Features: Inside diameter 16 mm, outside diameter 18 mm.thread is M16. Optical Power Meter FCAdapter Dust Cap ,Metal Cap Metal Chain. Red light Pen Metal for Head . Suitable for optical power meter probe is the kind of large.Take care and your meter contrast. brand new and high quality Optical power meter light source SC-ST-FC converter for head. Specifications: Product name: Optical fiber meter adapter Material: zinc alloy Features: good repeatability, strong stability Installation inner diameter: 8mm Note: Please allow 0-1cm error due to manual measurement. pls make sure you do not mind before you bid. Due to the difference between different monitors, the picture may not reflect the actual color of the item. Thank you Package includes: 1 x SC-ST-FC Optical Power Meter