Radiation resistant, radiation tolerant, rad hard or non-browning lenses are all terms used to refer to a lens designed for use in a radioactive environment. When exposed to Gamma or X-ray radiation most optical materials will change colour, typically dark brown, resulting in almost total loss of transmission. A new tech note provides an informative introduction to this topic and discusses how lenses made from cerium doped glasses can maintain throughput in environments subject to radiation even over long periods of time.
Read tech note: www.resolveoptics.com/why-use-cerium-doped-glasses/