and 1 dB/km at 1300 nm. Singlemode loses 0.35 dB/km at 1310 nm and 0.25 dB/km at 1550 nm. Very high quality singlemode fiber intended for long distance applications is specified at a loss of 0.19 dB/km at 1550 nm. Plastic optical fiber (POF) loses much more: 1 dB/m at 650 nm. POF is large core (about 1 mm) fiber suitable only for short, low speed networks such as TOSLINK optical audio or for use within cars. Each connection between cables adds about 0.6 dB of average loss, and each joint (splice) adds about 0.1 dB. Invisible infrared light (750 nm and larger) is used in commercial glass fiber communications because it has lower attenuation in such materials than visible light. However, the glass fibers will transmit visible light somewhat, which is convenient for simple testing of the fibers without requiring expensive equipment. Splices can be inspected visually, and adjusted for minimal light leakage at the joint, which maximizes light transmission between the ends of the fibers being joined. The charts Understanding wavelengths in fiber optics and Optical power loss (attenuation) in fiber illustrate the relationship of visible light to the infrared frequencies used, and show the absorption water bands between 850, 1300 and 1550 nm. Safety The infrared light used in telecommunications cannot be seen, so there is a potential laser safety hazard to technicians. The eye's natural defense against sudden exposure to bright light is the blink reflex, which is not triggered by infrared sources. In some cases the power levels are high enough to damage eyes, particularly when lenses or microscopes are used to inspect fibers that are emitting invisible infrared light. Inspection microscopes with optical safety filters are available to guard against this. More recently indirect viewing aids are used, which can comprise a camera mounted within a handheld device, which has an opening for the connectorized fiber and a USB output for connection to a display device such as a laptop. This makes the activity of looking for damage or dirt on the connector face much safer. Small glass fragments can also be a problem if they get under someone's skin, so care is needed to ensure that fragments produced when cleaving fiber are properly collected and disposed of appropriately. Hybrid cables There are hybrid optical and electrical cables that are used in wireless outdoor Fiber To The Antenna (FTTA) applications. In these cables, the