Blue and green connector, the difference is 8°

The main difference between APC and UPC connectors is at the end face of the connector, where the fiber enters inside the ferrule. APC connectors have fiber and connector that are polished at an angle of eight degrees. UPC connectors are polished without an angle. However, the UPC connectors are not exactly straight, but have a slight curvature for better alignment of the core. Another more obvious difference is the colour. The UPC connectors are blue, and the APC connectors are green.

The reflected light from the UPC connectors is reflected directly back towards the light source. The 8° angle on the end face of the APC connectors causes reflected light to reflect towards the fiber cladding and partially escape from the fiber. This causes some differences in return loss, which is a measurement of reflected light. It’s expressed as a negative dB value. Industry standards recommend that the return loss of an UPC connector should be -50 dB or higher and return loss for an APC connector should be -60 dB or higher.

Return loss is not the same as insertion loss. Insertion loss is the amount of power lost from transmitter to receiver. Losses occur from splices, connectors, bends and from cable length. UPC connectors tend to have better insertion loss properties than APC connectors. But the difference these days is starting to be insignificant.  

Other things to consider

Some devices are more sensitive to return loss than others and require APC connectors. Reflected light affects  signals in the longer wavelength ranges especially. Like for RF video signals (1550 nm), reflected light can adversely affect the signal. APC connectors are also commonly used in passive optical applications (both GPONs and XGS-PONs), as many of these systems also use RF signals to deliver video and in XGS-PON even longer wavelengths (1577 nm) to transmit data to the client. Even faster passive optical networks (NGPON2) and other WDM applications that use higher wavelengths over single mode fiber are also likely to require lower return loss generated by APC connectors.

It should also be noted that APC connectors are most often used in splitters. When using APC connectors, the connectors do not need to be connected to each other and still the return loss remains less than -50 dB. If the splitters have UPC connectors, then about -14.5 dB will be reflected from the unconnected connector.