The 55 mm URPRO Cyan (CY) filter utilizes URPRO's patented color-correcting technology. This filter is ideally suited for tropical blue salt water or clear fresh water at depths of 12 ft to 60 ft. We recommend the URPRO CY filter to divers traveling to a tropical island (i.e. Belize, Cozumel, Caribbean, Maldives, Bora Bora, etc.). All URPRO filters sold by SRP feature an ultra low-profile filter ring in order to limit mechanical vignetting. As a result, this filter can not receive other 55 mm filters - it has no female receiving threads.