The filter is a hafnium oxide interference filter! There are some absorption-based filters being studied in academic settings (as they'd be potentially cheaper to manufacture) but none are used commercially as far as I know

Honestly there aren't that many commercially available unfiltered KrCl lamps out there. I'm only aware of one and it's stupid expensive. Every other lamp you can actually buy is filtered, although some filters are a bit worse than others (though still pretty safe). Any lamp module that uses the Ushio Care222 emitter is certainly very well filtered because Ushio integrates the filter into the module.