You don't just "buy" a badge; you typically subscribe to a monitoring service. These providers send fresh badges monthly or quarterly and process the old ones in accredited labs. Film & tld badges - personnel monitoring - AEIL
: Unlike some digital methods, the physical film provides a long-term, verifiable record of exposure.
: Built-in metal filters (like lead or aluminum) allow the lab to distinguish between different types and energies of radiation, such as beta vs. gamma.