Sinotech Gem Identification Tools Chelsea Filter Cf-01 This Filter Is Designed To Transmit Only Deep Red And Yellow-green Light (Best Obtained Under Strong Electric Light) This filter is Designed to transmit only deep red and yellow-green light (best obtained under strong electric light). Just hold the filter close to your eye and look at the stones. Multiple stones can be viewed at once. It is helpful with identifying certain red, blue and green stones.