the material itself), have different absorption in infrared vs visible light. A very dark dye which appears black to visible light, can be transparent to infrared, even near infrared. For instance a 720nm filter itself: it's opaque when you look through it, but it's like a clear filter when an IR camera looks through it.
Thin cotton shirts are another example.
