EN 171 covers filters protecting against infrared radiation. They are required wherever there is incandescent material: metal casting, glass furnaces, ceramic kilns, forging.
Marking
An IR filter is marked with the prefix 4, a hyphen and a shade number from 1.2 to 10 — for example 4-5. The number should match the temperature of the source and the working distance from it.
Many IR filters also carry the R symbol, meaning the lens has enhanced infrared reflectance. That keeps the lens itself cooler and makes long exposures more comfortable.