Can sunlight affect freespace optical systems?

Due to the equipment operating in the near Infra red spectrum then yes it is possible for them to be affected by strong sunlight. However this would only be possible if the Infra red heads were mounted in such a manner whereby the sun could be directly behind one of the heads, e.g. if equipment was installed in the UK in an East or West orientation with one head on a low building (one or two storeys high) and the opposing end was on the top of a high building (ten or twelve storeys) then it is theoretically possible for the sun to be directly behind the head twice a year, start and end. The affect this would have would be that the natural IR signal generated by the sun would be received at the lower head and would give the impression of "cross talk". The sun, however, is only in this position for a period of about 20 minutes, after this time the heads would restart communications as normal.

As stated this is a theoretical situation. If the units are to be installed in an orientation where it may occur it is easy to install the higher head in a position whereby the sun cannot be directly behind it e.g. on the side of a wall of a roof top plant room instead of the top. In addition for the situation to occur you must also have a clear blue sky.

Another theoretical situation that could cause outage, is scintillation (heat haze). If freespace optical units are mounted in a situation whereby they are operating over an expanse of tarmac, or a surface prone to scintillation, the distance to be achieved is in excess of 500 metres and the height from that surface to the central part of the transmission signal is less than 20 feet then the transmission signal could suffer from an amount of refraction which would be enough to interrupt the signal. Again this is a theoretical situation and one which can be overcome at the time of assessment for an installation.